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afterburner
2006-Feb-08, 04:53 PM
ok, first of all...pardon my stupidity....:wall:
what i want to know is how this time difference happens

lets just say that we have two simple digital clocks(shows time, month,year), that run on AA batteries...now one stays on earth...and the other one goes on a trip aboard a space ship with a few astronauts...lets also say that scientists plan for the ship to travel for half a year in one direction (at the speed of light, or close to)...and half a year back(at the same speed)...now....the astronauts know they are going to be travelling for a total of one year...and we on earth know that they are going to come back in one year..so how in the world is one clock thats on the ship going to show a different time, when both clocks have been ticking for exactly one year? hope i made sence in explaining what i meant..and didnt get...

Argos
2006-Feb-08, 05:06 PM
>>so how in the world is one clock thats on the ship going to show a different time, when both clocks have been ticking for exactly one year?

Because of time dilation, which is a natural property of the universe. This property becomes noticeable when you approach the speed of light. It is described in the Theory of Relativity. Further info (http://casa.colorado.edu/~ajsh/sr/time.html).

Disinfo Agent
2006-Feb-08, 05:20 PM
ok, first of all...pardon my stupidity....:wall:
what i want to know is how this time difference happens

lets just say that we have two simple digital clocks(shows time, month,year), that run on AA batteries...now one stays on earth...and the other one goes on a trip aboard a space ship with a few astronauts...lets also say that scientists plan for the ship to travel for half a year in one direction (at the speed of light, or close to)...and half a year back(at the same speed)...now....the astronauts know they are going to be travelling for a total of one year...and we on earth know that they are going to come back in one year..so how in the world is one clock thats on the ship going to show a different time, when both clocks have been ticking for exactly one year? hope i made sence in explaining what i meant..and didnt get...The bolded sentence is where you're making a mistaken assumption. We do not know that the astronauts will be back in one of our years. In fact, according to Einstein's theory of special relativity, what we do know is that it will take longer than a year before we see the astronauts back on Earth. There's a formula (http://www.thebigview.com/spacetime/index.html) that gives the amount of time actually elapsed for observers on Earth.
This is why we call it relativity: the rate of passage of time is not absolute. It may be different for different observers.

the_bullet
2006-Feb-08, 05:36 PM
This Link (http://www.btinternet.com/~j.doyle/SR/sr7/sr7.htm) using a bouncing beam of light, shows that with a constant speed of light, time dilation will occur.

afterburner
2006-Feb-08, 06:16 PM
This Link (http://www.btinternet.com/~j.doyle/SR/sr7/sr7.htm) using a bouncing beam of light, shows that with a constant speed of light, time dilation will occur.

i have one major problem with that site and the experiment in general....althought it does APPEAR that light follows this triangular path...it doesnt ACTUALLY follow the path of that triangle...what it ACTUALLY does is go up and down between the mirrors. is this not true?

Kaptain K
2006-Feb-08, 06:47 PM
It is relative to your frame of reference. To a co-moving observer, it goes straight up and down. To a "stationary" observer, it follows the zig-zag path. That is the point of relativity. What you see (measure) depends on your frame of reference!

afterburner
2006-Feb-08, 07:08 PM
It is relative to your frame of reference. To a co-moving observer, it goes straight up and down. To a "stationary" observer, it follows the zig-zag path. That is the point of relativity. What you see (measure) depends on your frame of reference!

so does this mean that when the experiment starts the clocks (time) are the same...for the duration of the experiment our perception of the moving clock is that it slows down(when in reality it doesnt)... but after the experiment there should be no difference in time, since, like you said, it is only our(the observers) perception of what happens...its not what actually happens right? or wrong again?

i guess what im really saying is that when we synchronize our clocks and send one off, when it comes back...it still should show the same time...becuase it was only our perception that the time aboard the ship was moving slower...so when the ship slows back down and we check our clocks..what seemed like a slowdown in time actually wasnt.....please correct me if im wrong because this is killing me...oh and suppose we use normal clocks like i said in the original post, not

Disinfo Agent
2006-Feb-08, 07:14 PM
Relativity is not a matter of perception. You start with synchronized clocks, and end up with dissynchronized clocks, even though you don't notice any change in the clock near you during the experiment.

afterburner
2006-Feb-08, 08:02 PM
Relativity is not a matter of perception. You start with synchronized clocks, and end up with dissynchronized clocks, even though you don't notice any change in the clock near you during the experiment.

ok, consider this...distance / speed = time....right?
if here on earth we calculate that it should take one year for a round trip at the speed of light... why would the clocks prove us wrong?

The Supreme Canuck
2006-Feb-08, 08:29 PM
Because as you approach the speed of light (you can never reach it, by the way), things get strange. If we send out a mission that will return in one year from our point of view, the mission will take less than one year from the point of view of the mission's crew. This is because, from our point of view, the clocks (and everything else, including biological processes and thought) on the ship slow down. But, from the point of view of the crew, nothing changes.

ToSeek
2006-Feb-08, 08:30 PM
ok, consider this...distance / speed = time....right?
if here on earth we calculate that it should take one year for a round trip at the speed of light... why would the clocks prove us wrong?

If you're on the spaceship, the distance will appear shorter. The equation will continue to work just fine except that both the distance and the time will be less (in identical proportions) than what is measured from the Earth.

afterburner
2006-Feb-08, 10:13 PM
If you're on the spaceship, the distance will appear shorter. The equation will continue to work just fine except that both the distance and the time will be less (in identical proportions) than what is measured from the Earth.

ok this is actually the best answer i read so far (along with The Supreme Canuck) so thank you, however i still have a few more questions just to be sure

assume clock A is on earth and clock B is on the ship

1. when we compare clock A to clock B after the trip, they will show different times? true or false?

2. when we plan the one year trip, the astronauts will arrive on time based on clock A. so one year from departure time, again, based of clock A. true or false?

3. the astronauts on board will experience one year go by based on clock B. True or false?

4. now for the big question...so if both here on earth (clockA) and on the spaceship (clock B) one year goes by, but the clcoks show different times...where does the missing time go, or the missing distance? it just doesnt add up?

5. consider this experiment... (just a thought experiment)
">" - represents the ships position every second (93,000 miles aprox) as estimated from earth. (assuming its travelling close to the speed of light)
"_" - represents a series of sattelites that take pictures consecutively every second at the time that the spacesip is supposed to be at that location, again, as estimated from earth.
MY question is...will the sattelite capture the the spacecraft as its travelling through space assuming everything is timed perfectly ..if not please explain why

Earth I--------------Route-------------------------------I
/
O____>_____>_____>_____>_____>_____>_____>___________________
_ _ _ _ _ _ _

afterburner
2006-Feb-08, 10:15 PM
sorry...the little lines are supposed to be right under ">" <---this symbol

Disinfo Agent
2006-Feb-08, 10:28 PM
assume clock A is on earth and clock B is on the ship

1. when we compare clock A to clock B after the trip, they will show different times? true or false?True.

2. when we plan the one year trip, the astronauts will arrive on time based on clock A. so one year from departure time, again, based of clock A. true or false?If the ship travels away from Earth and back for a year (i.e., as measured by clock B), then when it arrives back on Earth more than one year will have elapsed, as measured by clock A.

3. the astronauts on board will experience one year go by based on clock B. True or false?True.

4. now for the big question...so if both here on earth (clockA) and on the spaceship (clock B) one year goes by [...]It does not. See above. Better yet, do some reading on the so-called Twins Paradox of relativity theory.

afterburner
2006-Feb-08, 10:36 PM
True.

It does not. .

well if clock B experiences one year..and clock A experiences one year..then the time does not add up...but anyways...im gonna read the twin paradox theory now....
oh and what about question 5?

Disinfo Agent
2006-Feb-08, 10:56 PM

I'm not the best person to answer question nr. 5, but my guess is that the pictures of the ship, taken by the satellites and sent to Earth, would not reach Earth at 1-second intervals. Rather, the time interval between pictures should be greater than 1 second, increasing above 1 s as the spaceship moves away from Earth, and decreasing back to 1 s as the ship comes back to Earth.

P.S. The paragraph above was wrong. See here (http://www.bautforum.com/showpost.php?p=678199&postcount=34).

afterburner
2006-Feb-08, 11:06 PM
It does add up, you just have to add it the right way (http://mathforum.org/library/drmath/view/56328.html). :)

I'm not the best person to answer question nr. 5, but my guess is that the pictures of the ship, taken by the satellites and sent to Earth, would not reach Earth at 1-second intervals. Rather, the time interval between pictures should be greater than 1 second, increasing above 1 s as the spaceship moves away from Earth, and decreasing back to 1 s as the ship comes back to Earth.

if what you're saying is true..and the sattelites DO capture the spaceship at the time that was predicted on Earth, does that not mean that the time dilation does not happen?

Disinfo Agent
2006-Feb-08, 11:08 PM
if what you're saying is true..and the sattelites DO capture the spaceship at the time that was predicted on Earth, does that not mean that the time dilation does not happen?What predicted time are you referring to?

afterburner
2006-Feb-08, 11:18 PM
What predicted time are you referring to?

5. consider this experiment... (just a thought experiment)
">" - represents the ships position every second (93,000 miles aprox) as estimated from earth. (assuming its travelling close to the speed of light)
this prediction...where we, from earth predict the spacecraft is going to be every second based on clock A
basically we know the speed, we know the direction...so we can estimate where the spacecraft is going to be every second, again, based on clock A

so if the sattelites that are programmed to take pictures, according to clock A, actually capture the spacecraft, then doesnt it mean that time moves the same on the craft and on earth?

"_" - represents a series of sattelites that take pictures consecutively every second at the time that the spacesip is supposed to be at that location, again, as estimated from earth.
MY question is...will the sattelite capture the the spacecraft as its travelling through space assuming everything is timed perfectly ..if not please explain why

Earth I--------------Route-------------------------------I
/
O____>_____>_____>_____>_____>_____>_____>________ ___________

Disinfo Agent
2006-Feb-08, 11:25 PM
To make a correct guess of where the ship will pass at a given instant, you need to use relativity theory, which assumes that time does not flow the same way for the ship and for Earth. So, I'm not sure what to say...

relativity of simultaneity (http://en.wikipedia.org/wiki/Relativity_of_simultaneity)

afterburner
2006-Feb-08, 11:33 PM
To make a correct guess of where the ship will pass at a given instant, you need to use relativity theory, which assumes that time does not flow the same way for the ship and for Earth. So, I'm not sure what to say...

relativity of simultaneity (http://en.wikipedia.org/wiki/Relativity_of_simultaneity)

sorry i dint specify...the prediction based on the basic distance / velocity = time formula ....so without using relativity knowing that the spaceship travels at c, or close to, and knowing how much it will travel in a second (clock A earth second)..you can predict where it should be in space, from earths perspective...

Dragon Star
2006-Feb-08, 11:44 PM
sorry i dint specify...the prediction based on the basic distance / velocity = time formula ....so without using relativity knowing that the spaceship travels at c, or close to, and knowing how much it will travel in a second (clock A earth second)..you can predict where it should be in space, from earths perspective...

It just doesn't work like that with that kind of speed, as said before, relativity=time not the same for A and B.

Argos
2006-Feb-09, 01:03 PM
so without using relativity knowing that the spaceship travels at c, or close to, and knowing how much it will travel in a second (clock A earth second)..you can predict where it should be in space, from earths perspective...

If you want correct results you have to employ the postulates of relativity in your calculations.

Nicholas_Bostaph
2006-Feb-09, 01:50 PM
sorry i dint specify...the prediction based on the basic distance / velocity = time formula ....so without using relativity knowing that the spaceship travels at c, or close to, and knowing how much it will travel in a second (clock A earth second)..you can predict where it should be in space, from earths perspective...

I'm by no means an expert, but let me take a shot at this. Someone please correct me if I'm wrong.

From Earth's viewpoint the ship will appear to pass another satellite every one second. However, Captain Cool of our imaginary ship, when looking out the front window, will see the satellites begin to 'squish' and appear to move closer together. When checking the ship's clock, he will see that he's actually passing another satellite every 3/4 of a second (or whatever the time dialation works out to be). So both Earth and the captain think that the ship is moving at x mph. Time is able to move slower for Captain Cool even though the speed remains consistent between the two reference frames because in his reference frame the space he is traversing has decreased.

Someone please verify that what I said was correct; I'm still learning myself.

Ken G
2006-Feb-09, 02:26 PM
Yes, that's quite right Nicholas. Captain Cool thinks she is passing the satellites in 3/4 of a second because they are close together, on Earth we'd say her clock reads 3/4 of a second, while the satellites read 1 second, because her clock is running slow. In response, Captain Cool says that the satellites take photos just as she passes them, and each reads a second later as she passes, but in fact they are running slow and they only take photos as she passes because they were not properly synchronized to begin with. They all read different times when she set out on her mission, even though on Earth we thought the satellite clocks were properly synchronized.

JohnW
2006-Feb-09, 04:30 PM
It's implied in the above two posts, but it's such an important point I think it needs to be spelled out: both time measurements are correct. There isn't a "true" time and a "perceived" time. And yes, I know it doesn't make sense (well, it does if you do the calculations, but it feels wrong). But these effects have been observed and, as far as we can tell, it's the way things work.

alainprice
2006-Feb-09, 05:05 PM
Gravity Probe A was able to measure the expected time dilation caused by the gravitational field of the Earth itself. It simply gained some altitude, the clock sped up, and then it came back down.

A quick and necessary proof that time is not absolute. If time is not absolute, yet the speed of light is, then distance is not absolute.

So a trip at 99.999% of c to an object 1/2 light year away(as measured from earth before departure), will NOT take 6 months. It would take days. Same for the return trip, it would take days.

Earth observers can take pictures of you, but you would never appear the same color. You would be redshifted as you travelled away, and blueshifted as you came closer.

If you left on your birthday, you would arrive back on your birthday the next year, yet you would actually only be roughly a week older, scratching your head as to why you have birthdays 7 days apart now.

NEOWatcher
2006-Feb-09, 05:21 PM
...If you left on your birthday, you would arrive back on your birthday the next year, yet you would actually only be roughly a week older, scratching your head as to why you have birthdays 7 days apart now.
Ooooh; I wish I were that baby being born on a ship going a little faster.
A birthday all year long.
http://www.cosgan.de/images/smilie/musik/n035.gif

Disinfo Agent
2006-Feb-09, 06:27 PM

Kristophe
2006-Feb-09, 06:46 PM
A good question for you to think about, afterburner, about absolute time:

How do you measure it? Whose clock do you determin to be the clock? When the space ship comes back, and the ship's clock and Earth's clock disagree, which one is wrong?

See, everything is moving relative to something else. We're crusing along at 30 km/s relative to the Sun, and the sun is motoring along at a similar speed relative to the neighbouring stars. If everyone's clock is running slow compared to everyone else's, how do you decide whose is going at the right speed?

The question posed by relativity is: What does it mean for two events to be simultaneous? How do you determine this? If Johnny and Jimmy are 300,000 km apart, and they shine flashlights at each other, how do they determine whether they're turning them on at the same time?

They could get Sally to stand between them, 150,000 km away from each, to say whether she saw each beam at exactly the same time. That seems like a good way to make the determination, right? But what about Sue, who is 600,000 km away from Johnny, but is 670820 km away from Jimmy? She'll see Johnny's beam almost a full quarter of a second before Jimmy's. Sally says that the boys are turning the flashlights on at exactly the same time, but Sue claims that Jimmy is lagging behind. Who's right? More importantly, who's wrong, and why?

You really have to consider these questions before you move on to time dilation and length contraction.

Argos
2006-Feb-09, 07:48 PM
Earth observers can take pictures of you, but you would never appear the same color. You would be redshifted as you travelled away,

And given the said Time Dilation you would appear to slow down as you redshifted.

Celestial Mechanic
2006-Feb-09, 09:08 PM
... So how in the world is one clock thats on the ship going to show a different time, when both clocks have been ticking for exactly one year? ...
I should have posted an answer to this earlier. The brief answer is that there is no absolute, universal time that any clock can (potentially) measure. Instead there is a local time, called proper time, that is local to each object and depends on that object's trajectory through spacetime. An ideal clock will measure proper time.

Let's imagine that we are travelling from Munich to Turin for the Winter Olympics. The actual distance from Munich to Turin will depend on the path driven. It will almost certainly be greater than the distance if you are lucky enough to fly between the two cities, and that distance is going to be greater than the great circle distance between the two, and certainly greater than the straight line distance (since that passes through the Earth). We have no trouble visualizing this, it's part of our everyday experience. There is no "triangle paradox" in Euclidean geometry.

But when the words "time" or "spacetime" are mentioned, a lot of people's minds turn into metaphysical mush. The problem is that we do not have any of the kinds of experience with time that we do with space, and it is very difficult to divorce ourselves from the notion of absolute time.

So as for the Earth-bound and spacefaring observers, each one has a different elapsed time dependent on the spacetime trajectory. Nothing is wrong with either of their clocks, neither clock runs slow. It is just like my travel example above, neither automobile runs short or long, each auto's odometer registers the correct mileage. (Kilometrage? :) ) The readings are different because the elapsed distances or times really are different.

I hope this helps. :)

astromark
2006-Feb-09, 10:25 PM
Good stuff, and measuring speed is the same. Time and motion are relative.
How would you or could you determine 'Stationary' When you measure the movement of galaxies. They are all moving. Are we stationary? Never. Is anything? Not likely. To determin the position of stars and other observed objects is of corse subject to the errors that movment tend to distort the actual position of the light sorce. Concidering also that some of these objects might be a long way from where they appear now.

