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Ragnar
2002-Feb-15, 11:00 AM
I am an amateur in the realm of relativity, but I think I grasp the basics. Here is my question: If you are travelling in a space ship away from earth near the speed of light, how would you in the space ship perceive your clock to be moving? I believe that since in your frame of reference, the Earth is moving away from you, so if you could look at a clock on Earth from your space ship it would slow down, therefore making your clock faster by comparison. Is this correct reasoning? On the other hand an observer on Earth would see you and your spaceship moving from his/her frame of reference and see the spaceship clock moving slower. My ultimate question is will the observer in the spaceship really see his clock move faster with respect to the Earth clocks? Thanks for your help. I have a dinner bet on this so don't let me down!

SeanF
2002-Feb-15, 01:18 PM
On 2002-02-15 06:00, Ragnar wrote:
I am an amateur in the realm of relativity, but I think I grasp the basics. Here is my question: If you are travelling in a space ship away from earth near the speed of light, how would you in the space ship perceive your clock to be moving? I believe that since in your frame of reference, the Earth is moving away from you, so if you could look at a clock on Earth from your space ship it would slow down, therefore making your clock faster by comparison. Is this correct reasoning? On the other hand an observer on Earth would see you and your spaceship moving from his/her frame of reference and see the spaceship clock moving slower. My ultimate question is will the observer in the spaceship really see his clock move faster with respect to the Earth clocks? Thanks for your help. I have a dinner bet on this so don't let me down!


This is correct. The observer on the spaceship would say the Earth-bound clock is running more slowly than the spaceship clock. The observer on Earth would say the spaceship clock is running more slowly than the Earth-bound clock.

Of course, it's not just the "clocks" -- the observers would say that time itself is flowing more slowly at the other point.

Russ
2002-Feb-15, 03:36 PM
I may have missunderstood the question but I believe you have it backwards. For the person in the ship time would slow down WRT time on Earth so the Earth clock would appear to be very fast. If the traveller came back to Earth, all of the people who stayed behind would have aged more than the traveller.

Now from the travellers own point of view time would pass normally. Earth bound people would see the traveller's clock seem to slow and traveller's aging would slow also.

This is sometimes refered to as the "Twins Paradox". It is not actually a paradox but..... what the heck? /phpBB/images/smiles/icon_wink.gif

GrapesOfWrath
2002-Feb-15, 03:42 PM
On 2002-02-15 10:36, Russ wrote:
I may have missunderstood the question but I believe you have it backwards. For the person in the ship time would slow down WRT time on Earth so the Earth clock would appear to be very fast.
No, not for the usual formulation of the problem.


If the traveller came back to Earth, all of the people who stayed behind would have aged more than the traveller.
That's true, though.


Now from the travellers own point of view time would pass normally. Earth bound people would see the traveller's clock seem to slow and traveller's aging would slow also.
Also true.

SeanF
2002-Feb-15, 04:08 PM
On 2002-02-15 10:36, Russ wrote:

This is sometimes refered to as the "Twins Paradox". It is not actually a paradox but..... what the heck? /phpBB/images/smiles/icon_wink.gif



Russ, check GrapesOfWrath's post above mine here. The ship observer sees the Earth-bound clock as running more slowly than his own while he's moving away from Earth and while he's moving back towards Earth. What he sees happening back on Earth at the time he stops and turns around has been the subject of much discussion here on this board, and comes into play when determining why more time has passed on Earth and not less.

In other words, it is a bit of a "common sense" paradox, but the math does actually work out . . .

Wiley
2002-Feb-15, 04:17 PM
On 2002-02-15 11:08, SeanF wrote:

Russ, check GrapesOfWrath's post above mine here. The ship observer sees the Earth-bound clock as running more slowly than his own while he's moving away from Earth and while he's moving back towards Earth. What he sees happening back on Earth at the time he stops and turns around has been the subject of much discussion here on this board, and comes into play when determining why more time has passed on Earth and not less.



Howdy folks,

I just wanna add a little to what SeanF said. Whether the spaceship is moving towards Earth or away from Earth, our intrepid traveller will see Earth's clock ticking at the same slower rate. The amount of time dilation is a function of the square of velocity, so regardless of whether you're coming or going, it's the same. (For those who know vector calculus and want to get picky, it's actually a function of velocity dotted with itself.)

Chip
2002-Feb-15, 04:28 PM
On 2002-02-15 06:00, Ragnar wrote:
"...My ultimate question is will the observer in the spaceship really see his clock move faster with respect to the Earth clocks?"

If he could see the Earth clock from his ship, the Earth clock would be moving slower than his spaceship clock as he approaches the speed of light. If he just looked at his clock inside the ship, it would appear to be running at the "normal" speed all the time, and not speeding up. His spaceship clock runs faster in relation to the reference frame of the Earthly clock as he approaches the speed of light.

<font size=-1>[ This Message was edited by: Chip on 2002-02-15 11:29 ]</font>

Phobos
2002-Feb-16, 02:19 AM
If he could see the Earth clock from his ship, the Earth clock would be moving slower than his spaceship clock as he approaches the speed of light. If he just looked at his clock inside the ship, it would appear to be running at the "normal" speed all the time, and not speeding up. His spaceship clock runs faster in relation to the reference frame of the Earthly clock as he approaches the speed of light.


I believe that we are mis-interpreting statements such as the one above.

When the observer looks back on Earth, I believe that from his perspective ALL movements on Earth will seem faster (like watching a video on fast forward).

As I read statements such as the one quoted above I am given the impression that the opposite was intended, but I find it difficult to believe that somehow I have got this thing backwards (if observer on the spacecraft actually saw movements slowing down on Earth this would make no sense at all).

To make myself clear I offer a simple thought experiment. Let us assume that the observer on board the spacecraft completes his journey in just 6 hours (Earth time). If we now assume that the clock on board the spacecraft has advanced just 3 hours, then the hands on the clocks on Earth must have been moving twice as fast as those on the spacecraft.

