mikelepore said:
In the method of Newton, gravity is visualized as an "action at a distance." The mass over here somehow knows that there's another mass out there. It's a force because one mass pulls on the other.
In the method of Einstein, gravity is a local effect. The existence of a mass causes a disturbance in the geometry of space and time, and that disturbance is spread outward to distant places. The mass that feels the presence of another mass does so because it's responding to the characteristics of spacetime in it's own local vicinity. There's no assumption about a force from the first mass reaching across a distance to pull on the second mass, so many people say that gravity isn't a force.
Gravity is still a force in the sense that a force is whatever causes a mass to accelerate. It's not a force in the classical idea of an invisible line that spans a distance and then pushes or pulls on something.
Let me elaborate on this. (Mikelepore, I'm not commenting on you, please consider this posting an independent elaboration.)
It's interesting to consider why the Coulomb force is referred to as a force
both in terms of classical physics and quantum physics, whereas in the case of gravitation it is pointed out that in terms of GR gravitation is not a force in the classical sense.
Let's see what the Coulomb force and gravitation have in common:
- Originally the Coulomb force was thought of as instantaneous action at a distance. Later there was a shift towards thinking of the Coulomb force as being mediated by a field, the electromagnetic field, and a change in one location propagates as a change of the field with a finite velocity: the speed of light.
- Prior to the introduction of relativistic physics gravitation was thought of as instantaneous action at a distance.
After the introduction of relativistic physics both the Coulomb interaction and gravitational interaction are thought of as being mediated by a field.
- For the Coulomb force: charged particles are not thought of as interacting
directly with each other, instead each charged particle is thought of as the source of an electromagnetic field that extends in spacetime, and other particles interact
with the local field. (The local field is thought of as the superposition of all fields that extend to that local region.)
- For gravitation: Inertial mass is thought of as affecting its local spacetime, and gravitational effects extend away from a source because spacetime warps adjacent spacetime, thus extending curvature over stretches of spacetime. In other words: the spacetime curvature acts as mediator of gravitational interaction between gravitational sources.
I hope I've made the similarities clear. So
why is the Coulomb force still considered a force, both in terms of classical physics and quantum physics, while at the same time it is argued that in terms of GR gravitation is not a force in the classical sense.
The difference - the only difference - is that the Coulomb force causes acceleration with respect to the local inertal frame of reference. That is why it's still qualified as a force.
The gravitation from the Sun causes the planets to orbit the Sun, but the acceleration that the Sun causes is not acceleration with respect to the planet's
local inertial frame. Instead the local inertial frame of a celestial body is an inertial frame of reference itself.
(More precisely, the local frame that is co-moving with the center of mass of a celestial body does feature tidal effects, and in that sense it's not an inertial frame of reference, but the tidal effects are comparatively small. To a good approximation the local frame is an inertial frame of reference.)
Cleonis
