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goldfinger7476
2007-Jul-05, 09:40 PM
I'm sure that there is a very simple answer to this that I have over looked but if anyone can explain it to me i'd appreciate it.

An asteroid is traveling through space, it does a slingshot around a single planet which causes it to accelerate, after years and years if it comes in no contact with any other gravitational pulls or debris to slow it down, what stops this asteroid from reaching light speed since there is nothing to slow its acceleration?

grant hutchison
2007-Jul-05, 10:11 PM
It doesn't continue to accelerate. It accelerates towards the planet, and then decelerates as it climbs away again. If it passes behind the planet in the planet's orbit (producing the "slingshot"), the asteroid can steal some of the planet's momentum during the interaction, so the small asteroid ends up moving a lot faster, and the large planet ends up moving a little slower. But that's it: there's no continuing effect after the interaction.

Grant Hutchison

danscope
2007-Jul-05, 10:12 PM
Hi, Shall we realize that as you accelerate a body in motion, it will be inclined to
travel in a straighter line. Therefore, if you continue to accelerate this body
in meaningful values, you are going to observe a distinct change in it's orbit.
As a body accelerates, it's orbit will move away from it's gravitational center.
It may require an acceleration input to get it realigned to come near the
slingshot - ing planet once again. Even allowing or shall I say dis-counting this, you will never achieve anything like a small percentage of light speed. Big difference from 300,000 miles per hour and 180,000 miles per second.
Best regards, Dan

goldfinger7476
2007-Jul-05, 10:24 PM
Ok, lets remove the slingshot effect out this, an asteroid for some reason is very slowly accelerating for some reason, whether it be from a near motionless rock that is struck by very small comet (too small to have a gravitational effect on the asteroid) and propels it in another directions. Is there anything restricting it from reaching the speeds of light if it were able to avoid all gravity fields and debris. Or is it completely impossible for it to avoid all gravity?

Noclevername
2007-Jul-05, 10:32 PM
Ok, lets remove the slingshot effect out this, an asteroid for some reason is very slowly accelerating for some reason, whether it be from a near motionless rock that is struck by very small comet (too small to have a gravitational effect on the asteroid) and propels it in another directions. Is there anything restricting it from reaching the speeds of light if it were able to avoid all gravity fields and debris. Or is it completely impossible for it to avoid all gravity?

That impact would still produce only a momentary acceleration. Speed comes from motion, motion is energy, and energy has to come from somewhere. It doesn't just increase on its own.

grant hutchison
2007-Jul-05, 10:36 PM
Ok, lets remove the slingshot effect out this, an asteroid for some reason is very slowly accelerating for some reason, whether it be from a near motionless rock that is struck by very small comet (too small to have a gravitational effect on the asteroid) and propels it in another directions. Is there anything restricting it from reaching the speeds of light if it were able to avoid all gravity fields and debris. Or is it completely impossible for it to avoid all gravity?Still not seeing what you're getting at: the single impact gives a single episode of acceleration; it doesn't generate acceleration that goes on forever.

But are you maybe just wondering why we can't propel something to lightspeed by accelerating it the same rate for a long time? Turns out (because of special relativity) that it requires more and more energy to maintain the same acceleration as our object approaches the speed of light, so that it would take an infinite amount of energy to get the object right up to lightspeed.

Grant Hutchison

Van Rijn
2007-Jul-05, 10:39 PM
Ok, lets remove the slingshot effect out this, an asteroid for some reason is very slowly accelerating for some reason, whether it be from a near motionless rock that is struck by very small comet (too small to have a gravitational effect on the asteroid) and propels it in another directions. Is there anything restricting it from reaching the speeds of light if it were able to avoid all gravity fields and debris.


Yes. It would take infinite energy for the asteroid (or any mass) to reach the speed of light in a vacuum. It can approach it, but never reach it. This is an asymptotic limit.

goldfinger7476
2007-Jul-05, 10:40 PM
the single impact gives a single episode of acceleration; it doesn't generate acceleration that goes on forever.


ok so afther the single impact it would accelerate to a point then maintain a constant speed? I suppose what I am getting at, is if there are no outside forces causing the asteroid to slow down why would it not keep accelerating.

Van Rijn
2007-Jul-05, 10:43 PM
ok so afther the single impact it would accelerate to a point then maintain a constant speed? I suppose what I am getting at, is if there are no outside forces causing the asteroid to slow down why would it not keep accelerating.

It can keep accelerating, and it can get closer and closer to the speed of light in a vacuum. It can go from 99%, to 99.9% to 99.99% and so on, but would never reach 100%.