Disinfo Agent
2006-Feb-10, 12:13 PM
I'm not the best person to answer question nr. 5, but my guess is that the pictures of the ship, taken by the satellites and sent to Earth, would not reach Earth at 1-second intervals. Rather, the time interval between pictures should be greater than 1 second, increasing above 1 s as the spaceship moves away from Earth, and decreasing back to 1 s as the ship comes back to Earth.I've given this more thought, and I think what I'd written in the previous page was wrong. The ship travels at constant speed, so there's no difference between the trip to one satellite and the trip to the next satellite.
Let's assume the ship travels at 99.9% the speed of light, in a straight line, and that the satellites are placed along the way at 1 lightsecond intervals. In a classical Newtonian universe, it would take the ship 1.001 seconds to get from one satellite to the next. But, in reality, from the point of view of the stationary satellites, it takes longer than that. The signals sent by the satellites arrive on Earth at constant intervals, but the time interval between consecutive signals is greater than 1.001 s, due to the time dilation effect.
From the point of view of the ship's crew, however, the trip between two consecutive satellites takes less than 1.001 s, due to the space contraction effect.

afterburner
2006-Feb-10, 02:31 PM
A good question for you to think about, afterburner, about absolute time:

How do you measure it? Whose clock do you determin to be the clock? When the space ship comes back, and the ship's clock and Earth's clock disagree, which one is wrong?

See, everything is moving relative to something else. We're crusing along at 30 km/s relative to the Sun, and the sun is motoring along at a similar speed relative to the neighbouring stars. If everyone's clock is running slow compared to everyone else's, how do you decide whose is going at the right speed?

The question posed by relativity is: What does it mean for two events to be simultaneous? How do you determine this? If Johnny and Jimmy are 300,000 km apart, and they shine flashlights at each other, how do they determine whether they're turning them on at the same time?

They could get Sally to stand between them, 150,000 km away from each, to say whether she saw each beam at exactly the same time. That seems like a good way to make the determination, right? But what about Sue, who is 600,000 km away from Johnny, but is 670820 km away from Jimmy? She'll see Johnny's beam almost a full quarter of a second before Jimmy's. Sally says that the boys are turning the flashlights on at exactly the same time, but Sue claims that Jimmy is lagging behind. Who's right? More importantly, who's wrong, and why?

You really have to consider these questions before you move on to time dilation and length contraction.

ok ok...i understand this....however, Sally is exactly between the two ....and Sue isnt...she is further away from one of the boys, and therefore it is no surprise that the light that she sees coming from Jimmy is lagging behind...I would say that, in this example, they are both right, however, Sally is in a better position to answer the question whether or not the boys are turning on the lights at the same time...you see? or maybe I dont see?

Ken G
2006-Feb-10, 02:51 PM
You are right, the problem with how to synchronize clocks is a lot more subtle than just time of flight issues like not being at the center point. Relativity is the bizarre stuff that remains after you have already corrected for the finite time of flight of light. For example, a frame where the two flashlights are stationary will not agree on where the center point is with a frame where the two flashlights are moving, nor will it agree that the flashlight clocks were synchronized to read the same time.

Grey
2006-Feb-10, 03:52 PM
ok ok...i understand this....however, Sally is exactly between the two ....and Sue isnt...she is further away from one of the boys, and therefore it is no surprise that the light that she sees coming from Jimmy is lagging behind...I would say that, in this example, they are both right, however, Sally is in a better position to answer the question whether or not the boys are turning on the lights at the same time...you see? or maybe I dont see?Actually, I think that this was a poor example (sorry, Kristophe!). As Ken G pointed out, the issue is a bit more subtle. I'd suggest that you start by reading this (http://www.marxists.org/reference/archive/einstein/works/1910s/relative/ch09.htm) passage, straight from Einstein himself.

Now, let's look at the situation in a bit more detail. I'll repeat some of Einstein's conclusions, but then we'll look at the same situation from the point of view of an observer on the train in detail. We have two observers, both of whom were in exactly the same place at the time the lightning strikes happened from the point of view of the embankment. The lightning strikes will leave burn marks on both the railroad cars and the embankment, so both observers can measure them out later and confirm that the two strikes happened an equal distance apart. Now, from the point of view of the embankment, the light from the two strikes arrives at the same time, but the observer on the train moves toward the forward lightning strike while that light is travelling, so that observer will see forward lightning strike first, and the other a bit later.

Now, all observers will always agree about what events actually took place, so everyone, including the observer on the train, will have to agree that the she sees one lightning strike first, followed by the other. And of course the observer on the train is perfectly justified in thinking that she is stationary, and the stormclouds are rushing past her. Now, we know that she sees the forward lightning strike first, and she can check later and confirm that the two lightning strikes happened an equal distance apart. So, there are two possible conclusions she could draw. First, maybe the speed of light coming from the two strikes was different. The light from the forward strike moved faster, so it got to her first, even though the strikes happened simultaneously. This is the assumption that seems most natural, but every experimental test to see the effects this would cause have failed. No matter how we're moving or how the light source is moving, the speed of light we measure seems to be exactly the same.

So Einstein simply started with that as one of his axioms, that the speed of light is the same for all observers. But then the observer on the train knows that the distance to the lightning strikes is the same, and the speed of light coming from those strikes is the same, so the time elapsed before she sees them must be the same. If the time elapsed is the same, but she sees them at different times, the events themselves must have happened at different times, in disagreement with what the observer on the embankment sees.

But we've already agreed that no inertial observer occupies a privileged position. What the observer on the train thinks happened is just as valid as what the observer on the embankment thinks happened. So, were the lightning strikes "really" simultaneous or not? The answer depends on the reference frame you choose to use. If you accept that the speed of light is constant regardless of how you or the source are moving, and that all reference frames are equally valid, you're forced to accept that whether two distant events are simultaneous or not is not an absolute.

It's certainly strange, and if you keep reasoning from here, you'll come up with all sorts of weird conclusions. However, if you test those conclusions, the experiments all work out, so we conclude that the universe is weirder than we thought.

JESMKS
2006-Feb-10, 05:05 PM
If we had a pound of tritium, with a half life of 12.3 years and took half of it on the trip, would the weights of the two halfs be different upon return?
Jack

ToSeek
2006-Feb-10, 05:14 PM
If we had a pound of tritium, with a half life of 12.3 years and took half of it on the trip, would the weights of the two halfs be different upon return?
Jack

Yes. The Earthbound one would have decayed more than the one on the spaceship.

Knowledge_Seeker
2006-Feb-11, 09:01 PM
You know what, after reading all of this, now im confused.

Can someone answer for me again why, the time is different. and why the clock on a ship would go slower (or the clock on earth go faster) when they both have clocks that regulate at the same time.

i just dont see how speed changes time.

i always knew, and believed time dilation but i just didnt see, how?

EDIT: I went to this link that someone on here mentioned http://casa.colorado.edu/~ajsh/sr/time.html and i saw the second animation.

i dont understand what light has to do with time. i see that it takes the light longer to move to a traveling object but if you had like a digital clock, what would light have to do with it.

Ken G
2006-Feb-11, 09:51 PM
Can someone answer for me again why, the time is different.

No one can tell you that. It just is. Why is there time at all? Why must it be the same in all reference frames? Those questions are no less valid, and no less impossible to answer.

i just dont see how speed changes time.

You're not supposed to see how, nobody knows. The universe obeys two rules: the speed of light is the same in all frames, and all you can measure is relative (not absolute) velocity. That's it, the weird nature of time derives from these statements. But why are they true? Why not?

i dont understand what light has to do with time.
The way I see it, light is a probe of time, that's all. It has nothing to do with time, and we'd have relativity even if we had no light sources to probe it. The tree falling in the woods.

Knowledge_Seeker
2006-Feb-12, 04:30 AM
thanx ken g i appreciate the answer!

nokton
2006-Feb-12, 06:56 PM
It's implied in the above two posts, but it's such an important point I think it needs to be spelled out: both time measurements are correct. There isn't a "true" time and a "perceived" time. And yes, I know it doesn't make sense (well, it does if you do the calculations, but it feels wrong). But these effects have been observed and, as far as we can tell, it's the way things work.
With respect, JohnW, if we follow Alberts teaching, there is neither true
nor perceived time, as you rightly state. Only relative time, depending
upon the speed of the observer and the gravity environment the observer
is in. Both speed and local gravity will determine an observers time frame
reference. A second observer at a different speed and gravity environment
would have a different time frame reference.
Just a thought JohnW, if the astronauts mentioned above took a CD
with them, how would it sound at half lightspeed....?
Nokton.

Duane
2006-Feb-12, 07:33 PM
You know what, after reading all of this, now im confused.

Can someone answer for me again why, the time is different. and why the clock on a ship would go slower (or the clock on earth go faster) when they both have clocks that regulate at the same time.

i just dont see how speed changes time.

i always knew, and believed time dilation but i just didnt see, how?

I'm going to take a stab at this, but if I have it wrong please feel free to correct me.

To answer the first question, the time is not dfferent for the reference frame of the viewer. On board the ship, the passengers would experience the passing of a year; that is the clock would tick out a year on board the ship.

On Earth, the observers would also experience the passing of a year.

Time dilation between the two reference frames is what would be different, as it relates to the observers aboard the ship and those on Earth. Those on the ship would be a year older but arrive back at an Earth that had seen a longer period pass. Those on Earth would see a year pass, but the ship would not arrive until some time later.

The how relates to Relativity. As a mass accelerates, time as it relates to that object passes faster for someone outside of the frame of reference of the accelerating mass.

teri tait
2006-Feb-12, 07:43 PM
I know science and medicine are adapting hardware to allow the physically challenged to do wonderful things like work a computer by thought with electrodes things on their heads, so this proves thought travels in some sort of wave. I wonder how far and fast would the same process work in space? Or in zero gravity? Just a loose thought, please do not feel compelled to respond.

nokton
2006-Feb-12, 08:33 PM
I'm going to take a stab at this, but if I have it wrong please feel free to correct me.

To answer the first question, the time is not dfferent for the reference frame of the viewer. On board the ship, the passengers would experience the passing of a year; that is the clock would tick out a year on board the ship.

On Earth, the observers would also experience the passing of a year.

Time dilation between the two reference frames is what would be different, as it relates to the observers aboard the ship and those on Earth. Those on the ship would be a year older but arrive back at an Earth that had seen a longer period pass. Those on Earth would see a year pass, but the ship would not arrive until some time later.

The how relates to Relativity. As a mass accelerates, time as it relates to that object passes faster for someone outside of the frame of reference of the accelerating mass.
Duane, you grasp Alberts concept, your last paragraph says all, you have.
But put aside those on earth, or on a 'ship'. Just time is relative to an
observer anywhere and wherever the observer may be at a given time.
Nokton.

grant hutchison
2006-Feb-12, 08:57 PM
I know science and medicine are adapting hardware to allow the physically challenged to do wonderful things like work a computer by thought with electrodes things on their heads, so this proves thought travels in some sort of wave.Yes, it's electromagnetic energy: just a change in the electric field of your scalp generated by electrochemical activity in you brain cells. The signal falls off very quickly, so is difficult to sense at any distance from the surface of the head. I can't think why it would behave any differently in space.

Grant Hutchison

NEOWatcher
2006-Feb-13, 01:29 PM
snip
i just dont see how speed changes time.

i always knew, and believed time dilation but i just didnt see, how?

If you take it as a mathematical concept...
speed = distance/time
or
time = distance/speed

Part of the problem is that there is no way to measure time without using distance as a factor. Even a digital clock has to deal with electrons moving, or a crystal vibration.

SeanF
2006-Feb-13, 02:38 PM
Just a thought JohnW, if the astronauts mentioned above took a CD with them, how would it sound at half lightspeed....?
It would sound exactly the same. If it didn't, that would disprove relativity! :)

Ken G
2006-Feb-13, 02:42 PM
It would sound exactly the same. If it didn't, that would disprove relativity! :)
That's right, everything is always "normal" in our own frame. Relativity is about how to change reference frames.

nokton
2006-Feb-13, 06:43 PM
If you take it as a mathematical concept...
speed = distance/time
or
time = distance/speed

Part of the problem is that there is no way to measure time without using distance as a factor. Even a digital clock has to deal with electrons moving, or a crystal vibration.
NEOWatcher, with respect, you traveling at half lightspeed, lightspeed
is till 186 thousand m per sec as you measure it. OH dear, why is
lightspeed not now 93 thousand m per sec? Because time is slowed by
your your speed. You will still clock light passing you at 186 m per sec
Respect,
Nokton

NEOWatcher
2006-Feb-13, 08:06 PM
NEOWatcher, with respect, you traveling at half lightspeed, lightspeed
is till 186 thousand m per sec as you measure it. OH dear, why is
lightspeed not now 93 thousand m per sec? Because time is slowed by
your your speed. You will still clock light passing you at 186 m per sec
Respect,
Nokton
Just the opposite...
The speed has stayed constant (186 m/sec)
The distance travelled has changed (remember things get "squished" in the direction of travel)
Therefore time must change.

NEOWatcher
2006-Feb-13, 08:23 PM

You're familiar with that "Objects are closer than they appear" mirror on your car.
Let's use that analogy but reverse the mirror so they appear farther than they are.

Now lets say the only way to see things while travelling in reverse is through that mirror.

I'm in my car, and can only go 600 feet per minute in reverse.
I start backing up, and in the mirror, the garage appears to be 30 feet behind me.
But if I were outside the car, I would measure the garage door as 60 feet behind me.

So, according to that magical mirror I travelled 30 feet.
I know the car goes 600 feet per minute, therefore:
time = 30feet / 600 feet/min
time = 1 ft / 20 ft/min
or 3 seconds

The person standing outside the car sees the 60 feet travelled, therefore
time = 60feet / 600 feet/min
time = 1ft / 10 ft/min
or 6 seconds.

Ken G
2006-Feb-13, 08:32 PM
Yes, the two observers will only agree on the speed, because relative speed is the key number for changing reference frames. What they won't agree on is either the time elapsed or the distance traveled, but both will think their own time is ticking by perfectly normally, so won't "notice" anything amiss about anything happening in their own reference frame.

afterburner
2006-Feb-13, 09:44 PM
Thank you Grey for giving that answer, im sure it helped a lot of people(and me) to grasp the concept that Einstein was trying to explain a little better.

this is a little lengthy, but please read and tell me if im still not getting it..

What's going through my head, however, is that in the train example, the train operator knows the speed at which the train is travelling at, she also knows that the speed of light is a constant. Now, when measuring the time of both strikes, she is going to measure the strikes as they happened NOT simontaneous at first, but then, realizing that she is moving, she is going to make the necessary adjustments, and conclude that the strikes DID indeed happen at the same time...

Now i guess what im trying to say is that if we knew, and tracked the position of everything in this universe (or an area big enough so satisfy our needs) with a few, of say, supercomputers. Would that now mean, that we know the absolute time? Because we know how everyone is experiencing things, taking into consideration that the speed of light is a constant.

Saying all this, would that not also mean that there really is no slower time for things that are moving? that time is indeed absolute? and that ACTUAL absolute time is from the perspective of the ones that know the position of everything in the universe aswell as the speed of light (not the position of molecules...just things big enough to matter in our calculations) ? Would that not also mean that the astronauts in my original post will not return younger, and that time travel is really impossible(at least through speed), when we look at what actually happens from the point of view of the absolute clock that i described above?

The only problem with this would be that everyone would have to be in constant communication with eachother, and our communication CANT go faster than the speed of light, not permitting everyone to "synchronize their clocks" sort of speak...

My conclusion: we will have to wait untill someone invents an instantaneous communication device ( whether a teleporter, or wormhole, or somehow using the other dimentions..i dunno enought about this to say, but it has to be relatively instantaneous, or at least as close to instantaneous as possible...definitely faster than light) in order to prove what im saying...so because thats not happening any time soon, i would like to thank everyone that contributed to this thread and helped me formulate this weird thought....

i dont know if this is still right...so PLEASE CORRECT ME if im wrong...

i guess my next question would be how does gravity fit into all of this? :D

Disinfo Agent
2006-Feb-13, 10:00 PM
Now i guess what im trying to say is that if we knew, and tracked the position of everything in this universe (or an area big enough so satisfy our needs) with a few, of say, supercomputers. Would that now mean, that we know the absolute time? Because we know how everyone is experiencing things, taking into consideration that the speed of light is a constant.

Saying all this, would that not also mean that there really is no slower time for things that are moving? that time is indeed absolute? and that ACTUAL absolute time is from the perspective of the ones that know the position of everything in the universe aswell as the speed of light (not the position of molecules...just things big enough to matter in our calculations) ? Would that not also mean that the astronauts in my original post will not return younger, and that time travel is really impossible(at least through speed), when we look at what actually happens from the point of view of the absolute clock that i described above?The problem is that you'd still have to explain all of this (http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html).

i guess my next question would be how does gravity fit into all of this? :DIt enters the stage in Act II: General Relativity. ;)

SeanF
2006-Feb-13, 10:12 PM
What's going through my head, however, is that in the train example, the train operator knows the speed at which the train is travelling at, she also knows that the speed of light is a constant. Now, when measuring the time of both strikes, she is going to measure the strikes as they happened NOT simontaneous at first, but then, realizing that she is moving, she is going to make the necessary adjustments, and conclude that the strikes DID indeed happen at the same time...
Are you sure? :) If the speed of light is constant, then the only way the light from two simultaneous strikes can reach you at different times is if one's farther away than the other, right? In this case, she'll receive light from the strike in front of her first, so if they were simultaneous, that one must have been closer.