Now if he was watching the clocks on Earth and the hands appeared to be moving slower throughout the trip, then how did the clocks on Earth end up showing times which were ahead of the times denoted by the clocks on board the spacecraft ?

Jeff


<font size=-1>[ This Message was edited by: Phobos on 2002-02-15 21:46 ]</font>

GrapesOfWrath
2002-Feb-16, 11:19 AM
On 2002-02-15 21:19, Phobos wrote:
Now if he was watching the clocks on Earth and the hands appeared to be moving slower throughout the trip, then how did the clocks on Earth end up showing times which were ahead of the times denoted by the clocks on board the spacecraft ?
That's why it's called a paradox, although it really isn't. Just seems counter-intuitive, I guess.

Twin paradox (http://mentock.home.mindspring.com/twins.htm) is my attempt at explaining it. And this (http://mentock.home.mindspring.com/twin2.htm), and this (http://mentock.home.mindspring.com/twinrdux.htm).

Phobos
2002-Feb-16, 06:35 PM
Your examples are well presented explanations of certains specifics about the twins paradox, but you have not addressed the example I gave of the 6 hour trip.

In this example there is only one possibility and that is that the observer on the spacecraft will see clocks on Earth move faster than clocks on board his spacecraft.

Phobos

GrapesOfWrath
2002-Feb-16, 08:01 PM
Just substitute "hours" for "years" at that first link. Then Ann, on Earth, ages 10 hours, while Bob ages 8 hours. And yet, while traveling, they both see each others clocks as slow.

Ragnar
2002-Feb-16, 08:53 PM
Another quick question.

A fellow brought up the following point when we were discussing time dilation. If you travel near the speed of light to a planet 100 light years away, the spaceship would get there in say 20 years. Now only 20 years have passed for our traveller, but he has reached a planet that should take a 100 years at the speed of light. He has no reference (assuming the planet has no one to greet him). It was brought to my attention that the traveller would be convinced that he must have travelled faster than the speed of light since only 20 years have elapsed for him. Weird. Am I missing something? Oh, and thaks for all your help on the previous post!

2002-Feb-17, 02:41 AM
Am I missing something? Oh, and thaks for all your help on the previous post!


Yes, you are missing a possibility. The guy in the space ship, during the turn around, sees the universe in the direction of his acceleration speed up. In other words, he all of a sudden is under the influence of a gravity-like force, except that this force has no mass associated with it. The universe in the direction opposite this "nonNewtonian" force has a rate of time that is speeded up relative to his own. The further that portion of the universe is, the bigger it speeds up.

Just repeat after me. An inertial frame is one where the laws of physics are simplest. An accelerated frame is one where the laws of physics are a bit complicated. The time dilation (the slow down) only has to apply to the inertial frame.

SeanF
2002-Feb-18, 12:59 PM
On 2002-02-16 15:53, Ragnar wrote:
Another quick question.

A fellow brought up the following point when we were discussing time dilation. If you travel near the speed of light to a planet 100 light years away, the spaceship would get there in say 20 years. Now only 20 years have passed for our traveller, but he has reached a planet that should take a 100 years at the speed of light. He has no reference (assuming the planet has no one to greet him). It was brought to my attention that the traveller would be convinced that he must have travelled faster than the speed of light since only 20 years have elapsed for him. Weird. Am I missing something? Oh, and thaks for all your help on the previous post!


Yep, you're missing something. Relativity not only involves a temporal "distortion," but also a spatial "distortion." While moving, the traveler would measure the distance between the two planets as being a little less than 20 light-years. Therefore, since it took him 20 years to travel less than 20 light-years, he would measure his own velocity as less than the speed of light.

When he's done and stopped, he could look at Earth 100 light years away and say, "Gee, only 20 years passed since I left there," but he would at no time during the journey have measured his own velocity as exceeding c.

SeanF
2002-Feb-18, 01:01 PM
On 2002-02-16 13:35, Phobos wrote:
Your examples are well presented explanations of certains specifics about the twins paradox, but you have not addressed the example I gave of the 6 hour trip.

In this example there is only one possibility and that is that the observer on the spacecraft will see clocks on Earth move faster than clocks on board his spacecraft.

Phobos


Phobos, in order to get back to Earth, the traveller needs to stop and turn around. He would "see" weird things happening with the clocks on Earth while doing so.

However, he would see the Earth clock as running more slowly than his own clock during the entire time he is simply moving relative to Earth.

sarnian
2002-Feb-21, 08:08 PM
On 2002-02-18 08:01, SeanF wrote:

Phobos, in order to get back to Earth, the traveller needs to stop and turn around. He would "see" weird things happening with the clocks on Earth while doing so.

However, he would see the Earth clock as running more slowly than his own clock during the entire time he is simply moving relative to Earth.



What if when he turns around in a large curve without slowing down? by the time he reaches earth there would be a huge discrepancy between the earth clock as he observes it and the actual time on the clock.The clocks must be syncronised when he stops so when and how does the clock as the shipboard observer sees it catch up to the clock as it realy is?
I have been thinking about this and would like to present a thought experiment which gives a different view on things:
On earth is a clock E1e;on a space ship are three clocks, S1 shows ship time, E1s shows earth time via an instantanious link to E1e,and E2 which is timed by a powerful laser pulsing at 1 second intervals fired along the course the ship will travel. The ship flies off into space and returns 10 years later at which time E1s shows 10 years and S1 shows 8 years.
Let us now look at the journey by freezing the action at certain points.(The figures are illustrative not exact)On the way out: 1 light year out, E1e reads 20 months E1s (because of its instantanious link) reads 20 months, S1 reads 16 months(80% earth time)and E2 reads 8 months(E1s-12months because of distance). 2 light years out, E1e reads 40 months E1s reads 40 months, S1 reads 32months and E2 reads 16 months(40-24) So far the earth clock E2 appears to be running slow but what about the eventual return trip? 2 light years out, E1e reads 80 months E1s reads 80 months, S1 reads 64months and E2 reads 56 months(80-24); 1 light year out, E1e reads 100 months E1s reads 100 months, S1 reads 80 months and E2 reads 88 months(100-12). In both directions regardless of speed E1e and E1s agree,On the way out clock E2 appears to run slow buton the way back clock E2 appears to speed up until when it finaly arrives back on earth E1e and E2 once again read the same.

