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Do all known quantum gravity theories lead to a true unification?

  1. Mar 31, 2014 #1

    ftr

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    I mean I don't see any theory actually derives gravitational interaction from actual fields of particles. I only see some manipulation of GR to make it look like a quantum field theory. So are these theories looked upon as one more step to actual unification or are they the FINAL unification?

    Any clarification for my misunderstanding is highly appreciated.
     
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  3. Mar 31, 2014 #2

    Simon Bridge

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    Define "true".
    ... the theories are designed with the end in mind - yes.

    However, such a treatment will yield virtual particles as part of the perturbation theory needed to work out the interactions. That will tell us what sort of particle to look for - what it's mass is etc. Then we can go look and, in principle, verify the particular theory.

    Since we know how a quantum theory must relate to a classical one, this gives us a way to investigate which is systematic.

    What you are proposing is to start with the particle, get the interaction, and see if it matches the predictions of GR. Then go look for it. The trouble with that is, there is no systematic way of doing this.
     
  4. Mar 31, 2014 #3

    ftr

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    Thanks for the reply.You seem to have defined "true" very well.

    Wouldn't that imply that there is something wrong with QFT, particle fields or the interaction?

    I am not sure why you are saying there is no systematic way since we are able to do it for all other forces. I agree that it is hard experimentally, but a lot of cosmology theories are done with with lots of modeling using indirect measurements.

    But I guess I am wondering why there is no attempt (at least that I am aware of).
     
  5. Mar 31, 2014 #4

    jtbell

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    How can we possibly know whether we have the FINAL theory of ANYTHING? :confused:
     
  6. Mar 31, 2014 #5

    ftr

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    I am not sure why you are confused. We are looking for a theory that unifies the forces in a coherent way. And since particles are quantum fields and particles attract, then gravity should arise inherently from them. The problem is that we model gravity being generated via the stress-energy momentum which represents matter but they are not the "same".
     
  7. Mar 31, 2014 #6

    Simon Bridge

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    I was attempting to use yours ;)

    We know there is something wrong with QFT, it's not a GUT for a start ... so what do you mean?

    Do you have a reliable reference for this assertion.

    IFAIK the "other forces" all got quantized the same way the as the attempts for gravity - treating the classical theory as what happens on average in the quantum theory ... then using perturbation theory to do the maths. Its also how relativistic QM gets taught.

    Tell you what, come up with a systematic way to start with the particle and derive the force, without knowing in advance what the particle should be ... then you should get an idea about why nobody is trying this approach.
     
  8. Mar 31, 2014 #7

    Simon Bridge

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    Note:
    ... you appear to be asking: "should we finally unify the four known classical forces under one umbrella theory, with that be the end of it?"

    Well, if it were not the end of the unification process, that would mean there is an unknown force somewhere yet to be discovered.

    But:
    How can we possibly know if there is something unknown?

    However, I suspect you mean to be asking if the proposed quantum gravity theories are the final stage to unification - and we just have to prove one of them true and that's the end of unifying the known forces.

    The GUT that results would be the final word and all that was left for physicists to do would be to measure physical constants more accurately.

    But:
    Empiricism (i.e. science) cannot prove anything true, only eliminate the untrue
    ... so it is unlikely to be the end of things.
    ... If the GUT is final, how would we know it is final?


    We may find ourselves in the situation some time ago when it was thought that Newtonian mechanics was the final word and all that was left for physicists to do was measure physical constants more accurately.
    Turned out to be wrong.

    jtbell is not confused really, he is telling you that in order to answer your question about the finality of Quantum gravity theories, we need to know how you would know you would have a final theory of anything.

    Without that bit of information the question makes no sense - which is why I ignored it.

    Note: we may have the final theory right now.
    Maybe gravity is just a different beastie to the other forces so there is no underlying model that covers everything? How would we know?
     
    Last edited: Mar 31, 2014
  9. Apr 1, 2014 #8

    jtbell

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    To my mind, it's not a final theory until it can explain all data from experiments that have already been done and that will be done in the future. So we can't have a final theory until we've done every conceivable experiment. I'm not holding my breath waiting.
     
  10. Apr 1, 2014 #9
    What would happen to the field of theoretical physics if a theory were found that unified all four interactions?
    Would everyone be out of jobs? Would eveyone have to become solid state physicists?
     
  11. Apr 1, 2014 #10
    There'd still be a lot of work trying to work out how it explained all the known phenomena and working out if it predicted new ones. There's also lots of theoretical physics that isn't concerned with quantum gravity at all. I'd expect a lot of attention would be turned towards quantum computing.
     
  12. Apr 1, 2014 #11

    ftr

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    You bring up a good point which I asked in another thread. The failure to find GUT is one indication, moreover, the system relies heavily on ansatz of symmetries where couplings are introduced by hand and not derived. Also, each energy sector has it "own" theory.

    That is why I am saying that the present quantum gravity theories make things even more complicated instead of resolving issues. If you look at the best in class like string theory there is even no clear equation for the electron.

    I am looking for a theory which is close to this one for example.

    http://arxiv.org/pdf/1003.1262v3.pdf


    But in general I will be satisfied with a theory which I defined as true, plus an explanation for the fundamental constants including masses and couplings. Then I think cosmology should fall out automatically (uniting space-time with particles/fields). In such a case I think the ontology will also be trivial. I don't think scientists( I share this view) believe there is anything beyond the physics as above, so no need to worry so much.

    I think we should be humble enough to find a theory that is satisfying in the same sense as I explained above.
     
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