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I was reading this brilliant answer by Luboš Motl, where he claims that

The field that communicates gravity is the metric tensor field..

Now, here he says that gravity is indeed a force, and that the field corresponding to the force is the metric field. Now, this makes sense, as the metric tensor can form a tensor field.

But, isn't this view going exactly against the original motive for General Relativity. The original reason for the theory to form was to show that even bodies accelerating under the so called force of gravity can be inertial frames. That is where the entire idea of gravity being the curvature of spacetime came from.

My question is: is gravity, in the modern sense, viewed as a force arising the metric field, or do we stick to the original GR: gravity being curvature of spacetime? If we do prefer the former, why? Because it seems to be going straight against the fundamental motives and principles of GR. Or, is it because this view allows us to introduce gravitons and helps a bit in the unification of forces?

PNS
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    why can't it be both? Effective force arising from curvature of spacetime. You can word it either way as long as it's based on the same theory. – Kosm Aug 09 '20 at 10:35
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    Actually, the original GR didn't say gravity was the curvature of spacetime. This is worth a read: https://arxiv.org/abs/physics/0204044. – John Duffield Aug 09 '20 at 11:29
  • @Kosm, I thought of it as being 'choose one forget the other', but now reading your comment and the answer, it dawned on me that both can co-exist. Thanks for your explanation. – PNS Aug 09 '20 at 13:17
  • @John Duffield thanks for the articles. I will try and read it as soon as possible. – PNS Aug 09 '20 at 13:18

1 Answers1

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My question is: is gravity, in the modern sense, viewed as a force arising the metric field, or do we stick to the original GR: gravity being curvature of spacetime?

One has to keep in mind that physics is modeled by several different theories, very successfully. Thermodynamics, statistical mechanics, Maxwell's equations for classical electromagnetism, quantum field theory etc.

In the region of overlap between models it can be shown mathematically that the models blend into each other naturally. Thermodynamics and its variables, for example , emerge from statistical mechanics with no contradictions.

The concept of Force, coming from everyday horses pulling carts, can be distilled to F=dp/dt. Whenever a function of momentum can be defined in four dimensional space time, the concept of force is defined. This is clear in the Feynman diagrams of QFT.

If we do prefer the former, why?

It is not a matter of preference, but of choosing the best of the models for the specific boundary conditions.

Because it seems to be going straight against the fundamental motives and principles of GR.

The motive for GR was for large masses and energies. When masses and energies are low Newtonian mechanics is sufficient to fit the data.

Or, is it because this view allows us to introduce gravitons and helps a bit in the unification of forces?

Unification of forces happens in the microcosm of particles, which uses dimensionsan immense order of magnitude away from general relativity requirements. It will be enough if one can show that the QFT model of gravity melds with the general relativity requirements at large masses and energies.But this has not much to do with the concept of force=dp/dt and its universal applicability classically and in QFT.

anna v
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