Gravitons always struck me as a goofy half-baked concoction (even though we've supposedly observed them now), and so did dark matter. Emergent gravity theories are very interesting.
Gravitons appear in several different families of theory.
We have not observed any of them; the interaction cross-section is so small (because gravity is so weak) that we have no hope of observing a single graviton, or failing to observe an expected one, any time soon.
Gravitational waves have spin 2 symmetry and are massless. A straightforward quantization of a gravitational wave leads to gravitons as a spin 2 massless gauge boson. Conversely, in almost any gauge theory that admits a massless spin 2 particle, that particle mediates a force almost exactly like gravitation, and is amenable to analysis under the Paramaterized Post-Newtonian Formalism.
Indeed, when gravity is weak, such theories essentially exactly reproduce General Relativity. It's only in strong gravity that non-negligible differences appear, and in this case strong gravity is when the quantum uncertainty in curvature distorts distances in ways that can't be worked around e.g by a change of basis or system of coordinates. Alternatively, gravity is strong when you have multiple loops of gravitons in a Feynman diagram.
So in the type of quantization above, the problem is that the momentum carried by an individual graviton is much much much much smaller than that carried by an individual low frequency photon. For example, a near-infrared photon has a momentum-energy on the order of about 1 eV/c^2, a 100 MHz photon has an energy on the order of 10^-7 eV/c^2, and a graviton will be less than 10^-22 eV/c^2 (LIGO collab., https://arxiv.org/abs/1602.03837, about six paragraphs before section VIII).
In other words, even the extremely weak signals LIGO's detected represent waves comprising huuuuuuge numbers of gravitons. But each graviton is so weak (or equilvalent long-wavelengthed) that we have no hope of detecting it individually. Think of how much fun you would have trying to isolate with your fingers an individual molecule of water in a surfers-beware ocean wave -- that'd be easier than isolating a graviton from a presently-detectable GW.
Observing gravitational waves is not really the same as observing individual gravitons. It's unlikely we can observe gravitons with any equipment we can actually build.
Gravitational wave doesn't carry mass. It travels at the speed of light, so it has to be massless. In fact, in 4D gravitational waves can arise even without any mass/energy source.
I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard.
And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.
> I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard.
This sentence is wrong. I'm not sure what you mind with "photon which can disappear into nothing, leaving just the energy", but gravitons [1] can do the same thing.
A gravitational wave is made of a huge amount of gravitons, it's like the electromagnetic wave produced by radio transmitter. I´m not sure if with your classification the electromagnetic wave produced by radio transmitter "can't ever vanish entirely".
[1] If they exist. I'm almost almost sure gravitons exist. There is still not a good quantum gravity theory, but I strongly believe that we will discover it in the future an it will include gravitons.
> can disappear into nothing, leaving just the energy", but gravitons [1] can do the same thing.
I'm not convinced they can vanish, unlikely photons there is nothing that can stop a graviton, they are able to travel through all matter.
So given they have unlimited range, how would one vanish?
> A gravitational wave is made of a huge amount of gravitons
That is not established at all!! Not to mention it's mathematically identical to saying gravity is made up of gravitons, and we certainly have not established gravitons exist even for plain gravity.
After all a gravity wave is just a modulation in gravitational force, not some new entity.
Gravitons (if they exist) will interact with any particle with mass, so a big heavy wall should stop them. Since the interaction is extremely small, the wall must be extremely heavy, probably bigger than a galaxy, not just a normal wall.
>> A gravitational wave is made of a huge amount of gravitons
> That is not established at all!!
Completely agree. My conviction that they exist is based in many assumptions. In particular that we will someday have a quantum gravity theory that will be correct and that it will integrate nicely with the other physics theories. It's a good guess, but it's possible that we will not know the answer for hundreds of years.
Can you elaborate on what you mean by "a gravitational wave has mass (carries mass is more accurate)"? That doesn't fit my understanding of gravitational waves.
To be precise, particles (with precise momentum) are certain modes or excitations of a quantum field which is often governed by something like a wave equation. However, not all quantum fields (and their associated particles) are force carriers. These are usually just the gauge bosons.
In any case, though, noone has managed to quantize gravity yet (i.e. describe gravity as a quantum field theory with gravitons as force carriers), so we've detected just gravitational waves but no gravitons.
Well, we've quantized gravitation extremely well - it's just perturbative quantum gravity, of which there are a couple flavours. In weak gravity it makes predictions identical to General Relativity, but is taken to be an effective field theory (in the Charles Wilson sense of "effective) in that we do not know how to deal with Feynman diagrams with more than a loop or two of gravitons, which only happens in strong gravity.
In these theories, gravitons are quantizations of the weak-field perturbations.
The non-renormalizability by power-set counting of gravity because it is a long-range force is a good result of perturbatively quantized gravity. Who knows if there is a workable way of renormalization by other methods? Not me.
Since we don't get strong gravity except close to a black hole singularity or in the extremely early universe, it's unfair and premature to say that the EFT approach has been unsuccessful.
Since QFT usually involves perturbation theory of a free field, whether or not particles are virtual (= intermediate products of interactions) is beside the point.
Gravitons are basically unavoidable. They may not be fundamental particles, but they'll always show up in the long-wavelength approximation of any theory of quantum gravity that extends general relativity.
