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In the twins paradox, the twin that travels away from earth comes back to find that his (her) twin has aged much more due to relativity's effect on time. Why can't we posit that it is not the one twin moving away from Earth at the speed of light, but it is the entire universe, the earth and his twin included, who are accelerating and moving away at the speed of light and then coming back? In this case shouldn't be the other twin to have aged less? (and, in fact, the whole universe) What is wrong with this way of thinking?

This has NOTHING to do with the other question. I am asking what is wrong in my thought-experiment in which the whole universe accelerates away at the speed of light and then comes back. There clearly is an asymmetry here I am missing, and asking what it is. The other question is asking to clarify the twins paradox, it is not addressing the asymmetry.

user
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    Something that is very important to understand: while pop-sci books always present this as a though-experiment involving people it has been tested repeatedly with unstable particles in accelerators. – dmckee --- ex-moderator kitten Feb 09 '16 at 22:07
  • @dmckee I don't doubt for a second that this is true and tested repeatedly. I am trying to understand what is wrong, and I know there is something wrong, in my thought-experiment. Because I know I am making a simple mistake but I cannot see it. – user Feb 09 '16 at 22:10
  • @gonenc I don't see how this is a duplicate. I am asking what is wrong in my thought-experiment in which the whole universe accelerates away at the speed of light and then comes back. There clearly is an asymmetry here I am missing, and asking what it is. – user Feb 09 '16 at 22:13
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    To whoever put a minus one, will you care to explain why? – user Feb 09 '16 at 22:14
  • It is an obvious duplicate that is why all the minus signs.... Look at this answer for example. – Gonenc Feb 09 '16 at 23:05
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    It should be apparent which of the two twins is really accelerating away and then accelerating back because he or she will feel acceleration effects that the other twin won't. So your argument breaks down because it assumes that it is valid for either twin to say that he or she is stationary and that the other twin is the one who is doing all of the accelerating and decelerating. –  Feb 09 '16 at 23:26

3 Answers3

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If you get accelerated forces act upon you. Therefore you can tell exactly that it is you who accelerates and not the whole universe.

Yukterez
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  • How do you distinguish a positive force acting on me, from (an opposite) one acting on the whole universe? – user Feb 09 '16 at 22:32
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    The same way you can tell a ball is falling down when you drop it, and you and the ground are not falling up. – knzhou Feb 09 '16 at 23:56
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One of the premises of special relativity is that observations made in different non-accelerating (or "inertial") frames are equally valid, and that there are certain quantities that all such observers agree on.

One such quantity is the proper acceleration $\vec{a}^2-a_0^2$. All inertial observers agree on whether an object is accelerating or not.

This is the asymmetry. If one twin stays in an inertial frame and the other one accelerates, the accelerated one ages less. Incidentally, if we make it truly symmetrical, and accelerate the whole universe one way and the twin the other way the same amount, both twins age the same.

Chris
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I agree with Симон Тыран. "If you get accelerated forces act upon you. Therefore you can tell exactly that it is you who accelerates and not the whole universe"

To answer the follow-up comments - Even if we can not distinguish between who has actually accelerated, the body that is accelerated, does feel the acceleration. And clock speed is influenced by that exertion of force/acceleration. So, whether we can distinguish or not, the clocks (time dilation) knows the stress of acceleration.

kpv
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