# [Physics] We Don’t NEED Quantum Gravity because Gravity isn’t Even A Force!

general-relativitygravityquantum-field-theoryquantum-gravity

Now, I understand the motivation for quantum gravity. I honestly want to work on a theory myself. However, gravity, according to General Relativity, is not a fundamental force of nature. To me, it's just a natural tendency, or an inertial tendency. (I call them this because of the equivalence between inertial mass and gravitational mass and inertia.) Inertia is one of these tendencies, the tendency to resist induced motion. Gravitation is just the natural tendency to follow a geodesic. Not a force. One could argue that It takes only a mass moving through space for inertia to take place, it takes two bodies for gravity to occur. However, this induced motion must have been caused by something else. Just as with the curvature of spacetime. Quantizing gravity is like quantizing inertia. I am aware that there is no "inertial radiation"; however the gravitational radiation needn't be quantized. It's just ripples in spacetime. If spacetime were quantized, then I am assuming that it'd be a field, with a value for every point in spacetime. With a QFT, we could determine EVERYTHING– every moment time, every point in space- where a particle is, where it's not. This would not only violate the Heisenberg uncertainty principle, but it would be the reason for non-renormalizability of gravity. Infinite solutions that can't be neglected.

On the flip side, one could argue about the high energy situations such as the Planck epoch, where gravity may need extreme modification. Plus, gravity is a field, so it should be quantized. However, as stated by the renormalization issue, this probably wouldn't be true.
We have a theory for the quantum realm and the three fundamental forces of nature, and we have a theory for the two "great inertial tendencies"- inertia and gravitation.

The Question:
Given the info provided above, do we really NEED a quantum gravitational theory, (save for the high energy scenarios)

P.s. What is wrong with putting QFT in curved spacetime? (which I am aware is one of the main needs of the theory, but we don't truly need a graviton, after all, all the fields in the standard model have the natural tendency to follow a geodesic, so this tendency should be added in anyways!)

#### Best Answer

Your reasoning is based upon gravity being different to the other forces. However all the forces can be formulated in a geometric way. See for example the question Can all fundamental forces be fictitious forces? and the links within it. So there is no reason to suppose gravity is fundamentally different to the other forces, and no reason to suppose it is fundamentally non-quantum.

As for not quantising gravity, the problem is that if matter can exist in a superposition of states, and matter is the source of the gravitational field, then the gravitational field must also have to exist in a superposition of states. This is the point Luboš Motl makes in his answer to What are the reasons to expect that gravity should be quantized?. This means a quantum description of gravity is required and not an optional extra.