All it needs is an experiment that can test it.
Meanwhile, there's interesting experimental action in low-energy physics, down near absolute zero. Many of the weirder predictions of quantum mechanics have now been observed directly. Look at the list of Nobel laureates in physics since 1990. A big fraction of them involve experiments with very low energy states, where thermal noise is small enough that quantum effects dominate. Some of that work led to useful technology. That's forward progress.
Astronomers can observe extremely energetic environments from a great distance.
It's not a controlled experiment, but sometimes they get lucky and see something that suggests new physics.
I have no idea what might be needed to provide astronomical evidence for string theory.
Even when we are able to operate at higher resolution sometimes the model makers still operate on their own scales. For example, I believe political science and economics ought to be studied from a biological perspective if they are to be fully understood, since gene by environment interaction determines so much. However, there seems to be little interest among political scientists and economists to study the nitty gritty of molecular and population genetics, and little interest among the molecular and population geneticists to study political science and economic theory. And because of this, seems to me such models will always fall short compared to models that operated on a perhaps more appropriate scale of inputs.
The TLDR is that you can never expect the same level of certainty when you don't have direct experiments, but you can still rule out _some_ hypothesis, and see how far other hypothesis take you. This is called theoretical physics. Just because you can't make an experiment doesn't mean you can't do anything.
Certainly internal self-consistency will take you a long way if you don't have experiments. Some people find beauty in this :-)
The other things you refer to are still Occam's razor: symmetry is handy because it eliminates symmetry-breaking entities even though we know they can happen in the standard model (Higgs) and "beauty" is really just another way of saying Occam's razor - you'd prefer your theory to not be full of dozens of free parameters because it starts to fit any possible outputs and be less predictive.
At all points the issue is that unless you've fully explored a simpler space with less entities, don't start adding them because you can always keep adding them to solve any problem but predict nothing (ala epicycles keeping geocentric solar models alive. You could probably run a space program assuming the Earth is the center of the universe, but it would be fiendishly difficult to model).
I think it's better to think of most real world models as being low dimensional-ish, where there is a decaying power law of eigenvalues, and most are quite small, though not zero. You can get quite far by looking at the largest modes and ignoring small ones, but you're not exact, so you're not seeing The Truth, or whatever. Forcing your self to use fewer parameters is a way of denoising, however, that is quite effective.
- Albert Einstein
Because you can write a lot of mathematics with no practical applications for generations (then whoops: number theory and cryptography!)
Of course physicists sometimes do make wrong predictions and it can take some time to figure out the hypothesis is wrong. However the goal is always to make something they can test to prove the hypothesis holds, which string theory has so far failed to do.
as long we have to do with a consistent string like theory.
Is my understanding correct?
Has it been rigorously shown that it can never be tested? Or is that your prediction?
But yes, not rigorous.
People talk about this as though it's an attempt at deception, whereas two people notionally working in string theory could in fact be proposing highly incompatible models which would be conclusively ruled out (and a lot of them have been in so far as that can be done - i.e. experimentation has put tight bounds on their possible parameters).
Yeah... except that hasn't been proven. That's just your belief, right?
I don't think anyone has proven that string theory can yield no testable predictions. I think if someone had, that'd be a big deal.
And I don't think we should pretend that an open problem is closed just because we don't like it.