# Speed of light and time

1. Sep 10, 2009

### Zman

What was Einstein’s reasoning behind the assertion that time travels at the speed of light?

Is it still considered to be the case that time travels at the speed of light?

2. Sep 10, 2009

### Bob_for_short

Time in physics is what your clock shows. It is a periodic independent process. It does not travel at all. The notion of speed is applicable to bodies: v=dR/dt, where dR is the body displacement during the time interval dt.

Last edited: Sep 10, 2009
3. Sep 10, 2009

### Jimmy

I've never heard of time moving at the speed of light. However, some popularizers of physics have stated that all objects move through space-time at the speed of light. Brian Green's Elegant Universe for example:
This could be the idea to which the OP is referring. If it is, I'm sure the resident experts on relativity can shed some light on the subject.

4. Sep 10, 2009

### A.T.

A question about the "speed of time" is posted every month on this forum.
Yes, that way around it makes more sense. It is a common geometrical interpretation. But it should be noted, that "space time" here refers to "space proper-time" and not the usual Minkowski "space coordinate-time". Here a diagram of space proper-time, showing time dilation and length contraction directly:

5. Sep 10, 2009

### Jimmy

That's great. Thanks for that. Demos like that certainly help one get a better grasp of what's happening. It makes sense to me that all objects move with the same, fixed speed in space-time. If that weren't the case, things would be pretty strange, would they not? Would "gravitational forces" no longer be consistent from one object to the next? Mass-Energy equivalence? The speed of light itself- or would that even change? And speaking of that, does the observed speed of light have anything to do with our constant speed? I'm sure I'm way off base...

6. Sep 10, 2009

### Fredrik

Staff Emeritus
It is of course complete nonsense to say that time moves at a certain speed. This is a recent thread about that subject.

I really don't like that Brian Greene quote. My comments about it are in this thread, starting with post #6 and ending with post #18.

7. Sep 10, 2009

### Jimmy

Sorry for the quote and thank you for the links.

8. Sep 10, 2009

### A.T.

Here is a similar one about the Schwarzschild metric (gravitational field of a sphere):
I think you have it a bit backwards here. All those geometrical models are merely an interpretation of the things we observe, not the cause of it. If the things we observe would be different, our models would be different, but so far we have not observed things that advance at different rates in space proper-time.
I think you are just misunderstanding Greene, because you assume he speaks about moving in the usual Minkowski space coordinate-time, while he probably means space proper-time. If you see proper-time as the time dimension the constant advance in space-time makes much more sense.

Last edited: Sep 10, 2009
9. Sep 10, 2009

### Jimmy

I think I misspoke then because I certainly don't believe the models are the cause of what we observe, or that things advance at different rates in space proper-time. Just speculating about the implications if the latter were true.

10. Sep 10, 2009

### A.T.

Well, different advance rates in space proper-time would mean that, you could have two identical clocks at the same place and at rest to each other ticking at different rates. Or two photons in vacuum moving side by side at different velocities. We have not observed this so far.