# Spacetime distortion - question

1. Dec 6, 2003

### Tail

Let's say there are two objects of the same mass. One of the is moving, the other not. Now let's look at both of them at a certain moment of time. In this MOMENT both are still, they occupy a certain space. Can you tell which one is moving based on spacetime distortion (theoretically)?

2. Dec 6, 2003

Staff Emeritus
"One of them is moving and the other not". Relative to what? Each of them will see the other as moving and apply reciprocal "distortions" to explain the data they receive. There is no "fixed space distortion" for you to examine.

3. Dec 7, 2003

### Tail

Damn. I've always hated relativity.

But didn't Einstein state that things in motion distort spacetime more = time flows slower around them?

4. Dec 7, 2003

Staff Emeritus
In General Relativity, momentum is one of the things, along with energy, that can warp geometry. The point, though, is that even more than Special Relativity, GR is slave to the frame. Einstein called it General Covariance; all your physics has to be true no matter what frame of view you adopt (up to smooth changes of coordinate). So it is still true that in one frame A is moving and in another it is B that is moving. This is all based on unaccelerated motion. Accleration is a whole nother question in GR, see the equivalence principle.

5. Dec 9, 2003

### HallsofIvy

NO, that's the error in one of Zeno's paradoxes (so it goes back a lot longer than than relativity!). At a given instant an object moving relative to another still has a velocity.

6. Dec 10, 2003

### ObsessiveMathsFreak

I think what tail was asking was that given simply, a freeze frame "shot" of the two objects and of the space/time distortion around them, can you tell how fast they are going, relative to one another of course.

Can you obtain velocity from the information about space/time distortion. Maybe you can. Isn't the inverse possible.

7. Dec 10, 2003

### Tail

Yes, that was exactly what I meant!