SeanF
2002-Feb-21, 08:36 PM
Sarnian,

All your numbers seem to be right, and there's no discrepancy in the times the clocks are reading at the end, so I'm not sure what your question is.

Something to think about, though -- ignore the E1s clock (since under current theory instantaneous communication is impossible) and figure out how a person on the spacecraft would determine the time of E1e based on what he's reading on E2.

Keep two things in mind: First, that as far as he's concerned he's sitting still and the Earth is moving; Secondly, that the speed of light and his own speed should remain constant for him.

Specific example: at the first point you mentioned (S1 reads "16 months") Earth's been moving away from him at 0.6c for 16 months (by his measurements), and he's receiving a signal from E1e that says "8 months". How long ago (S1 time) did that "8 months" signal leave Earth, and where was Earth at the time? What was the difference between S1 and E1e at that time?

Figure this out for various points on the outgoing trip, getting further from Earth each time. Then, figure this out for various points on the return trip (keeping in mind that Earth is moving towards the spaceship now).

What happens to the spaceship's measurement of Earth time close to the turn-around point?

grumium
2002-Feb-24, 04:31 PM
Excuse me but- I haven't heard anyone mention that there is a difference between the time dilation and our perception of the time dilation. Specifically, the question was "what would the observer see" that means we are asking for the latter, not the former. The reason I say this is because the signals which are travelling back and forth between the Earth and the ship will appear red shifted, or stretched. This causes the perception, on both ends, that the other end is running slow. Isn't that correct? And by extension that means on the return trip both sets of signals would be blue shifted, or in other words, speeded up.That means both ends would see the other's time as moving faster.

<font size=-1>[ This Message was edited by: grumium on 2002-02-24 12:12 ]</font>

GrapesOfWrath
2002-Feb-25, 09:54 AM
On 2002-02-24 11:31, grumium wrote:
The reason I say this is because the signals which are travelling back and forth between the Earth and the ship will appear red shifted, or stretched. This causes the perception, on both ends, that the other end is running slow. Isn't that correct?
If so, then how would it work out for the return journey? As you point out, it seems to reverse the effect--so that's a clue.

2002-Feb-25, 11:43 AM
<a name="20020225.5:09"> page 20020225.5:09 aka on Impulse
On 2002-02-25 04:54, GrapesOfWrath wrote: To: 5:10 A.M. PST

On 2002-02-24 11:31, grumium wrote:


LOCKTHRD = HUb' Ba INDX (http://www.badastronomy.com/phpBB/viewtopic.php?topic=183&forum=1#LOCKTHRD)
6:43
5:10
1:33 to convert PST to BaBBpt add one HOUR 33 min?

<font size=-1>[ This Message was edited by: HUb' on 2002-02-25 06:47 ]</font>

grumium
2002-Feb-26, 02:58 AM
Are you saying the signals would not be blue shifted on the return? Or that they would not appear speeded up? Or something else?

GrapesOfWrath
2002-Feb-26, 09:48 AM
On 2002-02-25 21:58, grumium wrote:
Are you saying the signals would not be blue shifted on the return? Or that they would not appear speeded up?
"What do you think special relativity says about it?" is what I meant. What is your impression of that theory, and how it treats that case?

2002-Feb-26, 01:12 PM
<a name="20020226.6:34"> page 20020226.6:34 aka two clocks
On 2002-02-26 04:48, GrapesOfWrath wrote: To? 6:35 A.M. 2-2-26
JD2452332 | 5 MUCUK 7 KAYAB | February 26, 2002 : 6:36 A.M.
On 2002-02-25 21:58, grumium wrote: too
Are you saying the signals would not be blue shifted on the return? Or that they would not appear speeded up?[/quote]
"What do you think special relativity says about it?" is what I meant. What is your impression of that theory, and how it treats that case?
[/quote] My guess about "Radio Signals" vs " Laser Beams " going
from Texas to the space station's that [yep]
turns the Guys Blue check the .jpg. So 4me
not only must you have a CB set but a Laser Mouse? i've both, but use neithor. {oh my}

Ring
2002-Feb-26, 11:46 PM
On 2002-02-25 21:58, grumium wrote:
Are you saying the signals would not be blue shifted on the return? Or that they would not appear speeded up? Or something else?


There are two different phenomenon going on here; time dilation and Doppler shift. Depending on the speed you could *see* the clocks running faster even though you would *measure* them on a gridwork of clocks to be running slower.

Special relativity is about measurements not about what you see.

grumium
2002-Feb-27, 02:38 AM
I get the impression that you still don't follow me. If you are moving toward Earth at .6C and recieving video transmissions, they would be coming in at about 2 1/2 times the rate at which they were broadcast. Your time dilation is only 20%. Why wouldn't they appear to be coming in faster than normal? And by the way ,this question IS about what you would SEE on those transmissions. On the subject of what I think of the theory, I think Einstein was a brilliant theoretician but he must have made a lousy research assistant. I have been studying his work for almost 30 years and it still awes me. But the scientist in me says there is something wrong with it. It frustrates me that we are still unable to experimentally verify its conclusions and what little data we really have to go on suggest the theory is close but inexact or otherwise incomplete. This doesn't surprise me since, upon scrutiny of his "lab technic " I was disturbed to find he comitted the kind of mistakes that render data and conclusions scientifically worthless. I don't have a problem with "thought experiments"; every scientist performs his experiments in his mind, but you must be careful to consider all physical laws- you can't throw out the ones that are inconvenient and you can't uncouple velocity from acceleration. Also, if you're travelling near the speed of light you can't change directions or " hop" onto or off of something travelling at near light speed to avoid the acceleration. Lastly, I don't accept some of his definitions. It seems he had trouble with this, too; there is something distinctly different about our frame of reference. If you think of gravity as a special type of acceleration, we are not in a system at rest.

