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The problem of time

  1. Feb 13, 2012 #1
    I've been discussing Lorentzian and Einsteinian relativity here on PF, as well learning about them elsewhere, and a question occurred to me, and I thought that this might be the place to post it.

    The problem of time, as I understand it, is based on the fact that quantum mechanics and Einsteinian relativity use different concepts of time; I've read that QM uses "a more Newtonian" concept. I'm just wondering if this issue would be the same under Lorentzian relativity, which also seems to incorporate a more Newtonian concept of time?

    Given that both Lorentzian and Einsteinian relativity are not distinguished by means of experimental evidence, would it be possible to unify QM with Lorentzian relativity; are there any such attempts?

    EDIT: I was going to add a point on the Wheeler-DeWitt equation, but I didn't think it was relevant, but if my understanding, that the mathematics of Einsteinian and Lorentzian relativity are the same, then perhaps it might be relevant.

    My basic understanding if the Wheeler-DeWitt equation is that time does not appear in it, suggesting that the universe is timeless; while Lorentzian relativity includes the notion of absolute time, it is a short distance from the notion of absolute time to timelessness.
     
    Last edited: Feb 13, 2012
  2. jcsd
  3. Feb 13, 2012 #2
    I like talking about time,

    I can't make a question out of "The problem of time, as I understand it, is based on the fact that quantum mechanics and Einsteinian relativity use different concepts of time; I've read that QM uses "a more Newtonian" concept. I'm just wondering if this issue would be the same under Lorentzian relativity, which also seems to incorporate a more Newtonian concept of time?"

    So what's up for discussion?
     
  4. Feb 13, 2012 #3
    Einstein defined time as the movement of a clock hand. In other words, the best he could come up with is that 'time is the measurement of time'. It struck me as completely unsatisfactory, and yet I've never heard any better. Hardest concept ever to define.
     
  5. Feb 13, 2012 #4

    russ_watters

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    Why is 'time is what clocks measure' any less satisfactory than 'length is what rulers measure'?
     
  6. Feb 13, 2012 #5

    jim hardy

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    hmmmmm.... and from that, clocks are variable but c is not?

    what became of those Cern neutrinos that stepped to a different drummer ?
     
  7. Feb 13, 2012 #6
    Then what do rulers do? Measure length? Seems a little hand-wavy to me? That's like in Thermodynamics when a book tries to define a state in terms of properties and then the next chapter defines the properties in terms of states. :tongue2: Just a thought, though. Maybe I am missing something (I usually do!).
     
  8. Feb 13, 2012 #7
    hear hear

    (of course ignoring the "human" aspect of the question)
     
  9. Feb 13, 2012 #8

    collinsmark

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    Hello mangaroosh,

    Don't be confused, but QM works, and works quite well, when combined with Einstein's special relativity. And any Lorentzian form of physics is trumped by special relativity; so there's no point in going backwards.

    Summary: relativistic QM and Einstein's special relativity use the same concepts of time (and spacetime, for that matter).

    Points of interest:
    • Combining special relativity with quantum mechanics give rise to many predictions such as antimatter. And with a little more work (quantizing the fields themselves) give rise to quantum field theory which leads to things like photons and other force particles.
    • In academia, non-relativistic QM (which is more Newtonian) is taught first, since it's a lot easier. But it doesn't mean that special relativity and QM don't work together, it just means that you haven't gotten there yet.

    It's QM and Einstein's general relativity that don't fit together well. QM, even relativistic QM, assume a flat spacetime (special relativity assumes a flat spacetime too), but with GR spacetime can be, and generally is, curved.

    But spacetime (and time) in GR is really more-or-less the same interpretation of spacetime (and time) in special relativity; it's just that spacetime in GR is curved is all.
     
    Last edited: Feb 13, 2012
  10. Feb 13, 2012 #9
    It's a topic I enjoy myself nit.

    Up for discussion is anything to do with the nature of time really, but the question in the above is whether or not it would be possible to unify Lorentzian relativity with QM and to resolve the "problem of time"?
     
  11. Feb 13, 2012 #10
    I would say that "length", like "time" is just a concept, and not a physical property of an object.

    The question I would ask is, how exactly does a clock measure time; that is, how is the physical property of time measured by a clock?

    If we take an atomic clock for example, it is the number of oscillations of a caesium atom which are measured (or counted), not some secondary physical property called "time".
     
  12. Feb 13, 2012 #11
    thanks Mark.

    are you familiar with the term "the problem of time"; just wondering what your interpretation of it is?
     
  13. Feb 14, 2012 #12

    collinsmark

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    Okay, I think I see what you're saying: things get weird near Planck scales. Under the hypothesis of quantum foam, spacetime is anything but flat at Planck scales. And since QM and GR don't play well together, GR isn't about to try and rescue anything.

    I guess my original point though was that above Planck scales, QM and special relativity are already together, so there's no point in taking a step backwards into Lorentzian aether. That was really my only point.
     
    Last edited: Feb 14, 2012
  14. Feb 14, 2012 #13
    Just to start with an extract from a paper:
    Does time exist in Quantum Gravity?

    Unfortunately, I don't fully understand the issue, but my understanding is that standard quantum theory doesn't incorporate the notion of gravity, while it is a central part of GR, to the extent that both are incompatible as they are i.e. quantum theory would require some formulation of gravity in order to be unified with GR - I'm not sure what would need to be changed in GR to marry it with quantum theory.

    I was more just wondering if the aforementioned "problem of time" could be resolved through Lorentzian relativity, because it also uses the concept of absolute time.

    From discussing it with some people on here, and according to the wiki entry (I haven't searched further yet) it appears that "neo-Lorentzian relativity" has effectively been stripped of everything but the concept of an undetectable, absolute rest frame - that includes the aether I think. Again, from discussing it with people, the postulation of this absolute rest frame appears to be one of the main reasons (the only one I have heard raised actually) as to why Einsteinian relativity is preferred over Lorentzian. I'm wondering, if the absolute rest frame were done away with, would it put Lorentzian relativity on par with Einsteinian, given that experiments do not distinguish between either?
     
  15. Feb 15, 2012 #14
    It's not any less satisfactory. "Length is the measurement of length" would be just as unsatisfactory. Length can be explained relative to points and to the other dimensions, but there is nothing in the same class as time to compare it to. "Time is the measurement of time" is basically saying "time is time".
     
  16. Feb 15, 2012 #15
    Did anyone ever try to define time in terms of (amount of) state change? From my armchair, that seems the most natural to me.
     
  17. Feb 15, 2012 #16
    There's ideas like that floating around out there, yes. Time and entropy, time and the second Law of Thermodynamics. (I would have to stand up and reposition my armchair to get a good look at them, and that's not going to happen.)
     
  18. Feb 15, 2012 #17

    russ_watters

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    Could you provide such a definition please? I can't see how what you describe is possible.
     
  19. Feb 15, 2012 #18
    Length is one of the three physical dimensions. It's at right angles to width, and they are both at right angles to height. All three together describe a volume. Any two points that are not congruent can be said to delineate a length. A length is not a point, though. (A point has no length. It's a dimensionless location.) Length can be measured, but it isn't automatically a measure. All these concepts help define the others, and they can be illustrated with tangible objects and any x y z coordinates.

    Time doesn't have a group of things in the same class that help define it.
     
  20. Feb 15, 2012 #19
    So if if you accept that time is the measurement of time, then then you should also accept that length is the measurement of length.
     
  21. Feb 15, 2012 #20
    I guess this hasn't happened yet because I've been staring at a ruler since you posted this and I still can't tell what time it is.
     
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