Edit:

This assumes there is some force that continues to accelerate it.

grant hutchison
2007-Jul-05, 10:43 PM
ok so afther the single impact it would accelerate to a point then maintain a constant speed? I suppose what I am getting at, is if there are no outside forces causing the asteroid to slow down why would it not keep accelerating.It would accelerate during the impact, as the impactor exerted a force on it. As soon as the force stopped, the acceleration would stop. Thereafter, with no outside forces acting on it, it would neither slow down nor speed up: it would move at a constant velocity. No force, no acceleration.

Grant Hutchison

Noclevername
2007-Jul-05, 10:49 PM
ok so afther the single impact it would accelerate to a point then maintain a constant speed? I suppose what I am getting at, is if there are no outside forces causing the asteroid to slow down why would it not keep accelerating.

Acceleration is an increase in speed, in other words, of energy; to make something accelerate you must continue to apply energy. Stop applying energy, acceleration stops. From then on, it's just coasting.

There's nothing slowing it down, but there's nothing speeding it up either.

goldfinger7476
2007-Jul-05, 11:01 PM
Makes sense, I knew it would be simple

Nowhere Man
2007-Jul-06, 12:05 AM
Also see what a certain I. Newton had to say (http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion).

There is a restatement of the third law that says, "For every vision, there is an equal and opposite revision."

Fred

RalofTyr
2007-Jul-06, 04:25 AM
What if, instead, you strapped a small nuclear thruster on that asteriod that would provide a constant acceleration? Would that not eventually, given the fuel supply, reach light speed?

Noclevername
2007-Jul-06, 04:34 AM
What if, instead, you strapped a small nuclear thruster on that asteriod that would provide a constant acceleration? Would that not eventually, given the fuel supply, reach light speed?

See post # 9. It's possible to get closer and closer, but for anything made of matter, it's never going to be reachable.

Twinsun
2007-Jul-06, 04:54 AM
anyway the particles in space decelerate it too ... BUT we can OVERCOME light speed in a fair simple manner I believe : shoot a beam of light while the source is moving in the same direction as light ... thus the light speed will be equal to the initial speed ( 300000000 km/h ) PLUS the small speed with which the source is moving ... if I'm wrong please correct me by all means :)

danscope
2007-Jul-06, 05:07 AM
What if, instead, you strapped a small nuclear thruster on that asteriod that would provide a constant acceleration? Would that not eventually, given the fuel supply, reach light speed?

Hi, "For every action, there is an equal and opposite reaction ."
****************
Have you seen any atomic reaction motors that do not accelerate some mass in the opposite direction to which the "ship" wishes to accelerate?
My point is that if you wanted to accelerate 5 ton ship to light speed,
it would require the thrusting of mass in the opposite direction. A lot of it.
You run into the problem of having to accelerate your ship, your self, and your fuel ( in the case of "Proven" chemical reaction rockets, which have done their best so far.
You appear to assume that a nuclear engine would yield unlimited thrust
without requiring any fuel other than what it started out with, and run for a thousand years. I don't believe there is any magic nuclear star drive that
will satisfy your craving for extreme velocities... with some scheme to
find genuine thrust without exhausting Mass.
Acceleration in space requires thrust ( apart from the borrowed acceleration
of orbiting bodies which you refered to earlier).
I think it may have been Dr. Robert Goddard ( who lived near by )
who when considering a method of obtaining extreme altitudes....must have thought....." There is no free lunch . " when it comes to methods of acceleration.
Best regards, Dan

01101001
2007-Jul-06, 05:11 AM
shoot a beam of light while the source is moving in the same direction as light ... thus the light speed will be equal to the initial speed ( 300000000 km/h ) PLUS the small speed with which the source is moving ... if I'm wrong please correct me by all means

Yeah, you're wrong (Wikipedia: Speed of light :: Constant velocity from all inertial reference frames (http://en.wikipedia.org/wiki/Speed_of_light#Constant_velocity_from_all_inertial _reference_frames)), and you shouldn't be offering up that much wrongness in a Q&A answer. Q&A is for mainstream answers, not for alternative physics (or even not for old Newtonian physics) answers.

Jens
2007-Jul-06, 05:13 AM
ok so afther the single impact it would accelerate to a point then maintain a constant speed? I suppose what I am getting at, is if there are no outside forces causing the asteroid to slow down why would it not keep accelerating.

It seems like you understand the reason, so I don't know if it's really necessary to use an analogy, but think of this. When you throw a baseball straight out (not up into the air), you must instinctively know that it will continue to go the same speed (or slow down, because of air resistance). But there is no way that it would keep speeding up. If you threw a ball in a vaccuum (a place with no air), doesn't it make sense that once it left your hand, it would keep going at the same speed? It wouldn't keep going faster and faster.

