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xkcd #2933: Elementary Physics Paths
(imgs.xkcd.com)
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==COSMOLOGY==> 'Uhhh ... how sure are we that everything is made of these?'
It's not untestable. It gives predictions and there has been tests for those predictions. The unfortunate part is that the predictions are often not very concrete, and the range of a lot of these predictions lies far beyond our capabilities. But people are looking to measure them indirectly in various ways. So it's not like it is untestable by design or anything like that.
AFAIK, every single idea from string theory that could be tested was rejected. And the theory was made more complex, less predictive so that it could still work without the testable idea.
These are very broad statements that are not very easy to comment on. "Every single idea", makes it sound like they are a lot, I would not say they are. "Was rejected", depends what you mean... " did not show positiv results", "no longer possible to motivate economically", sure, " refuted as bullshit", not so much. "Was made more complex", sounds like there is intent, and/or, depending on what you mean by complex, that it would be necessarily a bad thing to using more advanced maths to formulate things you could not before, and hence solve new problems.
I can mention two possible avenues of inquiry that are less than 5 years old that has sprung from string theory as possible support for it: signals of black hole structure in gravitational wave 'ring downs' of black hole mergers, and the exclusion of a positive cosmological constant. But if you know that these are untestable or rejected, I'd love to hear about it.
I thought the problem with string theory is that its predictions match up with what the standard model already explains. Maybe that’s only for the things we have the capability to mature any time soon.
No, the problem is very different. In string theory you have a lot of freedom to build various models, and they can provide the standard model, slight deviations from it, or something completely different. Before LHC we knew we had some version of the standard model, the hope was that the LHC would find that we have some particular deviation, like supersymmetry (susy) with such-and-such masses and particles. It did not. The prediction is susy, the problem is that the prediction (at least yet) is not exactly this type of susy. String theory says there is supposed to be a lot of extra stuff beyond the standard model, question is just how do you find it, which is made harder by string theory allowing for so many models.