<font size=-1>[ This Message was edited by: grumium on 2002-02-26 22:52 ]</font>

Ring
2002-Feb-27, 03:15 AM
On 2002-02-26 21:38, grumium wrote:
I get the impression that you still don't follow me. If you are moving toward Earth at .6C and recieving video transmissions, they would be coming in at about 2 1/2 times the rate at which they were broadcast. Your time dilation is only 20%. Why wouldn't they appear to be coming in faster than normal? And by the way ,this question IS about what you would SEE on those transmissions.


And I get the impression that you don't have the foggiest idea how to politely ask a question in such a manner as to induce someone to expend the effort to answer it.

grumium
2002-Feb-27, 04:06 AM
Perhaps so, we all have our flaws.I certainly did not intend to offend but I feel as though you can't question Einstein here without being attacked. Instead of answers I get questions.

GrapesOfWrath
2002-Feb-27, 12:23 PM
Questions aren't really the same thing as attacks--I hadn't realized until your later posts that you were questioning Einstein. I had thought that you were working it out within the context of Einstein's theories of relativity.

OTOH, I don't quite understand from your explanations in the Twin Paradox (http://www.badastronomy.com/phpBB/viewtopic.php?topic=606&forum=1&start=50) why you think that Einstein's thought experiments require the suspension of physical laws. Which physical laws?

I know that is another question, but I'm trying to understand your own question.



On 2002-02-26 21:38, grumium wrote:
I get the impression that you still don't follow me. If you are moving toward Earth at .6C and recieving video transmissions, they would be coming in at about 2 1/2 times the rate at which they were broadcast. Your time dilation is only 20%. Why wouldn't they appear to be coming in faster than normal? And by the way ,this question IS about what you would SEE on those transmissions. On the subject of what I think of the theory, I think Einstein was a brilliant theoretician but he must have made a lousy research assistant. I have been studying his work for almost 30 years and it still awes me. But the scientist in me says there is something wrong with it. It frustrates me that we are still unable to experimentally verify its conclusions and what little data we really have to go on suggest the theory is close but inexact or otherwise incomplete.

My impression is that we have quite a lot of data, not just a little. Dirac derived his famous quantum equation using special relativity--and the derived theory has been tremendously successful.


This doesn't surprise me since, upon scrutiny of his "lab technic " I was disturbed to find he comitted the kind of mistakes that render data and conclusions scientifically worthless.

You have found errors in this lab technique? Which of his lab experiments are you referring to?


I don't have a problem with "thought experiments"; every scientist performs his experiments in his mind, but you must be careful to consider all physical laws- you can't throw out the ones that are inconvenient and you can't uncouple velocity from acceleration.

Again, which physical laws are you referring to?


Also, if you're travelling near the speed of light you can't change directions or " hop" onto or off of something travelling at near light speed to avoid the acceleration. Lastly, I don't accept some of his definitions. It seems he had trouble with this, too; there is something distinctly different about our frame of reference. If you think of gravity as a special type of acceleration, we are not in a system at rest.

That's certainly one way to think about gravity. Einstein had some success thinking about it as not a special case.

Anyway, welcome to the BABB. You bring a certain nostalgic feel to it, reminds me of some of our former dearly departed posters.

Wally
2002-Feb-27, 12:58 PM
On 2002-02-27 07:23, GrapesOfWrath wrote:

Anyway, welcome to the BABB. You bring a certain nostalgic feel to it, reminds me of some of our former dearly departed posters.






Gee GoW. . .didn't realize you were the nostalgic type! Maybe if we ask really nice, the BA will, uhm, offer another chance to a few of those dearly departed individuals??? /phpBB/images/smiles/icon_wink.gif :0

SeanF
2002-Feb-27, 01:35 PM
On 2002-02-26 21:38, grumium wrote:

If you are moving toward Earth at .6C and recieving video transmissions, they would be coming in at about 2 1/2 times the rate at which they were broadcast. Your time dilation is only 20%. Why wouldn't they appear to be coming in faster than normal?



First off, if you're moving towards earth at 0.6c, the Doppler Effect would be about 1.6, not 2.5, wouldn't it?

But what that means is that you would expect to "see" Earth's clock running 1.6 times the rate of your own clock. If you actually "see" Earth's clock running 1.28 times the rate of your own clock, then Earth's clock is 20% slower than it should be.

So, you would see Earth's clock running 28% faster than your own, but 20% slower than "normal", because "normal" would be the expected 60% faster than your own.

At any rate (Hah! Get it? Sometimes I amuse myself /phpBB/images/smiles/icon_smile.gif ), when a Relativity-related experiment talks about how one observer "sees" another clock, it's usually understood to be a reference to how they would measure that other clock's actual time, not how they would literally "receive" any light-speed signals from that clock . . .


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-02-27 08:36 ]</font>

grumium
2002-Feb-28, 06:59 AM
Thanks for the warm(!) welcome. I'm going to have to break this into parts since it is very wordy and I'm still having trouble with the format. To GoW, I'm referring to the "lab" in Einstein's mind, and the mental mistakes I believe he made. More on this next time as it is very long and I will have to condense it. To SeanF, thank you for correcting my bad math. I confess I made these calculations many years ago (before we had computers), and being self-taught, no one ever checked my work. If I understand you, we have a semantics problem and essentially agree that the transmissions would appear speeded up. When I envision "seeing" what's happening back on Earth, I assume you would pick up television broadcasts which would be coming in slower on the outgoing trip and faster on the return. It that correct? GoW, are we right?

GrapesOfWrath
2002-Feb-28, 08:43 AM
On 2002-02-28 01:59, grumium wrote:
To GoW, I'm referring to the "lab" in Einstein's mind, and the mental mistakes I believe he made.