Twinsun
2007-Jul-06, 09:58 AM
oh sorry :(

astromark
2007-Jul-06, 11:06 AM
Goldfinger's question seems to have been just a failure of understanding thats clearly been explained by good logical explanations and I trust that Goldfinger will have a better understanding of energy and the cost of accelerating a mass of any object to light speed is just unavailable. Only particles of ( Photons ),or waves of light can and, must travel at C. Anything that has mass can not attain light speed. That suggestion of light emitted forward from an already speeding object must actually end up moving faster than light. No. It does not work like that as, light has no mass. so imparts no energy to travel at C.

drhex
2007-Jul-06, 11:41 AM
Perhaps Goldfinger intended the asteroid to keep encountering planets after the first slingshot so that it would keep accelerating.

goldfinger7476
2007-Jul-06, 04:58 PM
Well I meant for it not come into contact with any forces. The way that I was thinking about it is if any thrust was given no matter how small, if there was nothing to counteract the acceleration it had received, what would stop it from accelerating to light speed. But I suppose since there isn't constant thrust then there would be nothing pushing it faster and faster and can't go any faster than speed that it was pushed to in the first place.

danscope
2007-Jul-06, 05:07 PM
Hi, One of the difficulties of extreme velocities (anywhere....and space in particular..) is the probability and certain destruction of an object which travels
at these high velocities ......encountering even quite small objects and particles.
Imagine a stainless steel bullet the size of a peanut being fired at you at
100,000 miles per hour.....( you moving at 40,000 MPH and the particle moving toward you at 60,000 MPH....) . That little particle will blow through a nuclear submarine, never mind a thin walled space craft .
This is very inconvenient for space travel.
Best regards, Dan

goldfinger7476
2007-Jul-06, 05:10 PM
I don't mean this in any realistic terms of space travel, I was just pondering that if rock was thrusted in a path away from all objects in the universe would hit hit light speed, and since mass can't travel at light speed (something I didn't realize before), it won't. But I hadn't taken in account that it wouldn't keep accelerating.

John Mendenhall
2007-Jul-06, 05:34 PM
I don't mean this in any realistic terms of space travel, I was just pondering that if rock was thrusted in a path away from all objects in the universe would hit hit light speed, and since mass can't travel at light speed (something I didn't realize before), it won't. But I hadn't taken in account that it wouldn't keep accelerating.

Nice work, Goldfinger, you got the idea, which puts you head and shoulders above most ATM'ers.

A interesting fact that you can use to to impress your friends: if you can borrow my wonderful starship, which accelerates at a constant 1G without ever running out of fuel, you will be close to (but not at) light speed after one year. This means that for travelers on the starship, time is running very s l o w l y . If you keep accelerating, you can cicumnavigate the universe in your own lifetime, if you can find away to get around the impacting particle problem mentioned in a previous post.

Try the Special Relativiy and General Relativity articles on Wikipedia; they're both good.

goldfinger7476
2007-Jul-06, 11:10 PM
Anything that has mass can not attain light speed.


A follow up question to this, what law of science says that mass cannot travel at 299,792,458 metres per second or faster? If anything a link to the wiki would help if this is such a easy question that isn't worth explaining

grant hutchison
2007-Jul-07, 12:49 AM
A follow up question to this, what law of science says that mass cannot travel at 299,792,458 metres per second or faster? If anything a link to the wiki would help if this is such a easy question that isn't worth explainingYikes.
The "law" is Special Relativity (Albert Einstein, 1905). Here's a stab at describing (rather than "explaining") it:

It seems, from repeated experiments, that we always measure the speed of light to be the same. It doesn't matter how fast we're going, or how fast the object that emitted the light was going. It's always turns out to be the velocity you give above. Which is odd, but undeniable.

From this observation, Einstein constructed an explanatory theory. If every observer measures the speed of light to be the same, no matter what their state of motion, then some other stuff that we think of as "always the same" can't be. Observers in motion relative to each other will disagree about which events are simultaneous, about how fast clocks should run, and about how long objects are. Those disagreements conspire to make them always agree about how fast light moves in their own (moving) laboratories.

Dig a little deeper into those disagreements, and you find that moving observers disagree about acceleration, too. If I accelerate away from you at what I measure to be 1g, you will observe it to be less than 1g: infinitesimally so at slow relative speeds, but very significantly at high speeds. The closer I get to lightspeed, the more we disagree about my acceleration. You see it decay away towards zero; I feel like I'm still accelerating merrily at 1g.

The result is that no-one can ever observe any accelerating object reaching lightspeed. Not even in theory. It would take an infinite amount of energy to make that transition.

All of the above seems very strange. But Einstein's predictions about how the Universe ought to behave have been tested in great and complex detail, and he seems to be right.


