| It's generally expected that the spacetime geometry is emergent as some coherent state of gravitons Unpacking that statement a bit: - In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory. - The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>) - It's convenient to talk about states with a definite number of particles because of the way the math works out. - To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty. You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar). Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**]. So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number.
Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry [*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity [**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc |