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I was doing research into how $G$ forces are calculated, and I was told that they show all acceleration that can be "sensed" by humans (i.e. all non-gravitational acceleration).

This confused me though, because the notion that gravitational acceleration cannot be sensed was not intuitive to me - I can feel when I am falling after all.

This led me to do we research into what it means that an acceleration cannot be detected (according to the equivalence principle as I was told), and I stumbled onto the Newtonian idea, that in a falling, windowless laboratory, no experiment could determine that the laboratory is accelerating. I took this to mean that experiments are not affected by gravitational acceleration, because I am sure that any human in the laboratory would be experiencing 0 $G$.

To explain this to myself, I made the following prediction (please note, I know very little of general relativity as we have not covered it in school, so please correct any incorrect assumptions I make): "Gravity is not seen as a force in general relativity".

I made this prediction based on the following: I knew, from a very interesting Vsauce video titled "which way is down" that spacetime is curved. This leads to straight lines which appear curved (geodesics). My thoughts were as following: because these geodesics are in fact straight lines, objects will follow them under the evolution of time, according to newtons first law. I thought that because these geodesics are straight lines, no force is needed in order to make objects follow them.

After I made this prediction, I asked the following question on quora: "According to general relativity, is gravity a force". I was hoping for some solid "No"s, affirming that I had made a successful prediction, but I got a mixed bag.

One answer confirmed what I was saying, "No. Gravity operates by a distortion of the four dimensional space."

Other answers said something different though. One didn't say out right that I was wrong, but said that gravity was rather "the momentum inflow created by the inertial reaction of a mass M to the 4(pi)G cosmological acceleration field". I do not understand exactly what that means, but I believe that they are speaking of some sort of universal gravitational field which is different to my prediction.

Another answer, which I won't post here because it was long said that "yes", gravity was considered a force in general relativity, and spoke about geometry and the field equation, and how although geodesics trace the lines taken by objects in a gravitational field, they don't cause the motion, and that gravity is one of the four fundamental forces of nature identified by physics.

Obviously for someone trying to learn about general relativity, having such different answers is confusing. I hope that my explanation has made my question and lack of understanding more specific.

The core of my question remains why gravitational acceleration is not detectable by experiments, but I am absolutely interested in learning about the nature of gravity.

Many thanks, Hugo

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Your reasoning is all correct. Gravity is not viewed as a force in general relativity. Gravity is instead caused by the curving of space and time. All objects naturally travel on straight lines through space and time (when no force is applied), but those "straight" lines become curved geodesics when space-time is curved.

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  • $\begingroup$ I have one follow-up question. Consider me sitting on my chair now. I am moving along a geodesic which tends to accelerate me (I expect due to the fact that spacetime is more curved as I approach earth) towards the earth, but my chair exerts a normal force on me, which keeps me from accelerating. When you look at a freebody diagram of me, the only force acting on me is the normal force, but I am not accelerating upwards. I understand that you treat gravity as a force in Newtonian mechanics, but how can an active normal force be cancelled out by a passive gravity in general relativity? $\endgroup$
    – UnrulyTank
    Sep 20 '20 at 19:32
  • $\begingroup$ If you have no forces acting on you, you will feel as if you are in free fall. If a chair is exerting a force on you, then you are no longer in free fall. You will feel the force of the chair pushing you away from your natural path. The same thing happens when you stand on the surface of the earth. You feel the force of the ground preventing you from continuing on your natural free-falling path through space and time. $\endgroup$
    – Yachsut
    Sep 20 '20 at 19:52
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    $\begingroup$ @UnrulyTank Sitting on your chair, you are not moving along a geodesic in spacetime because you are not in free fall. $\endgroup$
    – gandalf61
    Sep 21 '20 at 8:05

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