OK, but you should be prepared for a similar judgements. I don't think the term "lab technique (http://blatchley.ssd.k12.ak.us/science/hayashi/hwebpg.html#intro_)" would apply to thought experiments. You seem to think that special relativity has had little empirical confirmation, but to my mind there is an immense set of confirming data. Which particular part of the theory lacks confirmation, to you? Or which data do you dispute?

By confirming data I mean, it confirms special relativity over the previous classical theories. There have been subsequent theories that have superceded special relativity--even Einstein's own--but that doesn't mean that there was a mistake or sloppy lab technique. But, I'm working on some of those new and improved theories myself. If you have specific examples, rather than general disagreements, we could address them.

2002-Feb-28, 11:14 AM
<a name="20020228.4:35"> page 20020228.4:35 aka Tomas E.
On 2002-02-28 03:43, GrapesOfWrath wrote: To: 7 CHUEN 9 KAYAB
I dont care much for FAR FETCHED .9999's etc
heres my report on LAB's .. I spent 2 years with an assayer
studing Quant? he had been at Menlo with Edision ..
Edison at the time "did not believe in RADIO"
Edison belived you neaded a WIRE. anyway E.Snyder left
menlo {4radio} nad I found him amoung Nez Perice{sp}
I can tel you theres that look in the eyes
When you know youv'e seen beyound the present {belief systems} in this case Chemistry & the Periodic Table of Elements {oh my} 4:41 A.M.

grumium
2002-Mar-03, 09:00 PM
OK, GoW, first let me clarify. I don't take your questions to be attacks, I need those to improve my own understanding of what Einstein is talking about. I am perfectly willing to accept everything he says if it all suddenly makes sense to me. I think this is a good example of how 2 people can look at the same thing and "see" it differently (get it?). To me the problem comes when Einstein introduces perceptions. If you Accept his assertion that all time is to be measured by the clock of course you reach his conclusions. But the mind doesn't work that way. We all know that if you are absorbed in something, time seems to pass very quickly, but if you're waiting for the other shoe to drop, time can pass very slowly. Our minds move much more quickly than our bodies do, and the speed of thought is not yet known. You can see this effect in astronauts in free fall. It may well be that clocks in his frame seem to move more slowly than he expects.

triskit54
2002-Mar-03, 11:46 PM
Hi! I was working on an astronomy lab for school and i stumbled across this site... and spent the last hour or so reading and reading and reading...

anyway, i just wanted to introduce myself!

~triskit~

Kaptain K
2002-Mar-03, 11:53 PM
Welcome triskit54. Hope you enjoy the site as much as I do.

triskit54
2002-Mar-03, 11:56 PM
thank you!

actually, considering how much time i spend asleep in my lecture i can't believe i actually kind of know what people are talking about in here... /phpBB/images/smiles/icon_smile.gif

<font size=-1>[ This Message was edited by: triskit54 on 2002-03-03 18:57 ]</font>

GrapesOfWrath
2002-Mar-04, 09:03 AM
On 2002-03-03 16:00, grumium wrote:
I am perfectly willing to accept everything he says if it all suddenly makes sense to me. I think this is a good example of how 2 people can look at the same thing and "see" it differently (get it?). To me the problem comes when Einstein introduces perceptions. If you Accept his assertion that all time is to be measured by the clock of course you reach his conclusions. But the mind doesn't work that way. We all know that if you are absorbed in something, time seems to pass very quickly, but if you're waiting for the other shoe to drop, time can pass very slowly.
Even Einstein made use of that, though. Remember his famous remark: "Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT's relativity."

He was talking about subjective relativity there, as you are. But as near as we can tell, there is an objective reality too that is governed by physical laws which we are patiently deducing, and no matter how fast or slow we think, the body seems to age, or decay, at the usual rate, one second per second.

Pi Man
2002-May-11, 02:16 AM
There is a paradox here, even though the clocks running slower than eachother from different points of view isn't a paradox. It is true that any observer can say that another is moving and he is not, but from the perspective of some third party, which clock will indicate more time has passed and visa versa? The clocks have to be brought together to compare times, so one or the other has to stop, turn around, and come back. By doing so, he is "admitting" that he is indeed moving and the other observer is not. Thanx for letting me give some input. /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-May-11, 10:20 AM
On 2002-05-10 22:16, Pi Man wrote:
There is a paradox here, even though the clocks running slower than eachother from different points of view isn't a paradox.

Then, what is the paradox?

John Kierein
2002-May-11, 02:26 PM
I'm very much interested in time dilation. I have a simple understanding of it as it relates to the red shift. Any help you can give is appreciated. I don't want to immediately say what I think it is, but there are people who say they see time dilation in one distant supernova, and others who say it isn't there in quasar variability. What exactly is seen in the SN and what isn't seen in the quasar that would be expected. I think I know what I would expect, but I want to be sure I've got it right.

kjavds
2002-May-13, 01:25 PM
On 2002-05-11 06:20, GrapesOfWrath wrote:


On 2002-05-10 22:16, Pi Man wrote:
There is a paradox here, even though the clocks running slower than eachother from different points of view isn't a paradox.

Then, what is the paradox?


Hey! I LOVE Pi Man's answer! it's TOPS!! That there isn't a paradox is a straightforward issue: there isn't a paradox because we've found that relativity is how nature functions, so the theory can't very well contain ANY inconsistency or contradiction or paradox. The folks on this board didn't coin the phrase 'twin paradox'. It is a SCENARIO that APPEARS to embody a paradox, but doesn't. Here's my take...

There are several conceivable answers to the twin paradox but first, to clarify the question, this question amounts to a 'paradox' only because the twin which has travelled and then come BACK has aged less than his Earthbound brother; it's the coming back part of the story that is key. Because, if the spacebound twin never came back, then NO CONCLUSION COULD BE ASSERTED as to how their ages might compare -- it would be meaningless under relativity, just plain meaningless. And in fact, if the Earthbound twin suddenly sped off to chase and eventually catch up with his wayward brother, then it's that later embarking twin who would be the younger when they again unite!

