speed of light and time

[blinkyblinky]

I understand that if you move away from a clock at the speed of light, the clock will appear frozen because the light from the clock face will never be able to overtake you (and hence appear to move forward). But this just seems to be about particles (photons) moving around in physical space. It doesn't seem to me to mean that time itself has changed. Am I misunderstanding this, or is that all there is to it?

[JudgeJ]

No, that is not all there is to it. The reason for the frozen clock that you put forward is not the reason why the clock appears to stand still.

The principle that “moving clocks run slow” was put forward by Albert Einstein in his theories on Relativity. They are too complicated  (a) for me to fully understand and (b) to explain on AB. The theories have been proved (to a degree) by comparing the progress of highly accurate atomic clocks, one travelling on a fast moving aircraft and one remaining stationary on the ground.

The aspect of this I always have difficulty with is that both clocks are moving (relative to the other) so how is it determined which of them is the slow runner ?!?!?!?

There are a number of good web sites which set out to explain this to mere mortals. If you are a simple as I am, however,  it will take you a while to get your head round it. 

 

[rojash]

The slow runner is the one movng RELATIVE TO THE OBSERVER

[JudgeJ]

Thanks rojash, but in the example of the two clocks there are two observers: one on the plane observing events on the ground and one on the ground observing events on the plane.

Both could be considered stationary (or moving) with the other moving (or stationary) relative to him. So whose clock runs slow?

I know from the experiement that was undertaken that the clock on the aircraft ran slow. But people in the aircraft could well argue that they are stationary relative to the clock on the ground, and it is the clock on the ground that is moving and should thus run slow.

If you remove the reference points (i.e. the earth and the stars, etc.) both arguments are valid. Both observers could argue that they are moving (or stationary) relative to the other. So whose clock runs slow then?

[NetSquirrel]

Ah, JudgeJ, you are having the same problem with relativity that I am, I see!  Next week I will ask Dr Steuernagel that very question, and pray I understand enough of the answer to relate to you.  In the meantime, perhaps you can gain some insight from an answer from a previous thread, given by Clanad.  http://www.theanswerbank.co.uk/Science/Question154541.html

[mibn2cweus]

I believe it has to do with which one is moving relative to the starting point.

[fo3nix]

JudgeJ: as rojash says, there is no such thing as a definitely slow or definitely fast clock. there is a slow one and a fast one *to you*, but it may not be the same to others.

i.e. there is no such thing as absolute time. how you see time moving may be different to how others see it.

[NetSquirrel]

But in the experiment, when the clocks were brought back to each other, the clock that was on the plane ran slow, and the one that "stayed still" kept "normal time".  Through what reasoning can you deduce that this is the logical outcome?  Why wouldn't the times be the other way around, for example? 

[mibn2cweus]

What if we thought of it this way?
For any two objects moving at the same velocity, (no motion relative to each other), it is the object which experiences the change in velocity for which time is dilated (as if it acquired a higher energy level by virtue of the energy used to accelerate it) until it is returned to the initial velocity of both (energy equilibrium is restored). 

[entropy]

The problem arises when you think of space and time as separate entities. In fact they are bound together in what Einstein called "spacetime". When you are stationary, all your movement is through time. As you accellerate, a proportion of this movement through time is diverted to movement through space. So the faster through space you travel, the less you travel through time. If therefore you could travel at light speed, all of your movent through spacetime would be taken up with your movent through space and there would be none left for your movement through time. Time would then stop for you. Light (photons) do not age because of this. Mad eh?

[mibn2cweus]

That implies that there are stationary points throughout space where clocks tic at a maximum rate. Since the universe is expanding, are these points moving relative to each other?

Or is it the point where the experiment begins that determines which clock runs slow?

Do energy and mass share a similar relationship? Can mass be considered as energy 'trapped' in a position in space and energy as mass �frozen� in time?

[entropy]

because of the universes expansion there is not a point at which we could be considered stationary. After all, we would be moving reletive to the expanding spacetime. I really reccomend any book by Brian Greene; he just has that excellent ability to explain complex concepts to dunces like myself!

[blinkyblinky]

Thanks to everyone who have replied so far. Special thanks to entropy for his post of the 25/10/05 - very useful to me. Incidentally, is there any particular Brian Greene book you recommend as an introduction to this particular subject?

[entropy]

The fabric of the cosmos... published by penguin. It gives clear, concise explanations of relativity and quantum physics, and is written for the layman without patronising him. One of the best popular science books I've ever read....

[blinkyblinky]

Thanks entropy, I'm going to get hold of that.

[incijalu]

does anybody know if the two atomic clocks used on the experiment continued to display the time discrepancy indefinately or did they "catch up" at some point( which could indicate that the apparent difference in time was an "illusion" rather than a real change - a bit like the way the "visual position" of an object and the "sound "position eventually catches up when the object stops moving.

Possibly a obvious question thats been dealt with but I don't seem to have been able to find the answer