# We Don't NEED Quantum Gravity because Gravity isn't Even A Force! [duplicate]

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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!)

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• Possibles duplicates: physics.stackexchange.com/q/6980/2451 , physics.stackexchange.com/q/10088/2451 , physics.stackexchange.com/q/52211/2451 and links therein. – Qmechanic Apr 4 '15 at 5:01
• I more want to know if my info and line of thinking is correct. – Damon Blevins Apr 4 '15 at 5:11
• "On the flip side, one could argue about the high energy situations such as the Planck epoch, where gravity may need extreme modification" I think this for me is the crux of the issue. From the standpoint of a nonspecialist like me, it seems very hard to accept that, with its singularities, GTR is the last word. GTR teaches us (actually, one can't even escape this conclusion in Newtonian theory) that "empty" space has definite properties, which can vary from place to place. So the question arises as to what the stuff of "empty" space is, and how it gives rise to what we know through GTR. – WetSavannaAnimal Apr 4 '15 at 6:22
• Otherwise put: to be satisfied that GTR is the last word is like being happy not to be allowed to ask the question of whether the trees and stones and people around us are made of continuums or ultimate particles. In asking such questions, we're simply following in the rather obvious footsteps of Democritus and Artistotle. – WetSavannaAnimal Apr 4 '15 at 6:26
• Very true...... I'm making a theory of time where in some classical limit, dimensional GR time reduces to QM/CM time. Same for probability as a dimension. It would reduce to what we see in QM. – Damon Blevins Apr 4 '15 at 6:31

## 1 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.

• Thanks!!!! I'm a 15 year old nerd, just to brag.... Energy is the source of the gravitational field... Not just matter- im asking this one just out of curiosity, but can other forms of energy exist in a superposition of states? – Damon Blevins Apr 4 '15 at 5:52
• I'm not very educated in the superpositions. – Damon Blevins Apr 4 '15 at 5:53
• @DamonBlevins: The source of the gravitational field is the stress-energy tensor. This includes matter/energy, but also include momentum and pressure. All physical systems can exist in a superposition of states, though for cats and other macroscopic objects the coherence time is immeasurably short. – John Rennie Apr 4 '15 at 5:57
• Oh yeah! I totally forgot About that! Duh! – Damon Blevins Apr 4 '15 at 5:59
• Plus.... Maybe as the energy density of an event decreases, time will seem less and less like a dimension and , in a classical limit, would reduce to the time we see in QM/CM, instead of the dimension it appears to be in GR. The same thing for probability as a dimension, 'reducing' to QM. This would mean energy density plays a role in how much space and time seem to be entwined with one another. – Damon Blevins Apr 4 '15 at 7:20