Can Quantum Physics and Relativity Co-Exist?

Can Quantum Physics and Relativity Co-Exist?
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As far as I know quantum physics and relativity theory will never get along. Does that mean one of them is basically wrong? 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:

Quantum physics and relativity theory get along just fine in most respects. Not always, but…

First of all, the prevailing quantum theory, quantum field theory, is fully relativistic from the onset. Relativity is “built into” the theory, so to speak. And it’s the only quantum theory that is fully causal (effect never precedes cause, for any observer) and can account for particle creation and annihilation.

When I say fully relativistic, I mean special relativity. But quantum field theory can “live” on the curved background of general relativity, too. Sure, things get interesting (in the general case, we have to fully embrace the field concept and give up on the notion of particles altogether) but the theory works.

So what doesn’t work? Well… Einstein tells us that matter is the source of gravity, through the stress-energy tensor. But in a quantum theory, this stress-energy tensor consists not of numbers but of so-called noncommuting operators. Does this mean that the gravitational field must also be described by a quantum theory? Well, maybe… but nobody succeeded with that. We do not have a viable quantum theory of gravity.

But do we really need one? There is a simple (almost too simple) modification that allows the two theories to coexist just fine: Instead of the quantum operators representing the stress-energy tensor, just use their so-called expectation values. Those are numbers that can be plugged into Einstein’s field equations. This approach, called semiclassical gravity, works very well; it accurately describes Nature everywhere except for the immediate moments after the Big Bang or the immediate vicinity of a singularity deep inside a black hole’s event horizon. In other words, places and times that we can never explore experimentally.

So perhaps semiclassical gravity is the answer? But many people find it deeply dissatisfying, a kludge if you wish. So there are philosophical reasons to go beyond semiclassical gravity. But perhaps philosophy misleads us. We won’t know until we know which will be… who knows when.

But it is not true that relativity and quantum physics do not get along. They get along just fine most of the time, almost all the time, as a matter of fact.

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