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At what point does gravity become incompatible with QM?

  1. May 10, 2013 #1
    Is there some fuzzy area between the macroscopic and atomic worlds where gravity "still sorta works," or is it a dramatic cut off? I never understood this whole idea that it suddenly stops making sense at the quantum level. Does it start making progressively more sense as you increase the scale?
  2. jcsd
  3. May 10, 2013 #2
    Gravity doesn't just stop making sense at the quantum mechanical level. We have no problem treating gravity quantum mechanically below energies of 10^19 GeV or so. (This is 10^15 times higher than the energy scale reached by the LHC). Above that point, the math tells us that the simplest quantum mechanical version of general relativity stops being valid, so something new must take over--perhaps string theory. Note that string theory is itself a quantum mechanical theory.
  4. May 10, 2013 #3


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    The problem with gravity is not so much that it doesn't make sense, but that it just doesn't need to be treated quantum mechanically: It is (1) very very weak compared to other forces in the systems and (2) doesn't change much at microscopic scales. In most microscopic processes, pretending gravity doesn't exist is an effectively exact approximation, and if it is not, then treating it at the level of F = m*g certainly is. (That's not even taking into account the effects at classical mechanics level....)
  5. May 10, 2013 #4


    Staff: Mentor

    Check out:

    'One can find thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

    The short answer is about the plank scale.

    The longer answer is gravity is not incompatible with QM - a perfectly valid quantum theory of gravity exists - it is just not renormalisable, which means a cutoff must be explicitly included, and the cutoff is about the plank scale. The difference between renormalisable and non-renormalisable theories is renormalisable theories have a magical property - what we observe does not depend on the cut-off. We know, QED for example, breaks down well before the plank scale so it really has a cutoff as well but it doesn't need to be explicitly included in the theory because of the magic property it has of renormalisability - the precise value of that cutoff doesn't matter. But the jig is up with gravity.

    Last edited: May 10, 2013
  6. May 11, 2013 #5
  7. May 11, 2013 #6


    Staff: Mentor

    Quantum theory does not absolutely guarantee anything has to be quantized. However given a field Lagrangian the fact you can is very telling and allows theories to be developed and predictions made, which is what science is all about.

    Feynman may be right, he may be wrong - exactly as it is with all speculation. What we need is a well developed alternate theory that makes predictions that can be tested. And that was pretty much Feynman all over - correspondence with experiment is his bottom line.

  8. May 14, 2013 #7

    Righttttttt !
    Last edited: May 14, 2013
  9. May 15, 2013 #8


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