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 do find it hard to place a lot of faith in Hossenfelder's videos when she's talking about something I don't know that well due to
1. her videos on subjects I do know more about (e.g. SR and interpretation of QM) that seem to stake out rather minority opinions as the only reasonable ones without, I think, fairly representing the logic behind the more consensus positions. (This isn't so bad if you're just talking to physicists, but this is a channel that's for the general public.).
2. her videos staking out strong positions on subjects where she clearly lacks expertise.
So it's useful that you pointed this one out as worthwhile.
