Do General Relativity and Quantum Mechanics even need to be unified? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Answer by Viktor T. Toth, IT pro, part-time physicist, on Quora:
Both general relativity and quantum physics describe Nature. We would like to have a description of Nature that is self-consistent. That’s what unification is: a self-consistent framework that works.
From a practical perspective, this unification may never be needed. Quantum field theory works just fine on a curved background. And classical gravitational theory describes that curved background with exquisite precision.
So the issue of unification only comes up when the geometry can no longer be treated as a mere background, or conversely, when the classical theory is no longer accurate. But these circumstances exist (as far as we know) in only two places: the earliest moments of the Big Bang, and the immediate vicinity of singularities hidden behind black hole event horizons.
Everywhere else, a theory called semiclassical gravity works just fine: it uses the so-called expectation value of quantum fields as a source of classical gravity, which in turn determines the curved background for those quantum fields.
So perhaps from a pragmatic perspective, this is all we need, as there will never, ever be an experiment that takes us beyond semiclassical gravity. But even then, it is (philosophically, perhaps) deeply unsatisfying that only such an imperfect marriage exists between the two theories.
And who knows, one day we may find out that the regime of quantum gravity is accessible after all to some clever observations/experiments, in which case the question acquires practical significance. Or conversely, perhaps a unification of the two theories might lead to novel applications, which may even lead to new kinds of engineering.
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