But if the one in front of her was closer than the one behind her, then that would mean that the two simultaneous strikes didn't occur until after the half-way point had passed.

That's still a contradiction with what the first observer measured, that the strikes occured exactly at the half-way passing.

afterburner
2006-Feb-13, 10:27 PM
Are you sure? :) If the speed of light is constant, then the only way the light from two simultaneous strikes can reach you at different times is if one's farther away than the other, right? In this case, she'll receive light from the strike in front of her first, so if they were simultaneous, that one must have been closer.

But if the one in front of her was closer than the one behind her, then that would mean that the two simultaneous strikes didn't occur until after the half-way point had passed.

That's still a contradiction with what the first observer measured, that the strikes occured exactly at the half-way passing.

ok, but like i said...if the traveller knows where everything is, and has constant, instantaneous communication with the other observer....then the traveller will know what, and i say this once again...ACTUALLY happened...not what the traveller thought, or saw what happened..but what ACTUALLY happened....so the strikes DID indeed happen at the same time...and it was only the perception of the traveller that mislead him to think otherwise.
basically, if the person on the train, worked back the timeline(once again knowing where the strikes originated)...he is able to realise that the strikes WERE in fact at the same time.... after seeing how far he must have tavelled towards the strike he saw first, and how much farther away from the strike he saw second...

SeanF
2006-Feb-13, 10:36 PM
ok, but like i said...if the traveller knows where everything is, and has constant, instantaneous communication with the other observer....then the traveller will know what, and i say this once again...ACTUALLY happened...not what the traveller thought, or saw what happened..but what ACTUALLY happened....so the strikes DID indeed happen at the same time...and it was only the perception of the traveller that mislead him to think otherwise.
basically, if the person on the train, worked back the timeline(once again knowing where the strikes originated)...he is able to realise that the strikes WERE in fact at the same time.... after seeing how far he must have tavelled towards the strike he saw first, and how much farther away from the strike he saw second...
First - there's no such thing as instantaneous communication, because there's no such thing as instantaneous.

Second, if the observer determines that the two strikes occurred simultaneously at the moment he was midway between them, but he received the light from one before he received the light from the other, then he would conclude that the two light beams were not traveling at the same velocity relative to himself. Relativity is based on the postulate that light will always travel at the same velocity relative to any given observer.*

*In an inertial frame, naturally. :)

Grey
2006-Feb-13, 11:02 PM
What's going through my head, however, is that in the train example, the train operator knows the speed at which the train is travelling at, she also knows that the speed of light is a constant. Now, when measuring the time of both strikes, she is going to measure the strikes as they happened NOT simontaneous at first, but then, realizing that she is moving, she is going to make the necessary adjustments, and conclude that the strikes DID indeed happen at the same time...How does the person on the train know what speed she is travelling at? We're assuming that this is a perfectly smooth-travelling train, with no bouncing or other inertial forces. As far as she can measure, she's standing still. And remember that one of our assumptions (guided, remember by experimental results) is that any inertial frame is as good as any other. You're suggesting that the embankment is somehow a special preferred reference frame, and we have no reason to believe that such a thing exists.

The only problem with this would be that everyone would have to be in constant communication with eachother, and our communication CANT go faster than the speed of light, not permitting everyone to "synchronize their clocks" sort of speak...If there were such a thing as "instantaneous" communication, that would in fact invalidate relativity, since we could indeed synchronise clocks as you suggest. Of course, the fact that experimental results match the predictions of relativity strongly suggests that such communication is impossible. And even if there turns out to be a way to do it, those experimental results still confirm that things like time dilation are real, so whatever theory replaced relativity would still ahve to incorporate these results.

Ken G
2006-Feb-14, 12:52 PM
I'm not sure that instantaneous communication, or the lack thereof, plays a key role in any of this. If both observers know relativity, they are both perfectly capable of calculating what the other will observe, so what does it matter if they can communicate this? In other words, I can synchronize my clock with any other observer by calculating what their clock is doing, but if I am continuously synchronizing my clock, I'm not measuring time. To measure time, I must synchronize my clock once, and then let it do what clocks do from there on. That will lead to a discrepancy in the clocks in short order-- why do I care if I can communicate instantaneously or not? That would seem to have more to do with the principle of causality, which is not invoked in a simple lightning-strike observation as we are considering here. It seems to me the real issue is that clocks that are at different places and are synchronized for one observer will not be synchronized for another observer in relative motion, and they can communicate this problem all they like-- it won't go away.

SeanF
2006-Feb-14, 03:16 PM
Ken, the lack of instantaneous communication is only important insofar as people learning Relativity always say, "but if the observers had instantaneous communication they could see what was really going on." :)

Sandoval
2006-Feb-14, 03:37 PM
That would seem to have more to do with the principle of causality, which is not invoked in a simple lightning-strike observation as we are considering here.
In fact, if superluminal communication was at all possible, violations of causality would be trivial to accomplish. Here are the details: Relativity, FTL and causality (http://www.theculture.org/rich/sharpblue/archives/000089.html)

Grey
2006-Feb-14, 03:39 PM
Ken, the lack of instantaneous communication is only important insofar as people learning Relativity always say, "but if the observers had instantaneous communication they could see what was really going on." :)Yes and no. It's pretty straightforward to show that the assumption that the speed of light is constant is equivalent to deciding when a distant event happened by timing a light beam bounced off of it, and taking the time of that event as halfway between when you sent and received the signal. But if communication was instantaneous, that wouldn't be the best way to decide when a distant event happened.

Similarly, two events that are spacelike separated have an indeterminate temporal relation to each other. But if you could communicate instantaneously between the two locations, you'd be able to establish an unambiguous time ordering to the events, just as you can for events that are adjacent.

Of course, as I pointed out, that fact that all of relativity's predictions work out is pretty strong evidence that superluminal communication is not possible. And even if it is, the theory that explains it would still have to account for why the universe looks like superluminal communication isn't possible, so it would no doubt be even harder to grasp.

teri tait
2006-Feb-14, 03:46 PM
So the theory of relativity boils down to
Eye told you, so?
;)

Ken G
2006-Feb-14, 03:54 PM
Similarly, two events that are spacelike separated have an indeterminate temporal relation to each other. But if you could communicate instantaneously between the two locations, you'd be able to establish an unambiguous time ordering to the events, just as you can for events that are adjacent.
This is an interesting insight, but I wouldn't say such events necessarily must have an indeterminate temporal relation. I would say the temporal relation depends on the choice of coordinates. Thus the concept of "instantaneous" is not immediately obvious, even if information could travel infinitely fast. However, due to the cosmological principle, there is a single concept of time that all observers can use unambiguously, which is the proper time since the Big Bang. We could then define "instantaneous" communication as communication that happens between people at the same age of the universe, over arbitrary distance scales. Of course this is not possible in practice, nor is it obvious that this would be the "right" definition of instantaneous, but it is at least a workable definition. The laws of relativity would continue unaltered even if it were somehow possible to communicate between all the other people at the same age of the universe as we find ourselves now, so I don't see why instantaneous communication is relevant to relativity-- after all, we routinely calculate what such people would "see", so what difference does it make if they tell us we are right? But there would be a violation of causality, which seems to me is something deeper than relativity. Relativity says that the Lorentz transformation preserves causal relationships, but it doesn't assert that there must be such relationships. I'm not sure exactly where causality originates.

Ken G
2006-Feb-14, 03:59 PM
Ken, the lack of instantaneous communication is only important insofar as people learning Relativity always say, "but if the observers had instantaneous communication they could see what was really going on." :)
Nobody should make this incorrect statement. Just because we have relativity does not in any way compromise our ability to see what is really going on. Quite the opposite-- relativity is precisely the mechanism for determining what is really going on. If we routinely moved at near c, then all our clocks would say different things and our perceptions of reality would be very different, so we would routinely use relativity to reconcile the differences, i.e., to figure out just what is really going on. Of course, we'd have to use a more sophisticated recognition that reality is really an equivalency class of experiences, not a single unique set of experiences.

Disinfo Agent
2006-Feb-14, 05:59 PM

nokton
2006-Feb-14, 06:53 PM
Thanx Grey, well expressed, concise and reasoned as always.
Not patronising, just remembering.
Nokton

SeanF
2006-Feb-14, 07:11 PM
Ken, the lack of instantaneous communication is only important insofar as people learning Relativity always say, "but if the observers had instantaneous communication they could see what was really going on." :)
Nobody should make this incorrect statement.
Afterburner made it in this very thread. It's a common thing when people are learning Relativity. I'm sorry, but it's simply wrong to say "nobody should make this statement." The statement is "common sense," after a fashion, and it needs to be dealt with when discussing Relativity.

Ken G
2006-Feb-15, 12:14 AM
It sounds like you are saying that a lot of people make the mistake of thinking you can say more about what is really going on than relativity allows. I would agree with that. I was just saying that the statement is wrong, and that's why people should not state it. What can be "common sense" about an incorrect statement? I'm afraid I don't understand your reasoning.

Ufonaut99
2006-Feb-15, 01:29 PM
I'm trying to understand when you can say something was before/after something else - what are the rules?

For example, event1 happens at a certain time at (x1,y1,z1), and another event at (x2,y2,z2). When can we KNOW that event2 happened after event1?

Is it that if the light of event1 has passed (x2,y2,z2) before event2, the we know event2 was after event1 (and vice-versa) - otherwise before/after is meaningless because different frames of reference can return different results?

Ken G
2006-Feb-15, 02:33 PM
Is it that if the light of event1 has passed (x2,y2,z2) before event2, the we know event2 was after event1 (and vice-versa) - otherwise before/after is meaningless because different frames of reference can return different results?
Yes, you have it exactly. This is what Grey meant by "spacelike" vs. "timelike" separations-- timelike separations can be connected by light signals, and Lorentz transformations will never change the order of timelike separated events, or else macroscopic physics could not be built on the cause/effect principle (and it is). However, if light cannot pass between the events, then different reference frames won't necessarily agree on the order of events, they are spacelike separated and this separation hopelessly monkeys with the synchronization of clocks. But it causes no contradictions, because there are no cause/effect connections either.

SeanF
2006-Feb-15, 05:29 PM
It sounds like you are saying that a lot of people make the mistake of thinking you can say more about what is really going on than relativity allows. I would agree with that. I was just saying that the statement is wrong, and that's why people should not state it. What can be "common sense" about an incorrect statement? I'm afraid I don't understand your reasoning.
Okay, if you don't think "common sense" can ever be wrong, then we clearly have different ideas of what the term means. There's probably no point in any ongoing discussion of that.

Nonetheless, anytime you're discussing Relativity with someone who is not already knowledgable on the subject, you need to be prepared to discuss instantaneous communication - because it will come up.

nokton
2006-Feb-15, 08:45 PM
I'm not sure that instantaneous communication, or the lack thereof, plays a key role in any of this. If both observers know relativity, they are both perfectly capable of calculating what the other will observe, so what does it matter if they can communicate this? In other words, I can synchronize my clock with any other observer by calculating what their clock is doing, but if I am continuously synchronizing my clock, I'm not measuring time. To measure time, I must synchronize my clock once, and then let it do what clocks do from there on. That will lead to a discrepancy in the clocks in short order-- why do I care if I can communicate instantaneously or not? That would seem to have more to do with the principle of causality, which is not invoked in a simple lightning-strike observation as we are considering here. It seems to me the real issue is that clocks that are at different places and are synchronized for one observer will not be synchronized for another observer in relative motion, and they can communicate this problem all they like-- it won't go away.
Ken, your concept of relative clocks agree with. Point is, concept is a
problem, not yours, but the ones who cannot grasp it.
Nokton

nokton
2006-Feb-15, 09:06 PM
A good question for you to think about, afterburner, about absolute time:

How do you measure it? Whose clock do you determin to be the clock? When the space ship comes back, and the ship's clock and Earth's clock disagree, which one is wrong?

See, everything is moving relative to something else. We're crusing along at 30 km/s relative to the Sun, and the sun is motoring along at a similar speed relative to the neighbouring stars. If everyone's clock is running slow compared to everyone else's, how do you decide whose is going at the right speed?

The question posed by relativity is: What does it mean for two events to be simultaneous? How do you determine this? If Johnny and Jimmy are 300,000 km apart, and they shine flashlights at each other, how do they determine whether they're turning them on at the same time?

They could get Sally to stand between them, 150,000 km away from each, to say whether she saw each beam at exactly the same time. That seems like a good way to make the determination, right? But what about Sue, who is 600,000 km away from Johnny, but is 670820 km away from Jimmy? She'll see Johnny's beam almost a full quarter of a second before Jimmy's. Sally says that the boys are turning the flashlights on at exactly the same time, but Sue claims that Jimmy is lagging behind. Who's right? More importantly, who's wrong, and why?

You really have to consider these questions before you move on to time dilation and length contraction.
OMG, does no one understand Albert?
Ok Kristophe, there is no absolute time, time is dependent on the observer.
The position of the observer is crucial to the observation. Time is relative
so cannot be absolute. Lightspeed is fixed is it not? If so it's an absolute.
Why then does the time differential change as we gather speed?
Nokton

nokton
2006-Feb-16, 05:59 PM
Grey,
Once again, thankyou for your reasoned and logical imput to the
queries presented here. First met you about two years ago. Respected
you then for your intellect, and your approach to science, still do.
Nokton

Sam5
2006-Feb-17, 02:11 AM
ok, first of all...pardon my stupidity....:wall:
what i want to know is how this time difference happens

lets just say that we have two simple digital clocks(shows time, month,year), that run on AA batteries...now one stays on earth...and the other one goes on a trip aboard a space ship with a few astronauts...lets also say that scientists plan for the ship to travel for half a year in one direction (at the speed of light, or close to)...and half a year back(at the same speed)...now....the astronauts know they are going to be travelling for a total of one year...and we on earth know that they are going to come back in one year..so how in the world is one clock thats on the ship going to show a different time, when both clocks have been ticking for exactly one year? hope i made sence in explaining what i meant..and didnt get...

That’s part of the 1905 “clock paradox” of the SR theory. To try to get rid of the paradox, Einstein wrote a new paper in 1918 in which he added gravitational fields to his original 1905 paper’s thought experiments, and he added atomic clocks, and one of the two clocks stayed in a strong gravity field longer than the other, and that’s how one of them slowed down more than the other. This happens with atomic clocks in a gravity field, but not with pendulum clocks. With pendulum clocks, the one that stays in the strong gravity field longer, speeds up. By 1918 he realized his original 1905 error. You may read his clock paradox correction paper in Volume 7 of “The Collected Papers of Albert Einstein”, Princeton Press, paperback edition, Pages 66-75, under the article title “Dialogue About Objections to The Theory of Relativity.” The original German version was published in Die Naturwissenschaften 6, (1918) pages 697-702. This rare paper was published in English only in 2002, so not many people have had a chance to read it yet.

Ken G
2006-Feb-17, 03:28 AM
That's an interesting point, that pendulum clocks expressly rely on gravity so are not generally good ways to measure time (for example, try using a pendulum clock to tell time on the Moon! It just won't work, it's not that time slows down on the Moon). It is surprising that he made that mistake even in his first attempt, but I guess nobody's perfect!

SeanF
2006-Feb-17, 03:01 PM
It is surprising that he made that mistake even in his first attempt, but I guess nobody's perfect!
What mistake?

Ken G
2006-Feb-17, 03:20 PM
According to Sam5, Einstein described the use of pendulum clocks to measure time in his early relativity efforts. Since they did not include gravity, it wasn't really a mistake, but it was a problematic way to think of time measurement in the general sense.

Fr. Wayne
2006-Feb-17, 04:04 PM
What is wrong with my analysis: The speed of light is a constant. Therefore time is not relative. If time were relative, speed would be relative. What I believe Einstein was saying is that time-space IS relative. This approach E=mass(space/time)^2 ergo √(E/m)= space/time ergo Time = Space/√(E/m) ergo the greater the distance from observer that m travels, the faster time must move to the observer (wherever he is) RELATIVE to real time of the m. That's how the observed edges of the Universe are 14 b yrs old to us and only a mere 1/2 million year old at furthest edge of m (if you were keeping time there). In other words, Light is a trickster. "He" is constantly making time out of the space "he" is limited to.

Disinfo Agent
2006-Feb-17, 04:15 PM
The speed of light is constant, but other speeds are not, so I think you have to be careful with that sort of reasoning.
And note also that a speed is a quotient, speed = displacement / time. It's quite possible for two different pairs (displacement, time) to have the same quotient: 8/4 = 5/2.5 = 3/1.5 = 2. So, you can have two observers measuring different displacements in different times, but ending up with the same speed.

SeanF
2006-Feb-17, 04:42 PM
According to Sam5, Einstein described the use of pendulum clocks to measure time in his early relativity efforts. Since they did not include gravity, it wasn't really a mistake, but it was a problematic way to think of time measurement in the general sense.
Ah. Einstein never used pendulum clocks to measure time in any of his relativity experiments, and I don't think Sam5 was intending to suggest that he did. I do see how his post could be read that way, though.

I'll let him clarify what he thinks Einstein's "mistake" was himself, if he so desires. It actually isn't a mistake, though. :)

Grey
2006-Feb-17, 05:25 PM
I'll let him clarify what he thinks Einstein's "mistake" was himself, if he so desires. It actually isn't a mistake, though. :)And, of course, since this is the "Questions and Answers" section, if Sam5 wants to try (again) to support his claim that there's a contradiction in special relativity that Einstein realized and corrected by introducing general relativity, he should really do so in a separate thread in the AtM section.