For some clarity, please see also www.ezrelativity.tk (a non-commercial site).

Basically, relativity is more than just length contraction and time dilation: it's also time dissynchronicity, as the face readings off two separated yet co-moving clocks will differ by a calculable amount, with the trailing clock ahead in time (yet both clocks are equally slowed). It is this third consequence of relativity that is just one way to explain the twin paradox. When the rocket ship turns around, that time dissynchronicity likewise reverses and applies to the whole span. This is one of the tougher ways to explain the twin paradox, sorry. Symmetry (with Earth's perspective) is lost because the space traveller knows unequivocally that he reversed direction, as he felt the deceleration. OR, the twin paradox can be restated withOUT deceleration, by saying that a space craft already moving toward Earth passes the oubound traveller at the would-be turnaround point, and there they quickly synchronize their watches. But again, you know unequivocally that directionality has reversed and so symmetry is lost. And the other space traveler's watch reading will belie the missing time when it eventually arrives at Earth.

One of the clearest explanations to the twin paradox is simply GR - General Relativity. The Earthbound twin experiences smooth constancy, yet the spacebound twin FEELS his turnaround as strong G-forces, as you can well imagine. Whenever G-forces are FELT, there arises an additional relativistic time dilation -- one which will more than compensate for the discrepancy accrued per Special Relativity's covariance.

And in answer to Ragnar's original question, every party senses time moving normally for himself regardless of his circumstances. The time dilation thing is wholly subjective, varying according to how each particular observer is 'situated'.

Hey PiMan, THANKS FOR A RESOUNDINGLY GREAT ANSWER!

honestmonkey
2002-May-13, 02:05 PM
I don't pretend to understand all this, but I would like to ask a question that has been bugging me for a while. When you all talk about the "Twin Paradox" (that isn't a paradox), you say something like:

Traveller goes out at (or near) the speed of light, STOPS for a smoke or bite to eat, then turns around and comes back at (or near) the speed of light.

The stopping throws a monkey wrench into things and must be taken into accout, I understand (the universe plays catch up with clocks and such).

What if the traveller doesn't stop? That is, he or she just makes a big loop, or uses black holes to curve the path so it comes back to the starting point without ever stopping. Pedal to the metal the whole trip. (Surely you've driven by someone's house to find their car not in the driveway, so you don't bother even getting out and ringing the doorbell.)

Well, that's my question. Not sure what it means, though.

GrapesOfWrath
2002-May-13, 02:48 PM
On 2002-05-13 09:25, kjavds wrote:


On 2002-05-11 06:20, GrapesOfWrath wrote:


On 2002-05-10 22:16, Pi Man wrote:
There is a paradox here, even though the clocks running slower than eachother from different points of view isn't a paradox.

Then, what is the paradox?


Hey! I LOVE Pi Man's answer! it's TOPS!! That there isn't a paradox is a straightforward issue:

OK, but Pi Man said there was one. Which is it?

Kaptain K
2002-May-13, 03:09 PM
honestmonkey,

The answer to your question lies in the definitions of velocity and acceleration.
Velocity is speed and direction.
Acceleration is any change of velocity.
Therefore if you make a big constant speed loop or whip around a black hole to change direction without changing speed, you have still changed velocity.

Wally
2002-May-13, 04:42 PM
On 2002-05-13 10:05, honestmonkey wrote:
I don't pretend to understand all this, but I would like to ask a question that has been bugging me for a while. When you all talk about the "Twin Paradox" (that isn't a paradox), you say something like:

Traveller goes out at (or near) the speed of light, STOPS for a smoke or bite to eat, then turns around and comes back at (or near) the speed of light.

The stopping throws a monkey wrench into things and must be taken into accout, I understand (the universe plays catch up with clocks and such).

What if the traveller doesn't stop? That is, he or she just makes a big loop, or uses black holes to curve the path so it comes back to the starting point without ever stopping. Pedal to the metal the whole trip. (Surely you've driven by someone's house to find their car not in the driveway, so you don't bother even getting out and ringing the doorbell.)

Well, that's my question. Not sure what it means, though.


Hey there HonestMonkey. It's not the stopping that causes the paradox (that isn't). It's the introduction of the 3rd inertial frame of reference. If it was possible for twin "B" to turn around and head back instanteously (i.e. no acceleration), the paradox still remains. That's why it works both ways. If twin "A" takes off and catches up with twin "B", who just kept going on his merry way, then twin "A" is younger, because he switched inertial frames of reference (again, assuming "instantaneous" acceleration for simplicity). Wally

_________________
". . .are you suggesting coconuts migrate???"

<font size=-1>[ This Message was edited by: Wally on 2002-05-13 12:43 ]</font>

rsa
2002-May-13, 04:59 PM
Since people often get confused by what happens at the turn-around, I just had a thought and wonder if this might make things easier to understand (or might be interesting anyway).

We don't know the shape of the universe. But one possibility is that it is finite and unbounded. If we just assume this for our thought experiment, wouldn't this eliminate the complicating issue of turn-around? IOW, the space traveler after his initial acceleration, could coast at some fraction of the speed of light and eventually end up back on earth by following the natural curvature of space. There would be no additional change in velocity caused by making a turn-around. Would this simplify the thought experiment?

GrapesOfWrath
2002-May-13, 05:37 PM
On 2002-05-13 12:59, rsa wrote:
We don't know the shape of the universe. But one possibility is that it is finite and unbounded. If we just assume this for our thought experiment, wouldn't this eliminate the complicating issue of turn-around? IOW, the space traveler after his initial acceleration, could coast at some fraction of the speed of light and eventually end up back on earth by following the natural curvature of space. There would be no additional change in velocity caused by making a turn-around. Would this simplify the thought experiment?

It's not necessary to go to the ends of the Universe--an asteroid follows the natural curvature of space, as do the planets. Or do you mean something different?

rsa
2002-May-13, 06:56 PM
On 2002-05-13 13:37, GrapesOfWrath wrote:
It's not necessary to go to the ends of the Universe--an asteroid follows the natural curvature of space, as do the planets. Or do you mean something different?

