John Carlos Baez on Nostr: What if we bite the bullet and *accept* the fact that naive quantum gravity is ...
What if we bite the bullet and *accept* the fact that naive quantum gravity is nonrenormalizable? What if we just try to work with this theory anyway?
Here's the problem: experts don't agree!
There's one school of thought called 'asymptotic safety', started by Steve Weinberg in 1978, which says it's not so bad:
https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_gravity
People in this school hope they can actually let the cutoff go to infinity, while correctly twiddling all the parameters describing n-graviton interactions, and get a well-behaved theory in the limit!
A bunch of them, like Marc Reuter, think they've succeeded. But since the number of parameters is actually *infinite*, the calculations are very hard, so these folks make brutal approximations. Many physicists remain unconvinced by these. And even if you grant that the approximations are good, there are a lot of arguments about what the resulting theory is like:
https://arxiv.org/abs/2004.06810
For example, I said that in QED, as you measure the electron charge using collisions at higher and higher energies, it keeps going up. This is called a 'running coupling constant'. The theory of the strong force, called quantum chromodynamics seems much nicer: the observed 'color charge' of quarks and gluons goes *down* at higher energies.
So what happens in naive quantum gravity? What would happen if you could measure Newton's constant G in higher and higher energy particle collisions? This is too hard to actually do, but never mind: I'm just asking what the theory predicts.
Some say G gets bigger, some say it gets smaller, some say this makes no sense. This is not just an argument about calculations. It's an argument about interpreting the calculations!
Here's the problem: experts don't agree!
There's one school of thought called 'asymptotic safety', started by Steve Weinberg in 1978, which says it's not so bad:
https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_gravity
People in this school hope they can actually let the cutoff go to infinity, while correctly twiddling all the parameters describing n-graviton interactions, and get a well-behaved theory in the limit!
A bunch of them, like Marc Reuter, think they've succeeded. But since the number of parameters is actually *infinite*, the calculations are very hard, so these folks make brutal approximations. Many physicists remain unconvinced by these. And even if you grant that the approximations are good, there are a lot of arguments about what the resulting theory is like:
https://arxiv.org/abs/2004.06810
For example, I said that in QED, as you measure the electron charge using collisions at higher and higher energies, it keeps going up. This is called a 'running coupling constant'. The theory of the strong force, called quantum chromodynamics seems much nicer: the observed 'color charge' of quarks and gluons goes *down* at higher energies.
So what happens in naive quantum gravity? What would happen if you could measure Newton's constant G in higher and higher energy particle collisions? This is too hard to actually do, but never mind: I'm just asking what the theory predicts.
Some say G gets bigger, some say it gets smaller, some say this makes no sense. This is not just an argument about calculations. It's an argument about interpreting the calculations!