Hamiltonian,? Grey, please tell me.It's a (perhaps very) small physics joke. In quantum physics, the Hamiltonian is an operator that gives you the energy of the system you're looking at. There are other operators that can give you other values, or that can change the wavefunction to a different one in some way. Two operators that are mutually compatible are said to "commute", so an operator that commutes with the Hamiltonian leaves the energy of the system unchanged, and energy is conserved under the application of such an operator. But the form of the statement also parallels something like a slogan for carpooling, where "conserve energy" has an entirely different meaning from the one used in physics. Someone might say something like, "Conserve energy. Commute with a friend.", suggesting that you should drive to work together in order not to use so much gasoline. I fear, like most jokes, that whatever humor value it might have had is quickly drained by being explained in detail, though. :)

Nicholas_Bostaph
2006-Feb-17, 05:32 PM
To answer the first question, the time is not dfferent for the reference frame of the viewer. On board the ship, the passengers would experience the passing of a year; that is the clock would tick out a year on board the ship.

On Earth, the observers would also experience the passing of a year.

Time dilation between the two reference frames is what would be different, as it relates to the observers aboard the ship and those on Earth. Those on the ship would be a year older but arrive back at an Earth that had seen a longer period pass. Those on Earth would see a year pass, but the ship would not arrive until some time later.

I'm still trying to grasp how one reference frame is the 'correct' one. Supposing we hop in our star cruiser and leave Earth at some relativistic speed. We travel in a big circle for one year before we arrive back at Earth. Now, one year has passed for us, while two years have passed on Earth. Correct?

However, since all reference frames are supposedly equally valid, why can it not be said that we stood still while the Earth (and the rest of the universe) flew away from us at relativistic speeds? In that case the Earth would have experienced one year while two years passed for us. How do we determine who (our star cruiser or the Earth) is actually moving when there is no absolute frame of reference? And without knowing who is the one actually moving, how do we know who time slows down for in relation to the other?

Fr. Wayne
2006-Feb-17, 05:41 PM
Conserve energy. Commute with the Hamiltonian.

Now I get it. You quantum guys are so principled in your uncertainty. You make me wanna sing..... CCR's My Suzy Q.... "I like the way you talk (de do do Da da)"...

grant hutchison
2006-Feb-17, 05:51 PM
How do we determine who (our star cruiser or the Earth) is actually moving when there is no absolute frame of reference? And without knowing who is the one actually moving, how do we know who time slows down for in relation to the other?Which one underwent the acceleration? Since the starship accelerates away from the Earth and then returns, while the Earth undergoes no acceleration, the situation is not symmetrical. The observer who underwent acceleration is the one whose clock is found to run slow when the two observers are reunited in the same inertial reference frame.

Grant Hutchison

Sam5
2006-Feb-17, 06:09 PM
According to Sam5, Einstein described the use of pendulum clocks to measure time in his early relativity efforts. Since they did not include gravity, it wasn't really a mistake, but it was a problematic way to think of time measurement in the general sense.

No, I’m sorry if I didn’t make myself clear. He used “balance wheel” clocks in the 1905 theory. Later, as of 1911, he switched over to using atomic clocks. That’s what he used in his 1918 correction paper because he realized by then that atomic clocks would change rates under gravitational or acceleration forces.

His 1905 paper and SR theory had no “forces” at all, and no gravity or acceleration, and he tried to use only “relative motion” as a means of slowing down his “balance wheel” clocks. After receiving a lot of criticism of his theory, and after having had the “clock paradox” of the theory pointed out to him – i.e. which clock is actually the one that “moves,” since both are moving “relatively” in the 1905 paper – he gradually, in lectures, began to add acceleration and gravity fields to his thought experiments to try to do away with the clock paradox. By 1918 the criticism was so vocal among many German physicists, he decided to write his 1918 article in which he used atomic clocks (fundamental oscillating atoms) and he tried to fit them into his 1905 thought experiments, and he added acceleration and gravity to those thought experiments, even though the original theory and original 1905 thought experiments do not contain acceleration or gravity.

By this means, he was able to add real “forces” to the atomic clocks in 1918, and that is what he claimed slowed down one clock more than the other. In the 1918 paper, he did not admit there was an error in the 1905 paper. He merely tried to present new thought experiments (with gravity and acceleration considered) as if they could be, retroactively, applied to the 1905 theory, after 13 years of his claiming in books and papers that the 1905 theory contained no consideration of gravity or acceleration.

I was just pointing out in my earlier post that different kinds of clocks react differently to environmental changes, such as pendulum clocks speeding up when placed in a strong gravity field and atomic clocks slowing down when place in a strong gravity field. So that no single kind of clock represents all of true time. His use of atomic clocks in the 1918 paper, rather than balance wheel clocks, was because it was fairly well known by then that atomic oscillation rates would slow down in a strong gravity field. So, his original 1905 SR theory clocks had been “balance wheel”, but in his 1918 correction paper he used atomic clocks (fundamental oscillating atoms), and he slowed them down by means of gravity and acceleration, rather than just by means of "relative motion" alone.

Nicholas_Bostaph
2006-Feb-17, 06:19 PM
Which one underwent the acceleration? Since the starship accelerates away from the Earth and then returns, while the Earth undergoes no acceleration, the situation is not symmetrical. The observer who underwent acceleration is the one whose clock is found to run slow when the two observers are reunited in the same inertial reference frame.

Grant Hutchison

Ahh...I finally see. Thank you. :)

Sam5
2006-Feb-17, 06:46 PM
Which one underwent the acceleration? Since the starship accelerates away from the Earth and then returns, while the Earth undergoes no acceleration, the situation is not symmetrical. The observer who underwent acceleration is the one whose clock is found to run slow when the two observers are reunited in the same inertial reference frame.

Grant Hutchison

You can’t use that to solve the "clock paradox" of the 1905 SR theory since the theory does not consider either acceleration effects or gravity effects. That’s what causes the clock paradox.

You can’t solve the SR error with a GR solution. There is no “clock paradox” in GR theory, but there is in SR theory.

SeanF
2006-Feb-17, 08:09 PM
Ahh...I finally see. Thank you. :)
Please keep in mind, though, that while it is acceleration that breaks the symmetry, it is not simply acceleration that causes the slow-down.

Consider where we have two clocks. One clock is left in the "original" inertial frame. The other clock is:

1) accelerated to 0.6c (relative to the first clock).
2) left to "cruise" at 0.6c for some amount of time.
3) decelerated back to 0.
4) reaccelerated towards the original clock back to 0.6c.
5) left to "cruise" at 0.6c for the same amount of time as in step 2)
6) decelerated back to 0 so it "stops" right next to the original clock.

The clock that accelerated out and back will lag behind, right? Simple enough.

But what if we have a third clock that undergoes the same process, with identical accelerations/decelerations in steps 1), 3), 4), and 6), but left to "cruise" for a longer time in steps 2) and 5)?

At the end of the experiment, how will this third clock compare to the second clock, considering that they were both subjected to identical accelerations (albeit in different locations)?

The third clock will lag behind the second because it spent more time in the other inertial frames (steps 2) and 5)).

SeanF
2006-Feb-17, 08:13 PM
You can’t use that to solve the "clock paradox" of the 1905 SR theory since the theory does not consider either acceleration effects or gravity effects. That’s what causes the clock paradox.

Two events separated in space by three light-years and time by five years when measured in one reference frame will be separated in space by zero light-years and time by four years when measured in a reference frame with a velocity of 0.6c relative to the first reference frame.

That is all Einstein's "peculiar consequence" involves, and it's not a paradox. There's no contradiction there.

Sam5
2006-Feb-17, 08:22 PM

Two events separated in space by three light-years and time by five years when measured in one reference frame will be separated in space by zero light-years and time by four years when measured in a reference frame with a velocity of 0.6c relative to the first reference frame.

That is all Einstein's "peculiar consequence" involves, and it's not a paradox. There's no contradiction there.

Hi Sean,

Your own thought experiment isn’t in the 1905 SR theory.

If there was no clock paradox or contradiction in SR theory, there wouldn’t have been a debate about it for the past 101 years and there wouldn’t need to be dozens of books and websites that try to straighten it out. Go read Einstein’s 1918 paper about it.

Disinfo Agent
2006-Feb-17, 08:27 PM
Not this again! Please. Leave it for the ATM forum. :hand:

Sam5
2006-Feb-17, 08:46 PM
Not this again! Please. Leave it for the ATM forum. :hand:

This is not “ATM”. This is Einstein’s own 1918 paper that you need to read and his own “solution” to the clock paradox.

SeanF
2006-Feb-17, 08:51 PM
Your own thought experiment isn’t in the 1905 SR theory.
Yes, it is. As I said, "my" thought experiment is Einstein's "peculiar consequence." I used different words, and gave specific time/distance numbers, but it is the same thing.

The first of my two events is the clock leaving A, and the second is the clock arriving at B. Since the clock that remains at B measures those two events as being spatially separated, it is in the first reference frame and measures the longer time between them. Since the other clock measures those two events as being colocated, it is in the second reference frame and measures the shorter time between them. Thus, they disagree on how much time passed.

Same thing.

(Agent, how is a thread with this one's title not a good place to discuss this? Whether I'm right or Sam is, one of us still still doesn't get it. :) )

Sam5
2006-Feb-17, 09:16 PM
Yes, it is. As I said, "my" thought experiment is Einstein's "peculiar consequence." I used different words, and gave specific time/distance numbers, but it is the same thing.

No, you are still trying to argue your opinion of his 1905 theory while totally ignoring his own 1918 correction paper, this one:

http://tinypic.com/view/?pic=nysb2r

...in which he added gravitational fields, acceleration, atomic clocks, and Kräfte.

I’m explaining what he added to his 1918 paper and how he modified his 1905 thought experiments after he realized the 1905 reason for his version of Lorentz’s 1895 time dilation phenomenon was wrong and after what he learned about atomic clocks, gravity, and acceleration.

Everyone who wants to discuss this subject should read the English translation of his 1918 paper, because it contains his own “solution” to the clock paradox, and that solution involves adding GR gravitational fields, acceleration, and atomic clocks, since the original 1905 SR clock paradox was not resolvable just by using the original terms of the 1905 paper.

Disinfo Agent
2006-Feb-17, 09:25 PM
(Agent, how is a thread with this one's title not a good place to discuss this? Whether I'm right or Sam is, one of us still still doesn't get it. :) )Good point!

Sam5
2006-Feb-17, 09:45 PM
(Agent, how is a thread with this one's title not a good place to discuss this? Whether I'm right or Sam is, one of us still still doesn't get it. :) )

Lol, I answered afterburner’s original post and explained why he “didn’t get it”. He didn’t get it because the paradox is unresolvable using only the terms of the original 1905 theory. This is why Einstein had to add GR terms to his 1918 paper and change the 1905 thought experiments to include gravity, acceleration, and atomic clocks.

Celestial Mechanic
2006-Feb-17, 10:13 PM
There is no "clock paradox" or "twin paradox" anymore than Euclidean geometry is "at the crossroads" because of a "triangle paradox". The reason that people do not get this even after a century of special relativity is that people do not have daily personal experience with the differences in proper time. It is all too easy to assume that we are each measuring some Newtonian absolute time when we are not.

Everyone understands from daily experience that the distance travelled depends on the path taken and that the shortest distance is a geodesic, which reduces to a straight line in a flat space. No one would expect that travel from A to B should be symmetrical with travel from A to C and then to B in a triangle, no one expects the distances to be the same, no one would resort to lengthening rulers as an explanation of the "triangle paradox". Everyone knows that there is no "triangle paradox" in geometry and they know it from daily experience.

What is difficult to understand is that the actual time elapsed also depends on the path taken; that the standard "Anne and Bob" scenario is also a triangle non-paradox where the straightest line (again a geodesic) has the greatest elapsed time. Nobody's clock is "running slow"; the elapsed times are really different.

SeanF
2006-Feb-17, 10:32 PM
No, you are still trying to argue your opinion of his 1905 theory while totally ignoring his own 1918 correction paper
You're still under the impression that General Relativity was a correction to Special Relativity when it was actually an expansion of it. If there's something wrong in his 1905 paper, there's no need to go to his 1918 paper to see it.

There's nothing wrong with the 1905 paper, it was just incomplete in that it didn't take into account the additional effects of gravity and/or acceleration. That doesn't mean the relative-motion effects it did deal with are incorrect. They're not.

Everyone who wants to discuss this subject should read the English translation of his 1918 paper, because it contains his own “solution” to the clock paradox, and that solution involves adding GR gravitational fields, acceleration, and atomic clocks, since the original 1905 SR clock paradox was not resolvable just by using the original terms of the 1905 paper.
Again, the 1918 paper is an extension of Relativity into the areas of gravity and acceleration. It is not a correction of the 1905 paper because there is nothing to correct. There. Is. No. Paradox.

Sam5
2006-Feb-18, 01:46 AM
That doesn't mean the relative-motion effects it did deal with are incorrect.

If relative motion alone would do it, would slow down one of the relatively moving clocks, he wouldn’t have had to add the gravity and acceleration forces to the thought experiments of his 1918 paper in order to try to resolve the clock paradox of the 1905 theory, and there wouldn’t have been a debate over the 1905 clock paradox for the past 101 years.

The original 1895 Lorentz time dilation theory has the forces, the 1905 theory does not, the 1918 paper has them added to the 1905 thought experiments to correct the 1905 error.

SeanF
2006-Feb-18, 02:33 AM
If relative motion alone would do it, would slow down one of the relatively moving clocks, he wouldn’t have had to add the gravity and acceleration forces to the thought experiments of his 1918 paper in order to try to resolve the clock paradox of the 1905 theory...
He didn't add the gravity and acceleration forces for that reason. He added them because the the original theory was incomplete - it only dealt with non-accelerating, non-gravitational situations.

...and there wouldn’t have been a debate over the 1905 clock paradox for the past 101 years.
That's ridiculous. It makes just as much sense to say that there'd be no debate over it if your interpretation were wrong. The debate is because people don't understand it.

Sam5
2006-Feb-18, 03:05 AM
People can read the 1918 paper in Volume 7 of “The Collected Papers of Albert Einstein”, Princeton Press, (paperback edition) and see what I am talking about. He tries to correct the clock paradox of the 1905 paper by adding acceleration and gravity effects and atomic clocks. In the 1918 paper one clock in his new modified version of the 1905 thought experiments experiences more gravity than the other and thus slows down. This gravity is not in the 1905 theory.

Ken G
2006-Feb-18, 07:22 AM
Sam5 and SeanF are not really disagreeing that much, it is really just what is meant by the difference between an error and an admitted incompleteness. What Sam5 is calling an error is actually more properly characterized as an incompleteness, as pointed out by SeanF. SR simply doesn't apply if you have gravity or acceleration, so it can't generate a paradox in those situations, it admits it just won't work. My car won't work if I don't put gas in it, is that an error in its design? Would I turn the key, hear it try to start with no gas, and think "what a paradox, my car worked yesterday but today it won't"? No, it is only designed to work when it has gas. The incompleteness of SR is fixed in the 1918 paper, which should be thought of as the culmination of Einstein's work on relativity, not a correction to it. Why would anyone characterize that as an error? But it certainly was an incompleteness, so what are we really arguing about here?

the_bullet
2006-Feb-18, 08:05 AM
Please keep in mind, though, that while it is acceleration that breaks the symmetry, it is not simply acceleration that causes the slow-down.

Consider where we have two clocks. One clock is left in the "original" inertial frame. The other clock is:

1) accelerated to 0.6c (relative to the first clock).
2) left to "cruise" at 0.6c for some amount of time.
3) decelerated back to 0.
4) reaccelerated towards the original clock back to 0.6c.
5) left to "cruise" at 0.6c for the same amount of time as in step 2)
6) decelerated back to 0 so it "stops" right next to the original clock.

The clock that accelerated out and back will lag behind, right? Simple enough.

But what if we have a third clock that undergoes the same process, with identical accelerations/decelerations in steps 1), 3), 4), and 6), but left to "cruise" for a longer time in steps 2) and 5)?

At the end of the experiment, how will this third clock compare to the second clock, considering that they were both subjected to identical accelerations (albeit in different locations)?

The third clock will lag behind the second because it spent more time in the other inertial frames (steps 2) and 5)).

What if you have 2 ships who fly many big orbits(using the term loosely) around a star but in opposite directions. Each would perceive the others clock to be ticking slower. Yet when they meet up on each orbit they will be able to check each others clocks via signal or whatever. Would this result in a paradox ?

