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Why is time relative?

  1. Jul 17, 2006 #1
    this may be posted in the wrong place, but can sombody help me understand why time is relative? why should it not be absolute?
  2. jcsd
  3. Jul 17, 2006 #2
    The line of reasoning that ends with "time is relative", starts with the postulate that the speed of light is the same for all observers. This postulate was itself based on the failure of certain experiments that were supposed to measure differences in the speed of light between observers.

    This behavior of light is different from that of a baseball thrown by someone on a moving train. The speed of the ball depends upon the situation of the observer. Someone on the train would give one speed for the ball, while someone on the ground watching the train go by would give a different speed.

    Speed is the ratio of space to time. If space and time are absolute, then the speed of light must be relative, depending upon the speed of the observer. The speed of light is absolute and does not depend upon the speed of the observer, therefore, either space or time must be relative. A more detailed algebraic calculation will show that both must be relative.
    Last edited: Jul 17, 2006
  4. Jul 17, 2006 #3
    You need to look up Einstein, Special Relativity, and Michelson-Morley. These two guys measured the speed of light in different directions, expecting a difference due to the earth's motion through space. They didn't find any. And since velocity is distance over time, if the velocity didn't change but the distance did, the time had to change as well.
  5. Jul 19, 2006 #4
    For a detailed explanation do what farsight said. But as a quick thought experiment ignoring most of the aspects of Einstein’s theories. First we synchronize our wrist watches. Suppose you stand 100m away from someone with a flash light and as soon as you see the person turn on the flash light you record the time on your watch. I will stand 200m away and will record the time from my watch when I see the flash light turned on. Assuming we were super accurate with our fast reflexes etc… when we compare times we do not agree on when the flash light was turned on. This is because light had to travel twice as far to reach me as it did to reach you. So time is relative to the observer. But light is so fast in reality we don’t notice that in our everyday lives as well as the other bizarre stuff that Einstein’s special relativity predicts.
  6. Jul 19, 2006 #5
    Simultaneity does not exist. Two events may seem simultaneous in one inertial frame, and not simultaneous in other; in fact observers will not generally agree on the order of events, depending on their relative motions.

    See, e.g. http://en.wikipedia.org/wiki/Relativity_of_simultaneity

    Another feature of SR - observers will not agree on elapsed time intervals. For instance, a fast plane circling the earth, returning to its starting point, will experience a slightly shorter time interval than an observer who never moved from the starting point. This is a feature of the geometry of Minkowski spacetime; the longest interval between any two spacetime points is a straight line! Exactly the opposite of Euclidean geometry. The crucial point is: obsevers do not agree on elapsed time interval, because their measurement of elapsed time depends on what trajectory they took in the meanwhile.
  7. Jul 20, 2006 #6
    S = D / T

    Speed and distance are both variable, and hence time is too :tongue2:

    A cop out explination but an explination nonetheless.
    Last edited: Jul 20, 2006
  8. Jul 20, 2006 #7
    Well isn't this a bit to broad?
    It is true that observers may not agree on the order of events but that does not imply that simultaneity does not exist.
    It really comes down to how you define it.

    For instance would we say, lenght does not exist since observers may not agree on the length of something, or duration does not exist since observers may not agree on the duration of something?
    Last edited: Jul 20, 2006
  9. Jul 20, 2006 #8
    It does come down to how you define it. What they meant was that universal simultaneity does not exist. Two events which are simulataneous in one frame will not, in general, be simultaneous in all frames. Simultaneity is subjective.

    Time is relative because the speed of light is constant. If one works out the consequences of the speed of light being measured the same in any frame, no matter the velocity, one finds that time and length must contract, and mass must increase. In fact, Einstein only postulated that the speed of light was independent of the speed of the emitter, but the constancy of the speed of light in any reference frame directly follows from that. Certain things in the universe are relative, and certain constant. It would be more intuitive for time to be constant, but it just isn't the case.

    Michelson & Morley is the most famous experiment which backs up relativity. I believe there was an astronomer which determined observationally that the speed of light as measured on Earth is independent of the speed of the emitter, but I haven't been able to find anything more specific.
  10. Jul 20, 2006 #9
    I meant simultaneity in the usual sense - for two events (t1, x1), (t2, x2), there is no consistent statement of the form "t1 = t2" except in the trivial case in which it holds in only one Lorentz frame (or else x1=x2, also trivial).

