For many years, editions here of This Week’s Hype were mainly devoted to bogus claims that someone had found a way to get a testable prediction out of string theory or other “evidence for string theory”. Recently there have been many fewer such claims, with consensus in the string theory community that there is now no hope to get a prediction from string theory about observable physics at accessible energies. One can watch the recent talks here on Steven Weinberg and his legacy to get a good idea of what this current consensus looks like: you can’t test string theory since string effects occur at much too high an energy scale, and Weinberg showed that such things will just look like the Standard Model sort of QFT at observable energies. In addition, Weinberg is also credited with the anthropic CC argument, taken as evidence for the otherwise unobservable string theory landscape. Taken together, the consensus of leading particle theorists has become that there’s no point to trying to do any better than the Standard Model, with the only answer available to anyone who asks questions about higher energies is “string theory, whatever that is”.
With particle physics abandoned, theorists have focused on quantum gravity as the only legitimate issue to study. For many decades the hope was that a consistent answer to the unknown question of what string theory really is (often called “M-theory”) would be found, and that would provide a final end to the subject of fundamental physics. This final theory would be untestable, but it would self-consistently explain why one could not hope to test it. In recent years though, after decades of no progress towards a consistent M-theory, string theorists have essentially given up on this hope.
This situation has lead to a recent trend in string theory research: instead of looking for positive evidence for string theory, try to find an argument that resistance is hopeless, string theory is the only theory possible. The arguments of this kind I’ve seen make no sense to me, but they are gaining in influence. One place I noticed this is in this recent white paper about the interesting topic of celestial holography, which has little to do with string theory. There the authors write:
A crowning achievement for the celestial holography program would be for it to determine concretely whether string theories are the only consistent theories of (asymptotically flat) quantum gravity.
Today Quanta magazine has more of this sort of thing, with an article whose title shows up on the web as A Correction to Einstein Hints At Evidence for String Theory. The sub-headline tells us that
In a quest to map out a quantum theory of gravity, researchers have used logical rules to calculate how much Einstein’s theory must change. The result matches string theory perfectly.
which sounds pretty impressive. The article starts off with quotes such as:
The hope is that you could prove the inevitability of string theory using these [bootstrap] methods,” said David Simmons-Duffin, a theoretical physicist at the California Institute of Technology. “And I think this is a great first step towards that.
Irene Valenzuela, a theoretical physicist at the Institute for Theoretical Physics at the Autonomous University of Madrid, agreed. “One of the questions is if string theory is the unique theory of quantum gravity or not,” she said. “This goes along the lines that string theory is unique.”
The paper at issue is this one which appeared on the arXiv nearly a year ago. 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. If the authors of this paper had found that the string theory bound was different than their bound, they could have written a paper arguing that they had finally found a way to falsify string theory (measure the coefficient, if it was found to be in the region allowed by general principles but not by string theory, string theory would be falsified).
The article does get right the motivations behind these claims:
Some physicists hope to see string theory win hearts and minds by default, by being the only microscopic description of gravity that’s logically consistent. If researchers can prove “string universality,” as this is sometimes called — a monopoly of string theories among viable fundamental theories of nature — we’ll have no choice but to believe in hidden dimensions and an inaudible orchestra of strings.
To string theory sympathizers, the new bootstrap calculation opens a route to eventually proving string universality, and it gets the journey off to a rip-roaring start.
and it gives a little space to skeptics:
Other researchers disagree with those implications. Astrid Eichhorn, a theoretical physicist at the University of Southern Denmark and the University of Heidelberg who specializes in a non-stringy approach called asymptotically safe quantum gravity, told me, “I would consider the relevant setting to collect evidence for or against a given quantum theory of gravity to be four-dimensional and non-supersymmetric” universes, since this “best describes our world, at least so far.”
Eichhorn pointed out that there might be unitary, Lorentz-invariant descriptions of gravitons in 4D that don’t make any sense in 10D. “Simply by this choice of setting one might have ruled out alternative quantum gravity approaches” that are viable, she said.
Another critique, though, is that even if string theory saturates the range of allowed α values in the 10-dimensional setting the researchers probed, that doesn’t stop other theories from lying in the permitted range. “I don’t see any practical way we’re going to conclude that string theory is the only answer,” said Andrew Tolley of Imperial College London.
I don’t at all understand why Quanta chose to cover this. All it does is help to spread hype and further the cause of the “resistance is futile” campaign from proponents of a failed research program.
Update: This kind of hyped story turns into the expected PR result:
Evidence for String Theory –In a quest to map out a quantum theory of gravity, researchers have used logical rules to calculate how much Einstein’s theory must change. The result matches string theory perfectly, reports Natalie Wolchover for Quanta.