Was that even remotely helpful? :)

Grant Hutchison

goldfinger7476
2007-Jul-11, 06:38 PM
Sorry to keep this thread alive but I thought about it again yesterday. So does that mean that objects can travel 299,792,458 metres per second or even faster since if something next to it is travelling 100,000,000 metres per second, the object will only appear to be travelling at 199, 792,458 mps? Or am I way off at this point?

m1omg
2007-Jul-11, 06:43 PM
Sorry to keep this thread alive but I thought about it again yesterday. So does that mean that objects can travel 299,792,458 metres per second or even faster since if something next to it is travelling 100,000,000 metres per second, the object will only appear to be travelling at 199, 792,458 mps? Or am I way off at this point?


no
you can accelerate to 299,792,487.99999999999999999999999999999999999999 9999999999999999999999999m/s but never to c
you can infinitely approach the c but never reach it
with more acceleration you will need more and more energy and time will dilate more and more and more...
so you can drive a circle around the galaxy but you need 0.9999999999... c not only 0.99 c because the time dilattion will be MUCH BIGGER at 0.999999999... c than at 0.99 c but AFAIK you will need about 1000000x energy to accelerate to 0.99999999 c... that that energy that will accelerate you to 0.99 c.
yes, that what you are saying will apply normally because all speeds EXCEPT c is relative.
lightspeed is all the same and absolute.
c=constant (lightspeed)

grant hutchison
2007-Jul-11, 06:59 PM
Sorry to keep this thread alive but I thought about it again yesterday. So does that mean that objects can travel 299,792,458 metres per second or even faster since if something next to it is travelling 100,000,000 metres per second, the object will only appear to be travelling at 199, 792,458 mps? Relative speeds aren't additive under relativity, so there's no way to find an observer who actually measures a speed greater than lightspeed.
If I see you travelling at 3/4c in one direction, and another observer approaching you at 3/4c fron the other direction, I might reason that each of you will see the other approaching at 1.5c. But you don't, because of those disagreements about length and time I alluded to earlier. It turns out that each of you sees the other approaching at only 96% lightspeed.

Grant Hutchison

Noclevername
2007-Jul-11, 07:03 PM
Sorry to keep this thread alive but I thought about it again yesterday. So does that mean that objects can travel 299,792,458 metres per second or even faster since if something next to it is travelling 100,000,000 metres per second, the object will only appear to be travelling at 199, 792,458 mps? Or am I way off at this point?

The closer to lightspeed an object gets, the more its relative mass will increase; more mass means more energy is needed to accelerate the object. Which again increases the mass, requiring even more energy. Etc. It's an unwinnable game of catch-up, because at some point the energy needed to keep increasing the object's speed reaches a point that is greater than the energy in the universe. In practical terms, of course, the actually reachable limit is probably far less.

EvilEye
2007-Jul-11, 07:10 PM
1. The Asteroid would never GAIN speed over time in a vacuum, unless acted upon by another force (gravity, or a bump from behind), but even then it has stolen the speed from the other object. It doesn't grow.

2. If the object DID happen to reach lightspeed, its mass would be infinite, and then it would be light itself. Pure energy. It would no longer be an asteroid.

goldfinger7476
2007-Jul-11, 07:27 PM
Ok that makes sense, I think my light speed hunger is over now

thanks

danscope
2007-Jul-12, 12:11 AM
Hi, I enjoyed your question and your curiosity. And it is an excellent opportunity to view the interesting phenomena of objects, acceleration,
and the speed of light. The pleasure has been shared.
Best regards, Dan

Noclevername
2007-Jul-12, 12:15 AM
Ok that makes sense, I think my light speed hunger is over now

thanks


You're welcome, and thanks for giving us something to babble about! :)

Robert TG
2007-Jul-12, 01:34 AM
Goldfinger7476, don’t let your light speed hunger be satisfied so quickly. Let me try and rekindle your curiosity.

The theory of relativity says you can not reach the speed of light but this will not stop man from trying. Just think of all the perpetual motion machines that ‘theory’ says can not be made and yet are presented to the patent office every year. (even if they don’t work).

There is a thought experiment of a person standing inside a stationary elevator on Earth and comparing this to another man standing inside an elevator in deep space that is accelerating at the rate of 1g. (1g is the acceration equal to the force of gravity.)
Both men would feel the same equivalent effect.

So, here is a 10 year flight plan for my imaginary astronaut. The flight plan is to go in one direction for two and a half years then spin the craft around 180 degrees to start to slow down for the return trip home. The next two and a half years is slowing the craft down, followed by the next two and a half years of increasing speed in the direction the astronaut wants to go. After being in space for 7 ½ years the astronaut swings his craft around once again 180 degrees to slow down, arriving back on earth in his 10 year travel time.

To add to his comfort on this 10 year journey we will supply him with artificial gravity by accelerating his spaceship at a constant rate of 1g.

