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Why time is relative.

  1. Sep 9, 2011 #1
    I was sitting in my bed thinking about how time can be relative after i heard about those experiments with atomic clocks slowing down on moving objects. My idea is:

    Since it is meant to be impossible to go faster then the speed of light, then as something moves faster, it should become progressively harder to continue speeding up as you get closer and closer to the speed of light.

    So if you put a clock, or a person on a train that is moving at half the speed of light. then it means all the molecules of the clock are already moving at half the speed of light, and so any other movement (such as the all the inner workings of the clocks molecules) , would necessarily have to slow down meaning the time measured by the clock or object would slow down proportionally to its velocity relative to the speed of light.

    So the equation for the rate that an object experiences time would be something like:

    rate of time experienced by object = (c - v) / c where c = speed of light and v = velocity of object.

    So as v increases, your experience of time decreases. If v = speed of light then c-v/c = 0 so the object moving at the speed of light wouldn't experience time at all.

    Can anyone who knows more about physics confirm if any of this actually makes sense please and tell me what to look up if its been written about before or if its just plain wrong lol.
  2. jcsd
  3. Sep 9, 2011 #2


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    Well, I'll give you points for a cute, original explanation. Unfortunately, it doesn't work.

    First, a technical point: you conclude (c-v)/c as a time dilation factor. This can be written (1 - v/c). The correct factor is sqrt(1- v^2/c^2).

    A bigger issue is that your idea is not isotropic. You might be able to rig it to explain 'resistance' to motion in the same direction as the overall motion, but why would that affect motion opposite or perpendicular? So a clock mechanism would behave in a non-isotropic way. That doesn't happen.

    If you tell us a little about your background and interest level, people here can readily suggest appropriate books to learn more.
  4. Sep 9, 2011 #3
    i guess in response to the isotropic movement you could stay, I think it would still be fair to assume that the molecules would still be moving slower even if they are moving in the opposite direction (decelerating) and yes then it is not perfectly isotropic but because the differences are so small it would be negligible since in that case you could say that for all objects there will be molecules moving in different directions and so nothing is isotropic anyway.

    If all other movement in the object is slowed down as its velocity is sped up, then any factor that would cause some of the molecules to move in the reverse direction would then also have to be slowed down by a proportional rate. So therefore molecules that are decelerating should be deceleration at a reduced rate relative to if they had done the same thing in a stationary object. Such as if someone drove a car backwards down a train moving near the speed of light, eg if its a car that can accelerate at 40mph per second on earth. To a stationary outsider, it would take longer then 1 second for the car to be moving 40mph less then the train.

    if you are going to say that in something as small as a clock, it is not isotropic because some parts will be moving in different directions, I don't see how that is any different from a stationary clock. Since even in a stationary clock there are molecules moving at different speeds and so by the same logic that clock wouldn't be isotropic either. But because the differences are so small they are negligible.

    I don't have much of a background in further education in physics other then at A level. So my interest level wouldn't be enough to read an entire book on the subject, something like a summary of paper that could explain it in less then a page or two would be the ideal thing im looking for.
  5. Sep 9, 2011 #4
    ps, why would the time dilation factor be squared. Is the rate that time slows down as objects move faster non linear?
  6. Sep 9, 2011 #5


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    Very non-linear.

    It will be very hard to try to explain all this to you if you are not willing to read more than a page or two.
  7. Sep 9, 2011 #6
    I might be willing to read a decent ammount just not a book if you get what i mean lol.
  8. Sep 10, 2011 #7
    Go ahead and suggest some reading material; this is incredibly interesting.

    I am currently considering physics as my major. I've had up through Calculus III, Diff. Eq., Matrix Algebra, Abstract Mathematics, and a little bit of Tensor Calculus. Any reading suggestions would be greatly appreciated. Thanks.
  9. Sep 10, 2011 #8
  10. Sep 10, 2011 #9


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    Taylor and Wheeler, Spacetime Physics, is the one most people recommend as a reliable first book for a physics student.

    Mermin, Space and Time in Special Relativity, is supposed to be another good one.

    (I haven't read either of these; I have very old books that I would not recommend as starting points).
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