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Theory of universe

  1. Apr 5, 2007 #1
    Hi friends!

    By considering the inconsistancies with Genereal relativity and Quantum mechanics, what will be the first step to postulate a grand unified theory of everything is the current research area for physicists now

    1. Is there really a law that describes the universe by whole? if so what will it predict or consist of?
    2. Is Quantum approach is correct for elementary particles? why General relativity is sidelined in Quantum? or what causes to neglect Gravity in Quantum?
  2. jcsd
  3. Apr 5, 2007 #2


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    The contradiction between GR and QM has been somewhat overplayed.

    At low energies, (on a human scale, normal energies) the two can live together as an "effective field theory". This is how, for instance, we are able to make predictions on the evaporation of black holes, an area involving both gravity and quantum mechanics.

    It is true that QM and GR are not compatible at high energies. Nobody knows the solution to this dilmena, but a large part of the problem is that we don't have any experimental data on the actual behavior of nature in this "high energy" realm.

    We don't know what is going to replace QM and GR at high energies. It could be string theory (but to be scientifically useful, the theory needs to start yielding more testable predictions than it has to date) - or it could be loop quantum gravity, or it could be something that we don't know about yet.

    I would suggest "Beyond the standard model" as a better forum for more information on these sorts of alternatives.
  4. Apr 6, 2007 #3


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    There are 3 questions here -

    1. Yes. QED theory agrees with experiment to high degree of precision.
    2. Most classical dynamics can be quantised using a standard procedure. But when applied to GR these methods give wrong answers ( infinities). Maybe because GR is not a dynamic theory ( a theory of energies).
    3. One can include curved space-time in quantum calculations, so to that extent ( as Pervect says) GR and QM can live in harmony.
  5. Apr 6, 2007 #4
    Apart from the simple cases of static solutions, which can simply be Wick rotated into Euclidean 4-space by expressing the metric in a time-independent form, can you give me one case where they live in harmony?

    We can don't even have an analytical solution of the combined gravitational field of two simple orbiting mass bodies, let alone a description of QM interactions on top of that.
    Last edited: Apr 6, 2007
  6. Apr 6, 2007 #5


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    Hawking Radiation ? Bekenstein horizon ?
  7. Apr 7, 2007 #6


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    The inescapable conclusion is that QM and GR are both incomplete [albeit highly successful within their domains]. Finding specific cases where the theories inexplicably break down is the logical approach - a 'devil is in the details' thing [or 'TOE' in this case] - which is why large sums of money are spent on projects like GPB and the LHC.

    By definition, the correct TOE will predict . . . everything. Specifically the behavior of particles, fields and combinations thereof at all energy levels. It should also eliminate apparent mathematical artifacts, like spacetime singularities, and replace [or justify] barbaric rituals like renormalization with more elegant, internally consistent methods.

    Quantum methods work amazingly well in many applications, but so does a sledge hammer. Neither is always the best tool. Is it more correct to quantize GR, or relativize QM? Other possible answers are 1] both, 2] neither.
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