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 Twin paradox (time travel and special relativity)

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SteveL
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PostSubject: Twin paradox (time travel and special relativity)   Tue Jun 22, 2010 3:04 pm

http://en.wikipedia.org/wiki/Twin_paradox

A topic I brought up recently.

One twin remains on earth while the other travels away at a speed close to that of light (e.g., 0.99 c). Each flashes a pulse of light towards the other at regular intervals, and each correctly calculates that the other ages more slowly than himself, because during the first twin's outward journey each twin is in fact moving away from the other in terms of relativity. When the traveling twin reaches his outbound destination, he reverses course and returns to earth, where it is revealed that he has aged less than his earth-dwelling brother. How can this be?

The answer, apparently, is that the traveling twin's change in direction (i.e., acceleration) negates his status as an "equal observer" (my astronomy textbook's term), which takes him out of uniform motion relative to his twin.

But what if the experiment is only one way and the twin only travels towards earth? Which twin would age less, if any? (We assume the traveler attains his speed instantly.) The answer might be that it would be impossible to establish a starting age for each of them because such information would have to be transmitted, and because this itself takes time, the experiment can't begin for each of them at the same time (the issue seems to be the limitation of information sharing due to the nature of the speed of light). In addition, relativity tells us that there is no such thing as a universal present moment; between people on earth (i.e., the twins before departure), this margin is small and can be accounted for, but when one twin starts the experiment at a distance, it would seem impossible to establish or coordinate a common start time. On the other hand, why can't the entire experiment be conceptually broken into two separate experiments, or legs: (1) the trip away, during which each ages less than the other, and (2) the trip home, the start time of which both twins can calculate?

P.S. Another oddity of special relativity seems to be that from the point of view of a photon (a light particle), which travels at speed c, no time passes at all. To a photon, it arrives at its destination instantly; to us it takes the necessary duration (e.g., 1 year to travel 1 LY).
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SteveL
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PostSubject: Re: Twin paradox (time travel and special relativity)   Tue Jun 29, 2010 4:32 pm

Here's another question:

Rather than a photon traveling at velocity c towards me, can't I be said to be moving at the same velocity towards the photon?

I have some thoughts, but they're probably wrong.

(This scenario avoids the acceleration aspect of the twin paradox.)
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