• Question: What do quantum theory and relativity disagree on? Why can't they both be applied across all scales?

    Asked by I'm Batman to Michele, Jackie on 8 Mar 2015.
    • Photo: Michele Faucci Giannelli

      Michele Faucci Giannelli answered on 8 Mar 2015:


      Just to be precise, the problem is to merge in a single theory both quantum theory and GENERAL relativity, special relativity is well integrated in quantum field theory.
      The answer to the first question is that they do not disagree that much because they are applied to completely different scales, so there are no macroscopic evidences that either is wrong. Quantum physics, and in particular the Standard Model of particle physics, is fantastic in describing Nature at the microscopic level. The discovery of the Higgs Boson is the final piece that prove that theory a success. At the same time General Relativity describe very very well the dynamics of large systems such as planets, stars and galaxies.
      So, coming to your second question, the problem is that for any actual application there is no need for a single theory because in any case a Theory of Everything in the limit of large or small scales would be essentially General Relativity or the Standard Model. I think the problem is more conceptual. So far we have managed to unify all theories but the gravity, and there are good reasons to believe that a single theory should exists to describe them all.
      As for the actual technical difficulties to unify all theories is that all quantum theory require to have a fixed space-time while we know that close to big masses, such as a star, that is not true. Quantum physics can still survive because all processes happen at such short distances that space-time can be approximated LOCALY to be constant or fixed. Still integrating the two will be a major challenge for the theorists.

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