In modern physics, material in the universe is made up of quanta or “particles” such as electrons, protons and neutrons. These units can be said to relate through various forces or fields (strong, weak, electromagnetic, gravitational) for which there are matching “field quanta” such as photons and gluons.

These quanta are usually understood as the particles that make up these fields, and while things are a bit more complex it is the right basic concept. We have a lot of experimental proof for these quanta, but there is one that’s often stated for which we have no experimental proofs, that’s the graviton.

One of the fundamental methods in quantum field theory is to begin with a wave form and then “quantize” it by the help of mathematical formalism. In this way you can display, for example, how photons ascend from the electromagnetic field. The same method can be applied with the gravitational field.

Begin with gravitational waves, and then quantize it to derive gravitons. But there are some glitches with this methodology. In quantum field theory all fields act inside a flat background of space and time (named Minkowski space). Gravitational waves interfere with space and time itself, so to derive gravitons it’s often supposed that the gravitational waves are a variation inside a background of Minkowski space. It this way you can take gravity as a field within flat space so that you can quantize it.

Of course, general relativity illustrates that is not how gravity works. Gravity is a result of space time curvature, so to quantize gravity you would have to quantize space time itself. Just how that might be done is one of the great unexplained mysteries in physics. So it’s probable that gravitons don’t exist. But it’s usually considered that they do, since most physicists ponder that in the end quantum theory will be at the heart of everything. The present key approaches to quantum gravity, such as string theory and loop quantum gravity, forecast the reality of gravitons with the similar characteristics we see in the simple “quantized wave” method.

Even if gravitons are there, it’s probable that we would never be able to perceive them. As one latest paper demonstrated, gravitons would interact so weakly with masses that you would require something like a Jupiter-mass detector circling a neutron star. Even then it would take more than a decade to perceive a single graviton. Even then the noise from particles like neutrinos would wash out your signal. If there’s no applied way to sense gravitons, does it make any logic to talk of them as a scientific model?

Perhaps, assuming they continue inside a robust model of quantum gravity, there may be secondary ways of proving their actuality. For now, though, they are totally hypothetical.