The acceleration required to equal earths gravity at sea level is as exactly 9.80665 m/s2 (approx. 32.174 ft/s2). This is equivalent to accelerating from 0 - 100 kph (62 mph) in 2.83 seconds. My small car can’t do this but this accerleration is easily obtainable with modern engines and rockets.

The spaceship will steadily increase its speed and within the first year (I estimate just under 354 days) the craft will approach light speed.

What happens at this point of his journey is curious. My understanding is that the astronaut will not feel anything strange about his journey. Relative to himself he has been constantly accelerating the whole time and all measurements appear quite normal.

It is the observer on Earth who is looking at light or radio signals from the spacecraft that will ‘see’ changes in the crafts behaviour. But careful measurements of the light received at Earth all show that the craft approached light speed and did not surpass it. The light or radio signal reaching Earth from the craft becomes more and more stretched which we interpret as ‘time’ slowing down. Sort of like slowing down the speed on a tape recorder, the sound on the tape is stretched over a longer period of time and 'to the observer' it sounds slowed down.

Are these changes ‘real’? Not according to the astronaut! His spaceship is still suppling the artificial gravity (accerleration). Time would seem normal to him and he could measure his spaceship with a ruler and see it has not shortened. His mass of fuel seems to be in proportion to the mass of his spaceship and see all seems quite normal for him to continue his journey. When his clock tells him, two and a half years has past, he spins his spaceship around, and still feels the comfort of artificial gravity as his spaceship is now slowing down.

Back on Earth, after two and a half year, the observers see that the spaceship has not spun around to decelerate. Of course, the light reaching them is light that left the spaceship quite some time ago and it will take years for the light showing the spin around to reach earth.

At the end of 5 years the spaceship has stopped slowing down and the astronaut is at the same velocity as when he left earth. He can look out the window and see that the galaxy appears normal, as it would on earth, except he is a different place. He looks around at the stars and gets his bearings and finds that he is more than 5 light years away from earth. So he now sends a ‘mid trip’ radio signal to Earth and it will take more than 5 years to reach earth because light travels at constant speed.

Back on Earth, they say… Time has slowed down for the astronaut, and that He never reached light speed. All their measurements prove this to be so. They wait patiently for a signal to arrive.

Our faithful Astronaut continues with the flight plan and the next two and a half years he is accelerating his way back toward earth. During this stage of his journey he listens to his radio to see if he will hear his own signal that he transmitted when he was stopped.

Still on schedule he spins his spaceship around at the seven and a half year mark and starts the final stage of his return home. At the end of ten years he reaches earth.

Time dilation does actually happen so he finds that everyone he knew has long since died. He says, I’m just ten years older and travelled far across the galaxy. The people on earth say his clock slowed down and he travelled for a very long time to get as far as he did.

Who is right? When does the ‘mid trip’ radio signal arrive on earth?

Thinking about this does confuse me. Is there a simple answer?

goldfinger7476
2007-Jul-12, 09:09 PM
Well I certainly hope one day we can figure out something to go the speed of light but, my taste buds are drooling at the thought of time diallation, I mean how can all that happen just because ur moving reallllllllly fast? I pretty much understand it but take your scenario for example. 2 human beings in an elevator for the exact same amount of time, but by the time the one in space reaches the other guy he will have long since been dead. What would happen if they were able to stay in constant contact?

EvilEye
2007-Jul-12, 10:22 PM
What would happen if they were able to stay in constant contact?


The movie "Frequency" would be made.

sorry... I couldn't resist.

grant hutchison
2007-Jul-12, 11:29 PM
Time dilation does actually happen so he finds that everyone he knew has long since died. He says, I’m just ten years older and travelled far across the galaxy. The people on earth say his clock slowed down and he travelled for a very long time to get as far as he did.

Who is right? When does the ‘mid trip’ radio signal arrive on earth?

Thinking about this does confuse me. Is there a simple answer?After 2.5 shipboard years accelerating at 1g, your astronaut reaches 0.988c and travels 5.46 light years, while 6.36 years elapse on Earth.
So the maximum distance from Earth is 10.92 light years, and the round trip takes 25.44 Earth years.
The midtrip signal is sent after 12.72 Earth years, and takes 10.92 years to reach Earth, so it arrives at Earth 1.8 years before the returning astronaut.

Grant Hutchison

Robert TG
2007-Jul-13, 12:56 AM
WOW! This is a fantastic forum!
Thank you very much Grant for working that out.
I'm amazed.

Can you share with me the formulas that you used to work these things out?