I don't think I'd call it the "ends" of the universe. But rather that if the universe is closed (spherical), there would be no net acceleration as an object that follows the natural curvature of space. (I think that's correct.) So rather than going off in one direction and then "accelerating" to reverse direction, just keep going in a "straight" line and you would end up back at the earth. The same as traveling in a straight line on the surface of the earth.

After I posted though, I think it just changes the problem from one of SR to one of GR. I think that it just changes the problem from one of acceleration to one of the curvature of space AND time. It probably doesn't make the twin paradox any easier to explain.

Sorry if this isn't clear or if I've mucked up the details.


<font size=-1>[ This Message was edited by: rsa on 2002-05-13 14:58 ]</font>

GrapesOfWrath
2002-May-13, 08:56 PM
On 2002-05-13 14:56, rsa wrote:
After I posted though, I think it just changes the problem from one of SR to one of GR. I think that it just changes the problem from one of acceleration to one of the curvature of space AND time. It probably doesn't make the twin paradox any easier to explain.

But, my question was, what about Venus for instance--it follows the natural curvature of spacetime, and "passes" us every so often (Venus revolves around the Sun in 224.701 days, whereas Earth takes 365.356 days). Wouldn't that work in your scenario?

rsa
2002-May-13, 09:24 PM
Yes, that would work I suppose. The only difference being that Venus follows a local curvature instead of a global one. But they are both following geodesics.

Of course, there wouldn't be much of a relativistic effect in your scenario. /phpBB/images/smiles/icon_wink.gif

Or am I missing some point that you're trying to make?

GrapesOfWrath
2002-May-14, 08:42 AM
On 2002-05-13 17:24, rsa wrote:
Of course, there wouldn't be much of a relativistic effect in your scenario.

Well, just give it time. /phpBB/images/smiles/icon_smile.gif

You didn't specify at what speed to take the tour of the universe, so just make it the same as the difference between Earth and Venus, and just let Venus go around a few times.

2002-May-15, 05:04 PM
<a name="20020515.8:58"> page 20020515.8:58 aka Moon Spin
On 2002-05-14 04:42, GrapesOfWrath wrote: To: 5 MANIK 1 ZIP

ZIP (CLOUD SERPENT)
& MANKIN = deer
for my take its Moon Spin
and any satalite in orbit /with spin
around the moon
{too mayn Boo's to count}

traztx
2002-May-17, 04:49 AM
I don't understand relativity at all. Help me out here.

Here's a way to view the problem without worrying about acceleration effects:

How about if we define a giant "clock" as the position of our planets in their orbits around Sol.

A space ship with an awesome telescope then passes the earth, heading for polaris. The ship is heading at 20% light speed relative to the Earth. In 5 earth years they will travel 25 lightyears away.

Point A.
As it passes Earth, they note the time on the ship's clocks. This is year 0. They record the positions of the planets relative to the stars.

They radio Earth, telling us they will emit pulses of light in 25 year and 50 years.

The ship uses some sort of propulsion system to maintain 20% light speed relative to the earth, regardless of gravitational forces.

Point B.
According to the ship's clock, it's been 25 years. Using the telescope pointed at the Solar system, they note that the planets have moved in position relative to the stars by 20 years. This makes sense because the Earth is now 5 light years away so the telescope is giving them a view of 5 year old light.

So from the ship's perspective, the Solar system clock is moving 20/25 or 80% the ship's clock speed.

At this point, the ship emits a massive pulse of light due south, back towards the solar system. At 5 light years distance, the light arrives at Earth 5 years later. If someone echos the pulse back north, the ship will eventually see this pulse when the solar system's planets were at the 30 year mark (by viewing the telescope at the time the pulse arrives and knowing that both lights were moving at the same speed).

Someone on the Earth eventually receives the light and notices that her calendar indicates 30 years have passed (her clock is syncronized with the planetary positions). This light tells her that the ship's clock was at it's 25-year mark when it emitted it. Therefore, from her perspective, the ship's clock seems to be running at 25/30 or about 83% speed.

Did I get this right?

Point C.
It's been 50 years. The ship is 10 light years away. The telescope shows 10 year old light, so the solar system clock displays 40 years in their telescope. Still seems to be running at 4/5 the speed of the ship. They send out the 2nd pulse.

The pulse arrives at the Earth and they echo it back to the ship. The ship eventually receives it noting that the solar system clock was at the 60 year position when they got the pulse.

Earth notes that 60 years have passed when the light arrives. The ship's clock appear to be running at 50/60 or about 83% speed. Same as before.

Now this ship passes another ship at this point. This ship is heading due south, towards the solar system at 20% relative light speed and also using propulsion to maintain this speed regardless of gravitational factors. It's telescope sees 10 year old light too, showing the planets in the same position as the 1st ship. The 2nd ship finds out from the 1st ship that this planetary position means solar clock was at 40 year mark.

Back to Point B.
The 2nd ship arrives at point B 25 years later. When they look at the solar system "clock" in the telescope... the planetary positions appear at the 70 year mark. According to this ship, the solar system clock has moved 30 years when the ship's clock moved 25 years. Therefore the solar system's clock seems to me moving 30/25 or about 120% the speed of the ship's clock.

Back to point A.
The 2nd ship arrives at point A another 25 years later. Now the solar system "clock" is at 100 years, since they are viewing the position from within the system. So according to the ship, the solar system moved from the 70 year mark to the 100 year mark in 25 year. 30/20 or 120% speed again.

The age of the people on the 1st ship was 50 older when the ships crossed paths. The age of the people on the 2nd ship was 50 older when they arrived at the solar system. The solar system shows that 100 years have passed.

The reason I use 2 ships is to eliminate acceleration from the problem which simplifies it.

This is how it adds up to me. I don't see any real dilation here. Just people not accounting for the age of light when they read each other's clocks.