    The point of "does simultaneity 'exist'" seems entirely semantic to me. It is not uniquely defined, and has no usefullness, so in my narrow, pragmatist worldview I'm not concerned.
  11. Jul 20, 2006 #10
    It is obvious that simultaneity exists in the universe because otherwise the universe could not have a causal structure.
    Last edited: Jul 20, 2006
  12. Jul 20, 2006 #11


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    There is still a single objective truth about whether one event lies in a second event's past light cone, that should be enough to give the universe an objective "causal structure", I don't see why simultaneity (which involves events with a spacelike separation that cannot possible affect one another) is relevant to this.
  13. Jul 20, 2006 #12
    Feel free to explain why you think simultaneity involves only events with a spacelike separation. To me that simply does not make any sense.
  14. Jul 20, 2006 #13


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    Because there is no inertial reference frame where two events with a timelike or lightlike separation happen at the same time-coordinate, whereas for any pair of events with a spacelike separation, there is always an inertial frame where they are simultaneous.
  15. Jul 20, 2006 #14
    Huh? :confused: Simultaneous MEANS "at same time". Timelike-seperated events occur at DIFFERENT time coordinates, in all frames. SR has a causal structure - things are either in the past lightcone, or the future lightcone, or causally seperated. Simultaneity has nothing to do with this.
  16. Jul 21, 2006 #15
    Two space-time events X and Y can be timelike separated and many worldlines can travel between it, which means that they are arriving at Y simultaneously. What their individual proper times indicate is really irrelevant!
    Last edited: Jul 21, 2006
  17. Jul 21, 2006 #16


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    Perhaps this is just a terminological issue, but that's not what physicists are talking about when they use the word "simultaneity". They are only talking about whether the two different events have the same time-coordinate in a given reference frame, the question of a signal going from one event to the other doesn't enter into it. Obviously if two signals converge at the same point in space and time they do so simultaneously (in all reference frames), but then the two events that you're saying happened simultaneously both occurred at Y (meaning there is no separation between them, timelike or spacelike), you aren't even talking about the event X of either signal being emitted.
  18. Jul 21, 2006 #17
    Worldlines have nothing to do with this! X and Y are not simultaneous; one happened before the other, and that is consistent in all frames. That is what "timelike seperated" means.

    'they are arriving at Y simultaneously"
    Yes, the event Y is simultaneous with itself. That's all that means.
  19. Jul 21, 2006 #18
    I don't think it is true that X has happened before Y in all frames. In flat space-time yes, but not in curved space-time.
    It seems to me that in curved space-time it is all but guaranteed that X happens before Y in all frames.
  20. Jul 21, 2006 #19


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    Even in curved spacetime, there is still an objective coordinate-independent truth about whether X lies in the past light cone of Y. Still, perhaps you could design a weird coordinate system where X and Y happen at the same time-coordinate, I'm not sure what the limits on allowable coordinate systems are for GR.
    Wait, isn't that the opposite of what you just said above?
  21. Jul 21, 2006 #20
    Posted by Rach3
    Worldlines have nothing to do with this! X and Y are not simultaneous; one happened before the other, and that is consistent in all frames. That is what "timelike seperated" means.
    I suspect you’re contradicting yourself by not staying clear on the differance between "spacelike separated" and "timelike separated".
    To make what Rach has said clearer, remember if X and Y are only "timelike separated" then there exists a reference frame moving fast enough that X and Y will occur at the same PLACE in that frame. Note that frame also sees the time between X and Y to be the largest - all other frames will see shorter time intervals but always in the same order. As he said if they are "timelike separated" they cannot be simultaneous in any valid reference frame.

    But if X and Y are “spacelike separated” there is no reference frame where X and Y are measured as being in the same place, but they can be simultaneous. Example you and a friend snap fingers simultaneously across a room – not even a reference frame moving at the speed of light from your fingers to their fingers can put them those events at the same place.
    But all other frames other then you and our snap partner’s will disagree about who snapped first and by how much.

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