[cryptonector]

That gravity is curvature of spacetime is one view of two equivalent views, but it is the standard view. The other view is that you have flat spacetime with different distortions (of things other than spacetime) than the distortions you get in curved spacetime. The Schwarzschild metric essentially lets you do exactly that projection of curved spacetime to flat, and vice-versa. When you watch an animation like https://www.youtube.com/watch?v=hF7zltx7Ecc or https://www.youtube.com/watch?v=E1mD4C7dBKc you're watching a flat spacetime representation of GR's effects, and the reason for using flat spacetime in these representations is <drum-roll/> that that is what us humans understand.

So if you take the gravity curves spacetime view, then gravity is not a force and all that. But if you take the alternative view then gravity is a force. Now, I'll leave what the distortions are that gravity produces in flat spacetime for another time, or for the reader. But I'll say this: this view is both controversial (perhaps replies will show this) and not (see above -and many other- animations).

[NitpickLawyer]

I last took a physics course when Pluto was a planet, so excuse my possibly outdated question, but isn't the detection of gravitational waves proof of gravity being a force?

I follow a few educators/communicators in this field and I have a feeling they're using this "gravity isn't really a force" to bridge the gap between their deep understanding and us mortals that don't poses the language / understanding to get the entire meaning behind it. Is that feeling correct or am I missing something?

[ithkuil]

Fwiw gravitational waves were predicted by Einstein himself (Einstein, Albert, Ueber Gravitationswellen, 1918) as a consequence of general relativity.

The core idea is that when you move a mass, its contribution to the spacetime geometry changes, but the effects of the change of the geometry doesn't apply instantaneously to all the universe but instead the change propagates at the speed of light.

So that explains why any sudden movement of a mass creates a "crest" that moves through space at the speed of light.

Furthermore, the sources of fast movement of extremely heavy mass just happen to involve an object that wiggles back and forth in a periodic way because those events involve heavy objects orbiting other heavy objects.

That's the reason we can measure a wave with multiple crests and we can talk about a wave length of the gravitational waves: the wave length of the gravitational waves matches the period of the orbit of the heavy mass.

[HappMacDonald]

s/movement/acceleration/g

[Wololooo]

So the main issue here is how people were presenting it, in Quantum field theory, as stated by other people, each force is associated with a field and has at least one force carrier, the exact number is linked to the specifics of the mathematical framework underlying it

To that extent you can build 3 fundamental forces, electro magnetic, weak (that are called together electroweak) and the strong force. You have an extra force carrier through the Higgs that allows you to give mass to everyone.

Now you need to consider gravity because you know that gravity exist and since everything under the sub is quantised, well so should gravity.

The main issue with gravity is that it is interpreted so far as a curvature of space time, it's mainly fine for big items, but the implications for quantum field theory is that you should modify the small integral element that you use (space shouldn't have the same size) except that you look locally at space that is mainly flat... And changing the integral does not lead to well behaved behaviours.

You can start to introduce new fields but doing so also causes an issue...

Funnily enough even in the standard model something is missing, everything mostly fits, but that's the trick, mostly, neutrinos have mass and this in itself is a problem because the Higgs mechanism doesn't provide mass to them ...

Long story short, people take shortcut when explaining the messy gritty part of it, which is "fine" but not really, and from a simple standpoint one would like to have a simple field from which gravity is born, which might be but so far, to my simpleton understanding, this hasn't been too successful, unless some form of string theory is realised. But the pre requisite for this is a form of supersymmetric theory existing which is currently disfavored, but could exist in the unproved energy scales from here to the plank energy scale.

Sorry this ended being a tad long and I'm not sure this is clarifying things.

[cryptonector]

> The main issue with gravity is that it is interpreted so far as a curvature of space time,

Yes, that is the main issue. It doesn't have to be that way though. If you look at the Einstein field equations, and solutions like the Schwarzschild and Kerr metrics, the key component is a metric tensor that is nothing more than a mapping from flat spacetime to curved spacetime. We have the ability to choose which interpretation to use. The metrics are nothing more than Mercator-like projections.

If you take the curved spacetime view then you get distortions of spacetime. If you take the flat spacetime view then you get other distortions like that the speed of light -though always seen as the same locally- varies according to the gravitational potential (there are other distortions as well).

We seem to have a bit of a fetish for the curved spacetime view. But oddly when you look for animations depicting interactions with black holes and photons or particles / small bodies what you almost invariably find are of two types: a) flat spacetime representations, or b) the funnel representation, and (b) often comes with a flat spacetime representation above the funnel. How do you think the authors produce the flat spacetime representations? A: By applying the metrics to go from curved spacetime to flat! And why do they use flat spacetime for their animations? A: Because it's easier for humans to understand!

The reality is that flat and curved spacetime are two sides of the same coin. If curved spacetime is the sticking point for quantizing gravity, then switch to flat spacetime.

[AnimalMuppet]

> neutrinos have mass and this in itself is a problem because the Higgs mechanism doesn't provide mass to them ...

The Higgs mechanism doesn't provide all mass, even of the things that it provides mass for. They each have a "bare" mass, that is, a mass without any Higgs interactions. They just have a much greater mass because of the Higgs interaction. (And maybe that's why neutrinos have so little mass...)

[Wololooo]

Uuuuh... No...

The W Z have only mass from the Higgs and nothing else for example.

But to answer your point completely here is an answer which is in the two following links making the point for the Dirac fields

https://physics.stackexchange.com/questions/607435/what-part...

https://cds.cern.ch/record/292286/files/B00008237.pdf at page 46

Or have I misunderstood your point?

[AnimalMuppet]

I stand corrected.

[stouset]

My naïve understanding is that you can model gravity as a force in a flat, static spacetime. Equivalently you can model gravity as a forceless distortion of curved spacetime. Both models can be translated faithfully into one another, so you can solve problems related to gravity in either domain.

[trevordixon]

My naive understanding is that forceless spacetime distortion predicts somewhat different things than the old model. That's how general relativity finally explained the procession of Mercury's orbit for example.

[cryptonector]

GP means that you can take the Einstein field equations (and their solutions) and use the metric tensors to map between flat and curved spacetime, with either way being equivalent. GP did not mean that those tools map from Newtonian flat spacetime to curved.

[AnimalMuppet]

Consider centrifugal force. In a non-rotating reference frame, it doesn't exist. In a rotating reference frame, it does, that is, it shows up as a term in the equation of motion.

You wouldn't expect to find quantums of centrifugal force in a rotating frame of reference, and no quantums of centrifugal force in a non-rotating frame of reference. They're both describing the same situation; either quantums exist in both frames, or they exist in neither.

So either gravity really exists as a force, or it doesn't. If it does, then I would expect gravitons, and expect them in all frames of reference. If it doesn't, then I would expect no gravitons in any frame of reference.

Except... If I understand correctly, static electric and magnetic fields are not carried by photons - they just sit there. It's only changing E/M fields that are carried by photons. So maybe only changing gravitational fields are carried by gravitons, and the static fields are just curved space-time?

[whatshisface]

It's a bit of an internet meme, gravity can take momentum away from one object and transfer it to another, and that's what Newton said a force was. The meme is that the way it happens makes "changing momentum" (3-momentum, the one Newton was talking about) and "going straight" (geodesically, in curved space-time) hard to separate in English.