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I am not a physicist but still I love the subject very much. A thought recently popped up in my mind while studying and I'm still confused about it.

"If a person falls freely he will not feel his own weight"

This is a remark made by the great physicist, Albert Einstein.

So the situation is like

Suppose you acquired some net charge and were sitting on a chair in space and being pulled by the electromagnetic forces rather than the gravitational forces, would you "feel a force on your back" despite the fact that you weren't accelerating?

Similarly, if I took the chair away, would you start accelerating despite the fact that you were not feeling any force?
(I'm assuming here that the net charge on your body is uniformly distributed.)

What makes gravity so special that it does in fact act on all matter (and light) whereas forces like electromagnetic force do not?

I'd be very happy if someone answers this question.

Thanks a lot :)

Community
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आर्यभट्ट
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2 Answers2

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Other then gravity being described by the mediation of a massless spin 2 particle (all other forces are mediated by spin 1), gravity is so special because it is the only force that is capable of curving spacetime itself.

The short answer for why gravity is unique is that it is the theory of a massless, spin-2 field. To contrast with the other forces, the strong, weak and electromagnetic forces are all theories of spin-1 particles.

Why is gravity such a unique force?

None other force we know of can do this. Why is this so special? Because it changes the structure of spacetime itself, the very thing all matter and energy (and fields) exist in, thus it can effect equally all matter and energy just based on simply their common intrinsic quality, that is stress-energy. And this is very special, because this is the only thing that all known particles have in common, and is interchangeable (mass energy equivalence), stress-energy.

It is very special, because if no other force acts on them, all known particles must follow geodesics, and must follow spacetime curvature. It is not trivial, but gravity causes GR time dilation too, thus effecting for all known particles the relative passage of time. Thus, the circle is complete, space and time are both effected by gravity, and are joined into a inseparable entity called spacetime. This is GR.

Árpád Szendrei
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  • This what my teacher said Well general relativity doesn't treat gravity as a real force it's an inertial/apparent force. That's a bit of a paradigm shift :) One of the key observations is that inertial mass is the same as gravitational mass very suspicious. So GR redefined universe in a way where gravitation is an inertial force and it works marvellously. Indeed, SR handles gravity-as-a-real-force just fine -GR throws this explanation out of the window, and in doing so, explains a few suspicious things as well as some pretty annoying things like why would light bend in a gravitational field – आर्यभट्ट Jan 12 '20 at 04:22
  • This just changes the question to: why doesn't the electromagnetic force curve spacetime? – user253751 Jan 17 '20 at 11:40
  • Why the downvote? – Árpád Szendrei Jan 17 '20 at 15:44
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Your last sentence is what's relevant here. In your electromagnetic free fall scenario, you would not feel a force as there is nothing pushing against you as the ground does when we stand on the surface of the earth. In gravitational free fall, if you release an object in your possession, it will just hang there alongside you because it is falling with the same acceleration. That's why whatever container you were in would be an inertial reference frame, indistinguishable from being in the same container but far away from any source of gravity. In the electromagnetic free fall scenario, if you release an uncharged object, or a charged object with a different charge from what is on you, it will not fall with the same acceleration.

Not_Einstein
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