Ken G
2006-Feb-18, 01:24 PM
Very interesting question, the bullet. The answer must be that they will not perceive each other's clocks moving slowly, since it would indeed result in a paradox by the symmetry of the situation. It is analogous to the twin paradox (which isn't really a paradox) in a closed universe where one twin went all the way around and met the other from the back side. In the latter case, I'm sure the answer must also be that the twins will be the same age, but the reason is not obvious. It must come from the fact that both twins wuold be in a gravity field, so both would be noninertial reference frames. In such a frame, the whole rest of the universe and all its contents are accelerating, so the other clock is not only moving and not only accelerating, but it is accelerating in a changing gravitational field as the rest of the universe accelerates too. This latter effect must compensate the slowing of time due to the other two, in some way that is completely beyond my ability to do general relativity, which was already limited before the bullet raised this question!

grant hutchison
2006-Feb-18, 04:09 PM
Hm. Let me think about this one in terms of space travellers leaving and returning: two period of coasting, interspersed with periods of acceleration. The usual scenario.
With reference to a space-time diagram: It seems to me that the fact clocks are no longer synchronized when a star-traveller returns to Earth is because of a shift in the line joining simultaneous events in the star-traveller's frame of reference. In the Earth observer's reference frame, the star-travellers lines of simultaneity slope backwards into Earth's past as he recedes from us, and then forward into Earth's future as he approaches us. As the star-traveller decelerates at his most distant point and then accelerates towards Earth, his lines of simultaneity swing swiftly into the future at Earth: a relatively short elapsed time under acceleration shifts the simultaneity lines through a wide span of time on the distant unaccelerated Earth.
If we imagine two spacecraft separating in opposite directions from the Earth, and then returning, both will do the same trick with their lines of simultaneity as they turn around at their most distant point. Both will find the same time-keeping mismatch with Earth's clocks on return to Earth. But their concepts of simultaneity will have done equal and opposite tricks with each other, both shifting through a broad span of distant time in a short span of on-board time, and so their clocks will remain synchronized.
It seems to me that each should see the other's clock running slow while they are in unaccelerated relative motion, so that some compensatory change must occur while they are under acceleration, but my brain is letting me down at that point.

Grant Hutchison

SeanF
2006-Feb-18, 04:44 PM
Sam5 and SeanF are not really disagreeing that much, it is really just what is meant by the difference between an error and an admitted incompleteness...But it certainly was an incompleteness, so what are we really arguing about here?
No, we really are disagreeing. :) Here's an analogy of the situation:

Einstein, Inc., produces a new satellite phone technology. The initial production, however, only works when the skies are perfectly clear. Since that is virtually impossible, the system is virtually useless in the real world. They make some modifications to the technology so that it works even with unclear skies, and the system is very successful.

Sam5's argument, however, is that the "clear skies" thing is a misunderstanding, that the initial product did not and could not work at all, even allowing for perfectly clear skies. He is not arguing that the original release was incomplete, he is arguing that it was wrong, and that whatever modifications they made would have been necessary regardless of the clarity of the atmosphere.

What if you have 2 ships who fly many big orbits(using the term loosely) around a star but in opposite directions. Each would perceive the others clock to be ticking slower. Yet when they meet up on each orbit they will be able to check each others clocks via signal or whatever. Would this result in a paradox ?
I think Ken's right - they would actually measure each other as ticking at the same rate. I suppose it is possible that they would measure the other clock as speeding up and slowing down so that it ends the same, but I don't think so.

In my post that "the bullet" referenced, you would expect the clocks to end up the same if the "cruising time" were the same. That's obvious if they're both moving in the same direction. But if they're moving in opposite directions, they'd be in relative motion to each other the entire experiment! In this case, they would see the other as ticking more slowly as they were in inertial motion. However, during the acceleration/deceleration phases, they would measure each other as ticking much faster - enough to offset the slowdown measured doing the inertial motion.

[ADDED: grant posted while I was typing this. I do believe you've got it exactly right, grant. The "compensatory change" you're looking for during the acceleration is the swing of the simultaneity line you mentioned. It's interesting in that when "Clock A" is accelerating, he sees "Clock B" as ticking faster and his own clock as maintaining the same rate, but the actual change that's occuring is at Clock A - it is his measurement of the universe around him that is changing]

the_bullet
2006-Feb-18, 05:07 PM
I think Ken's right - they would actually measure each other as ticking at the same rate. I suppose it is possible that they would measure the other clock as speeding up and slowing down so that it ends the same, but I don't think so.

You may be correct and both your and Ken's GR and SR knowledge far exceeds mine. However, the clocks have no idea they will meet up again. All they see is the other clock moving in relation to it and so therefore "should" see the moving clock ticking slower. There is obviously something simple I am overlooking here.:confused:

Sock puppet
2006-Feb-18, 05:49 PM
You may be correct and both your and Ken's GR and SR knowledge far exceeds mine. However, the clocks have no idea they will meet up again. All they see is the other clock moving in relation to it and so therefore "should" see the moving clock ticking slower. There is obviously something simple I am overlooking here.:confused:
I'm in a hurry, so this'll be a short post. Maybe someone else will elaborate on it:
For half of each orbit, the ships are accelerating towards each other, and each one sees the other's clock running fast. for the other half, they are accelerating away from each other and will see the others clocks running slow. The two situations exactly balance since the duration and magnitude of the acceleration will be equal in each direction. There will be no GR effect due to being in the gravity well of the star as both are at the same depth in it.

Sorry if this isn't clear. If someone hasn't explained this better by monday, I will.

Edit: the above is actually wrong :( Apologies.

nokton
2006-Feb-18, 06:40 PM
And, of course, since this is the "Questions and Answers" section, if Sam5 wants to try (again) to support his claim that there's a contradiction in special relativity that Einstein realized and corrected by introducing general relativity, he should really do so in a separate thread in the AtM section.

It's a (perhaps very) small physics joke. In quantum physics, the Hamiltonian is an operator that gives you the energy of the system you're looking at. There are other operators that can give you other values, or that can change the wavefunction to a different one in some way. Two operators that are mutually compatible are said to "commute", so an operator that commutes with the Hamiltonian leaves the energy of the system unchanged, and energy is conserved under the application of such an operator. But the form of the statement also parallels something like a slogan for carpooling, where "conserve energy" has an entirely different meaning from the one used in physics. Someone might say something like, "Conserve energy. Commute with a friend.", suggesting that you should drive to work together in order not to use so much gasoline. I fear, like most jokes, that whatever humor value it might have had is quickly drained by being explained in detail, though. :).
In the same way two electrons can 'communicate', then separate, but each
electron knows where the other one is.... Grey, not in contest with you.
Respect you.
Nokton

the_bullet
2006-Feb-18, 07:22 PM
I'm in a hurry, so this'll be a short post. Maybe someone else will elaborate on it:
For half of each orbit, the ships are accelerating towards each other, and each one sees the other's clock running fast. for the other half, they are accelerating away from each other and will see the others clocks running slow. The two situations exactly balance since the duration and magnitude of the acceleration will be equal in each direction. There will be no GR effect due to being in the gravity well of the star as both are at the same depth in it.

Sorry if this isn't clear. If someone hasn't explained this better by monday, I will.
Thanks for the reply Sock Puppet. My knowledge is limited so apologies if my questions seem naive. If the spaceships were in "true" orbits around say a black hole so that they might reach significant speeds, aren't these orbits just straight lines through space-time and so they are moving unaccelerated ? Thanks...

Ken G
2006-Feb-18, 08:25 PM
I think the answer to that last question is that by the equivalence principle, you can replace acceleration by the effects of gravity, and therefore have unaccelerated motion but in a curved spacetime. So you don't escape the ramifications for clock desynchronization, just as you can have a twin paradox where one twin goes off, half-orbits a black hole, and returns. Grant's scenario is a good one to think about-- imagine triplets, where two twins go off in opposite directions and both stay young even though they think their traveling sibling's clock is even slower than the one that stayed behind. The key seems to be that acceleration (or gravity) causes desynchronization, but not acceleration alone-- the farther away the acceleration occurs, the greater the desynchronization (an accelerating clock and a nonaccelerating clock at the same point do not
begin to desynchronize until they physically separate.) Each twin thinks the other's clock is quite slow until they notice they are passing into a gravity field that causes distant clocks to go quite rapidly. In the case of the two orbiting twins, each thinks the other clock is slow until they separate, at which point the acceleration (or gravity) effect dominates and they think the other clock speeds up to compensate by the time they come together again. Thus I suspect (but don't know) that the orbiters think their own clock gets ahead at first, then falls behind at maximum separation, then catches up again as they approach. This is a bit like what Sock puppet was saying, but I don't think the direction of the acceleration is the key, I think it's more the displacement of the acceleration. Perhaps the direction plays a role too, I wouldn't rule anything out without knowing how to really do the calculation!

nokton
2006-Feb-19, 07:41 PM
And, of course, since this is the "Questions and Answers" section, if Sam5 wants to try (again) to support his claim that there's a contradiction in special relativity that Einstein realized and corrected by introducing general relativity, he should really do so in a separate thread in the AtM section.

It's a (perhaps very) small physics joke. In quantum physics, the Hamiltonian is an operator that gives you the energy of the system you're looking at. There are other operators that can give you other values, or that can change the wavefunction to a different one in some way. Two operators that are mutually compatible are said to "commute", so an operator that commutes with the Hamiltonian leaves the energy of the system unchanged, and energy is conserved under the application of such an operator. But the form of the statement also parallels something like a slogan for carpooling, where "conserve energy" has an entirely different meaning from the one used in physics. Someone might say something like, "Conserve energy. Commute with a friend.", suggesting that you should drive to work together in order not to use so much gasoline. I fear, like most jokes, that whatever humor value it might have had is quickly drained by being explained in detail, though. :)
Grey, thankyou.
The quantum physics scenario, vis a vis Hamilton makes sense
to me. Your description is not of a joke but real science. The energy of any
quantum system is related to that of the observer. Observe it is to change it.
The change is defined by the energies of the observer and the observed.
If that is so, if equilibrium of energy is maintained between the two, then
an observer would see the truth.
Nokton.

Ufonaut99
2006-Feb-20, 09:25 AM
Let's ignore GR (acceleration and gravity), and assume everything's instantaneous (or, for the perfectionists, we accelerate in 1 second to 99%c :) - the effect on the clocks will be negligible - right?).

So, 2 of our triplets take off in opposite directions from Earth, coast, then decelerate to Earth's FoR - where they find that they're 1 light year away (2ly from eachother). They stay there for <insert big number> years, then reverse the procedure to return to Earth.

Now, while each ship is heading away from Earth, they see Earth's clock running slow, and the other ship's clock running slower. When they coast inn Earth's FoR, they eventually see the other ship also stop and enter the same FoR. When they look at that ship's clock, they see it's now advancing at the same rate, but a long time back.

When both ships return to Earth, they find they're a lot younger than the Earth-bound triplet, but (since they're performed the same manoovers) their clocks must agree. So each ship must see the other's clock advancing fast as they come closer.

grant hutchison
2006-Feb-20, 10:19 AM
When both ships return to Earth, they find they're a lot younger than the Earth-bound triplet, but (since they're performed the same manoovers) their clocks must agree. So each ship must see the other's clock advancing fast as they come closer.I don't think it's valid to exclude the acceleration phase, since any given change in velocity will change the slope of the lines of simultaneity by the same amount, no matter whether it's achieved slowly or quickly. Those swinging lines of simultaneity during acceleration drive the desynchronization of the clocks.
The ships might see each other's clocks advance quickly during the approach phase (because they're running into oncoming light), but once they'd allowed for that, each would deduce that the other's clock was running slow, just as they did during the separation phase. Their relative velocities are the same; only the relative positions of the spacecraft have changed to make this encounter an approach rather than a recession. Given that the relative velocities are the same in both situations, SR predicts the same slowdown effect. (it's a v-squared term in the equation, so it doesn't care whether your sums feed it a positive or negative velocity.)
I think the compensatory change in observed rate must occur during the distant de/acceleration phase. During that time interval, one observer will judge her own time to be simultaneous with a broad span of time aboard the other ship, including not just the other ship's de/acceleration phase but a chunk of its coasting phase, too. This suggests that either accelerated observer would deduce that the distant ship's clocks ran fast so long as her local acceleration continued. My slight hesitancy about this is that I don't know enough to actually write down the formulae, so there's a distinct degree of handwaving about the logic.

Grant Hutchison

Edit: In your example when the distant ships observe each other for a while before returning to Earth, I believe they'd find their clocks showed the same elapsed time (once they'd compensated for the distance separating them and the light travel time across that distance). The simultaneity changes during their deceleration to rest would have compensated for the slow-down observed during free flight.

Ken G
2006-Feb-20, 11:16 AM
Yes, acceleration coupled with displacement causes desynchronization. The farther the displacement, the greater the desynchronization. Great acceleration for a short time has the same effect as gradual acceleration for a long time, what really matters is the change in speed and the displacement where it happens. I believe that each and every point made by Grant is correct. So in RobA's example, as the opposite ships separate, each thinks the other's time has lagged theirs. If they decerate to a halt, the other's clock quickly catches up. Then if they accelerate back toward Earth, the other's clock actually gets ahead, but they catch up with the other's clock during the return trip. Something similar must happen when both are orbiting and they meet up-- for awhile they lead the other's time, then they fall behind by the time they reach opposite orbital points, then they catch up on the return.

Sam5
2006-Feb-20, 02:00 PM
I don't think it's valid to exclude the acceleration phase, since any given change in velocity will change the slope of the lines of simultaneity by the same amount, no matter whether it's achieved slowly or quickly. Those swinging lines of simultaneity during acceleration drive the desynchronization of the clocks.

That’s a basic problem with the 1905 theory. It leaves out any consideration of acceleration, leaving only “relative motion” as the “cause” of the clock slow-down, and that was an error which he corrected in his 1918 paper. The “relative motion” alone places no “force” on either clock, and with no “force” neither clock has a physical reason to slow down. But with acceleration (or gravity) considered, then the slow-down is caused by the forces placed upon the individual clock that experiences the forces.

Sam5
2006-Feb-20, 02:14 PM
So in RobA's example, as the opposite ships separate, each thinks the other's time has lagged theirs. If they decerate to a halt, the other's clock quickly catches up. Then if they accelerate back toward Earth, the other's clock actually gets ahead, but they catch up with the other's clock during the return trip. Something similar must happen when both are orbiting and they meet up-- for awhile they lead the other's time, then they fall behind by the time they reach opposite orbital points, then they catch up on the return.

I think this is merely a Doppler-effect illusion. The clocks don’t actually change rates. The distant clock appears to each observer at each of two separating clocks to be ticking more slowly while separating because of the constantly increasing transmission lag time of the light signals from the clocks to the observers. Then when approaching each other, the distant observed clock appears to be ticking faster due to the decreasing transmission lag time of the light signals from the clocks to each other’s observer.

Then if we have any kind of different gravity or acceleration effect on either atomic clock, that would physically slow down that clock, and that slow-down is a real effect, not a Doppler illusion. If the gravity or acceleration forces are equal on each clock, then both would physically slow down the same amount.

Grey
2006-Feb-20, 02:55 PM
SR simply doesn't apply if you have gravity or acceleration, so it can't generate a paradox in those situations, it admits it just won't work.This isn't actually correct. Special relativity cannot deal with gravity, but it can and does work under circumstances where something is accelerating, contrary to Sam5's claims. You do this by considering a series of reference frames, each of which is inertial, but which are all moving relative to each other, chosen such that the accelerating object is stationary with respect to each one in series. And of course, you then take the limit as the relative velocity goes to zero and the number of intermediate reference frames goes to infinity. For any of the various scenarios described above, you could work out what special relativity would predict, and it always works out without paradox.

You may be correct and both your and Ken's GR and SR knowledge far exceeds mine. However, the clocks have no idea they will meet up again. All they see is the other clock moving in relation to it and so therefore "should" see the moving clock ticking slower. There is obviously something simple I am overlooking here.:confused:The thing you're overlooking is the above, that neither of these two spaceships remains in a single inertial reference frame, so the calculations aren't quite so simple. If people think it would be helpful, we could probably pick one of these cases and work out what happens.

SeanF
2006-Feb-20, 03:08 PM
That’s a basic problem with the 1905 theory. It leaves out any consideration of acceleration, leaving only “relative motion” as the “cause” of the clock slow-down, and that was an error which he corrected in his 1918 paper.
Two clocks. In motion relative to each other. At the instant they pass each other, when they are in the exact same place at the exact same time (negligible difference, anyway). The only difference between them is the relative motion.

Those two clocks will measure the universe around them differently - and I'm not just talking about them measuring each other as ticking slowly, either. They will also disagree, for example, on the amount of time that passed between some distant event and the two clocks passing each other.

Therefore, if a single clock has it's relative motion changed, that single clock will measure the universe around it differently after the change than before.

Since everything before that "Therefore" was explicitly in the 1905 paper, what comes after the "Therefore" is implicitly there.

Thus, while the 1905 paper does not deal with what that clock sees while it is accelerating, it is incorrect to suggest that it "leaves out any consideration of acceleration." It quite clearly does not.

Sam5
2006-Feb-20, 11:59 PM
Thus, while the 1905 paper does not deal with what that clock sees while it is accelerating, it is incorrect to suggest that it "leaves out any consideration of acceleration." It quite clearly does not.

From the 1905 paper:

“a uniform motion of parallel translation with velocity v along the axis of x in the direction of increasing x is then imparted to the rod”

“let a constant velocity c be imparted in the direction of the increasing x”

“two systems of coordinates in uniform translatory motion”

“§ 3. Theory of the Transformation of Co-ordinates and Times from a Stationary System to another System in Uniform Motion of Translation Relatively to the Former”

“a uniform motion of parallel translation with velocity v along the axis of x”

“moving relatively to the system K with velocity v”

“viewed from a system in uniform motion”

“the clock at A is moved with the velocity v along the line AB”

Sam5
2006-Feb-21, 12:09 AM
A. Einstein, 1916:

“Relativity: The Special and General Theory

Part II, The General Theory of Relativity

XVIII

Special and General Principle of Relativity

The basal principle, which was the pivot of all our previous considerations, was the special principle of relativity, i.e. the principle of the physical relativity of all uniform motion.”

Jeff Root
2006-Feb-21, 01:12 AM
Sam5,

Sean and Grey understand all that perfectly. In the same posts
that you replied to, they clearly addressed the objection you raise,
and explained how to deal with it.

-- Jeff, in Minneapolis

SeanF
2006-Feb-21, 03:37 AM
Sam5, did you even read my post?

