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We know that Quantum Theory should be considered as a framework in which all other theories/forces (Strong, Weak, EM and Gravity) exist.

For example, we have the Quantum Chromodynamics, Quantum Flavordynamics (Electroweak), Quantum Electrodynamics (but still no Quantum GR).

When I think about this, it then strikes me why gravity, and specifically special relativity, is part of the framework itself (because QFT is based on QM and SR). Why would a theory like SR (and maybe in future GR) be part of the framework? This looks like a circular logic.

I would appreciate it if someone can explain?

My understanding is that gravity is different because it exist everywhere, and are not a result of a charge (electric, color or flavor). That's why we cannot speak about a quantum theory of EM without considering gravity, but we can do it without considering strong or weak force. Is this correct?

EDIT: it would make more sense to me to see Gravity/GR considered only as a framework (a geometric one) rather than a force, or it can be considered a fictitious force, as with the centrifugal force. see first paragraph of first answer here.

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Special relativity and general relativity are quite different beasts. SR is a group of symmetries that get embedded in the quantum theory, and gives us RQFT. Thus, you can have a field and therefore a quantum field theory on top of SR with no need to quantize SR. You can do quantum mechanics of EM, for instance, with no need to talk about gravity (remember, SR is a group of symmetries only).

General relativity / gravity adds dynamical degrees of freedom, and while you can do quantum field theory on top of GR (e.g., hawking radiation), it is necessarily incomplete.

Here's a previous question on why gravity needs to be quantized

Long story short: imagine doing a double-slit experiment with very heavy particles. Since the particles couple to gravity, we would expect their gravitational back-reaction to exhibit quantum behaviour, and, therefore, gravity itself.

Unfortunately, just adding gravity as another field and plodding along doesn't work. There has been a lot of promising progress in string theory though. Some would say too much progress ;)

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