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Grand unified theories and quantum mechanics

  1. Feb 20, 2009 #1
    Why is it that general relativity and quantum mechanics are so different and that physicists are having difficulty combining them.

    Please give me a simple answer. I scoured the whole net and each website has its own opinion.
     
  2. jcsd
  3. Feb 21, 2009 #2

    malawi_glenn

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    hehe and you do not think that people answering here will give his/her opinion? ;-)

    To me, the most difficult thing is that the quanta of gravity must have spin 2 and be massless, and you can't construct a quantum field theory which is Lorentz-invariant and renormalizable. This is wienberg-witten theorem.
     
  4. Feb 21, 2009 #3
    I agree with malawi_glenn, and another important reason is that classically the equations of General Relativity develop singularities (black holes) within a finite time. The classical analogues of our quantum fields for the color force, weak force, and e&m do not have this behavior.
     
  5. Feb 21, 2009 #4

    malawi_glenn

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    Mass has not anti-mass in the same way as we have positive and negative charges etc.
     
  6. Feb 21, 2009 #5
    Could you put that clearer? Do you find it difficult that
    - quanta of gravity must have spin 2 and be massless
    and
    - can't construct a quantum field theory which is Lorentz-invariant and renormalizable

    If so, why do you find it difficult?
     
  7. Feb 21, 2009 #6

    malawi_glenn

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    You can show that gravitons must have that property within the standard model and then the rest follows from Wienberg-Witten theorem.

    The difficultly is that you can't construct a quantum field theory which is lorentz invariant and renormalizable with that property of the graviton.
     
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