The journalBob Jones adds:Foundations of Physicshas been promising a special issue on “Forty Years of String Theory: Reflecting on the Foundations” for quite a while now, with a contribution first appearing back when it really was 40 years since the beginnings (more like 43 now). The final contribution has now appeared, an introductory essay by the editors (’t Hooft, Erik Verlinde, Sebastian de Haro and Dennis Dieks).

The overall tone of the collection is one of defensive promotion of the subject. The fact that string theory’s massively overhyped claims to give a unified theory of particle physics have led to miserable failure is mostly completely ignored.

Most people would say that string theory is an idea about quantum gravity. ... You can complain all you want about the lack of testable predictions in particle physics, but these objections seem pretty irrelevant since most string theorists aren’t trying to do phenomenology and since string theory has achieved so much success in other areas…I may be stupid, but I fail to see that string theory has anything to do with quantum gravity.

The best theory of gravity is general relativity. String theorists nearly always assume that no gravity is present. To account for gravity, they sometimes just say that the underlying space could satisfy the equations of general relativity. Relativity uses a 4-dimensional spacetime, and string theorists use 10 or 11 dimensions, so they extend the equations. But that's all. They do not quantize gravity. They claim to have a theory of everything with quantum field theory have gravity, but they have no quantum gravitational fields. They say that the theory has a graviton, but quantum mechanics is about observables and the graviton is impossible to observe.

I wonder how a non-physicist would understand the excuse that "string theorists aren’t trying to do phenomenology". Wikipedia defines:

The term phenomenology in science is used to describe a body of knowledge that relates empirical observations of phenomena to each other, in a way that is consistent with fundamental theory, but is not directly derived from theory.Someone might ask how one could be a scientist and not care about phenomenology. The answer is that string theorists have never been able to relate their theories to phenomena, so they ignore phenomena.

Leonard Susskind writes:

Just to be precise about what constitutes string theory, let me give a narrow definition — no doubt much too narrow for many string theorists. But it has the virtue that we know that it mathematically exists. By string theory I will mean the theory of supersymmetric string backgrounds including 11-dimensional M-theory and com-pactifications that preserve some degree of supersymmetry. These backrounds are generally either flat (zero cosmological constant) or anti de Sitter space with negative cosmological constant.Steven B. Giddings tries to summarize the outlook for making string theory a quantum gravity theory:

With that definition of string theory, there is no doubt: string theory is not the theory of nature — the world is not supersymmetric, and it has positive cosmolog-ical constant. Exactly how the definition has to be expanded in order to describe the observed universe is not known. Nevertheless string theory has had a pro-found, and I believe lasting, influence on how gravity and quantum mechanics fit together.

To summarize the situation, string theory has been a continuous source of new ideas in mathematics and physics, and showed a lot of initial promise for resolving the problems of quantum gravity. However, the more profound problems are yet to be convincingly addressed, and there are deep puzzles about how they might be addressed by string theory. ...Keep in mind that these are top string theorists trying to defend the theory. But it is painfully that any connection between string theory and quantum gravity is wildly speculative.

We seek a consistent framework for describing quantum processes, in which spacetime locality emerges in an approximation. This, together with the requirement that it produce an S-matrix (and other local dynamics) with familiar properties of gravity seems a very tall order. This is actually encouraging, as it suggests the problem is sufficiently constrained to guide the resolution of this profoundly challenging set of problems. It remains to be seen what role string theory plays in this, and whether it can provide further clues.

Update: I see Lumo has a long tortured explanation of whether string theory makes presumptions about gravitational fields. There is no simple answer.

"I wonder how a non-physicist would understand the excuse that "string theorists aren’t trying to do phenomenology"."

ReplyDeleteWell using the Wikipedia definition, this non-physicist says string theorists are not doing science.

I think some non-physicists have been suspicious of string theory for a while, but it seems to be defended very effectively.

"I fail to see that string theory has anything to do with quantum gravity."

ReplyDeleteLet me try to explain. One of the most important theoretical breakthroughs in physics of the past thirty years was the development of something called the holographic principle. This principle basically says that any description of gravity on a (D+1)-dimensional spacetime manifold is equivalent to a quantum field theory on the D-dimensional boundary in the sense that there's a "dictionary" for translating between the two theories. Over the years, physicists have developed many concrete models of the holographic principle, particularly in the contexts of string theory and pure (2+1)-dimensional quantum gravity, but it's important to realize that the holographic principle is a very general mathematical statement about the gravitational field which is independent of the particular features of these models. The reason string theory is important is that it provides concrete realization of the holographic principle and serves as a useful toy model for testing out ideas about quantum gravity.

"String theorists nearly always assume that no gravity is present."

I'm not sure why you're still saying this. We've had this discussion before, and I explained that it is not true.

"They do not quantize gravity."

String theory is not a naive quantization of the Einstein-Hilbert action based on the usual prescriptions of quantum field theory (this approach leads to a non-renormalizable theory), but it is definitely a theory of quantum gravity in the sense that it is a consistent quantum mechanical theory which reproduces ordinary general relativity on large distance scales.

"Someone might ask how one could be a scientist and not care about phenomenology."

Of course all scientists should care about phenomenology, but that doesn't mean that all scientists have to work on phenomenology in order to contribute something of value to science. Some theoretical physicists, such as string theorists, work on more mathematical problems. They develop new computational techniques and explore the mathematical relationships between different theories. The previous commenter is correct in saying that this sort of work is not science per se, but that doesn't mean that it doesn't have important applications in science.

"string theorists have never been able to relate their theories to phenomena, so they ignore phenomena."

Here you have to distinguish between the different applications of string theory. If you're thinking of string theory as a theory of particle physics, then it's true that string theorists have been unable to use their theory to make predictions at accessible energies. But the statement that string theory is unrelated to real-world phenomena is completely false. For example, string theory is one of the most important tools for understanding ordinary quantum field theory at strong coupling. A few years ago, it was famously used to compute the viscosity of the quark gluon plasma, an exotic state of matter produced in particle accelerators, and the string theory calculations were in good agreement with observations.