Question

in twin paradox it is said that if one of the twin say A goes to a distant galaxy at speed close to light's and returns on earth then he would have aged less than the other say B, one who was during all that time on earth...now according to equivalence postulate,both frames are equivalent...so we can see in the frame of reference of the A that B is moving far from him and then returning..so shouldnt he see B as being young??...then by symmetry,none should have aged less..

Answer 1

No, they are not equivalent. Because one changes speed and the other does not.

In special relativity, all INERTIAL frames are equivalent, but twin A in not travelling in ONE inertial frame throughout its round-trip. (He/she must feel acceleration when changing direction.)

Answer 2

@quarkine: do you know expression for 'ageing'? (or whatever name it is given in English)

Answer 3

nope..
in time dilation it said that clock runs slowler in moving frame.so how does acceleration affect the running speed of clock??

Answer 4

I am working out an answer, but I have to recollect bits and pieces of what I know about sp relat., and I need to draw something. It might take a while for me to answer. Have you checked Wikipedia? They might have a good way of lifting the paradox.

Answer 5

in the explaination, i think i saw that it said something about the acceleration to high speed while taking off from earth occured in negligible amount of time and so did the final deceleration during return..so i assumed that the age difference was due to the movement during close-to-speed-of-light and not the brief acceleration time..now it do make a bit more sense when we impose the inertial frame condition,but still symmetry still exist..we can always say that earth was accelerating away from A's spaceship and returned to it,couldn't we??

Answer 6

Is time dilation not more an issue of `absolute' velocity (if there is such a thing) than of relative velocity?

Suppose I'm in a train going past a station. To me it appears that the train is standing still and the station is moving with a huge speed past the train. So would that mean that the station has e.g. more kinetic energy from my point of view?

As long as person B is not moving at an `absolute' speed near light-speed, he should age faster than person A, who is moving at an `absolute' speed near light-speed.

Answer 7

Wikipedia's page http://en.wikipedia.org/wiki/Twin_paradox has it.

If you scroll down to: "Resolution of the paradox in special relativity", you will read the same explanation I gave you :-)

Now, what I was looking for was the Minkowski diagram of the trip. It also lies in that section of the page. Have a look at it, and we can discuss it later.

Answer 8

@slotema but the very beginning of the book starts with discarding of the idea of absolute speed...

Answer 9

It is not the acceleration itself that makes you age, but the fact that it gives you such a speed that time dilation will apply.

Answer 10

Imagine we are blindfolded and we run a race from start to finish.
You're B, I'm A.

If at one point I swerve out out the way, the people from the finish line see me, they shout, and I swerve again aiming at them, obviously I will have travelled a greater distance than you, ok?

What I want to point out, is that the extra length I have to run does not happen at the very point where I change direction, but that it is nevertheless a consequence of that change of direction.