Einstein's Theory: The Speed of Light and Time

[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?

 

[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.

 

[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:

Einstein proclaimed that all objects in the universe are always traveling through space/time at one fixed speed - that of light. This is a strange idea; we are used to the notion that objects travel at speeds considerably less than that of light... We are presently talking about an object's combined speed through all four dimensions - three space and one time - and it is the object's speed in this generalized sense that is equal to that of light... If an object is sitting still (relative to us) and consequently does not move through space at all, then ... all of the object's motion is used to travel through one dimension - in this case, the time dimension. Moreover, all objects that are at rest relative to us and to us and to each other move through time - they age - at exactly the same rate or speed. If an object does move through space, however, this means that some of the previous motion through time must be diverted... the object will travel more slowly through time than its stationary counterparts, since some of its motion now is being used to move through space... We see that this framework immediately incorporates the fact that there is a speed limit to an object's spatial velocity: the maximum speed through space occurs if all of an object's motion through time is diverted to motion through space. This occurs when all of its previous light-speed motion through time is diverted to light-speed motion through space. But having used up all of its motion through time, this is the fastest speed through space that the object - any object - can possibly achieve... Thus light does not get old; a photon that emerged from the big bang is the same age today as it was then. There is no passing of time at light speed. (p50-51)

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.

 

[A.T.]

I've never heard of time moving at the speed of light.

A question about the "speed of time" is posted every month on this forum.

However, some popularizers of physics have stated that all objects move through space-time at the speed of light.

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:
http://www.adamtoons.de/physics/relativity.swf

 

[Jimmy]

Here a diagram of space proper-time, showing time dilation and length contraction directly:
http://www.adamtoons.de/physics/relativity.swf

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...

 

[Fredrik]

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.

 

[Jimmy]

Sorry for the quote and thank you for the links.

 

[A.T.]

That's great. Thanks for that. Demos like that certainly help one get a better grasp of what's happening.

Here is a similar one about the Schwarzschild metric (gravitational field of a sphere):
http://www.adamtoons.de/physics/gravitation.swf

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?.

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.

II 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.

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.

 

[Jimmy]

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 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.

 

[A.T.]

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.

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.