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Time and Space

  1. Jun 17, 2009 #1
    Are time and space independent of an object, or are they properties of an object?
  2. jcsd
  3. Jun 17, 2009 #2


    Staff: Mentor

    What do you mean physically. In other words, what kind of experiment could you perform to distinguish the two options?
  4. Jun 17, 2009 #3
    Statements about time and space are just statements about measurements of distance and duration, and measurements of distance and duration are reducible to observations of objects.
  5. Jun 17, 2009 #4
    There is a space-time distortion felt by objects in motion...but is this effect felt only by the objects in motion or is this distortion extended to local space-time?
  6. Jun 17, 2009 #5


    Staff: Mentor

    Again, what would that look like physically? How would you experimentally tell if the distortion was "felt only by the objects" or "extended to local space-time"?

    The reason I ask is because if there is no experiment you can use to determine it then you are asking a philosophical question rather than a physics question. Not that there is anything wrong with philosophical questions, but just to realize that they are different from scientific questions.
  7. Jun 17, 2009 #6
    If time is slowing down for an object traveling at high speeds, and if this is only felt by the moving object, then how would we interpret the situation: would it be that the object is distorting time (time is independent of the object), or is the object's time distorting (time is a property of the object)?
  8. Jun 18, 2009 #7


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    You have to clearly say what you want to measure. For example: You have to identical clocks going at the same rate when placed side by side at rest to the observer. When one clock is moving relative to the observer it is measured by the observer to tick slower than the clock at rest to the observer.

    You always need two clocks to define time dilation.

    Does this interpretation change any measurable quantities? If not: pick the one you like more. Physics doesn't care. My favorite is that the clock advancing in space, advances less in time.
  9. Jun 18, 2009 #8


    Staff: Mentor

    Hi Gear300, let me try and guess what I think you are asking and propose an experiment to measure it. If that is not what you are asking then maybe you can refine or correct the experiment:

    Consider 3 identical ideal clocks: the reference clock, the rest clock, and the moving clock. The reference clock and the rest clock are far away from each other, at rest wrt each other, and synchronized via the standard Einstein synchronization procedure. The moving clock is moving inertially at relativistic speeds and passes the rest clock as close as possible without colliding (never coming close to the reference clock). After the moving clock departs, are the rest clock and the reference clock still synchronized?

    Is that what you were getting at? If not, can you propose a different experiment?
    Last edited: Jun 18, 2009
  10. Jun 18, 2009 #9


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    Homework Helper

    Nice post DaleSpam :smile:
  11. Jun 18, 2009 #10


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    In special relativity, you can move objects around without changing the properties of spacetime (metric). So different positions of objects correspond to the same spacetime.

    In general relativity, different positions of objects correspond to different spacetimes.

    In both cases, the properties of spacetime are inferred by using objects.
  12. Jun 18, 2009 #11
    Yes...you have it more or less on the spot...but, based on the other posts, I'm assuming this is left to little more than interpretation?
  13. Jun 18, 2009 #12


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    In the experiment DaleSpam came up with, special relativity predicts that the rest clock and the reference clock would still be synchronized after the moving clock departs.

    That entails ignoring gravity, of course, since special relativity doesn't deal with gravity. If you take into account the gravitational forces that the clocks exert on each other, then general relativity predicts that the rest clock and the reference clock would not quite be synchronized. The rest clock would be slightly behind the reference clock due to gravitational time dilation. But that would occur even if the moving clock were sitting still - it's due to the moving clock's mass, not its motion.
  14. Jun 18, 2009 #13


    Staff: Mentor

    Relativity makes definite predictions about this experiment, how you choose to interpret the prediction is up to you. Provided there is no significant gravitation, the rest clock and the reference clock remain synchronized. If the moving clock is attached to a gravitating mass then the clocks will not remain synchronized.

    EDIT: I see that diazona was quicker :smile:
  15. Jun 18, 2009 #14
    I see...Thanks for the replies.
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