What did I get wrong? This doesn't seem to match the predictions of relativity that the people on the ships would be younger or something.

--Tom

Chip
2002-May-17, 09:14 AM
Hello traztx,

You cover many interesting problems. I'd like to address just a small one near the beginning of your examples.

You said:
"The ship is heading at 20% light speed relative to the Earth. In 5 earth years they will travel 25 light years away."

Are you saying the ship is moving 20% of the speed of light? That is, it has accelerated toward the speed of light (called "c") by 20% of "c". If so, there might be an error in the example which could lead to further problems later.

If the ship could somehow travel at the speed of light, (there are many problems with this, but let's overlook them,) it would take 25 years (time on earth) to reach a point 25 light years (a measurement of distance, not time.) This would be true for an observer watching the ship from earth. (For the crew on the ship, the trip would be instantaneous if they could travel at the speed of light.)

But you stated they are traveling much slower at 20% of the speed of light. At that speed, the ship would not take 5 years (time) to travel 25 lightyears (distance). My late night math may be off too, but one light year is about 9,500,000,000,000 kilometers, so it would take them many thousands of years to go 25 lightyears distance from earth.

Or did you mean they are going 20% less than the speed of light, or 80% of the speed of light? Either way, it would take them a considerable time as viewed from earth. From their point of view time would be normal, but in relation to their friends on earth, I think relativistic effects could be detected above 80% of the speed of light.

I understand that your main topic was concerned with viewing the planets in our solarsystem, but I just wanted to clarify something. Sorry if I didn't understand what you meant.

Chip



<font size=-1>[ This Message was edited by: Chip on 2002-05-17 05:19 ]</font>

GrapesOfWrath
2002-May-17, 09:40 AM
On 2002-05-17 05:14, Chip wrote:
But you stated they are traveling much slower at 20% of the speed of light. At that speed, the ship would not take 5 years (time) to travel 25 lightyears (distance). My late night math may be off too, but one light year is about 9,500,000,000,000 kilometers, so it would take them many thousands of years to go 25 lightyears distance from earth.

As near as I can tell, our late night math is off! /phpBB/images/smiles/icon_smile.gif

I think traztx meant to say that the ship travels 5 lightyears in 25 years--notice at point C the ship is 10 lightyears away in 50 years.



On 2002-05-17 00:49, traztx wrote:
What did I get wrong? This doesn't seem to match the predictions of relativity that the people on the ships would be younger or something.

Of course it doesn't. You only used Newtonian mechanics. In order to match the predictions of relativity, you'd have to use relativistic mechanics--which do not depend upon simple clock reading errors.

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-05-17 05:51 ]</font>

2002-May-17, 03:55 PM
On 2
Universe shape ? Changing [x] UNchanging [ ]
I may have another point, if i can think about it?
oh its the point i call "RUMPLE"
and relates to such ideas as
"we cannot detect the motion of the earth"
to me its true; with an exception
but let me refraze it in a smooth airplane ride
you cannot detect the motion of the plane
but IF your in "RUMPLED" weather you can?
On 2002-05-17 00:49, traztx wrote:
What did I get wrong?
HUb' says 7:46 A.M. PST maybe your math chip
does not match your softWHERE

Donnie B.
2002-May-17, 07:15 PM
Hmmm, HUb', what do you mean by Rumple? Is it the irregularity of spacetime caused by the presence of masses (like stars)? Those irregularities are very real and observable, and lead to phenomena like the gravitational lensing that shows us magnified (and sometimes multiple) images of very distant galaxies.

So in that sense, you're absolutely right: "rumpled" spacetime allows us to better perceive the motion of the cosmos (expansion) by giving us a clearer view of those faraway galaxies...

2002-May-20, 01:52 PM
<a name="20020520.5:41"> page 20020520.5:41 aka db
On 2002-05-17 15:15, Donnie B. wrote: To: HUb' {in Math Hour}?
Hmmm, HUb', what do you mean by Rumple? [ 5:44 A.M. ]
an aside ..Looking at Lorentz & Maxwell i get 2 4's SO:
=count down=====& S Flare:=================== my wersion of 4's
:to solar:flare|::::::::::::::|:::::::::::::::|::::::: ::::::::|
:and start of::|:|/::::::::::::::::::::::::::::|:::::::::::::::|
::"RUMPLE1"::::|:|::::::::::::|:::::::::::::::|:::::::::::::: :|
:8::7::SIX:5,4:|/::::::::::::::|:::::::::::::::|:::::::::::::::|
theres my 1st version of a rumple..its a comic strip {characters later}

<font size=-1>[ This Message was edited by: HUb' on 2002-06-01 09:43 ]</font>

2002-May-20, 01:54 PM
<a name="20020520.5:42"> page 20020520.5:42 aka 2 db
On 2002-05-17 15:15, Donnie B. wrote: To: HUb' {in Math Hour}?
Hmmm, HUb', what do you mean by Rumple? [ 5:44 A.M. ]
an aside ..Looking at Lorentz & Maxwell i get 2 4's SO:
============================================= my wersion of 4's
:::::::::::::::|:::::::::::::::|:::::::::::::::|:: :::::::::::::|
::::TRICKEY::::::|::::::::::::::::::::::::::::::|: ::::::::::::::|
:::OOPPSI:::::::|:::::::::::::::|:::::::::::::::|: ::::::::::::::|
:::6:12 A.M.::::::|:::::::::::::::|:::::::::::::::|::::::: ::::::::|
THIS WAS enterd twice: because of delayed timing? a miss Stake but left as FUN ? Y

<font size=-1>[ This Message was edited by: HUb' on 2002-05-20 10:16 ]</font>

Pi Man
2002-Jun-30, 02:52 AM
About my paradox that isn't a paradox. The paradox lies in the *admittance* of one's state of motion. Both can say that they are stationary, but the moment that one goes to compare clocks, his decision instantaneously(did I put in an extra syllable?) makes his clock be the slower of the two.