4.1 The "Twin Paradox" (Relativity and FTL Travel)
Description
This article is from the Relativity and FTL Travel FAQ, by Jason W. Hinson jason@physicsguy.com with numerous contributions by others.
4.1 The "Twin Paradox" (Relativity and FTL Travel)
The twin paradox deals with the question of "who's clock is running
slower?" The story goes as followers: Two twins (say Sam and Ed) are both on
Earth when one of them (say Sam) decides to leave the Earth by very quickly
accelerating to a speed close to the speed of light. We then consider the
two frames of reference after Sam has reached a constant velocity. According
to special relativity, in Ed's frame of reference, Sam's clock is running
slowly, while in Sam's frame of reference, it is Ed's clock which is running
slowly.
Now, as long as the two are apart, it is not to hard to argue that the
question is strictly dependent on your point of view. By this I mean that we
can argue that there is no correct answer to the questions--that who's clock
is running slower depends completely on what frame of reference you are in.
However, how would we continue this argument if we added the following to
the story:
At some point after Sam begins his trip away from the Earth, one of the
twins decides to go meet with the other twin. Either Ed decides to
accelerate away from the Earth and catch up to Sam, or Sam decides to
accelerate back towards the Earth to go back and meet with Ed. We then ask
this question: when the two twins are standing next to one another again,
which one is older?
With the above addition to the story, there must be a definite answer
to the final question. So, how can we continue to say that the answer
depends on your frame of reference? Well, as we will see, the final question
does have a definite answer, but the question of how this came about IS
dependent on who you ask.
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