Also, can you expand a little on why the astronaut who (in his time frame) travels for 5 years and reaches a maximum distance from earth of 10.92 light years, would not consider himself as having traveled faster than light speed?

danscope
2007-Jul-13, 02:40 AM
Yes.... and .....by what method does a ship have.....and accelerate enough fuel
to accelerate at one G for 2 1/2 years; and Decelerate at one G for 2/1/2 yrs
and do so again? Have you Any idea how much fuel that would require?
Trust me........it is an exponential function of fantasy. The more fuel you require for the longer amount of time, the more mass you have to accelerate
which means more fuel. This is quite simply a non-function of reality.
You will find that you are up against the numbers, and they are considerable.

Best regards, Dan

Ufonaut99
2007-Jul-13, 03:14 AM
Hey, the fantasy's fun ! Trust me, all I learned about physics I learned from Star Trek, and they didn't have any problem!

Seriously, yes, we don't have the technology at the moment to launch these sorts of ships, but exploring the thought experiments is good in it's own right. Also, time dilation does have a real effect - the GPS system had to be built using it, it affects the particle accelerators (where electrons, etc are accelerated to near-c), etc.

It is reality we're exploting here :)

EvilEye
2007-Jul-13, 03:24 AM
The real problem would be outisde the ship, would it not?

Outside of the ship the thrust would slow as you neared lightspeed.... and at lightspeed it would cease (with time).

So even at 99.9999etc.... of lightspeed would be the closest you could get.

You'd be "putt-putting" through space with your thrust getting there....stopping, and then going....and then stopping....etc...............

Ufonaut99
2007-Jul-13, 03:38 AM
Here’s my take on it:

Imagine Alice, Bob and Charlie standing at the kerb, when a car races past at 100kph. Bob and Charlie jump into their cars and accelerate to 90kph. They now see Alice receeding behind them at 90kph. The car is also receeding in front of them at 10kph.

Charlie now accelerates by the same amount again. He now sees Alice receeding at 180kph, Bob receeding at 90kph, and the car ….. well, it’s eating his exhaust !

Now let’s go to relativity.

Imagine Alice, Bob and Charlie standing at the kerb, when a light beam goes past at c. Bob and Charlie jump into their cars and accelerate to 0.90c. They now see Alice receeding behind them at 0.90c. However, when they look ahead, they see the light beam is STILL receeding in front of them at c !

Charlie now accelerates by the same amount again. Looking back, he sees Bob receeding at 0.90c. However, Alice isn’t receeding at 1.8c, but 0.9something c.
Looking forward again, Charlie sees the light beam receeding from him STILL at c !

As you can see, it’s impossible to catch the light beam travelling at c, since no matter how fast you accelerate, the light beam will ALWAYS be receeding from you at c.


(ps: Yes, I know they can't really "see" the light beam in front of them!)

EvilEye
2007-Jul-13, 03:45 AM
The answer to the age old question is answerable.

If you were traveling at the speed of light, and you turned your headlights on, would they do anything?

Yes. depending on your perspective.

If you were in the ship, then you would see them.

If you were outside the ship... they never turned them on.

Robert TG
2007-Jul-13, 04:38 AM
I have read that scientists have made two identical atomic maser clocks, which are very accurate to one part in 30 million million and placed one of those clocks on a jet plane and flew it around. When it landed and the clocks were placed together again, the times were slightly but measurably different.

I find facts like this uncomfortable, but the universe wasn’t made for my comfort.

One second of our time lasts 1.7 seconds on a spaceship travelling at 80% of the speed of light.

grant hutchison
2007-Jul-13, 09:40 AM
WOW! This is a fantastic forum!
Thank you very much Grant for working that out.
I'm amazed.:) It didn't take much effort: I've long had the formulae set up in MathCAD so that I can play with them.


Can you share with me the formulas that you used to work these things out?The Relativistic Rocket (http://www.math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html), from Physics FAQ, lays them out for you.


Also, can you expand a little on why the astronaut who (in his time frame) travels for 5 years and reaches a maximum distance from earth of 10.92 light years, would not consider himself as having traveled faster than light speed?If he looked out the window and observed the stars, he'd see that he was moving at just 0.988c at turnover. He'd also see that the Universe was shortened in his direction of travel: a lightyear would be just 0.15 lightyears in length to our traveller at turnaround. So, in effect, the distance he crossed during his journey was shorter than the final distance he achieved after slowing down.

Grant Hutchison

grant hutchison
2007-Jul-13, 10:02 AM
2 human beings in an elevator for the exact same amount of time, but by the time the one in space reaches the other guy he will have long since been dead. What would happen if they were able to stay in constant contact?Imagine they exchange signals regularly. While the traveller accelerated away from his Earthbound colleague, they'd each receive the other's signals at wider and wider intervals. When the traveller came to a halt at the farthest point of his journey, they'd exchange signals at a normal rate, but each would be viewing the other's past, because of the delay caused by the travel time of the signals. As the traveller returned, he'd run into the signals from Earth more frequently, and his signals (each travelling a shorter distance than the previous one) would also arrive at Earth more frequently.
So both observers would see the other's signal rate slow down and then speed up, as the distance between them first increased and then decreased. But the traveller would find that he had time to send off fewer signals than the Earthbound observer during the whole journey time; he would experience less time during the trip.

