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Is planck time the same in all reference frames?

  1. Jun 24, 2011 #1
    Quick question: Is planck time the same in all reference frames? Is it different at, say, half the speed of light than at a relatively stationary point? What about in a severe gravitational field, like a blackhole?
     
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  3. Jun 24, 2011 #2

    Dale

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    Planck time is a unit of time. It follows the same rules as seconds. If you have a clock which undergoes time dilation and it is displaying seconds then it will still undergo the same amount of time dilation if you change the display to show Planck times. There is nothing particularly special about Planck time compared to any other unit of time.
     
  4. Jun 24, 2011 #3

    Bill_K

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    More specifically, the Planck time is √(hG/c5) which is a universal constant.
     
  5. Jun 24, 2011 #4

    bcrowell

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    I think the answers so far may be somewhat of an oversimplification. The definition of the Planck time is frame-invariant, and in fact if you were going to be doing a lot of quantum gravity calculations, you'd probably want to pick units where the Planck time is equal to 1. Clearly the value of the number 1 isn't going to be frame-dependent. However, if you have in mind theories of quantum gravity in which spacetime is discrete, like LQG, there is an issue, because naively you'd imagine that if there's a lattice or something, the lengths of the edges should equal the Planck time, but then you'd expect the lengths of the edges to undergo time dilation, etc., in which case you'd think you could pick a frame in which the length of an edge was, say, 10-20 of the Planck time. But that wouldn't make sense, because in those theories, the Planck time is supposed to set a minimum scale. I think what this shows is that the way you'd naturally visualize a theory like LQG is a little too naive, but I don't know enough about this to be able to supply a good explanation of how it really works. I'll kick this thread into the BTSM forum, and I'm sure the experts there will be able to help more.
     
  6. Jun 24, 2011 #5

    atyy

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    I believe this is not understood in LQG. Some references are:
    http://arxiv.org/abs/0708.1721
    http://arxiv.org/abs/0708.2481
    http://arxiv.org/abs/gr-qc/0205108
     
    Last edited: Jun 24, 2011
  7. Jun 24, 2011 #6

    Chronos

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    There are theories where planck time is invariant, but, has a degree of intrinsic uncertainty. The sum of these uncertainties at the macroscopic level is proposed as an explanation for relativistic effects. One piece of evidence possibly supporting this idea is the expected cutoff energy of high energy cosmic rays - which is not observed. Richard Lieu discusses this in http://arxiv.org/abs/astro-ph/0202443
     
  8. Jun 24, 2011 #7

    bcrowell

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    Wasn't this kind of thing also part of the motivation for doubly-special relativity? http://en.wikipedia.org/wiki/Doubly-special_relativity But I think I've heard that DSR is turning out to have insuperable problems -- I'm sure there are others who could post more knowledgeably about that.
     
  9. Jun 24, 2011 #8

    atyy

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  10. Jun 24, 2011 #9

    martinbn

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    Probably I am missing something, but I thought that in quantum theories with minimal length, the idea is not that there is a lattice with the minimal length as a distance between the nodes (or something like that), hence the puzzle 'what happens in a fast moving reference frame?'. But that length, area, volume are quantum observables with discrete spectrum. In other words whenever you perform a measurement of length you get as a result which is an integer multiple of the minimal length.

    As I said, I know next to nothing about this, so I would be glad if someone gave a clarification.
     
  11. Jun 24, 2011 #10

    marcus

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    Right. Which is compatible with Lorentz invariance, analogous to the fact that angular momentum has discrete spectrum and is compatible.
     
  12. Jun 25, 2011 #11

    Chronos

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    I believe DSR was based on the planck length. It is unclear if that is different from a fundamental unit of time. I suspect it is the same argument. I am still intrigued by the cosmic ray energy cutoff - or lack thereof.. It's one of those observations that gives pause to question fundamentals. And this would not be the first time Lieu has gone out on a limb. He does, howebver, have interesting ideas.
     
  13. Jun 25, 2011 #12

    Fra

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    I think interesting distinction is:

    1) DSR is implementing the maximum energy scale in the classical theory. Ie. before "quantiztion", by deforming the transformations between observers. Thus replacing SR. Thus this presumed extended symmetry is note infered (concluded from measurement), it's taken as classical baggage.

    2) I think at least as far as I understand rovelli's LQG logic, the analogy of discrete spectrum of observables refers to a quantum theory.

    I think the differenct logic between these ways are different and important. I suspect Marcus knows more about the distinction in LQG development. I didn't pay that much attention to it withing LQG.

    /Fredrik
     
  14. Jul 26, 2011 #13
    Seems like //arxiv.org/abs/1106.0313, which atyy referenced, says that

    If momentum space is flat, Planck scale stuff is invarient; if momentum space is experimentally shown to be curved geometry, Doubly Special Relativity will be confirmed.

    Do you guys think that reasonable??

    I don't understand enough about momentum space to have an opinion about whether it is a physcial, detectable, observable, entity; Obviously the authors think so.

    And what about the broad view here that spacetime is an artifice, not a "fabric", not a physical entity. If that's a widely held view, how can momentum space be an observable entity. (I'm the dumbell who was "beaten up" here several years ago when I proposed spacetime WAS something physical.)

    thanks

    For reference, here are a few excerpts from the paper which I think capture it's essence:

     
    Last edited: Jul 26, 2011
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