Comment by cauch

First, it is incorrect to say that "any set of data is consistent with string theory". For example, string theory is death in water if we observe a single Majorana fermion with no partner at a comparable scale. This is an observation that fully falsifies string theory.

But more importantly, "falsifiable" means "observations reduce the landscape". It does not matter if the landscape never fully disappear, as long as the observations constrain the parameters of the theory, then the theory is falsifiable. This is the real condition for a theory to be unfalsifiable. Astrology is not falsifiable: whatever the observation, the full astrology theory remains exactly the same, nothing has been constrained.

Why is it the correct definition? Because your definition leads to illogical situations. For example, I can divide "string theory" into "string theory of region 1", "string theory of region 2", "string theory of region 3", ... which are all in different parameters space. In this case, suddenly, "string theory of region 1" can be fully excluded. But it is illogical to pretend that "string theory of region 1" is suddenly "good falsifiable science" just because you renamed it while the physics is 100% the same. Another example of illogical situation is "an observed theory suddenly qualifies as falsifiable" (cf. the example with the electromagnetism below).

Also, "try again with more energy" is very common in beyond standard model physics. It is the case for lepton compositeness, heavy bosons, fourth lepton generation, SUSY, all of Effective Field Theory, ... None of these theories are abandoned because "unfalsifiable". The argument that string theory is bad because unfalsifiable feels like a bad faith argument while plenty of other physicists activities have the same characteristics but people who are against string theory does not care when physicists work on those.

It's also the case with common theories like electromagnetism or weak force: we are happy with them because we have observed them, but theoretically (in their current formulation), their parameters are free: if the alpha constant was 10 to the power -100, electromagnetism would have been undetectable up to now (the world would be quite different, but still). And if in this parallel universe a physicist would have came up with the theory and tried to observe it and failed, they could have always said "well, it just means that the alpha constant is even smaller". So, do you agree that electromagnetism is a non falsifiable theory because the alpha constant is a free parameter and could have been arbitrarily small? Because if not, you are saying that exactly the exact same theory is sometimes falsifiable, sometimes unfalsifiable, depending on where the free parameters are: right now string theory is unfalsifiable, but if we observe cosmic superstrings, suddenly, the theory qualifies as falsifiable.

Finally, the more you eliminate part of the landscape, the more the credibility of the theory decreases. And even if some part of landscape remains, the theory is abandoned when it is virtually useless.

String theory necessitates observable objects to remain viable as an useful theory (for example, SUSY particles, specific inflationary signature, ...). If these are not observed, the theory is not 100% rejected, but it becomes "useless": it will prove that the reality needs another fix than the string theory to explain some of the problem we observe in the current standard model. In other term, "string theory" can always remain a possibility, but "string theory as the answer of the current problems" would be 100% rejected and physicists will start working on other theories.

The idea that you pretend that physicists will still work on string theory when all the "juice" would been removed due to constraints is just so surreal to me. The reason string theory is popular amongst physicists is because they want to understand the current problems and that, according to them, string theory is a good candidate to explain them. It is crazy to imagine that when we have demonstrated that string theory cannot explain these problems, people whose main motivation is to explain these problems would say "we know we will not get anything of what we want if we work on this subject, but, hey, it's theoretically not fully excluded, so, I have to continue for eternity before passing to a more promising subject".