eli5 it has been established that the curvature of space-time results in gravity. So what exactly is the Graviton?
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The graviton is a hypothetical particle created in order to try and account for gravity at a quantum level.
Right now, quantum mechanics and relativity do not play well with each other. In situations where you have to use both (e.g. black holes and Big Bang stuff), you get nonsensical answers; the laws of physics break down.
Gravitons are one attempt to try and resolve this.
First, understand that these theories are mathematical formulas that predict what you’ll see when you make a measurement. Einstein’s theory treats space-time as A geometrical shape and he uses that shape to explain what we feel as gravity.
When scientist speak of gravitons, they have reformulated the The math such that gravity is not described as a geometrical shape, but as a force with gravitons being a theoretical particle.
Gravitons are quanta of gravitational wave (wave of the space time) just like photon are quanta of electromagnetic wave.
Being a quanta of a wave means that they are created and destroyed locally in a discrete fashion and then propagate as a wave. All waves are the sum of a finite integer number of quanta, but if it’s very large, it’s easier to just consider the wave rather than the quanta.
In the case of gravity, we are unable to make any experiment in which a single graviton as an effect because the effect of a single graviton are always to weak. The gravitational we can observe (and we have been able to only recently) comes from black hole collision and contain an extremely large number of gravitons.
Ordinary light is also an extremely large number of photons, but we are able to do single photon experiments.
The graviton is supposed to be the particle that makes gravity work in quantum field theory. It hasn’t been proven to exist, but there is no better explanation of how gravity works for quantum mechanics. General relativity is really good at explaining how gravity works on every scale except quantum and is inherently incompatible with quantum field theory.
The graviton is theorized because the other 3 fundamental forces (electromagnetic, weak nuclear, and strong nuclear) have particles associated with how matter “communicates” those forces across space. In order for the electromagnetic force to happen, a photon has to go from one place to another. For the Weak force, W^+ W^- or Z^0 bosons need to move from one place to another. For the Strong force (which holds quarks together to make Hadrons (baryons and mesons) and holds atomic nuclei together) gluons need to move from one place to another. So far quantum theory has shown symmetry for all of the particles in we know of, so why would gravity be left out?
So far, this symmetry has held up very well, we knew about up(+), down(-), and strange(-) quarks, and the strange quark was called that because why was there this extra negative quark, but no positive one. Then we found the charm(+) quark, named so.because it was charming, it restored symmetry. But then if we look at the leptons, there electrons and neutrinos, and each has 3 forms, electron, muon, and tau. Why do leptons get 3 forms, but quarks only get 2? So then they theorized 2 more quarks, top(+) and bottom(-), and now there are 3 types of each quark, and 3 types of each lepton, and there is symmetry. We also know it stops there because to create anything higher up on the list would take more energy than the universe has.
Now, all forces in the universe act at the speed of light. Before it was known as the speed of light, it was called the speed of causality, which is why we use c to denote it. It was proven that it was the fastest anything could affect another thing. And this was first shown with light, which us just an electromagnetic wave, so it must move as fast as things can be affected electromagnetically. Im not sure how we proved it with the other 2 forces, but with gravity, it was theorized by Einstein with general relativity suggesting the existence of gravitational waves traveling at the speed of light, which we just proved existed a few years ago and do infact travel at c. If we could prove the existence of the graviton, it would confirm what we know about quantum field theory and offer us a bridge to connect it to general relativity, and bring us closer to a grand unified theory. If we can disprove the existence of the graviton, then either everything we know about gravity is wrong (highly unlikely) or most of quantum field theory would need to be rewritten (slightly less highly unlikely).
Now, you may have heard of the Higgs Boson, which is necessary to give particles mass, but this doesn’t necessitate gravity, this is just for inertial mass. For most things, inertial mass and gravitational mass are the same thing, but light has inertia, while having no mass, so there are exceptions that we haven’t figured out. Another possible exception is the electron neutrino, which we know has inertia (otherwise it never would have been discovered) but we don’t know if it has mass or not (we do have an upper limit on how much mass it has) and we’re not even sure if it travels at the speed of light or not as a result.
Fun fact: one of the proposed names for the top and bottom quarks was truth and beauty, but it was rejected by the naming committee.
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