[wayn3]

- its not my reasoning, i'm just trying to explain quantum field theory.

- whether you call it "gravitational force" or "curvature of spacetime along whose geodesics massive objects slide" - the effect has to be mediated by a "particle". for popular media, "particle" is too big a word, because people tend to think of protons or atoms. subatomic particles are just excitations of quantum fields. little blips of localized energy, of which we are only able to see the top layer.

^ this has nothing to do with general relativity. general relativity describes the macroscopic world pretty well. it generally breaks down on very small scales.

how planets move is described very well by general relativity. how they mediate the involved forces is not described at all.

edit: i just thought about that straight line statement. there seems to be a misconception that a geodesic is a "generalized straight line". That is not remotely true. Geodesics, in mathematics, are "shortest paths". While that happens to coincide with what a straight line does in a plane, generalizing that meaning in the other direction doesn't work.

In general relativity, we talk about geodesics when we mean "out of all the possible paths we can take, we are choosing the one that minimizes energy loss". That is, then, a geodesic. But a geodesic is far from a straight line in terms of movement. Its the path of least resistance in the energy picture.

If you ask "whats the difference?" - the difference is that a straight line in energy space is not a straight line in regular space. Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun. There is nothing "straight line" about it.

When you fall into a black hole, you travel along a geodesic. But it wont feel like a straight line to you at all.

That you happen to be travelling along a straight line in the absence of forces is just a tautological truth. Applying differential geometry to that statement just makes it way more complicated to state the obvious.

[cygx]

there seems to be a misconception that a geodesic is a "generalized straight line". That is not remotely true. Geodesics, in mathematics, are "shortest paths".

Geodesics being generalized straight lines is exactly true. Also note that they are not necessarily shortest paths: In the framework of affine connections, they are defined as autoparallels.

Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun.

Earth is in free fall around the sun, so accelerometers will read 0. That's the whole point of General Relativity: Geodesic motion is not a consequence of Newton's second law, but the first one.

[sampo]

> whether you call it "gravitational force" or "curvature of spacetime along whose geodesics massive objects slide" - the effect has to be mediated by a "particle"

Why are you so convinced that curved spacetime is only an appearance and that there has to be particles behind it that create the appearance? Why cannot curved spacetime be the fundamental explanation itself?

[danharaj]

The equation that defines curvature in GR depends on the stress-energy tensor which describes the distribution of matter/energy in spacetime. This depends exactly on position and momentum of the matter involved, which is in direct contradiction with the quantum mechanical nature of the stuff.

Normally this doesn't matter because gravity is so weak compared to the scale where QM effects dominate but it is a mathematical inconsistency that becomes very relevant towards the extremes of both theories, in particular: the very early universe just after the big bang and the dynamics near black holes.