Whom do we consider as moving in special theory of relativity?

[jason_bourne]

hey guys... i have understood the part of special theory of relativity where they explain why the time slows down for a guy moving with greater speed.. or atleast i think i have understood. But the problem is that suppose two guys are in the space one guy is moving at a high speed and the other one is standing still .. so according to the theory the one whos moving his clock should go slow than the one who is standing in one place... but the point is as to which person should i considered moving or standing or moving according to normal relativity concept.. i may consider the one standing as moving and vice versa .. so wen they compare their watches whose watch should have delayed? how one decides this?

 

[Fredrik]

There's no preferred rest frame. They are both right when they say that the other guy's clock is ticking at a slower rate. (If you think this can't possibly be true, you are wrong. Check out the threads about the twin paradox if you want to know more).

By the way, what a clock measures is the "length" of the path it takes through space-time, with "length" defined in a funny way. You add up contributions of the form $\sqrt{dt^2-dx^2}$ along the path, so movement in "space" makes the path shorter.

 

[HallsofIvy]

The whole point of "relativity" is that it does not matter. The laws of physics are the same and will give the same result no matter which frame of reference you take.

(Notice, here, each person saying that the other person's clock is ticking slower is the "same result".)

 

[granpa]

the reason that each is able to see the other as time dilated is that there is a loss of simultaneity.

i wrote a long explanation of this but nobody responded.
https://www.physicsforums.com/showthread.php?t=236978

 

[Mentz114]

the reason that each is able to see the other as time dilated is that there is a loss of simultaneity.

i wrote a long explanation of this but nobody responded.
https://www.physicsforums.com/showthread.php?t=236978

If it requires a long explanation it's probably wrong. Simultaneity is an observer dependent phenomenon and is an effect of relative motion and position. It is not a cause of anything.