Now that we know gravitational waves are real does this mean that the graviton has moved from hypothetical to real also? And if not what else would constitute gravitational waves?

It depends on what you mean by “confirmed”. Gravitational waves are presumed to be composed of a stream of real gravitons, just like electromagnetic waves are composed of a stream of real photons. But one can conceive of a wave that doesn’t consist of particles at all. I don’t think that any serious physicist thinks it likely that gravitation waves are not composed of particles, but it’s not something we can directly rule out.

To most physicists, “confirmation of gravitons” would mean detecting an individual graviton, which is something that we probably will never do (and yes, I mean that literally: It is probable that the descendants of H. sapiens, over the course of however many billion years, will never have sufficient technology to detect individual gravitons).

Now, there might be some sort of indirect effect, some detectable manifestation of quantum phenomena with respect to gravity, that would be enough to be considered evidence of the graviton even short of a direct detection. But we can’t even begin to speculate about what such an indirect effect would be without a working theory of quantum gravity, which we do not currently have, and it is unknown when we will have it.

Could there be any useful implications associated with confirmation of gravitons existence?

Well, the first useful implication I can think of is that it would let our Type III civilization show off to the Type III civilization over in M31 that we’re better than them, sort of like the space race between the US and the USSR.

Beyond that, any confirmation of the graviton’s existence would go hand in hand with the development of a quantum theory of gravity, and we have no clue whatsoever where that could lead.

The answer is it isn’t really proof of gravitons. To take and obvious analogy the existence of em radiation was demonstrated long before the existence of photons were. The question of whether there must be gravitons making up gravitational waves is a little less straight forward though.

Expanding on a very recent post I made about quantum gravity, one method of reaching quantum gravity is the covariant perturbation method. Essentially using this method you can reformulate general relativity so that rather than gravity being the result of the curvature of spacetime, it is the result of a field defined in a background spacetime. Assuming the background spacetime is a flat Minkowski spacetime, such as in special relativity, then that field can easily be seen to be a massless self-interacting spin-2 field and when quantitized, the quantum of that field is the graviton.

There are variations on this approach, for example string theory takes the background as the product of a Minkowski spacetime and a compact Calabai-Yau space, rather than Minkowski space (though the aims of string theory are broader than quantum gravity). Due to their use of a background spacetime approaches to quantum gravity that take this road are called background-dependent.

However as I said in the linked post, many see this as heresy. A key feature of general relativity is diffeomorphism invariance and by choosing to use a background spacetime as the starting point you are essentially ignoring this very important property. They would also say that general relativity can be cast into the correct form to be quantitized without the need to introduce background spacetimes. These approaches are called background-independent.

In background-independent approaches like loop quantum gravity, gravitons cannot be fundamental particles. This as gravity, like in general relativity, is a property of spacetime, so it cannot be thought of as the result of a field with an associated particle inhabiting that spacetime. Still though you would expect that gravitational waves would exhibit quantum behaviour, and you would expect gravitons to be an emergant property in certain circumstances.

For comparison, sound is not usually thought of as composed of particles. And yet, under certain circumstances, it is convenient to treat sound as being composed of “phonons”, quantized particles analogous to photons. It’s even possible to have a gas composed of phonons, and to then have waves of variations of the pressure and density of that phonon gas, which is *not* the same thing as the original sound (insert “yo dawg” here).

It’s also worth noting that one can do the background decomposition trick with almost any background, not just Minkowski flat space. In practice, gravitational waves are almost always treated as being just a perturbation on top of some background (the nonlinearity makes any other approach madness), but that background might be Schwarzschild or some other known metric which closely matches the actual physical scenario (for instance, if you have gravitational waves passing by some massive object).

Yes, in fact you can do the ‘background trick’ with any spacetime: a spacetime (M,g) is a smooth manifold M with a Lorentzian metric g defined on it. You can then re-define in terms of a background spacetime (M,g’) and a field h, such that g = g’+h. The key thing is that the manifold M stays the same.

When this trick is useful in general relativity is when g’ is a well-known and easy to work with metric such as η and h is ‘small’ by comparison so that the higher order terms can be (selectively) ignored. However this is where one of the problems of “background-independence” comes into it. Generally there’s no way to ensure h is always ‘small’ for a chosen background metric, which in itself is not problematic for quantum gravity unless you are trying to approximate GR linearly, but for example (assuming M = R[sup]4[/sup]), we could decompose g into η+h, but equally we can also decompose it into η’+h’, where η and η’ are both Minkowski metrics and h ≠ h’. The choice of background is therefore generally arbitrary.

I’m not sure we’ve ever directly observed single photons of kilohertz radio waves.

Nope, and that illustrates one of the problems: The energy of a single particle is proportional to the frequency, and gravitational waves tend to have extremely low frequencies (we don’t know of any source higher than the few kilohertz you mention).

The other problem, of course, is that gravity is so much weaker than electromagnetism.

Why?

Oh.

[Honest transcription of me quickly reading this thread.]

This question amuses me a bit, because when Young demonstrated that light had wave properties, it was taken as obvious proof that there was no such thing as a particle of light. Now, of course, everything is understood as having both wave-like and particle-like properties (though oftentimes one aspect dominates the situation - there’s not much point in talking about the wavelike properties of a baseball), but that was a hugely radical idea, so it is interesting to see that these days people can go from “We’ve discovered a wave” to “Where’s the related particle?” so quickly.

Well yes, when Michelson-Morley discovered *no* waves everybody went nuts too.

Random note.

I actually designed and used a measuring device that utilized a Poisons (sp?) spot.

Well, by designed I mean I grabbed a cheap assed HeNe laser that was around, expanded the beam, passed it by a nickel (literally an honest to goodness nickel), threw in a lens and put an imaging detector with a narrow band filter 30 feet away.

Poisson a.k.a Arago spot