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Perception vs. Reality-relativity of time

  1. Jul 13, 2009 #1
    I am new to physics, and have been especially persistent in trying to understand relativity. After a few days of research and Q & A, I understand relativity, and its effects on observations, however one thing continues to bother me. A few people that I have talked to all say that time really does slow down when the observer is traveling at greater velocities; that they age more slowly.

    More specifically, this is my question; there are two twins, one standing on earth, and one who becomes an astronaut. The astronaut travels through space for a significant amount of time at a very high speed. When he returns, will he and his twin be the same age? In other words, does velocity effect the reality of aging?

    Most of the people I have asked have said that the astronaut will be younger than his twin, but I cannot wrap my mind around that. I am becoming confused and frustrated with this concept, please help me sort this out. As I stated earlier; I am new to physics, so it would help me a lot if the replies use simple terminology so that I do not have to research the more complex vocabulary that is used by more experienced and versed physicists. Thank you =)
  2. jcsd
  3. Jul 13, 2009 #2


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    Science Advisor

    If one twin moves inertially (doesn't change speed or direction) while the other moves away from him for a while, then accelerates to turn around and returns, the twin who accelerated will be younger when they reunite. This is known as the "twin paradox", there's a lot of detailed info here:

  4. Jul 13, 2009 #3
    What your problem specifically addresses is time dilation. Where speeding objects observe time differently when compared to a stationary reference frame.
  5. Jul 14, 2009 #4
    the answer is 'the sheep are black on one side'.

    most confusion with relativity is a result of not understanding 'relativity of simultaneity'.
    Last edited: Jul 14, 2009
  6. Jul 14, 2009 #5


    Staff: Mentor

    I like the link provided by JesseM above. It is important that you also know that this is not optical illusion, but instead has been verified experimentally to high precision: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

    The way I prefer to think of this is in geometric terms. You know that if you have two points on a piece of paper then the shortest path between those two points is a straight line. Similarly with relativity, where a straight line is an inertial objest's path. But the distance formula is Minkowski's instead of Euclid's so that the longest interval between two events is a straight line. Clocks measure this interval, not "coordinate time".
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