Grant Hutchison

Robert TG
2007-Jul-13, 12:38 PM
:)
The Relativistic Rocket (http://www.math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html), from Physics FAQ, lays them out for you.


Thanks for pointing me in the right direction. :) I think it will take me a few days, and a bit of luck, to read and understand the information in that link. But I'm going to try.

Robert TG
2007-Jul-15, 10:17 AM
OK, let me see if I have this right.

The Andromeda galaxy is about 2 million light years away. As nothing can go faster than light, sending a signal to there will take 2 million years to arrive at Andromeda.

I can get in a Relativistic Rocket with a 1g acceleration and arrive at Andromeda in only 28 years without going ‘faster than the speed of light’.

I arrive at Andromeda, which is now 2 million years older as is the Earth that I left behind. So, observers on Earth and Andromeda agree that the trip took 2 million years at less than light speed.

My rocket and I have only aged 28 years because time has slowed down for me because of my ‘high but less than’ light speed. I have travelled 2 million light year distance in 2 million years but have only felt that the trip took 28 years.

So, ‘Relativity’ makes my accelerating rocket unable to reach light speed but actually turns my rocket into an effective (one way) TIME machine.

At the halfway mark of this trip to Andromeda, the traveller would look out his front window and measure the distance to Andromeda as only approximately 14 light years away. Looking out the back window the traveller would see and measure the distance to Earth as approximately 14 light years. The traveller would still measure the speed of light as the same speed as he did on Earth, the difference is that the units of distance that the traveller uses would seem much shorter to a stationary observer outside the rocket.

Robert TG
2007-Jul-15, 06:38 PM
Time slows down as you approach light speed.

Can I correctly conclude...that Time would actually stop at light speed?

Would it follow that, in the time frame of a photon, the photon never ages?
That Time does not exist for a photon?
And that, every photon created anywhere in the universe travels to anywhere else in the universe (to it's final destination) instantly within it’s own time frame?


I have read that the general usage of the speed of light "c" is through a vacuum and that Light travels thru a medium (like air or glass) at a slower speed,
How slow can light travel?
And is it possible for matter to travel faster than ‘slow light’?
What happens to a ‘photons time frame’ when it’s speed is slowed down by a medium?

grant hutchison
2007-Jul-15, 06:48 PM
At the halfway mark of this trip to Andromeda, the traveller would look out his front window and measure the distance to Andromeda as only approximately 14 light years away. Looking out the back window the traveller would see and measure the distance to Earth as approximately 14 light years. The traveller would still measure the speed of light as the same speed as he did on Earth, the difference is that the units of distance that the traveller uses would seem much shorter to a stationary observer outside the rocket.
That all seems fine to me, apart from two points:
1) The apparent distance between Andromeda and the Milky Way galaxy at turnaround (when the ship is travelling at 0.9999999999995c) would be only about a 1.9 light-years; it would be longer both earlier and later in the voyage.
2) Purists would object to your use of the word "stationary", since relativity doesn't admit any reference frame that can be considered the absolute standard of rest; so you need to use some sort of awkward circumlocution like "at rest relative to the common reference frame of the Andromeda and Milky Way galaxies".

Grant Hutchison

grant hutchison
2007-Jul-15, 06:59 PM
Time slows down as you approach light speed.

Can I correctly conclude...that Time would actually stop at light speed?It's one way of looking at it. The equations of SR fall into a mess of infinities and zeroes when you reach lightspeed, reminding us that physical objects can't reach that velocity.


And is it possible for matter to travel faster than ‘slow light’?

What happens to a ‘photons time frame’ when it’s speed is slowed down by a medium?It is. It's what causes Čerenkov radiation, that eerie blue glow in the water jackets of nuclear reactors. (If you care to Google for this, use the accentless "Cerenkov" or the transliterated "Cherenkov".)

As for what the photon's up to while it's travelling slower than c ...
One interpretation describes it as flitting from atom to atom of the medium through which it is passing, briefly participating in what's called the "dressed state" of each atom, and then passing onwards at c until it next "hangs up" in the quantum state of the next atom it encounters.
KenG, who knows about this stuff, prefers a view that doesn't require us to imagine unobservably specific quantum interactions, and instead describes the photon as a quantum wave which propagates more slowly than c because of its interaction with the general medium through which it passes.

Grant Hutchison

EvilEye
2007-Jul-16, 01:34 AM
A photon does not age. You are right.

That's why we see baby galaxies.

