# Special relativity and frames of reference

1. Oct 8, 2004

### winhog

I have learned that objects moving at high velocity experience time dilation, among other things, and that there is no ultimate frame of reference in the universe. If this is so...say two galaxies pass by each other at near the speed of light...in which one would time be moving slower? There's no way to tell unless there is an ultimate frame of reference, right?

2. Oct 8, 2004

### jcsd

Your close to getting it right, which is commenable as this is an issue that confuses many people. The answer is that galaxy A sees galaxy B's clock slowed down wheraes galaxy B sees galaxy A's clock slowed down this is due to the fact that time itself is relative. We can never say which inertial observer's clock has slowed down in an absolute sense.

3. Oct 8, 2004

### winhog

In that case...why would a person on a spaceship moving quickly age slower than those on earth? (like in the grandfather paradox) Is it simply because the spaceship accelerated while the earth didn't? Couldn't that also be considered the earth accelerating away from the spaceship?

4. Oct 8, 2004

### jcsd

The Grandfather paradio is something different (it's a time travel paradox), your thinking of the twin paradox (which isn't a real paradox). Yes your right accelartion destroys the symmetry; accelaration in special relativty is absolute (well not stricctly as 3-acceleration is not absolute, but the important thing is that if someone is acclerating then all inertial observers agree that they are acclerating and in special relativty there is only absolute relativity between inertial observers i.e observers wo are not accelerating).

5. Oct 9, 2004

### winhog

Aha, that makes sense. Thanks for clearing that up!

6. Oct 10, 2004

### rcgldr

Yes.

No, because the spaceship was accelerated due to a force. It's the acceleration due to a force that counts.