The 1905 paper says that if Clock A and B are at rest relative to each other, they will measure space and time the same. If they are in "uniform motion" relative to each other, they will measure space and time differently. That's the entire concept of "reference frames" in a nutshell right there.

So if Clocks A and B are in motion relative to each other, and Clock C starts out motionless relative to A and then changes so that it is motionless relative to B, we can determine how Clock C's measurement of time and space will differ after the change compared to before - using nothing but the 1905 paper - since C is in a state of "uniform motion" relative to A and/or B in both states.

What we can't determine is how C will measure time and space during the change. That's what the later papers added.

It's really that simple.

woodguard
2006-Feb-21, 04:54 AM
Time dilation. I use this simple model.
Don’t think of time as time, because everything you know about time is wrong. Time is not a fix thing. You only think time is fixed because everything you see moves with you and at the same speed or close to it.

As you move faster the distance between seconds get longer.

If one clock is not moving(in relationship to the first person). So the distance between seconds is normal to them. If you looked at both clocks, they are moving thru time at the same rate.

You start running down the street away from the first person and you are running very fast.(There is smoke coming from your shoes) As you and your clock move faster the distance time travel between seconds is long. You don’t see this because you are the one moving.

But you and the clock are moving at the same speed and you see the clock as normal. If you looked over you shoulder at the first person’s clock, you would see it running faster. It not the first persons clock that is run fast but you are traveling slower thru time.

The faster you go the more distances between the seconds and at the speed of light(or very near), the distance is infinite(or too far to travel) and you cannot get from one second to the next.

If you throw your clock to the first person and they caught it, without killing themselves with the kinetic energy. Your clock traveled slower thru time but now that it is stopped, both clocks would move thru time at the same rate.But yours would be offset to an early time. The first person clock lived normal, but your clock lived shorted because it travel thru time slow relative to the other clock.

But you trip and fall and die. So you time is up!

Einstein role over three time with that description. :o

Grey
2006-Feb-21, 06:44 PM
What we can't determine is how C will measure time and space during the change. That's what the later papers added.Again, a small nitpick. We can determine this from special relativity. We consider the motion of C at each moment as a distinct reference frame, and we can describe what C observes using a succession of such frames. It's more complex to work out the math than in the simple uniform motion cases, but the calculations are all essentially the same. What we cannot do with special relativity is figure out what C would observe while in a gravitational field, and what general relativity assumes is that it will be exactly the same as what special relativity would predict for an an accelerating object without a gravitational field.

nokton
2006-Feb-21, 07:24 PM
Sam 5,
Relative motion is what Albert was all about and realised.
Never mind discourse about his old theories. There is no 'force'
per see, just reason, logic, and math.
Nokton.

SeanF
2006-Feb-21, 07:29 PM
Again, a small nitpick. We can determine this from special relativity. We consider the motion of C at each moment as a distinct reference frame, and we can describe what C observes using a succession of such frames. It's more complex to work out the math than in the simple uniform motion cases, but the calculations are all essentially the same. What we cannot do with special relativity is figure out what C would observe while in a gravitational field, and what general relativity assumes is that it will be exactly the same as what special relativity would predict for an an accelerating object without a gravitational field.
Yes, you're quite right, Grey. :)

Sam5
2006-Feb-21, 11:38 PM
Sam 5,
Relative motion is what Albert was all about and realised.
Never mind discourse about his old theories. There is no 'force'
per see, just reason, logic, and math.
Nokton.

Then why do you think Einstein changed the reason for the clock slowdown from just “relative motion” in his original 1905 paper to “a gravitational field” in his 1918 “solution” to the 1905 “clock paradox”?

Ken G
2006-Feb-22, 01:10 AM
There was no "change" in the reason for the clock slowdown. In the twin paradox, both forms of time change occur-- first, the moving twin concludes that the stay-at-home twin's clock is moving slowly, for just the usual special relativity reason, but then while the twin is turning around, the stay-at-home clock will be perceived to run very quickly indeed. Whether this latter bit is part of special relativity or not seems to be a point at issue in this thread, as Grant and Grey have posted arguments where you can see the desynchronization of the clocks as simply due to a changing sense of simultaneity between the frames. But if we use a gravity field to do the turnaround, then it's clearly a general relativity effect, even though it gets the same answer by the equivalence principle. So I think you may actually argue that the GR paper sets up GR just so as not to change the answer when gravity is involved (i.e., the equivalence principle).

SeanF
2006-Feb-22, 03:24 AM
Then why do you think Einstein changed the reason for the clock slowdown from just “relative motion” in his original 1905 paper to “a gravitational field” in his 1918 “solution” to the 1905 “clock paradox”?
What makes you think his 1918 paper was intended as a "solution" to some "paradox" in the first place? You're asking why he did this, but there's no reason to believe that's what he did.

Sam5
2006-Feb-22, 03:30 AM
There was no "change" in the reason for the clock slowdown. In the twin paradox, both forms of time change occur-- first, the moving twin concludes that the stay-at-home twin's clock is moving slowly, for just the usual special relativity reason, but then while the twin is turning around, the stay-at-home clock will be perceived to run very quickly indeed. Whether this latter bit is part of special relativity or not seems to be a point at issue in this thread, as Grant and Grey have posted arguments where you can see the desynchronization of the clocks as simply due to a changing sense of simultaneity between the frames. But if we use a gravity field to do the turnaround, then it's clearly a general relativity effect, even though it gets the same answer by the equivalence principle. So I think you may actually argue that the GR paper sets up GR just so as not to change the answer when gravity is involved (i.e., the equivalence principle).

No. What you are posting are internet message board ideas and personal opinions from modern books and magazine articles. These aren’t in the SR theory or the 1918 paper. There is no “turn around” in the clock paradox of the first “peculiar consequence” thought experiment in the 1905 paper, and acceleration is not considered, as I have shown in the Einstein quotes. There are no “twin” humans in the SR paper, and no “blast off” from the earth in a rocket or “turn around” in space and no zoom back to the earth. These are modern pop culture ideas. These have nothing to do with the SR theory, or Lorentz’s original time dilation idea or Einstein’s 1905 version of it or Einstein’s 1918 correction paper or the original controversy regarding the clock paradox.

Nobody disagrees about accelerating an atomic clock out into space and accelerating it in a turn-around and accelerating it back to earth and accelerating it to slow it down when back at the earth. Of course that clock will slow down its tick rates during each of its accelerations, but that is not a “clock paradox” and that is not related to the 1905 theory in any way.

Sam5
2006-Feb-22, 03:33 AM
What makes you think his 1918 paper was intended as a "solution" to some "paradox" in the first place? You're asking why he did this, but there's no reason to believe that's what he did.

Read the paper and you’ll see that’s why he did it.

grant hutchison
2006-Feb-22, 12:56 PM
We consider the motion of C at each moment as a distinct reference frame, and we can describe what C observes using a succession of such frames.I certainly recall having to do exactly this, as an assigned exercise, some 20 years ago, to derive the elapsed time, velocity and distance covered by a "starship" with constant acceleration in its own reference frame (or its own succession of instantaneous reference frames, to be precise). It all came out pleasantly with an unexpected crop of hyperbolic trig functions.
I can confidently state that the calculation was based entirely on SR, since I'm utterly ignorant of GR to this day. :)

Grant Hutchison

AstroSmurf
2006-Feb-22, 02:05 PM
No. What you are posting are internet message board ideas and personal opinions from modern books and magazine articles. These aren’t in the SR theory or the 1918 paper. [...] These have nothing to do with the SR theory, or Lorentz’s original time dilation idea or Einstein’s 1905 version of it or Einstein’s 1918 correction paper or the original controversy regarding the clock paradox.
But neither is acceleration necessary to explain the results of the 'peculiar consequence'. You are seeing the combined effects of time dilation, length contraction and simultaneity differences. This will be the case regardless of whether SR or GR are used to calculate the effects, since they are identical for this experimental setup.

Nobody disagrees about accelerating an atomic clock out into space and accelerating it in a turn-around and accelerating it back to earth and accelerating it to slow it down when back at the earth. Of course that clock will slow down its tick rates during each of its accelerations, but that is not a “clock paradox” and that is not related to the 1905 theory in any way.
That doesn't suffice as an explanation. The difference in elapsed travel time after the round-trip is dependant on the length and speed of the constant-velocity phases; this will be the result even if the experiment is set up so that no acceleration whatsoever takes place.

Grey
2006-Feb-22, 02:19 PM
...and acceleration is not considered, as I have shown in the Einstein quotes.Let's look at one of your quotes from Einstein.

...a uniform motion of parallel translation with velocity v along the axis of x...Now let's check the whole text from this section of the original paper.

Let there be given a stationary rigid rod; and let its length be l as measured by a measuring-rod which is also stationary. We now imagine the axis of the rod lying along the axis of x of the stationary system of co-ordinates, and that a uniform motion of parallel translation with velocity v along the axis of x in the direction of increasing x is then imparted to the rod.Hey, look at that. The rod starts stationary, and then motion is imparted to it. That sounds like a change in velocity. Don't physicists have a special name for a change in velocity? :)

Or in the section talking about the "peculiar consequence".

It is at once apparent that this result still holds good if the clock moves from A to B in any polygonal line, and also when the points A and B coincide.

If we assume that the result proved for a polygonal line is also valid for a continuously curved line...Hey, look at that. We're talking about an object that's moving in something other than a straight line. Looks like the same issue. So your quotes look to me like you're deliberately pulling excerpts out of context to try to mislead those reading your post.

SeanF
2006-Feb-22, 02:35 PM
Nobody disagrees about accelerating an atomic clock out into space and accelerating it in a turn-around and accelerating it back to earth and accelerating it to slow it down when back at the earth.
I bet if you realized what GR really says about that clock, you'd disagree with it.

Read the paper and you’ll see that’s why he did it.
Which sounds like he never actually said that's why he did it, you just think that's why he did it. And it seems that you think that's why he did it because you think there's a paradox that needs fixing. Circular logic, and not very persuasive.

Ken G
2006-Feb-22, 02:48 PM
I certainly recall having to do exactly this, as an assigned exercise, some 20 years ago, to derive the elapsed time, velocity and distance covered by a "starship" with constant acceleration in its own reference frame (or its own succession of instantaneous reference frames, to be precise).
And if I understand what you are saying correctly, this calculation must include two elements, must it not? First is the time dilation when you want to map back to the time for the stationary Earth, but then there must also be a continuous desynchronization effect due to the fact that the velocity is changing at some distance from Earth. I've never seen how that gets included, there must be some trigonometry there yes?

Ken G
2006-Feb-22, 02:54 PM
Of course that clock will slow down its tick rates during each of its accelerations, but that is not a “clock paradox” and that is not related to the 1905 theory in any way.
What do you mean that the accelerated clock slows down in each of its accelerations? From the point of view of Earth, I would have said that the clock first slows down as it is accelerated away from Earth, then does not chance its rate when it turns around, then speeds up as it is decelerated on arrival at Earth. How is it "of course" slowing down its tick rate during each acceleration?

SeanF
2006-Feb-22, 03:23 PM
What do you mean that the accelerated clock slows down in each of its accelerations? From the point of view of Earth, I would have said that the clock first slows down as it is accelerated away from Earth, then does not chance its rate when it turns around, then speeds up as it is decelerated on arrival at Earth. How is it "of course" slowing down its tick rate during each acceleration?
It would change during the turn-around in that it would speed up as it decelerates to a stop (so that it is ticking the same as the Earth clock when it is stopped), then slow down again as it reaccelerates, wouldn't it?

Nonetheless, Sam5's "understanding" of GR is simply that acceleration slows down clocks. You've got to realize that he believes that once the acceleration ceased and the clock was just cruising with a "uniform motion," the clock would immediately return to ticking at the "normal" rate (assuming, of course, the acceleration didn't permanently damage it somehow!) - and not only does he believe that, but he believes that's what GR says.

What I think is really interesting is what the accelerated clock observes the Earth clock as doing during the accelerations. That's primarily what I was referring to when I said that Sam5 would disagree with GR if he knew what it actually says.

grant hutchison
2006-Feb-22, 03:32 PM
And if I understand what you are saying correctly, this calculation must include two elements, must it not? First is the time dilation when you want to map back to the time for the stationary Earth, but then there must also be a continuous desynchronization effect due to the fact that the velocity is changing at some distance from Earth. I've never seen how that gets included, there must be some trigonometry there yes?That's where my anxiety concerning desynchronization comes from: I've never done that calculation, so I was unprepared to hang my hat on it, and was cheered by SeanF's support. As you say, the shift in simultaneity lines would have to be incorporated into the time dilation, so that we could say how much time actually elapsed in the distant body during the spacecraft's acceleration manoeuvre.
The calculations I was referring to were a simpler assignment, in which we looked at the spacecraft from an unaccelerated Earth: the slope of the simultaneity lines was therefore constant, and I had only to integrate through the spacecraft's successive instantaneous inertial frames.
It seemed to me that if we can deal with acceleration using only SR while looking from unaccelerated to accelerated, we should be able to deal with the reverse view without invoking GR. I may be wrong on that, of course, and I certainly don't have the maths skills in my head at present to deal with the more complicated calculation involving shifting simultaneity lines.

Grant Hutchison

Ken G
2006-Feb-22, 03:33 PM
It would change during the turn-around in that it would speed up as it decelerates to a stop (so that it is ticking the same as the Earth clock when it is stopped), then slow down again as it reaccelerates, wouldn't it?

I was imagining the turnaround was very abrupt, for simplicity. But if you resolve the turnaround, I think you're right, the instaneous speed will always determine the ticking rate from the point of view of Earth.

What I think is really interesting is what the accelerated clock observes the Earth clock as doing during the accelerations.
Yes, it is interesting that the total elapsed times have to work out, but there's no agreement on during what part of the trip the time discrepancy primarily occurs. Earth thinks the discrepancy accumulated gradually sue to time dilation effects, the spaceship thinks most of the discrepancy happened very suddenly during the turnaround due to desynchronization effects.

Sam5
2006-Feb-22, 06:38 PM
Hey, look at that. The rod starts stationary, and then motion is imparted to it. That sounds like a change in velocity. Don't physicists have a special name for a change in velocity? :)

Yes, I know that. :) But his calculations are based on the steady speed v. He doesn’t consider any acceleration effect. There are no math terms in the theory for an acceleration effect. The terms are based on the steady speeds of the moving clocks.

You can consider the “imparting of the motion” to be instantaneous, therefore lasting for an infinitely small amount of time, therefore stopping the clock for an infinitely small amount of time. But he doesn’t even consider that. His theory and calculations are based on the length of time the rod is moving at a steady speed relative to the other rod. That is “relative motion” only, and acceleration is not considered.

In order to consider the time dilation effects of acceleration we must accelerate the rod continuously and figure it’s time rate slowdown as occurring ONLY during the time it is accelerating, but NOT during the time it is moving at a steady speed/velocity relative to the other rod. But when we do that, then we are considering the GR effects, not any SR effects. There is no clock paradox in GR, but there IS a clock paradox in SR.

And in the SR paper he didn’t consider any acceleration effects of the “closed curve” thought experiment. Look how he leads up to the closed curve, through a series of straight lines and turns, with NO acceleration being considered during the turns. The “closed curve” is an infinite number of these turns, without acceleration being considered.

He later realized the shortcomings of the SR paper and this led him to develope the GR theory. He mentions this often in some of his papers written between 1906 – 1913.

Here are some of his comments about the older SR theory, in one of his 1916 GR papers:

A. Einstein, 1916:

”The Foundation of the General Theory of Relativity

#1 Observations on the Special Theory of Relativity

If a system of co-ordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws also hold good in relation to any other system of c-ordinates K’ moving in uniform translation relatively to K. This postulate we call the ‘special principle of relativity.’ The word ‘special’ is meant to intimate that the principle is restricted to the case when K’ has a motion of uniform translation relatively to K, but that the equivalence of K’ and K does not extend to the case of non-uniform motion of K’ relatively to K.”

Here’s what Wolfgang Pauli said about it in his 1921 book:

“We can neglect the effect of acceleration on the clock, so long as we are dealing with a Galilean reference system. If we take the special case where C2 is moved along the x-axis to a point Q and then back gain to P, with discontinuous velocity changes at P and Q, then the effect of the acceleration will certainly be independent of t and can easily be eliminated.”

You are trying to imply that the acceleration caused by the “imparting of the motion” or the so-called “turn-around” accounts for the full time dilation effects in the SR theory, but this is clearly not true, since acceleration is ignored as being the time dilation cause. Only the steady v speed of the relative motion causes the time dilation effects during the “relative motion” of the SR theory.

Sam5
2006-Feb-22, 07:03 PM
But neither is acceleration necessary to explain the results of the 'peculiar consequence'. You are seeing the combined effects of time dilation, length contraction and simultaneity differences.

Hi :)

After receiving some criticism about his “relative motion” causing “length contraction” in the 1905 paper, Einstein felt compelled to respond to this legitimate criticism in his 1907 follow-up paper, “The Relativity Principle and the Conclusions Drawn From It,” in which he said:

“From this it follows that the laws of geometry determine the possible arrangements of rigid bodies in non-accelerated motion always in the same way, independent of their common state of motion. Assertions about the shape of a body in non-accelerated motion therefore have a direct meaning. The shape of a body in the sense indicated we will call its ‘geometric shape.’ The latter obviously does not depend on the state of motion of a reference system.”