The light from them is the same as it was when it left. :)

grant hutchison
2007-Jul-16, 10:36 AM
A photon does not age. You are right.

That's why we see baby galaxies.

The light from them is the same as it was when it left. :)Well ...
We see very distant galaxies as they were many millions of years ago; but that's just because their light takes many millions of years to reach us.
It has nothing to do with the agelessness (or otherwise) of photons.

Grant Hutchison

grant hutchison
2007-Jul-16, 11:24 PM
1) The apparent distance between Andromeda and the Milky Way galaxy at turnaround (when the ship is travelling at 0.9999999999995c) would be only about a 1.9 light-years; it would be longer both earlier and later in the voyage.As an addendum to that, I might point out that, if you travel at 1g in the way we've been discussing, the measured distance from your departure point to your destination at turnover is always a little under two light years. :)
This is because, by measurements aboard your spacecraft, you never get more than 0.97 light years from home during the acceleration phase of the journey; the Lorentz contraction offsets the distance you've covered in the reference frame of your departure point, and you see it fall behind more and more slowly, asymptotically approaching something called the "Rindler horizon", astern. But up ahead, Lorentz contraction reels in your destination pretty quickly; so you just have to keep accelerating until your destination is as far ahead as your departure point has fallen behind, and then turn over to begin decelerating. Since (at 1g) your departure point gets stuck just under a light year behind you, you always end up turning over when the destination is just under a light year ahead.
Once you're in the deceleration phase, you find the situation reverses itself: your destination appears to creep closer only very slowly, while home falls away very quickly.

Grant Hutchison

EvilEye
2007-Jul-17, 01:42 AM
Well ...
We see very distant galaxies as they were many millions of years ago; but that's just because their light takes many millions of years to reach us.
It has nothing to do with the agelessness (or otherwise) of photons.

Grant Hutchison


ahhhh... but we see the light as it "was" because as it reaches us, it has not changed.

We know that at the very speed of light... time stops. Therefore, the photon is exactly as old as it was when it was sourced. It only traveled billions of years from YOUR perspective. From its perspective it was instantaneous.

You are the one that had to wait for it.

It didn't wait to meet you.

grant hutchison
2007-Jul-17, 08:22 AM
ahhhh... but we see the light as it "was" because as it reaches us, it has not changed.Neutrinos and electrons travel slower than light, and they don't change, either.
A stable message-carrier is what's required, not necessarily an ageless one.

Grant Hutchison

Robert TG
2007-Jul-17, 11:04 AM
I imagine that in the 'Photon time frame' every photon must exist a minimum of time, as in Planck time, but at the speed of light can any photon exist for a longer period of time (in it’s own time frame)?
Or have I completely missed something?

grant hutchison
2007-Jul-17, 01:02 PM
It just seems odd to me to talk about what a photon is or is not "doing" during its flight from emission to absorption. It's not as if we can observe it in any way during that interval; and it seems the photon, during its flight, is best described as a quantum probability wave that tells us how likely we are to absorb a photon at some particular point, if we choose to try the experiment.
We can write down the equations for how this quantum entity evolves with time in some chosen reference frame; but is it useful, or even possible, to say how time elapses "aboard" such a probability wave?

Grant Hutchison

Robert TG
2007-Jul-18, 06:55 AM
... but is it useful, or even possible, to say how time elapses "aboard" such a probability wave?


I remember seeing a cartoon presentation of Einstein's thought experiments where our hero Albert was sitting on top of a light beam... travelling at the speed of light.
Since Einstein's own equations say matter cannot travel at that speed, this thought experiment still did not contradict his conclusions and observations.

A photon does travel at the speed of light, and it does seem to me that for anything in that time frame, time itself would stop.

Our Universe has four dimensions of space and time. Everything that exists in our Universe needs a length, width, height and time to exist. Without out one of these dimensions the object would simply not exist.

So, a photon, must exist in time, but it seem to me, would not experience time passing.

Considering the Lorentz contraction as a reduction of ‘length’ as an object approaches light speed, what would be the length of an object that did reach just under light speed? Since it must have some length and time to exist, otherwise the object would be a massive object that exists in two dimensions. Would an object reach the limit of a Planck length and time or just cease to exist in our universe?

grant hutchison
2007-Jul-18, 07:41 AM
I remember seeing a cartoon presentation of Einstein's thought experiments where our hero Albert was sitting on top of a light beam... travelling at the speed of light.
Since Einstein's own equations say matter cannot travel at that speed, this thought experiment still did not contradict his conclusions and observations.Except his conclusion that a massive object cannot travel at lightspeed. Einstein's thought experiment of travelling with a beam of light was set up in order to point out that it would let us observe something that we never do observe: light that is stationary in our reference frame.

Grant Hutchison