If relativity is truely relative, then why, if I race away from Earth at
the speed of light and return years later, why has everyone else aged more
than me.  Why have I not aged more than them.  Shouldn't which clock slows
down be based on whose reference frame you're in?  Isn't myself racing away
at the speed of light really just the same thing as the Earth racing away
from me at the speed of light?

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Request for Question Clarification by mathtalk-ga on 24 May 2004 18:56 PDT

The issue you raise is commonly referred to as the Twin Paradox.

In the theory of Special Relativity, where there is no acceleration
and no gravity, this cannot happen.

Therefore the simple answer would be to say that the extension of
relativity to cover acceleration and gravity is taken up in the theory
of General Relativity.

Experiments have shown that the effect predicted (slower "aging" by a
clock that leaves Earth and travels rapidly before returning) is real.
 Therefore a more detailed answer would go into why, from the
standpoint of General Relativity, such an effect does not contradict
the "no preferred reference frame" tenet of relativity.

Hint:  Accelerating your own body away from Earth is not the same as
accelerating the Earth (and the rest of the universe) away in the
opposite direction.  However movement away from Earth is equivalent to
the Earth (and the rest of the universe) moving away from you in the
opposite direction.

regards, mathtalk-ga

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Request for Question Clarification by hedgie-ga on 25 May 2004 11:58 PDT

bbergan

   As usual, mathtalk is correct, and perhaps even wise not 
'going into' the dificulty of answering question.

 I, perhaps being less wise, am willing to try.

So, my question (RFC)  is: 
are you happy with the comment, or do you want an answer,
if so, do you want references
, and what level of math and physics can you tolerate?

Please. look at

http://answers.google.com/answers/threadview?id=31905

which is related, before answering this RFC.

hedgie

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Request for Question Clarification by hedgie-ga on 25 May 2004 12:05 PDT

and, I forgot to add, 

you may want to look at this answer as well

http://answers.google.com/answers/threadview?id=33716

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Clarification of Question by bbergan-ga on 28 May 2004 13:09 PDT

I am happy with the answer.  I had thought it might have something to do
with acceleration.  Thanks for the insight.

Hi, bbergan-ga:

The issue you raise is commonly referred to as the Twin Paradox.

In the theory of Special Relativity, where there is no acceleration
and no gravity, this cannot happen.  Special Relativity deals only
with "uniform motion".

If you've been in a train or subway car beside another train going in
the opposite direction, you may have experienced a momentary
uncertainty about whether at slow speed it was your car or the other
which was moving.  Once acceleration kicks in, though, it removes the
ambiguity.

Therefore the simple answer would be to say that the extension of
relativity to cover acceleration and gravity is taken up in the theory
of General Relativity.

Experiments have shown that the effect predicted (slower "aging" by a
clock that leaves Earth and travels rapidly before returning) is real.
 Therefore a more detailed answer would go into why, from the
standpoint of General Relativity, such an effect does not contradict
the "no preferred reference frame" tenet of relativity.

Hint:  Accelerating your own body away from Earth is not the same as
accelerating the Earth (and the rest of the universe) away in the
opposite direction.  However movement away from Earth is equivalent to
the Earth (and the rest of the universe) moving away from you in the
opposite direction.

Gravity of a rotating body has some very small and strange effects,
according to the General Theory of Relativity, called "frame
dragging".  An experimental test to measure these effects was launched
earlier this year by NASA, after half a century in the planning!  Read
more about it here:

[Satellite to test Einstein theory]
http://news.bbc.co.uk/2/hi/science/nature/3596499.stm

regards, mathtalk-ga

This is exactly why it is called the twin "paradox" it is not really a
paradox but often thought to be as the questioner is indeed doing here
"why is my motion moving away from earth not equivalent to the person
on earth moving away from me?"

The essential difference and the difference you would feel is that if
you head off in a spaceship, turn around and come back again, when you
did the turning around bit something very different happened to you
than happened to the rest of us on earth. You decelerated and
accelerated back in the direction of the earth - you would feel this,
(as Feynmann says "all kinds of unusual things happen - the rockets go
off, things jam up against one wall and so on"), on earth we would
feel nothing.

So the two motions are NOT equivalent.

As stated above it is a question that really starts to make you think
in terms of general relativity, but at a simple level special
relativity can answer it and calculate the age differences see for
instance:

http://www.phys.unsw.edu.au/~jw/twin.html

I think a very good discussion of the twin paradox is here...

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html

Here's another wierdity:  You can always tell which spaceship is
accelerating, but you can never tell which spaceship accellerated.  
That is, once all rocket motors are turned off, there is no reason to
say which one is moving and which is sstanding still. All you have is
uniform relative motion

Hmmm... if they're twin bothers and they're in air force spaceships, I
guess you could call it "unifomed relatives' motion!"