In other words, NO “length contraction” due only to “relative motion”. Lorentz invented the concept of “length contraction” in his 1895 book, and his length contraction had a true physical cause, a “force” placed on atoms during their motion through a field or caused by acceleration, like trying to push an inflated balloon rapidly through air. It deforms and “length contracts” because of the force placed on it. But the SR theory had no such forces, so in the 1907 paper Einstein had to backtrack about the “length contraction” of the true geometrical shape of a body moving only “relative” to another body.

You need to study the true history of the SR and GR theories, by studying their original sources. There has been too much confusion and too many errors injected into the clock paradox debate over the years by modern writers. I strongly recommend that everyone should read the Lorentz theory, and the various Einstein follow-up papers written about the SR theory, including the 1918 paper.

SeanF
2006-Feb-22, 07:16 PM
There is no clock paradox in GR, but there IS a clock paradox in GR.
Are you sure about that? (Whoops - just noticed the typo. I'm assuming you meant "no paradox in GR, yes paradox in SR")

I don't know how many times I've asked you to give the GR equation for dilation of an accelerating atomic clock, and you've never answered. It is math, after all, and you've admitted you don't understand it.

There is a "clock paradox" in GR, at least as much as there is in SR (not surprising, I guess, since SR itself is "in" GR). However, the fact that Einstein never translated the math into explicit words in his GR papers like he did in the SR paper - coupled with your refusal to accept anybody else's translation of the math into words - means you don't realize it's there.

Grey
2006-Feb-22, 08:06 PM
Yes, I know that. :) But his calculations are based on the steady speed v. He doesn’t consider any acceleration effect. There are no math terms in the theory for an acceleration effect. The terms are based on the steady speeds of the moving clocks.Sure. That's the part where we're making things simple for clarity.

You can consider the “imparting of the motion” to be instantaneous, therefore lasting for an infinitely small amount of time, therefore stopping the clock for an infinitely small amount of time. But he doesn’t even consider that. His theory and calculations are based on the length of time the rod is moving at a steady speed relative to the other rod. That is “relative motion” only, and acceleration is not considered.Correct. And amazingly, when we perform experiments, these calculations give us exactly the right answers.

In order to consider the time dilation effects of acceleration we must accelerate the rod continuously and figure it’s time rate slowdown as occurring ONLY during the time it is accelerating, but NOT during the time it is moving at a steady speed/velocity relative to the other rod. But when we do that, then we are considering the GR effects, not any SR effects.I suppose we could, but then we'd get the wrong answers. For example, you could have two objects which are accelerated and decelerated in exactly the same way, but have a different "cruising time" where they are travelling relative to an observer at the same constant speed, but for a different amount of time. Special relativity correctly predicts that we'd see different values on the clocks in this case, whereas if we were to take your suggestion of considering only the period of acceleration, we'd get the wrong result.

There is no clock paradox in GR, but there IS a clock paradox in GR.I assume you mean that second one to be SR? That statement is simply not true.

afterburner
2006-Feb-22, 10:46 PM
Ok…here is my understanding of the theory…
PLEASE CORREC TME LINE BY LINE WHERE I WENT WRONG AFTER READING THE WHOLE THING>>>THANK YOU<<<<

1. Lets pretend that we are moving at the speed of light (on my magic spaceship) towards a clock that is floating in space…This clock sends a signal every second towards the spaceship also at the speed of light…. now…If the spaceship were stationary it would receive that signal every second (like its supposed to)…right? Now, because the spaceship is moving at the speed of light towards the signal, the ship will in fact see the signal every half a second, because the time it takes for the signal to reach the ship is halved by the fact that the ship itself is moving at the speed of light toward the clock…therefore the ship will think that the time outside the ship, and the time that the clock is experiencing is moving faster, or, depends on how you look at it, the time on the spaceship is moving slower…is this correct?

Moving on…

2. Now, lets look at it from the clocks perspective…lets also assume that the ship has a clock on its dashboard that also sends a signal every second towards the stationary clock. The stationary clock will NOT see the ships clock, because the light from the ships clock cant move faster than the speed of light, and therefore will not reach the stationary clock, until the ship gets there. Is this correct?

Now…

3. Lets do a little thought experiment (refer to the diagram below)

...........D
......._______
........../....\
......../........\
......./...........\
....../..............\
...../.................\
.../.....................\
../........................\
./...........................\
A--------------------B--------------------C--------------------S

First of all, the distances AB, AD, BD, BC, and CS are the same, and are equal to 300,000 km (or 299 792 458 meters). Point A is a stationary object with a clock (call it Earth if you wish). Point B is a bright light bulb that is going to turn on one second before the ship (traveling from right to left at the speed of light) is going to reach point C, so when the ship is at point S. The ship is moving at the speed of light, and will see the light from the bulb at the precise moment that it reaches C, so the observer at point A will see the light from the bulb at the same time as the observer onboard the ship. Both observers from A and the ship agreed, before the experiment, to synchronize their clocks at the precise moment that they see the light (which is at the same time…SIMONTANEOUSLY J) for the very reason that when the ship gets to point A, the two clocks can be compared as to how much time went by, since the moment they both saw the light to the point when the ship arrives safely at A. You with me so far? Now, as a control, we also added point D, which is a simple mirror. When the light bulb is turned on, the light from the bulb will travel to point D (as well as A and C), and will get reflected to point A. Because of the distances (look above) this should take 2 seconds. Because the ship is also traveling at the speed of light, it will reach point A two seconds after it saw the light at point C. Also the ship will arrive at point A at the same time that the reflected light from the bulb will arrive. Correct?

If I’m not missing anything so far... This tells me that point A recorded two seconds before the ship arrived. NOW the question is …how much time did the ship record before IT arrived…you are probably going to say that it recorded less time because of time dilation…and I’m fine with that…but consider the following….

Remember the first question…when I described what the ship would see when traveling towards the stationary clock? Ok…now if we pretend that we are on that ship, traveling at the speed of light…it would not be possible for us to extend our hand toward the nose of the ship because that would mean that we are moving faster than the speed of light... correct?

Now…. because of what I just said (and its impossibility), lets now pretend that we are on the ship moving at half the speed of light. Now it IS possible to extend our hand toward the nose of the ship right?

My explanation…
So why do our clocks dilate? (Even if we use electronic ones, obviously atomic and mechanical ones as well) well, lets start from the beginning…our wrist watch sends signals inside at the speed of light (don’t correct me here, I said this just for the sake of the argument)…Now, when we are moving at half the speed of light, that watch still sends signals at the speed of light, but because WE (the ship) are moving at half the speed of light, the signals sort of slow down…. is this correct?

Now, when we are aboard the ship…TIME ITSELF doesn’t actually go slower, but our instruments do…so when we finally invent something that can go at half the speed of light or even faster and we want to know the correct time when traveling on this ship…we should actually adjust our watches to send signals more often (instead of every second, depending on the speed we are traveling at) to counter the fact that we are going faster and that our clocks signals cant move faster than light, so we decrease the intervals between the seconds. So basically the technology can’t maintain accurate time at such speeds, not time itself is going by at a different rate….

So is there REALLY time dilation…I personally think not…its just our instruments…

Now, when we go back and look at the third (thought experiment) question, where I asked how much time went by for both observers (A and the ones on the ship)…my answer is 2 seconds for both, but the instruments on the ship are wrong

What do you guys make of this?

Sam5
2006-Feb-23, 01:07 AM
I assume you mean that second one to be SR? That statement is simply not true.

Yes I did. Thanks. I went back and changed it.

Let me ask you this.... you’ve probably heard about “superluminal” galaxies, reports about the distant galaxies that show such high redshifts they are apparently moving away from us at “c” or faster than “c”. Do you believe these reports?

SeanF
2006-Feb-23, 01:33 AM
Let me ask you this.... you’ve probably heard about “superluminal” galaxies, reports about the distant galaxies that show such high redshifts they are apparently moving away from us at “c” or faster than “c”. Do you believe these reports?
Way to change the subject, man.

Ken G
2006-Feb-23, 02:02 AM
Now, when we go back and look at the third (thought experiment) question, where I asked how much time went by for both observers (A and the ones on the ship)…my answer is 2 seconds for both, but the instruments on the ship are wrong

What do you guys make of this?
This is what I make of it. There is no principle more fundamental to the physical sciences than the principle that says the instruments are not allowed to be wrong unless they're faulty in some way. Science is empirical, not philosophical. It is what you measure it to be, no more and no less. When our "common sense" from experience, and the results of our instruments, don't agree, it is always the instruments that are right. Science tries to make sense out of what our instruments say about the universe, not out of how we might like the universe to be.

Ken G
2006-Feb-23, 02:06 AM
It seemed to me that if we can deal with acceleration using only SR while looking from unaccelerated to accelerated, we should be able to deal with the reverse view without invoking GR. I may be wrong on that, of course, and I certainly don't have the maths skills in my head at present to deal with the more complicated calculation involving shifting simultaneity lines.

I've often heard that GR deals with gravity, not acceleration "per se", but I've never fully understood that. This thread is helping, but still, does not the equivalence principle stipulate that gravity and acceleration have similar effects? It can't simply be that GR says you can deal with gravity by doing what you would do with acceleration in SR vai the equivalence principle, it would not have taken ten years to figure out. So it must be related to non-constant gravity-- gravitational divergences and whatnot.

Jeff Root
2006-Feb-23, 02:28 AM
Afterburner,

1. Lets pretend that we are moving at the speed of light (on
my magic spaceship) towards a clock that is floating in space...
This clock sends a signal every second towards the spaceship also
at the speed of light.... now...If the spaceship were stationary
it would receive that signal every second (like its supposed
to)...right?
OK so far. Postulating a spaceship which can travel at the
speed of light relative to some observer is problematic, but
I think we can accept the idea in this case. However, it
won't do you any good. :(

Now, because the spaceship is moving at the speed of light
towards the signal, the ship will in fact see the signal every
half a second, because the time it takes for the signal to
reach the ship is halved by the fact that the ship itself is
moving at the speed of light toward the clock...therefore the
ship will think that the time outside the ship, and the time
that the clock is experiencing is moving faster, or, depends
on how you look at it, the time on the spaceship is moving
slower...is this correct?
No. If the spaceship is moving at the speed of light relative
to the destination, then its time dilation is 100%. That is,
it will experience zero proper time from the beginning of the
trip to the end. It will leave and arrive at the same instant,
with no time for anything to be observed in between.

If you imagined a slightly slower spaceship, there would be
time for something to be observed.

2. Now, lets look at it from the clocks perspective...lets also
assume that the ship has a clock on its dashboard that also sends
a signal every second towards the stationary clock. The stationary
clock will NOT see the ships clock, because the light from the
ships clock cant move faster than the speed of light, and
therefore will not reach the stationary clock, until the ship
gets there. Is this correct?
Yes.

3. Lets do a little thought experiment (refer to the diagram below)
A--------------------B--------------------C--------------------S

First of all, the distances AB, AD, BD, BC, and CS are the same,
and are equal to 300,000 km (or 299 792 458 meters). Point A is a
stationary object with a clock (call it Earth if you wish). Point
B is a bright light bulb that is going to turn on one second
before the ship (traveling from right to left at the speed of
light) is going to reach point C, so when the ship is at point S.
With some modifications, this is doable. You can't guarantee
ahead of time that it will work as you say, but that isn't
important here. The big problem is having the ship move at
the speed of light. Revise the scenario to use a slower ship.

You never made use of point D in your thought experiment, so you
might as well eliminate it.

Now, when we are aboard the ship...TIME ITSELF doesn’t actually
go slower, but our instruments do...
This is an extremely confusing subject. It is difficult for
almost everyone. You, too, are confused.

Time is relative. But your proper time can be said to apply
absolutely to you and anything which is in your inertial frame.
Your proper time is the time which you measure, of events in
your inertial frame. My proper time is the time which I measure,
of events in my inertial frame. If you and I are in different
inertial frames, then we need to use the equations of special
relativity to calculate the differences between our proper times
and convert from one to the other.

Your proper time never changes for you, nomatter how fast you
move relative to anything else. My proper time never changes
for me, nomatter how fast I move relative to anything else.
But when you and I are in motion relative to each other, I see
your time changed, and you see mine changed. Our instruments
are not affected. We are not affected. It is the relationship
between us which changes, and it affects our measurements of
each other's proper time.

so when we finally invent something that can
go at half the speed of light or even faster
The electrons in a TV or computer monitor CRT move at more
than half the speed of light relative to the world around them.

and we want to know the correct time when traveling on this ship...
Unlikely that anything resembling a spaceship will ever carry
humans at anything close to half the speed of light. Sorry.

we should actually adjust our watches to send signals more often
(instead of every second, depending on the speed we are traveling at)
to counter the fact that we are going faster and that our clocks
signals cant move faster than light, so we decrease the intervals
between the seconds. So basically the technology can’t maintain
accurate time at such speeds, not time itself is going by at a
different rate.
No. Completely wrong.

The technology is fine. Instruments are not affected. There is
from Earth while traveling away from Earth will be redshifted and
slowed down. Likewise, signals you receive while traveling toward
Earth will be blueshifted and speeded up. And since you're the
one undergoing large accelerations, more time will have passed
for the people on Earth than for you.

So is there REALLY time dilation...I personally think not...its just
our instruments...
You need to know what you are expressing an opinion about
in order for the opinion to have validity. In this case,
you didn't understand what time dilation is, so an opinion
of whether it really exists or not is rather pointless.

Time dilation was predicted several decades before it was
first measured directly. When it was measured, using atomic
clocks developed in the 1950's, it was found to be exactly
as predicted. So there isn't a whole lot of room to disagree
on whether it exists.

-- Jeff, in Minneapolis

Jeff Root
2006-Feb-23, 02:53 AM
galaxies, reports about the distant galaxies that show such high
redshifts they are apparently moving away from us at "c' or faster
than "c". Do you believe these reports?
I believe the reports, but you described them incorrectly.
You conflated two very different things. Plasma jets have been
observed rushing out of quasars or active galactic nuclei, which
seem to be superluminal. They actually are not. The apparent
speed is simply the geometric effect of light being emitted by
the plasma at different distances from us, resulting in shorter
travel times for light emitted closer to us. In other words,
Doppler effect.

Galaxies have been theorized to be moving away from us at
superluminal speeds, but they are beyond our horizon because
of those superluminal speeds, so can never be observed.

-- Jeff, in Minneapolis

AstroSmurf
2006-Feb-23, 09:32 AM
After receiving some criticism about his “relative motion” causing “length contraction” in the 1905 paper, Einstein felt compelled to respond to this legitimate criticism in his 1907 follow-up paper, “The Relativity Principle and the Conclusions Drawn From It,” in which he said:

“From this it follows that the laws of geometry determine the possible arrangements of rigid bodies in non-accelerated motion always in the same way, independent of their common state of motion. Assertions about the shape of a body in non-accelerated motion therefore have a direct meaning. The shape of a body in the sense indicated we will call its ‘geometric shape.’ The latter obviously does not depend on the state of motion of a reference system.”
Would you care to provide references to those criticisms? As far as I know, length contraction has never been removed from either SR or GR. As for the Einstein quote itself, it's too ambiguous out of context to show what he was trying to say.

And no, length contraction isn't a result of forces, no more than time dilation is. It's merely a relativistic geometric effect.

Frankly, Sam5, I couldn't care less about who said what and so forth. The exact process of how the theory of relativity developed is interesting from a historical perspective, but not from a physical one. The origin of a theory does not affect its truthfulness.

SeanF
2006-Feb-23, 12:44 PM
I believe the reports, but you described them incorrectly.
You conflated two very different things. Plasma jets have been
observed rushing out of quasars or active galactic nuclei, which
seem to be superluminal. They actually are not. The apparent
speed is simply the geometric effect of light being emitted by
the plasma at different distances from us, resulting in shorter
travel times for light emitted closer to us. In other words,
Doppler effect.
No Sam5's talking about galaxies that have been observed with redshifts greater than z=2. He thinks those galaxies disprove Relativity because a redshift of greater than 2 (using the classical Doppler equation) indicates a velocity greater than c.

What he doesn't understand (and he's certainly not the only one) is that Relativity modifies the Doppler equation. Under Relativity, z approaches infinity as the velocity approaches c, so a high redshift by itself does not indicate the galaxy is moving at a velocity greater than c.

Modern cosmology does predict that very distant galaxies are receding from us at velocities greater than c (due to the expansion of space). However, the reasons for that concept have nothing to do with those galaxies' high redshifts, which Relativity would explain as sub-c velocities anyway.

Jeff Root
2006-Feb-23, 01:36 PM
Sam5's talking about galaxies that have been observed with
redshifts greater than z=2. He thinks those galaxies disprove
Relativity because a redshift of greater than 2 (using the
classical Doppler equation) indicates a velocity greater than c.
Ah. I knew the equations were different-- I've even seen them
both derived within the last two months-- but I wasn't aware
they could be abused that way.

-- Jeff, in Minneapolis

afterburner
2006-Feb-23, 02:41 PM
This is what I make of it. There is no principle more fundamental to the physical sciences than the principle that says the instruments are not allowed to be wrong unless they're faulty in some way.

how about this...would the fact that they are not designed to go at the speed of light, or enen close to be a fault in design...it would be similar to trying to weigh an elephant with a scale designed for weighing humans...i mean, as i mentioned earlier, if somehow these instruments WERE able to send these signals faster than the speed of light(or instantaneously, even better)...then there would be no time dilation at all...someone mentioned earlier that this would affect causality...but i dont see how this is true since light is only a "messenger"

Grey
2006-Feb-23, 02:43 PM
Let me ask you this.... you’ve probably heard about “superluminal” galaxies, reports about the distant galaxies that show such high redshifts they are apparently moving away from us at “c” or faster than “c”. Do you believe these reports?Yes, but we've addressed this issue in other threads. Those recession velocities are all at cosmological distances, and we're already aware that anything involving that kind of distance requires us to use general relativity, not special relativity. That's now a flaw in special relativity except in the same sense that it's a "flaw" that you cannot use special relativity in the presence of strong gravitational fields. It's a special case theory that cannot deal with the curvature of spacetime, but it can be used successfully when that the curvature of spacetime is small enough that it can be neglected (that cutoff depends on the accuracy we desire for a particular experiment), and general relativity reduces to special relativity in the case of very small curvature.

