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Overlapping Quantum Theory & Relativity

  1. Aug 2, 2004 #1
    Question from an amateur:

    I often read that Quantum Theory can explain very accurately the very small but not the big, General Relativity very accurate for the big but not for the very small.

    How "wide is the band" in which both overlap, being both accurate?
    i.e how big is the size of things which can be accurately described using any of both?
    or perhaps in more technical words, how much do we have to "turn the parameter knobs" in each of them so that they start yielding incorrect solutions?
  2. jcsd
  3. Aug 2, 2004 #2


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    The question you are raising is one of the major problems in current physics. Where quantum theory and Gen. Rel. have to both hold together, the results are nonsensical. The best example is trying to describe what happens in a black hole. GR has everything collapsing to a point. Quantum theory says this is impossible. (Actually it is more complcated, but this gives you some idea of the problem).
  4. Aug 2, 2004 #3
    Next big problem : in QM position and momentum as wel as energy and time are uncertain. A particle's position is never exact so a particle is never pointlike. In GTR the position is exact.

    What we would like to have is a gravitational uncertainty priciple.

    But do we have that geniusses out there ????

    marlon (not a genius though)
  5. Aug 2, 2004 #4
    I'm not asking about merging both theories into one !! I know that's the major challenge in theoretical physics today.

    I just asked if there is some band of parameters for which both theories still give results matching the observations (even when beyond those parameters they start giving wrong predictions).

    My own guess is that it's not actually size what makes one theory or the other the best choice (even if it's frequently stated like that). But that QM can actually explain everything, small or big, except that it can not explain gravity. (of course let alone phenomenons such as complexity, consciousness etc !)

    So instead of saying "QM explains the very small and GTR explains the very big" as often quoted, it should be better said "QM explains everything except gravity (and gravity is only noticeable in the very big)". Isn't it?
  6. Aug 2, 2004 #5
    your final statement is true.
    If i am not mistaking then we can probe up till distances of about 10^-15 meters. At lower scales the gravitational effects can become important, but certainly not at the indicated distance, because then they would already have been seen by our accelerators !!!

  7. Aug 3, 2004 #6


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    I think that what Mathman was trying to explain his response was that there already are some areas in which the two theories to overlap.

    The question of, "on what scale of distance is GR important ?", can only be responded to with the question of how strongly gravitational field is. The stronger the gravitational field, the less amount of space you need to look at before gravity becomes a significant factor. Here on the surface of the earth, the difference between the force of gravity I am currently experiencing upstairs and the gravitational influence I will field in a moment when I go downstairs is negligible. On the surface of a neutron star, the difference in force over this 10-ft difference would be much more significant. Near the center of a black hole, the force of gravity changes significantly over the distance of a nm. It is roughly at this scale where quantum mechanics rules (for all things about the size of a molecule).
  8. Aug 4, 2004 #7
    Thanks LURCH !

    Became very clear now !!
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