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A tear-down of this article by the Columbia physicist Peter Woit can be found here: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.colu

A tear-down of this article by the Columbia physicist Peter Woit can be found here: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.columbia.edu/~woit/wordpress/?p=12641 It’s not about string theory or about conventional quantum gravity in four space-time dimensions. The topic is graviton scattering in maximally supersymmetric theories in ten flat space-time dimensions, and the argument is that the basic principles of supersymmetry, Lorentz invariance, analyticity and unitarity imply a bound on the coefficient of the lowest order correction term. The only relation to string theory is that a string theory calculation of this correction coefficient satisfies the bound (as expected, since string theory is supposed to satisfy the assumed basic principles). Much is made of the fact that in string theory one can get any value of the coefficient consistent with the bound. This is taken as evidence for the “inevitability” of string theory, but I don’t see this at all. It’s more accurately evidence for the usual problem with string theory: it’s consistent with anything. Also, see John Baez's comment: https://www.math.columbia.edu/~woit/wordpress/?p=12641#comment-239945 https://www.math.columbia.edu/~woit/wordpress/?p=12641#comme...

dekhn 5y ago
(I am a person who knows some physics but not a lot). If any of this stuff turned out to be true, what would the lab experiments and positive outcomes look like? I'm accustomed to particle accelerators and all sorts of other physical experiments, but I really struggle to relate these sorts of theories to anything that could be done in the lab, and if the theories were predictive, what sort of applied science could be done? No wide-eyed "we could make space elevators and travel FTL", just realistic extrapolation.
- tsimionescu 5y ago
  One clear requirement for at least some versions of String Theory is Supersymmetry - the existence of supersymmetric partners to some of the particles in the SM at some energy level. This energy level is not really pinned down, except that it must be higher than anything we've tried so far (since we've been unable to find these particles). The sky is the limit on what is a maximal bound - probably all the way down to the Planck scale, or close to it. The same is true of the extra dimensions - those are detectable, but again only at energy levels that far exceed what is plausible not just for the near future, but probably for the entire future of life in the universe.
zachf 5y ago
There's a good reason why most people who specialize in quantum gravity work in string theory: violating any axioms of string theory leads to theories which are self-evidently inconsistent with themselves and/or experiment. It's worth spending time trying to understand why, but unfortunately a lot of air is sucked out of the room by Woit's petulant negativity. It makes for fun podcasts but I'm glad researchers are willing to actually sit and think about the consequences of physics that doesn't match our intution, even when it's hard. And indeed this quote misses the point. The point is that there aren't "additional" effective theories of gravity that are self-consistent but couldn't be explained by string theory. This isn't a smoking gun that string theory is correct, but if the opposite were true, it would have suggested that string theory was missing something. So it's incorrect to conclude from this evidence that string theory is "consistent with everything"; it's merely consistent with the range of things that are consistent, and no more and no less. (Suspicious! But not conclusive.) One can complain that this isn't predictive, and indeed if you want to look for predictive power you look at other physical quantities. String theory makes very precise and unique predictions for the way that e.g. gravitons scatter off each other. The fact that humans don't (yet) have the ability to do such experiments is regrettable from a fundamental physics perspective, but it's a problem with humans, not string theory. So string theory continues to be a demonstrably self-consistent, (in principle) falsifiable, and stunningly beautiful theory--and dismissing it out of hand is a tragically common mistake.
- deleted 5y ago
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dekhn 5y ago

(I am a person who knows some physics but not a lot). If any of this stuff turned out to be true, what would the lab experiments and positive outcomes look like? I'm accustomed to particle accelerators and all sorts of other physical experiments, but I really struggle to relate these sorts of theories to anything that could be done in the lab, and if the theories were predictive, what sort of applied science could be done? No wide-eyed "we could make space elevators and travel FTL", just realistic extrapolation.

tsimionescu 5y ago
One clear requirement for at least some versions of String Theory is Supersymmetry - the existence of supersymmetric partners to some of the particles in the SM at some energy level. This energy level is not really pinned down, except that it must be higher than anything we've tried so far (since we've been unable to find these particles). The sky is the limit on what is a maximal bound - probably all the way down to the Planck scale, or close to it. The same is true of the extra dimensions - those are detectable, but again only at energy levels that far exceed what is plausible not just for the near future, but probably for the entire future of life in the universe.

zachf 5y ago

There's a good reason why most people who specialize in quantum gravity work in string theory: violating any axioms of string theory leads to theories which are self-evidently inconsistent with themselves and/or experiment. It's worth spending time trying to understand why, but unfortunately a lot of air is sucked out of the room by Woit's petulant negativity. It makes for fun podcasts but I'm glad researchers are willing to actually sit and think about the consequences of physics that doesn't match our intution, even when it's hard. And indeed this quote misses the point. The point is that there aren't "additional" effective theories of gravity that are self-consistent but couldn't be explained by string theory. This isn't a smoking gun that string theory is correct, but if the opposite were true, it would have suggested that string theory was missing something. So it's incorrect to conclude from this evidence that string theory is "consistent with everything"; it's merely consistent with the range of things that are consistent, and no more and no less. (Suspicious! But not conclusive.) One can complain that this isn't predictive, and indeed if you want to look for predictive power you look at other physical quantities. String theory makes very precise and unique predictions for the way that e.g. gravitons scatter off each other. The fact that humans don't (yet) have the ability to do such experiments is regrettable from a fundamental physics perspective, but it's a problem with humans, not string theory. So string theory continues to be a demonstrably self-consistent, (in principle) falsifiable, and stunningly beautiful theory--and dismissing it out of hand is a tragically common mistake.

deleted 5y ago
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