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Under what conditions will quantum effects become important for gravity?

  1. Jul 29, 2005 #1
    I have to write a short essay answering this question (1st year university level). I am finding it very hard to get any decent info on the net at all. If someone could help me out with this I would be very appreciative. Thanks.
    Regards
    Graeme
     
  2. jcsd
  3. Jul 29, 2005 #2

    Stingray

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    Look up something called the Planck length (also Planck mass, Planck energy, etc.). These give limits on when quantum gravity effects almost certainly appear. They may in fact be significant under much more mundane conditions, but nobody knows.
     
  4. Jul 29, 2005 #3
    thanks, i'l look it up. also what about blackholes? the normal rules of physics are thrown out the window when it comes to blackholes, would quantum gravity play a part here?
     
  5. Jul 29, 2005 #4
    A priori yes, specialy for mini-black holes. Some theories concerning the quantum gravity are of course trying to describe black holes (Lorentz one euclidian one,...). But I think it is not a first year level.
     
  6. Jul 29, 2005 #5
    Gravity effects manifests itself at the Quantum level, when the Energy Density, Charge Density, no longer transfer Photons from a 4-D spacetime.

    Non-Relativistic Gravity is Dimension dependant.
     
  7. Jul 29, 2005 #6

    Stingray

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    The singularity inside of a black hole would be affected by quantum gravity, but the vast majority of a (macroscopic) black hole's properties are described perfectly well by classical GR.

    Another popular place to look is at the very early universe.
     
  8. Jul 29, 2005 #7
    The WIMPs (weakly interacting massive particles) that have been proposed to explain dark matter might exhibit quantum effects in a gravitational potential well.
     
  9. Jul 29, 2005 #8

    Stingray

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    No more than anything else in the universe.

    You bring up a good point, though. I've been talking about quantum effects on gravity itself. There are, however, many situations where gravity can be treated classically, but where it is still strong enough to significantly modify quantum mechanical processes.

    These are two very different things. The latter one is much better understood, and less out of experimental reach (although less is a relative term!) than full blown quantum gravity. Hawking radiation from black holes would be an example of this. So would the Unruh effect.
     
  10. Jul 29, 2005 #9

    ZapperZ

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    I think that there's something conspicuously missing in this discussion so far here. Quantum effect of gravity HAS been observed!

    <shock spreads through>

    Note that in the Schrodinger equation, the "V" term for the potential can be ANYTHING. I could write the gravitational potential in there and no one can fault me. So how can I do that and observe "quantum effects" due to the gravitational potential? By applying it in situation with the suitable boundary conditions that can quantize the gravitational fall! This has been done.

    The cold neutron drop experiment of a couple of years ago showed that there are discrete gravitational states.[1] I'd say that this qualifies at quantum effects of gravity. It may not be the exotic quantum gravity that everyone is enamoured with, but I'd buy it.

    Zz.

    [1] V.V Nesvizhevsky et al., Nature v.415, p.297 (2002).
     
  11. Jul 29, 2005 #10

    Stingray

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    Yes Zz. But I think it's pretty much always implied in these kinds of threads that what's of interest is something linking quantum mechanics and the uniquely-GR aspects of gravity.
     
  12. Jul 29, 2005 #11

    ZapperZ

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    That's probably true. But if you go by JUST the title that was given, that isn't automatic. Plus, he has to write an ESSAY. So it means there is more of a larger freedom to explore the subject. I'd say that if I were the instructor and a student wrote on the neutron drop experiment, it would be a delightful surprise since it fits the topic. Not only that, it showed that the student did some homework and didn't go for the easy and obvious approach.

    Zz.
     
  13. Jul 29, 2005 #12

    NateTG

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    Really, the way the question is phrased, the answer is really something like:
    In experimental physics, gravitational effects become important in quantum mechanics when they are necessary in order to predict experimental results, that is, when there are experiments that can verify gravity on a quantum scale; in theoretical physics quantum gravitational effects are important whenever some researcher turns his or her mind to them.

    In your paper, try to identify conditions where quantum-scale effects of gravity would be noticable.
     
  14. Jul 29, 2005 #13

    Haelfix

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    This is a good question. The first place you want to look at is semiclassical gravity. All other QG things tend to shoot the scale lower (but not always)

    Namely take the Einstein Hilbert action, and quantize it.

    Quantum effects should show up as first order loop diagrams, scaled by Newtons coupling constant (G).

    It turns out, its about 6 orders of magnitude above the Planck scale or so where you might start to get significant experimental departure from tree level. Where would you measure such a thing?

    Well blackholes would probably be a good point, but also you might expect to see signatures in the cosmic microwave background. Small anisotropies that are diluted in inflation but still in principle experimentally visible. You might also see it in a variety of other cosmic situations.. Gravitational waves in quantum like objects (neutron stars, but especially some of the exotic hybrids thereof), and possibly cosmic strings.

    I could give you references, but perhaps its not quite suitable for a 1st year student.
     
  15. Jul 30, 2005 #14
    Alledged WIMPs would exhibit gravitational effects in their quantum behavior much more than ordinary matter because they are unaffected by electromagnetism and nuclear forces. For example, if there is any condensed phase of WIMPs we can observe somewhere in the universe, its spectrum (in gravity waves) would have absorption/emission lines determined by the quantum structure of "wimpium", analogous to the photon spectrum lines for normal matter.
     
  16. Jul 30, 2005 #15

    Gokul43201

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    This is more along the lines of what Zz mentioned. Let me add that gravitationally induced (phase) changes to the wavefunction of a neutron beam have been experimentally observed over a quarter of a century ago.

    The first, I believe, was the 1975 neutron interference experiment by R. Colella, A. Overhauser and S. Werner [1].

    Let me add that the OP's question is terribly worded. What does it mean for something to be important for gravity ? The "gravity" of a neutron star, for instance, is a function of its mass distribution, which may not be explained satisfactorily without QM. But then, should the OP be instead talking about QG rather than the application of QM to specific aspects of a problem ?


    [1]"Observation of Gravitationally Induced Quantum Interference," R. Colella, A. W. Overhauser and S.A. Werner, Phys. Rev. Lett. 34, 1472-1474 (1975).
     
  17. Jul 30, 2005 #16

    Stingray

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    Ok, I suppose that might be true. I'd just never heard of anyone talking about condensd WIMPs before (or observing gravitational spectra!). But I guess those things aren't any more 'out of reach' than the examples I gave.
     
  18. Jul 31, 2005 #17

    vanesch

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    Last edited: Jul 31, 2005
  19. Jul 31, 2005 #18

    ZapperZ

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