[justin_vanw]

I'm not an expert, but I simply don't understand your reasoning here.

For example, lets suppose some mass is moving at a constant inertial velocity through empty space. To me it seems that your reasoning would require gravitons to communicate to the mass that there are not other masses nearby and so to 'tell it' that 'straight line' for it means to go in a euclidian straight line. Otherwise, how does it know?

I think baked into your logic is that there is something special about geodesics in the presence of masses and so you need to tell the moving mass to 'curve', but to the mass it is just going straight, even if to an external observer it appears that it is curving or even orbiting.

[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.

[lmm]

> to the mass it is just going straight, even if to an external observer it appears that it is curving or even orbiting.

You're hiding a lot of assumptions in this idea. That model of curved geodesics sounds sensible in isolation, but it's completely contrary to how all other known forces work and seems incompatible with quantization (which is again how all other known physics works). I mean sure the universe conceivably could have three fundamental forces that work via particle exchange and one that works by curved spacetime, or magic, or the hand of god. But that doesn't seem very likely, and that kind of inconsistently would go against the history of physics up to this point.

[pasquinelli]

But we know gravity is by far the weakest force, 10^-36 that of electromagnetism compared to 10^-7 of the next weakest force. It's so out of scale with the other four fundamental forces, maybe it is fundamentally different.

[justin_vanw]

I disagree, I think you are assuming far more.

My description is, from my limited understanding, just describing General Relativity, which is a totally accepted and highly verified by observation and experiment.

Where you say it is incompatible with quantization, you are assuming that gravity has any quantization to begin with, which has never been observed. You are literally 'begging the question' here, assuming that gravity quantization needs to be explained by gravitons, when neither quantization of gravity nor gravitons have ever been observed.

[lmm]

> My description is, from my limited understanding, just describing General Relativity, which is a totally accepted and highly verified by observation and experiment.

Grandparent is indeed describing GR. I wanted to try to give a more outside perspective; I think we forget just how weird GR is just because we're used to it. I wonder how we'd be thinking about quantum gravity if we'd discovered QM (which has been verified far more rigorously than GR) first.

> Where you say it is incompatible with quantization, you are assuming that gravity has any quantization to begin with, which has never been observed.

Sure, but as I said quantization is how all other known physics works. While indeed we haven't observed quantization of gravity, fundamentally gravity happens in the same universe as the rest of physics, so something has to give. (And continuous approximations to quantized reality are again exactly how the rest of physics works, whereas I struggle to even imagine how you could recover quantum behaviour from a continuous underlying theory - though I'd be fascinated to hear about any such efforts).

[justin_vanw]

So what about my initial point? How does a mass moving in the absence of other masses know that a geodesic for it is a euclidian straight path?

I would say that it doesn't, and that inertial motion in the absence of other masses is meaningless, and I think this is a key insight that allowed the development of GR in the first place (since any attempt to introduce absolute coordinate systems breaks causality because you can have multiple outcomes from the same boundary conditions, at least in the Hole Paradox).

I would say in addition that every other part of physics that I am aware of can be described locally, for example you can detect if you are in an magnetic field (at least in theory) even at the level of a single proton (since it has a magnetic moment). There is NO test you can do locally to detect that you moving inertially in the presence of a gravitational field. If you can't detect it locally, either there is a huge coincidence (in this case the coincidence is that inertial mass and gravitational mass are identical) or you can't expect to find any messenger particles, since why would you need to 'tell' a mass that it is in a gravitational field when it can't even detect it to begin with.

[lmm]

I would say you can't have anything that would have mass in the first place without a field, which in turn means you have space and time.

[wayn3]

hes simply confounding the mathematical terminology with which we describe physics - with the actual physics.

which is in no way his fault. media present it exactly that way.