Special Relativity Question

Aug 2009
I just started special relativity in school and, being somewhat confused, I found myself asking this question.

An observer A is travelling at a substantial fraction of the speed of light relative to observer B. Since they are both moving at that speed relative to one another, how do we know which one experiences time dilation/length contraction?

I know it must have something to do with one of the observers having had to accelerate (I think), but I don't understand exactly why.

Thanks in advance


PHF Hall of Honor
Jul 2009
Yes, the observer that travels near the speed of light relative to a stationary observer experiences special realitivistic effects.
Aug 2009
Right, but my question involves the fact that A is moving at some speed relative to B, but B can be considered to be moving at that same speed relative to A. What determines which one we choose to regard as stationary?

I'll rephrase it in this idealized form: 2 observers, A and B, in empty space are travelling relative to one another at a substantial velocity v and with no reference points except themselves and one another. Each observer considers the other to be moving while he himself is stationary. Which observer experiences the relativistic effects and why? And if it has to do with whichever was initially accelerated, what is it specifically about his acceleration that causes the effects to apply to him alone?


PHF Helper
Feb 2009
You are right we can't decide who is moving as long as they travel with uniform velocity with respect to each other beacuse uniform motion is relative not absolute. So what happens is this.

A in his own ship eperiences no relativistic effects, but when he looks at B, he finds that B experiences them,since from A's stand point, it is B who is moving with relativistic velocity.
And vice-versa for B.

Special relativity states that the speed of light is constant in all inertial frames of reference and that the laws of physics also have the same form in all inertial frames of reference.

The price that has to be paid for this is that things like mass, time and length which were always taken to be absolute are no longer so in relativity.

However if any of them accelerates, he will feel a force. So accelerated motion can be detected and is not absolute. However for this problem, we need not bring in acceleration .

The postulates of special relativity are not valid for accelerated frames of reference.
  • Like
Reactions: swashbucklord