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Can time contract?

  1. May 2, 2008 #1
    My question is simple. I made a bet with my physic teacher. :cool:
    I believe time can contract. When two bodies are moving away, one of each other, time expands for both. each one will see the ohter's time expanding.
    When they are becoming more and more close, they will see each other more in the "present". (the distance between them will be shorter, so the time light takes to make the path will be shorter. thus, time have to contract because, in my opinion, is the only way to see the other in a shorter distance in TIME).
    That is why i belive time can contract.
    I will wait your answer. Thanks
  2. jcsd
  3. May 2, 2008 #2


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    Well, I'm afraid that you've lost the bet with you physics teacher :wink:

    Even for two bodies moving towards one another, time still dilates. There is no time contraction. It can be seen from the Lorentz transformations that the time measured in a reference frame moving relative to two events (which occur in the same reference frame) will always be greater than the proper time.
  4. May 2, 2008 #3
    thanks. but take this example:
    you go on a sheep that emits light every 1 second
    you are going in the oposite direction of the observer.
    he will see that you emit light in less than 1 second. how do you explain that?
  5. May 2, 2008 #4
    If you mean by time "contracting" that a clock is speeding up, yes, it can, but not for the reason you think. Moving clocks in an inertial frame always run slow whether the distance between the two observers is increasing or decreasing. It's easy to see why this is because in the Lorentz factor, "v" is always squared, so any sign reflecting a negative or positive direction is negated. When an object accelerates or decelerates, however, the clock of the rest of the universe speeds up. So while, for the spacefaring twin's entire journey home, the Earth and everyone on it appeared to be aging more slowly than he was, as he decelerates the Earth ages rapidly and he arrives to see his twin old and gray.
  6. May 2, 2008 #5


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    Yes he will, so in other words the time between two 'ticks' of the observer's clock is greater that the time between the 'ticks' of the sheep's clock, hence time dilation.
  7. May 2, 2008 #6
    I understand what you are saying. But think this way. a ship emits a image of a clock every second. the observer sees the space between the images horter.
    can he say time is contracting.
  8. May 2, 2008 #7
    I don't think that the question is closed
  9. May 2, 2008 #8
    Time dilates!

    I think the usage itself is wrong! You shouldn't say time contracts. But it is clearly time dilation with reference to special relativity where time dilation is reciprocal. Since the observer is stationary and the person in the ship emitting light is in motion it is clearly time dilation.
  10. May 2, 2008 #9
    even if the observer see the time between the light emited by ship shorter than any person in the ship?
    I can't understand why time isn't faster. I talked to a university teacher. whatever he tells me i will post
  11. May 2, 2008 #10
    You can't exactly say that time is faster or slower.It always just appears so to the observer in time dilation according to special theory.Time will appear to go faster for the person in contact with the frame of reference which emits the light, for the observer, that is, only the observer will feel that time is going faster for the person in the ship.
  12. May 2, 2008 #11
    atention: I am NOT saying that, because he sees time going slower on the outside he thinks his time is contracting. Please don't think that. he sees his time as always. he takes his clock as reference. he sees the outside going faster. as you see, the observer sees less time between the light emissions
  13. May 2, 2008 #12
    Yes, you are absolutely right.The observer sees less time between the light emissions because the ship is moving and he is not.That is time dilation.You are right almost completely except for the part that time "contracts"! Sorry,anyway if I had offended you in any way, because all I wanted to do is to have a healthy discussion with you. Anyway, wait till you get more answers you might get a clearer idea !
  14. May 2, 2008 #13
    Hey man! thank you! I am portuguese, so many times i think i am not expressing myself reight. sory, i don't mean to be rude.
    a question: seing time faster on the outside is dilation? what do we call then when we time on the outside slower?
  15. May 2, 2008 #14
    Are you asking that, when we observe that time is faster it is dilation, but what it is called when time is noticed to go slower?
  16. May 2, 2008 #15
    I am asking the opposite. when we see that our time is larger than the outside's time, when we object in slow-motion a dilation?
    thus, when we see objects in a fast.motion shouldn't we call that contraction?
  17. May 2, 2008 #16
    In the special theory of relativity , time dilation is observed only by the other party, that is , it will not be observed by the person who is in the same frame of reference as the light emitter .Of course, the emitter of light must be in motion with respect to the other.

    But, in the time dilation, according to the general theory of relativity, gravitational time dilation, that is, time dilation will be accepted to be uniform by all observers.

    So do you get it now. Time dilation has nothing to do with observing the time to go slower or faster. To generalise, time dilation, when a person in motion notices time to go fast for the stationary person and the person stationary observes time to go slower for the person in motion, time dilation is occuring.
  18. May 2, 2008 #17
    hey! thank you very much. i finally understood that i had a problem with language
  19. May 2, 2008 #18


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    I think you have misunderstood relativity. The time dilation effects occur even after accounting for all of the "appearance" effects caused by the finite speed of light.

    For example, say there is a clock moving away from you at .5c and emitting pulses every 1s in your frame, and say that it is 10 light seconds (ls) away at t=0s when it emits the first pulse. You will receive the pulse at t=10s, use the known distance and speed of light, and determine that the pulse was emitted at t=0s. The clock will emit the second pulse at t=1s, and you will receive it at t=11.5s (since the clock has moved). You will again account for the known distance and speed of light, and determine that the pulse was emitted at t=1s. This process of accounting for the travel time of light is sometimes called an "intelligent observer".

    Special relativity is not about "appearances".

    Now, in your example you are talking about a clock moving towards an observer. In this case, the observer will use the distance from the clock at each point in time to calculate when the clock actually emitted the light in the observer's frame. When this is done, the intelligent observer will determine that the moving clock runs slow by the time dilation factor regardless of what direction the clock is moving.
    Last edited: May 2, 2008
  20. May 2, 2008 #19


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    To go a bit further, the "appearance" effects are described by the relativistic Doppler effect, which is different from the classical Doppler effect precisely because of time dilation.
  21. May 3, 2008 #20
    time in relativity

    think the fact that time dilate or contract depends on the way in which cloks in the involved inertial reference frame are synchronized
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