> The empirical evidence is simply the fact that gravity exists; if there's no graviton then how does gravity work?
According to GR, gravity per se is a kind of illusion caused by the movement of particles in a curved space-time. The more basic effect is spacetime curving itself around mass(/energy).
Now, whether this interaction itself is quantized or not (if the curvature comes in integer multiples) remains unknown. It is very much possible that gravity/space-time curvature is a different kind of effect than the other three fundamental interactions, and that it is not in fact qunatized, so there is no graviton.
And unto string theory, the problem with it is that it doesn't explain anything new. It posits some ideas of how gravity might work in a qunatized form, but any actual tests of the correctness of those calculations requires studying a black hole up close, which won't happen very soon. And of course, it infamously does so at the cost of adding lots of new elements to physics (extra dimensions, extra particles) none of which are testable or observed. And it still doesn't fix other problems of QM/QFT, most notably the measurement problem.
Plus, there is no actual need for String Theory to say that it's the best option we've got for X. For whatever we want to model, we can use either QFT or GR.
> According to GR, gravity per se is a kind of illusion caused by the movement of particles in a curved space-time. The more basic effect is spacetime curving itself around mass(/energy).
Right, but you can't actually do physics (that is, QFT) in that model. You can model a universe made up of particles that have mass and no other properties, and, uh, that's it.
> Now, whether this interaction itself is quantized or not (if the curvature comes in integer multiples) remains unknown. It is very much possible that gravity/space-time curvature is a different kind of effect than the other three fundamental interactions, and that it is not in fact qunatized, so there is no graviton.
I mean, yes, but in that case what is it? I would love for someone to come up with a non-graviton model that describes gravity while remaining compatible with QFT. But unless and until someone actually comes up with that model, gravitons are the best we can do.
> And unto string theory, the problem with it is that it doesn't explain anything new. It posits some ideas of how gravity might work in a qunatized form, but any actual tests of the correctness of those calculations requires studying a black hole up close, which won't happen very soon.
Directly observing both mass and non-mass aspects of particles at the same time is hard, sure. But we know that particles exist and have both mass and charge (and colour and all that)! They must in fact be subject to both QFT and gravitational effects at the same time. Yes, I can't immediately show that up on my desk in the lab, but I'm deeply uncomfortable with "ignore gravity unless you're working with big stuff, and then if you're working with big stuff ignore the whole rest of known physics and use this radically different theory (that, again, fundamentally contradicts the entire rest of known physics) to model what it's doing". I mean, yes, it does work for all the experiments we can currently do. But it's not at all a satisfactory model of how the universe actually works.
According to GR, gravity per se is a kind of illusion caused by the movement of particles in a curved space-time. The more basic effect is spacetime curving itself around mass(/energy).
Now, whether this interaction itself is quantized or not (if the curvature comes in integer multiples) remains unknown. It is very much possible that gravity/space-time curvature is a different kind of effect than the other three fundamental interactions, and that it is not in fact qunatized, so there is no graviton.
And unto string theory, the problem with it is that it doesn't explain anything new. It posits some ideas of how gravity might work in a qunatized form, but any actual tests of the correctness of those calculations requires studying a black hole up close, which won't happen very soon. And of course, it infamously does so at the cost of adding lots of new elements to physics (extra dimensions, extra particles) none of which are testable or observed. And it still doesn't fix other problems of QM/QFT, most notably the measurement problem.
Plus, there is no actual need for String Theory to say that it's the best option we've got for X. For whatever we want to model, we can use either QFT or GR.