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If gravity is just the result of the bending of space-time, how does that make it a force? Isn’t it just an interaction between celestial bodies?

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    $\begingroup$ What do you mean "it's a force"? a "force" is part of the model of Newtonian mechanics. All "forces" are interactions of bodies. In the model of GR, gravity is an interaction of mass/energy with spacetime. These are different ways of describing. They are models of reality. $\endgroup$
    – James K
    Commented May 5 at 5:18
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    $\begingroup$ There are several questions on this topic on our sister site. Here's a recent one: physics.stackexchange.com/q/811815/123208 & an old one physics.stackexchange.com/q/3009/123208 $\endgroup$
    – PM 2Ring
    Commented May 5 at 7:49

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The semantic meaning of the term force is now quite confusing. Perhaps, it has always been. We still say the four forces of nature are the weak force,the strong force, electromagnetism, and gravity. (Or, the three forces are electroweak, strong, and gravity.) So, gravity is a force of nature.

It is often useful, when solving problems, to define a force by $\vec{F} = m\vec{a} = m\frac{d\vec{v}}{dt}$, and gravity still causes 3d velocity vectors to change. Astrophysicists commonly use the term force in this way when describing gravitational effects. But it is understood that, in the theory of General Relativity, the change in velocity happens indirectly by the bending of space which makes trajectories curved in 3-space.

However, in some quantum gravity theories, gravity is an interaction mediated between particles by gravitons and is not a bending of space time. It therefore could be, fundamentally, a force. In other words, we don't know yet.

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  • $\begingroup$ You left out the Pauli force. It must be quite confusing to you to be stuck on Earth's surface ツ $\endgroup$
    – John Doty
    Commented May 7 at 16:26
  • $\begingroup$ @JohnDoty Yes. How exactly the anti-symmetric wavefunction of fermions results in a force is confusing, as is all of quantum mechanics. $\endgroup$
    – eshaya
    Commented May 8 at 16:02
  • $\begingroup$ I don't find it particularly confusing theoretically, and in any case that part of the theory emerges from simple, everyday, concrete observations of matter. $\endgroup$
    – John Doty
    Commented May 8 at 16:14
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It is considered as force in Newtonian physics, which's property is to attract objects which are smaller. But in modern time , According to Einstein's General Relativity theory, gravity is defined as bending of space-time. Objects curls the space-time with it's mass resulting bending of space-time and object that are in contact with that object will get closer due to the bending.

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    $\begingroup$ "which's property is to attract objects which are smaller. " What do you mean? In Newtonian physics, the gravitational force exerted by the Earth on you has the same magnitude (but opposite direction) as the gravitational force that you exert on the Earth. $\endgroup$
    – PM 2Ring
    Commented May 5 at 17:23
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Energy (and mass) "suck" spacetime (I don't think that "warps" conveys the picture well). It creates an inward flow of spacetime, which is an elastic fabric. A smaller object (small mass) interacts with a bigger object being pulled by the stronger flow. Elasticity of spacetime means that if you disturb it, it stores energy (like a compressed spring), so disturbances reach equilibrium as "forces" counteract. So, masses do attract each other, keeping both Newton and Einstein happy, except that Einstein's model allows for disturbances traveling through spacetime.

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A force gauge is one of the simplest physical instruments to construct and use. Such instruments are the true foundations of physics. Despite the vanities of theorists, mathematical abstractions are not fundamental: they are stories we tell about measurements using things like force gauges. Since you can measure gravity with a force gauge, it must be considered a force.

The "four forces" story is kind of a mess. You can't measure the strong or weak nuclear forces with a normal force gauge, although it is perhaps reasonable to generalize the notion to include the subatomic processes used to probe these. The commonality of electromagnetism and nuclear forces is the microscopic abstraction that relates these to bosonic fields, so there are some who have redefined "force" in bosonic terms. That's problematic for gravity: although it superficially looks like it might be modelable as a bosonic field, there is no successful bosonic model of gravity. And then there's the leptonic Pauli force that keeps you from falling through your chair: easily measurable with a force gauge, but not one of the bosonic forces.

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