John Carlos Baez on Nostr: One reason for the Great Stagnation was physicists' near-obsessive desire to find a ...
One reason for the Great Stagnation was physicists' near-obsessive desire to find a theory of quantum gravity that's 'renormalizable'. This led them to focus huge amounts of energy on theories with no obvious connection to the real world, like supergravity and superstrings. Ironically, none of these theories were ever proved to be renormalizable, and the obsession slowly died away. Nobody claims M-theory is renormalizable. But we're stuck living in the ruins of a dream.
It's understandable how we got into this mess. In the early days, like the 1940s, everyone was scared of the infinities that quantum field theory gives if you let particles interact by exchanging virtual particles with arbitrarily high energies. They desperately sought to avoid those infinities - and they discovered how to do it for 'renormalizable' theories.
The idea is roughly this. You start by saying no virtual particle can have energy² - momentum² bigger than some chosen number called the 'cutoff'. This makes all the infinities go away. The question becomes: what happens as you change the cutoff?
A quantum field theory is renormalizable if you can let the cutoff approach infinity, while simultaneously rejiggering the parameters in your theory, in such a way that answers to questions about physics at low energies actually converge!
But what about theories that aren't renormalizable, like the simplest theory of quantum gravity? What actually happens with these, as you let the cutoff approach infinity? It's surprisingly hard to get a straight answer! You'd think this would be settled by now. But experts are still arguing about it!
(5/n)
It's understandable how we got into this mess. In the early days, like the 1940s, everyone was scared of the infinities that quantum field theory gives if you let particles interact by exchanging virtual particles with arbitrarily high energies. They desperately sought to avoid those infinities - and they discovered how to do it for 'renormalizable' theories.
The idea is roughly this. You start by saying no virtual particle can have energy² - momentum² bigger than some chosen number called the 'cutoff'. This makes all the infinities go away. The question becomes: what happens as you change the cutoff?
A quantum field theory is renormalizable if you can let the cutoff approach infinity, while simultaneously rejiggering the parameters in your theory, in such a way that answers to questions about physics at low energies actually converge!
But what about theories that aren't renormalizable, like the simplest theory of quantum gravity? What actually happens with these, as you let the cutoff approach infinity? It's surprisingly hard to get a straight answer! You'd think this would be settled by now. But experts are still arguing about it!
(5/n)