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 Subject: How relative is relativity? Category: Science > Physics Asked by: bbergan-ga List Price: \$10.00 Posted: 24 May 2004 17:02 PDT Expires: 23 Jun 2004 17:02 PDT Question ID: 351408
 ```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?``` 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``` 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``` 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``` 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.```
 Subject: Re: How relative is relativity? Answered By: mathtalk-ga on 28 May 2004 14:34 PDT
 ```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!"```