Question

"The famous twin paradox"
http://en.wikipedia.org/wiki/Twin_paradox

Whenever we study the above concept, we consider one twin to be left on Earth (and hence get emotionally attached to the reference frame :P)

Let's consider that both the twins travel in a space ship (in opposite directions), return back (and experience the so called "asymmetrical acceleration when switching direction") and finally meet on some reference frame.

How can we predict there ages?

Answer 1

PS: I am not actually looking for a comparison of their ages. All I want is a theoretical analysis of relativity in this particular case.

Answer 2

you could predict their ages by knowing how old they were when they went into space and knowing how long they were gone for

Answer 3

In the original theoretical concept the two twins were of the same age (IDK if there can be twins of different ages)

As time is frame dependent, I can see no way of determining the time of their journey, although they took off at the same time (if that's what you wanted to ask)

Answer 4

I believe is the only thing that matters would be speed achieved. Assuming both twins have exact same path in opposite directions I would expect them to arrive to earth being equally old.

Answer 5

Why are we examining speed (or acceleration) with respect to the earth. What is so special about this frame of reference?

Answer 6

Nothing really, I just used it as ending point because they started from it. I should've used imaginary static point with 0 velocity - then twins would have same age upon arrival.

Practically speaking I think if the twins leave and arrive to earth one might end up a bit older depending on which direction they went to according to earth own velocity, unless they traveled perpendicularly away from earth velocity vector.

Answer 7

actually no silly me - i dont think reference point matters as long as each twin has exact same movement relative to said reference point.

Answer 8

But then, each twin will see the other to be at a relative motion with respect to himself/herself. So they both will expect the other guy/gal to be younger.

Answer 9

I am not good at minkowski space, but I believe that twins leaving and arriving to 0 point mentioned above will actually see each other in real time.

Twins that leave and then arrive at Earth might see one part of the trip at slower pace, and return at faster speed or other way around.

Answer 10

The moment you have acceleration, u need to use general theory of relativity.. besides.. acceleration is not relative.. so.. i feel the retricemetrical thingy you are talking about, will hold good for both of them, since both are being accelerated (and this is independent of frame of reference)..

Answer 11

I know that general relativity asserts an equivalence between pseudo force and gravitational force. What I cannot understand is with whom reference do we talk about acceleration (I know it is not relative as experimentally we have calculated the angular velocity of earth).

Also, how would light appear from a non inertial frame of reference? Will it appear to have a uniform velocity or will it appear to have retardation? (Compare it with the standard classroom example that is taught to explain that velocity of light will appear to be same in all inertial reference frames.)

The modified question of twin paradox which I asked is a cumulative of all these doubts of mine.

Answer 12

^^ @Mashy

Answer 13

Lol.. this i think is outside my domain :D .
but since you know that acc is absolute how does it matter which reference we took, shouldn't we get the same answer ? (if the answer is no.. and that is understood in general relativity then just tell me, i ll stop ok? :P)

Also second one, i think the answer is deep, here is alink that discusses this

http://physics.stackexchange.com/questions/33816/does-the-speed-of-light-vary-in-noninertial-frames

like i said, no clue in general relativity.. so don't ask me doubts about that link ok :D

Answer 14

Knowing the reference frame(s) for acceleration is important because (beside the fact that they would be the only Newtonian Inertial frames) then we can decide which among the two twins changed their (family of) reference frames and entered the reference frame of other.

As the one who changes his (family of) reference frames experiences acceleration, he/she can be called a time traveler and would be younger than the other.

Answer 15

Btw thanks for the link, I think I should post this question there.

Answer 16

For those who care to know the answer of this question, visit - http://physics.stackexchange.com/questions/29921/why-isnt-the-symmetric-twin-paradox-a-paradox

@Mashy
Do you know any other (good) physics forum?