[zachf]

There are exactly zero free parameters in string theory [0]. The details of why string phenomenology is hard is a difficult subject, but the characterization you've given of it is not correct. If you have a proof that string theory is not self-consistent, you should publish it, because there is no such proof in the scientific literature today. (Source: my PhD in physics.)

Unfortunately, there is a ton of misinformation about this topic on the web. For example, people love to say that string theory predicts anything and everything. But it predicts (and rejects) a lot; it’s just that all of the known predictions happen to fall into the categories of (1) predicting things that are very hard for humans to measure (behavior of black holes at long time scales, graviton scattering, etc) or (2) retrodicting things we already know are true (e.g. gravity, Lorentz invariance, etc.). This state of things isn’t by design of nefarious string theorists designing their theory to be untestable, it’s just cruel fate of what comes out of the math. Hopefully someday we can find some other type of prediction, but string theory isn’t easy.

[0] See e.g. https://indico.cern.ch/event/630393/contributions/2890113/at...

[mcmoor]

I thought lots of variants of string theory do predict things inside human means. But they've all failed, leaving only variants that predict things outside of it.

[zachf]

If you have something specific in mind, I’m happy to address it! But I’m not quite sure what you’re referencing.

[mcmoor]

Isn't SUSY one of them? That's the first thing I can remember

[zachf]

You would probably learn more by listening to Cumrun Vafa [0] than anything I could say. It’s hard to say much about string theory without space time supersymmetry not because it doesn’t exist (we know it does) but because it’s so hard to calculate anything…physicists are very reliant on a few tools, supersymmetry is a big one, and without it it’s really hard to say anything concrete, yet.

[0] https://www.youtube.com/watch?v=yppqz12ngbM&t=654s

[pyinstallwoes]

How is string theory useful?

[zachf]

To whom? To other branches of physics? Look up AdS/CFT. To the general public? Dunno, I guess the pursuit of understanding the universe is its own reward for some.

[pyinstallwoes]

How is it understanding the universe if it’s not applicable/useful in the general case?

[zachf]

You mean, if string theory does not turn out to describe the universe, how could it be useful? Well, by giving extraordinarily powerful tools for understanding things that are well established to be useful, like quantum field theory. AdS/CFT gives us the only tool we have to analytically understand quantum field theories in the so-called “strong coupling regime”. This is useful for discovering new properties of quantum matter in systems where you would otherwise need simulations. You can think of it intuitively as string theory providing a glue between two descriptions of the quantum matter, like a “type cast” in programming where you start with one kind of object but reinterpret it as another. The thing that is incomprehensible in one representation is simple in the other. This was discovered by studying limits of string theory in interesting geometries.