[whatshisface]

Our ability to solve integrals is much more limited when the dx represents a slight change in a function, rather than a small change in a real number. As a result, a lot of things that are easy to say in English such as "quantized curvature in spacetime," or "strongly coupled gauge theory," turn into a big mess when they're written down more precisely. One of the consequences of this limitation is that we have a model for quantized vibrations in spacetime that only works when they do not interact with each other. General relativity says that no, gravitational fields do interact with each other - so the picture we have at present is incomplete. The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.

String theory and loop quantum gravity fit into this picture by trying to replace the integral over something we can't handle with an integral that matches it at large scales, but turns into something more tractable at small scales. Maybe the fact that we still can't make sense of the integral is Nature's way of telling us that she does not do the integral either...

[Aardwolf]

> The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.

Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).

I don't intend to nitpick the sentence, just trying to understand the theory and I don't even know if particle means self interaction or the opposite and can't parse it here either...

If the answer is graviton is for non-self-interacting: what is the model for the other case (where gravity does self interact) and what would cause that self interaction if not the graviton?

[FeepingCreature]

> Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).

The point is, we know gravitation does self-interact. But our best model, the graviton, doesn't model self-interaction. So the model is probably accurate in regimes where you'd expect little self-interaction anyways.

[Aardwolf]

Would self-interaction mean something like:

just like the massless photon, the massless graviton would be bent by the gravity of black holes... hence self interaction?

[KolenCh]

Photon-photon interaction is photon self interaction. Gravity/graviton self interaction then means graviton-graviton interaction. In general relativity, all form of energy would have an effect of gravity, and also react to gravity. Since all matter, including photon and graviton, has energy, then they should self interact.

In QED, photon and photon do interacts too and you can calculate its effect to be small. In GR, you can expect self interaction is small if the space time curvature is small.

(Photon is the particle that mediates QED, and graviton is the hypothetical particle that mediates gravity.)

[gradschoolfail]

This needs to be emphasized more, by the TFA too — most (theoretical) physicists think that detecting gravitons is an engineering exercise that has no implications* for quantum gravity (as understood by the public)

>The model of non-self-interacting gravity is a particle we call a "graviton,"

This needs to be emphasized even more, because it has

>when the dx represents a slight change in a function

*see the discussion around sharikous’ comment below

https://news.ycombinator.com/item?id=42003116

[lazide]

Because they don’t want to run the risk of being wrong, eh?

[gus_massa]

Probably because the number of detection will be too few to test old theories or make new theories with the experimental results.

Physicists love to be wrong! If there is an experiment that disagree with the current theory, then is like the will west and everyone can publish their own pet theory that "fix" it. It's like raining free paper for them, their graduate students and everyone. Also, it's fun!

When experiments and theory agree, they have to use imagination to get a new "interesting" tweak that can be published. In some case the the tweak may be interesting, but most of the times it's not.

I remember a talk about a 2-sigma "particle". There was a small disagreement in some experiment, so someone did a thesis about a possible fix adding a new particle. A lot of hard work and hard calculations. It was a nice talk, and someone asked what what happened then. The sad new was that later the 2-sigma disappeared, it was only a fluke :( . This kind of work is important, but it's more boring that looking for new particles.

[gradschoolfail]

Haha i remember! friends got caught up in that! (Wasnt it 3-sigma tho?)

Theorists would also love for experimentalists to be wrong, but the most efficient way to do it would be by inventing new maths… [sad but still fun]

[gus_massa]

> The model of non-self-interacting gravity is a particle we call a "graviton," and it probably describes reality very well when the gravitation involved is so weak that its self-interaction is undetectable.

I disagree with that part. For the strong force we have the "gluons" and they are considered particles and they have a strong self-interaction. The strong self interaction makes it a huge mess and a lot of things that involve gluons are impossible to calculate.

It's more like:

fake quote> Let's pretend for 30 minutes that the strong field don't self-interact, so we have this nice particles call gluons. Now we add this interaction to the Lagrangian to make gluons interact with other gluons, and now we have a problem.

I agree that that when gravity is small enough, then gravitons give an easy to calculate aproximation. IIRC at high enough energies calculations with gluons get not impossible to calculate too.

[whatshisface]

The thing is, no particle is defined when it interacts. At our present level of understanding the only defined particles are the individual green's functions that appear in perturbation expansions, the lines in Feynman diagrams.

We see interacting particles in detectors, but since nobody can write down what field configuration they mean by "a photon," I can defend my phrasing - but you can defend yours too because I know what a bird is even if I don't know how they work.

[actionfromafar]

I don't know if it was just sheer luck that your comment fit my particular flavor of ignorance perfectly, but it struck me as great writing! I think I learned a little thing today. When I read the article I thought, too bad I can't ever understand anything of this, but now my personal model of the universe is just a little bit richer.

[Iolaum]

Even with gravity being "self interactive" can't we have stable particles ("gravitons"?) that behave like solitons do?

https://en.wikipedia.org/wiki/Soliton

[gradschoolfail]

If you’re young (or if you’re old but your mind remains flexible) i urge you to think hard about this problem.. i do and i will! my mind is already quite inflexible so i’m not going say anything definite about this question out of fear of saying something dumb/misleading. Your q hides monsters ready to snipe any serious mind of the planet!

That said, an easier question to ponder (and many have tried) might be:

  Do photons self-interact? With each other? In free space?

After thinking thru this question yourself, you might be more prepared to consider gravitons (=“spin-2 massless bosons”) ditto for me!