Physics often "goes out on a limb", developing theories that nobody can test - and often these limbs come crashing down. String theory and loop quantum gravity are crashing down as we speak, but there are plenty of earlier examples.
Neither of these theories truly make testable predictions. That is, starting from the core theory, without adding ad hoc assumptions or making vast leaps of logic, these theories predict essentially *nothing* that we see around us - much less make new predictions that we can test with experiments today.
However, physicists are not shy about making ad hoc assumptions and making vast leaps of logic!
So, string theorists have a long track record of 'predicting' particles and other effects that are just out of reach by current detection methods, and claiming that future experiments will see them. When these experiments are carried out, these predictions are always falsified. But string theorists argue that the core of string theory is not falsified, only the extra ad hoc assumptions and leaps of logic. So string theory carries on.
Hossenfelder gives a good explanation in the video.
And yet, despite having installed string theorists in top positions worldwide, string theory is gradually fading. Physics departments are less likely to hire string theorists than they were 10 years ago - and that was also true 10 years ago. So it seems the tree branch is slowly breaking off the tree, and will eventually crash onto the forest floor, opening up a bit more light for new plants to grow.
Loop quantum gravity has similar problems, but far fewer people work on it, and it hasn't managed to dominate institutions of higher learning in the same way.
@johncarlosbaez I listened to an interview of Cumrun Vafa by @seanmcarroll about the swampland program that almost made it sound like one could get down to predictions, but I think it falls into your characterization of making assumptions and leaps in logic to get predictions, and then just choosing a different set when your predictions fail.
@johncarlosbaez Interesting. None of my research involved high energy physics, so much of what I heard about string theory was the nonsensical stuff you get through popular science reporting, and I was totally unaware of this thread of discussion.
@internic - if you don't read Woit's blog you don't get the intelligent skeptic's careful eye on what's up with string theory. I recommend starting around 2000 and working your way forward. 🙃
@johncarlosbaez What about criticisms of LQG? That's something I've seen and heard a lot less about (but maybe I would have if I'd spent more time reading/listening to string theorists).
@internic - it's hard to find really good criticisms of loop quantum gravity because, roughly speaking, fewer people have studied it, and the people who do tend to be the people advocating it. A lot of the "pop" criticisms I've seen misunderstand the problems. An intelligent and completely damning critique can be found in the last section here:
https://arxiv.org/abs/hep-th/0601129
but this may be too technical for you to understand. It agrees with what I said, at least as far as loop quantum gravity goes:
"Neither of these theories truly make testable predictions. That is, starting from the core theory, without adding ad hoc assumptions or making vast leaps of logic, these theories predict essentially *nothing* that we see around us - much less make new predictions that we can test with experiments today."
But the authors might *not* agree with this statement about string theory.
@johncarlosbaez @internic
> if you don't read Woit's blog you don't get the intelligent skeptic's careful eye on what's up with string theory. I recommend starting around 2000 and working your way forward. 🙃
We all appreciate it when the recommended reading list is very short and simple like this.
@internic - The swampland program attempts to study the vast and mucky realm of quantum field theories that aren't limits of string theories, and make some general claims about them. It was already a thing back in 2005 - try this article from 2018:
https://www.math.columbia.edu/~woit/wordpress/?p=10188
It's not getting any interesting results as far as I can tell. Nonetheless Harvard has a "Harvard Swampland Initiative" where they invite people to give talks and do research:
https://www.math.columbia.edu/~woit/wordpress/?p=14034