But what does this have to do with your earlier claims? Really, if you want to bring up all these old subjects, you should really start a separate thread for it, so that we can keep this thread as a place to try to help afterburner understand the theory. If you think the theory is flawed, that's an argument separate from trying to understand the theory itself.

Ken G
2006-Feb-23, 02:53 PM
how about this...would the fact that they are not designed to go at the speed of light, or enen close to be a fault in design
What do you mean "go close to the speed of light?" Relative to what? That's the problem relativity addresses-- how are you going to specify which clock is going fast and which isn't? What if a clock is built entirely in a reference frame that is "going fast", from components that were also going fast. Why is it going to be faulty? (The clocks you are looking at right now were built at speeds of hundreds of km/s relative to the cosmic background radiation). You may be saying that the accelerations could be hard on a clock, but relativity has to work even when there are no accelerations exerted on the clocks. But here's an even more fundamental point-- it's not just clocks, its anything. After all, we have atomic clocks, which use motions inside atoms. Those motions also show time dilation. Or, elementary particles created in an accelerator live much longer than they "should" because they are moving so fast. Is the design of fast atoms, or fast particles, faulty because atoms and particles were not meant to go that fast? I'm afraid the only reasonable conclusion is that time itself really does slow down for those objects, relative to our sense of time (note that in their own sense of time, nothing is strange, just as nothing would be strange if you or I were moving that fast. Relative to distant galaxies, we are!).

Sam5
2006-Feb-23, 04:23 PM
I believe the reports, but you described them incorrectly.
You conflated two very different things. Plasma jets have been
observed rushing out of quasars or active galactic nuclei, which
seem to be superluminal. They actually are not. The apparent
speed is simply the geometric effect of light being emitted by
the plasma at different distances from us, resulting in shorter
travel times for light emitted closer to us. In other words,
Doppler effect.

Galaxies have been theorized to be moving away from us at
superluminal speeds, but they are beyond our horizon because
of those superluminal speeds, so can never be observed.

-- Jeff, in Minneapolis

Hi Jeff,

I’m not talking about “superluminal jets”. They don’t seem to be “superluminal” and they are incorrectly named.

I’m talking about “superluminal galaxies”, as per the Davis/Lineweaver paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070.pdf

Sam

Sam5
2006-Feb-23, 04:28 PM
Would you care to provide references to those criticisms?

Sure. You can find the paper in Volume 2 paperback edition of “The Collected Papers of Albert Einstein”, Princeton Press, page 252 – 311.

Sam5
2006-Feb-23, 04:38 PM
Yes, but we've addressed this issue in other threads. Those recession velocities are all at cosmological distances, and we're already aware that anything involving that kind of distance requires us to use general relativity, not special relativity.

1905 SR theory says nothing about being restricted to any “limit” of “distance”. Observation of the superluminal galaxies proves the 1905 theory to be wrong about the limiting speed of "c" and about time dilation being due to "relative motion" only. Einstein’s new, improved, and updated 1915-1916 GR theory is needed to explain them. Likewise, his 1918 paper was required to update the 1905 theory and solve the clock paradox with gravity and acceleration effects instead of just "relative motion" alone.

Case Closed.

:)

SeanF
2006-Feb-23, 05:33 PM
1905 SR theory says nothing about being restricted to any “limit” of “distance”. Observation of the superluminal galaxies proves the 1905 theory to be wrong about the limiting speed of "c" and about time dilation being due to "relative motion" only. Einstein’s new, improved, and updated 1915-1916 GR theory is needed to explain them. Likewise, his 1918 paper was required to update the 1905 theory and solve the clock paradox with gravity and acceleration effects instead of just "relative motion" alone.

Case Closed.

:)
Only if you think that "it doesn't work in all situations" and "it doesn't work in any situations" are the same thing.

Ken G
2006-Feb-23, 06:07 PM
Likewise, his 1918 paper was required to update the 1905 theory and solve the clock paradox with gravity and acceleration effects instead of just "relative motion" alone.

Perhaps it would help if you carefully described what you mean by "the clock paradox", why you consider it to be paradoxical in SR, and why you need GR to "solve" the paradox. Then I could understand better what you are really claiming here.

Grey
2006-Feb-23, 06:14 PM
1905 SR theory says nothing about being restricted to any “limit” of “distance”.Perhaps not, but it does say that it's limited to situations not involving gravitation. The cosmological issues you're referring to are gravitational in nature (that is, they arise directly from the fact that a flat Minkowski metric cannot be used to describe the universe at this scale). That's why we need general relativity to talk about cosmology, because the fact that spacetime can be curved affects the universe as a whole at very large scales.

Observation of the superluminal galaxies proves the 1905 theory to be wrong about the limiting speed of "c" and about time dilation being due to "relative motion" only.This is flatly wrong.

Einstein’s new, improved, and updated 1915-1916 GR theory is needed to explain them. Likewise, his 1918 paper was required to update the 1905 theory and solve the clock paradox with gravity and acceleration effects instead of just "relative motion" alone.This is likewise flatly wrong. If you want to try to support these clearly nonstandard views of relativity, take it to AtM, please.

01101001
2006-Feb-23, 06:48 PM
If you want to try to support these clearly nonstandard views of relativity, take it to AtM, please.

Please. This was been hashed out many time, in a more appropriate forum. It doesn't need another flogging, especially here. There are ATM assertions being made. There is advocacy of ATM. This forum is for earnest questions.

Moderator?

grant hutchison
2006-Feb-23, 07:19 PM
)...someone mentioned earlier that this would affect causality...but i dont see how this is true since light is only a "messenger"The causality violation created by faster-than-light travel arises because of the disagreement between observers about elapsed time and distance predicted by special relativity (and confirmed by many many experiments). The faster the relative motion of two observers, the larger their disagreement about time and space. However, they will all agree about cause and effect: if one event causes another, with the signal passing from cause to effect at the speed of light or slower, all observers will agree that the cause preceded the effect, no matter what their relative state of motion.
But if a signal could go faster than the speed of light, then there could exist observers, in particular states of motion, who would see the effect preceding the cause. This is generally agreed to be a Bad Thing.

Grant Hutchison

worzel
2006-Feb-23, 08:51 PM
Perhaps not, but it does say that it's limited to situations not involving gravitation. The cosmological issues you're referring to are gravitational in nature (that is, they arise directly from the fact that a flat Minkowski metric cannot be used to describe the universe at this scale). That's why we need general relativity to talk about cosmology, because the fact that spacetime can be curved affects the universe as a whole at very large scales.Sorry Grey, but while I agree that SR is not adequate to do cosmology, I don't like the assertion that that is why superluminals are ok. This is a bit OT, but Sam5 asked:

If we imagine a line of galaxies all receeding from each other in a row, and a person in each galaxy extending a tape measure to the next galaxy to measure its relative velocity, then each could measure the next galaxy receeding at 0.5c. In this way we could add all these relative velocities and arrive at superluminal velocities.

We could then extend our tape measure past each galaxy in a row and measure each galaxy's recessional velocity relative to us rather than adding up each neighbours answer. Without contradiction we would measure those superluminals at sub c.

The former sense is what cosmologists mean by superluminal. The latter sense is the sense in which relative velocities cannot exceed c. We could do the same in pure SR simply by taking a row of space ships all receeding from their neighbour at 0.5c (according to them). If we then define a ship's velocity as the sum of all the relative velocities between neighbours as measured by them then we would be doing something very similar to what cosmologists do and we would arrive at superluminal spaceships in SR with no gravity and no contradiction: we just changed our definition of velocity.

Not being as good at math as Grant, I am less reticant to post this without being able to back it up mathematically. This is my intuitive understanding from Ned Wright's Cosmology Tutorial (http://www.astro.ucla.edu/~wright/cosmo_02.htm)

SeanF
2006-Feb-23, 08:59 PM
Worzel, I think you're mistaken on this. Current cosmology is that distant galaxies are actually receding away from us at a velocity greater than c, not that they're moving at sub-c relative to a galaxy that's moving at sub-c relative to a galaxy that's moving...etc.

It's not a violation of Relativity because the recession velocity is primarily a factor of the expansion of space between us. They are not moving through space at a supraluminal velocity relative to us, but they are receding away from us at a supraluminal rate.

worzel
2006-Feb-23, 09:01 PM
Hi SeanF. Did you look at Ned's page I linked to? (I added it after you posted I think).

SeanF
2006-Feb-23, 09:22 PM
Hi SeanF. Did you look at Ned's page I linked to? (I added it after you posted I think).
You must've been editing to add the link while I was posting. :)

I think Ned's saying what I said, though, isn't he? :think:

worzel
2006-Feb-23, 09:36 PM
I don't think so. Look at his two spacetime diagrams and notice how the superluminals' light cones are tilted over, and how they're normal in the one below where the superluminals are actually sub-c.

In these variables, velocities greater than c are certainly possible, and since the open Universes are spatially infinite, they are actually required. But there is no contradiction with the special relativistic principle that objects do not travel faster than the speed of light, because if we plot exactly the same space-time in the special relativistic x and t coordinates we get: [ a spacetime diagram where more distant galaxies are increasingly Lorentz contracted, and are all moving at sub-c ]

SeanF
2006-Feb-23, 10:11 PM
I don't think so. Look at his two spacetime diagrams and notice how the superluminals' light cones are tilted over, and how they're normal in the one below where the superluminals are actually sub-c.
I'm definitely going to have to go through that again, when I have time. :)

worzel
2006-Feb-23, 10:19 PM
Do let me know what you when you have. Loads of people who are way more knowledgable than me regarding relativity (like yourself for instance) seem to have what I believe is this misconception about superluminals so I half expect to be totally wrong and I'd really appreciate any comments you have.

Grey
2006-Feb-23, 10:19 PM
Sorry Grey, but while I agree that SR is not adequate to do cosmology, I don't like the assertion that that is why superluminals are ok.We've touched on this before, but, though I respect Ned Wright a lot, I disagree with him on this. More on that below.

If we imagine a line of galaxies all receeding from each other in a row, and a person in each galaxy extending a tape measure to the next galaxy to measure its relative velocity, then each could measure the next galaxy receeding at 0.5c. In this way we could add all these relative velocities and arrive at superluminal velocities.

We could then extend our tape measure past each galaxy in a row and measure each galaxy's recessional velocity relative to us rather than adding up each neighbours answer. Without contradiction we would measure those superluminals at sub c.

The former sense is what cosmologists mean by superluminal. The latter sense is the sense in which relative velocities cannot exceed c. We could do the same in pure SR simply by taking a row of space ships all receeding from their neighbour at 0.5c (according to them). If we then define a ship's velocity as the sum of all the relative velocities between neighbours as measured by them then we would be doing something very similar to what cosmologists do and we would arrive at superluminal spaceships in SR with no gravity and no contradiction: we just changed our definition of velocity.No, that's not what's going on. Note that if we had a string of such observers here locally, and I then directly measured the velocity of the last one relative to me, it would still be subluminal, as you suggest. That is, the rate at which the distance between us was increasing would be less than the speed of light. But in the case of superluminal galaxies, the distance between us and the most distant galaxy really is getting larger at rate faster than light speed. It's not just that we're adding up a bunch of subluminal velocities, and ignoring special relativity's rule on the addition of velocities. Note that that it would not be possible, even in principle, to extend a string all the way to these superluminal galaxies to measure their velocity directly, and that's why it's not a violation of special relativity: there is no inertial reference frame which can really encompass both galaxies. Why couldn't we stretch such a string?

Take a closer look at the diagram using special relativistic coordinates, and the accompanying text. In particular,

But in special relativistic coordinates the velocities are less than c. We also see that our past light cone crosses the worldline of the most distant galaxies at a special relativistic distance x = c*to/2. But the Hubble law distance Dnow, which is measured now, of these most distant galaxies is infinity (in this model).It's true that converting to special relativistic coordinates gives us subluminal velocities, but it also ends up putting these galaxies at infinite distance. I'll maintain that, although it's possible in principle to use such coordinates, that they really aren't that useful or meaningful under these circumstances.

Ken G
2006-Feb-24, 03:09 AM
But in the case of superluminal galaxies, the distance between us and the most distant galaxy really is getting larger at rate faster than light speed. It's not just that we're adding up a bunch of subluminal velocities, and ignoring special relativity's rule on the addition of velocities. [

On the contrary, I think that worzel, and Ned Wright, are completely right here. It is that we are adding subluminal velocities algebraically, and getting something superluminal. This is because velocity is the rate of change of distance, which means you need a definition of both distance and time before you have a velocity. Those definitions are absolutely the crux of the matter. In SR, they have to be measured by a single clock and a single ruler (or clocks and rulers in the same inertial frame, which is essentially the same thing). But in comoving coordinates, you use the distance as the simple sum of all those tape measures laid out at the same age of the universe, and the time is just the local age of the universe. In those coordinates, you do simply add speeds algebraically, and it is fine to be superluminal, because they are not the SR coordinates. These coordinates also have a different formula connecting redshift z and velocity v, because velocity is simply different.

It's true that converting to special relativistic coordinates gives us subluminal velocities, but it also ends up putting these galaxies at infinite distance. I'll maintain that, although it's possible in principle to use such coordinates, that they really aren't that useful or meaningful under these circumstances.
No, I don't think that's right (but that we can debate it shows how subtle all this is!) The distance is not infinite, because in SR coordinates, the distance to the farthest things we can see is just the age of the universe times the speed of light, plus the age of the universe times the SR speed (to account for how much farther they have moved since they emitted the light we see). So that's approximately 2*c*age for the distance to the most distant things we could see. I don't see anything useless about those coordinates, they just don't take advantage of the cosmological principle the way comoving coordinates do. What does happen that is a little weird is that in SR distance coordinates, the density of galaxies goes infinite as you get to the distance 2*c*age, because of length contraction in the frame where you can apply the cosmological principle. Thus the whole universe gets packed into a finite distance (but we only observe the finite part that has had enough time for the light to get to us).

Grey
2006-Feb-24, 03:56 PM
Since this could be an interesting discussion, but is likely to distract from afterburner's request to help him understand special relativity just as much as Sam5's tangent, I'm going to take the advice I gave to him, and start a new discussion here (http://www.bautforum.com/showthread.php?p=689435#post689435).

Sam5
2006-Feb-26, 04:44 PM
Einstein’s new, improved, and updated 1915-1916 GR theory is needed to explain them. Likewise, his 1918 paper was required to update the 1905 theory and solve the clock paradox with gravity and acceleration effects instead of just "relative motion" alone.

This is likewise flatly wrong. If you want to try to support these clearly nonstandard views of relativity, take it to AtM, please.

You apparently haven’t read the 1918 Einstein paper or you wouldn’t stay this. Your ideas belong on the ATM section since you are going against what Einstein said. My opinion agrees with Einstein’s opinion.

SeanF
2006-Feb-26, 04:57 PM
You apparently haven’t read the 1918 Einstein paper or you wouldn’t stay this. Your ideas belong on the ATM section since you are going against what Einstein said. My opinion agrees with Einstein’s opinion.
No, it doesn't, it just agrees with what you think Einstein's opinion was.

But even if you're right, "mainstream" isn't defined as "Einstein's opinion." It's more accurately defined as "the opinion of the majority of today's scientific community" - hence main stream. And it is your opinions that are against that mainstream.

hhEb09'1
2006-Feb-26, 05:01 PM
There is no “turn around” in the clock paradox of the first “peculiar consequence” thought experiment in the 1905 paper, and acceleration is not considered, as I have shown in the Einstein quotes.This is not true, as Grey points out (we've discussed this before).

A change in direction is an acceleration, and that is exactly what was considered in that original 1905 paper.

Sam5
2006-Feb-26, 05:11 PM
There is no “turn around” in the clock paradox of the first “peculiar consequence” thought experiment in the 1905 paper, and acceleration is not considered, as I have shown in the Einstein quotes.

This is not true, as Grey points out (we've discussed this before).

A change in direction is an acceleration, and that is exactly what was considered in that original 1905 paper.

No, he has no math term in the paper for acceleration. He has terms only for velocity. Only “relative velocity” slows down his 1905 clock. He disregarded the effects of the accelerations of the starts and stops. This is why the first part of his 1905 paper is titled the “Kinematical Part”. Do you know why? Because he was considering the results of the Michelson-Morley experiment and it contained no consideration of acceleration.

hhEb09'1
2006-Feb-26, 05:29 PM
No, he has no math term in the paper for acceleration. He has terms only for velocity. Only “relative velocity” slows down his 1905 clock. He disregarded the effects of the accelerations of the starts and stops.You're misrepresenting the paper. The "peculiar consequence" definitely treats an example with an acceleration. He did not disregard it at all--in fact, it relies upon the acceleration to produce the consequence, which we now call the "clock paradox."

Nereid
2006-Feb-26, 07:00 PM
Sam5, you clearly have either a deep misunderstanding of the modern theory, or a (personal?) version of that theory which is most certainly at odds with the mainstream theory.