A layman question: Regarding string theory, is it really true that there’s a total dearth of any testable empirical predictions? At least for the foreseeable future of our experimental reach?Aaronson answers:
What I think is uncontroversial is this: there are various phenomena that, if they were observed (either at the LHC or in cosmology), would provide an enormous boost to string theory. The most important and plausible of those is supersymmetric particles; but there are also more exotic possibilities, like large extra dimensions and cosmic strings. The central problem is that, if those phenomena are not observed (as indeed they haven’t been, despite searches for them), that doesn’t falsify string theory at all. For it’s perfectly compatible with string theory that these phenomena would only show up at much higher energies than are currently accessible to us.This is like saying that there are experimental findings that would support telekinesis, and they are never seen, but that's no reason to doubt telekinesis.
Mathematician Greg Kuperberg tries to answer:
There is a “dearth” of *directly* testable empirical predictions in string theory, but it’s misleading to use the word “dearth”. It’s been acknowledged from the beginning that it’s difficult to make any new predictions from any reasonable model of quantum gravity. What theorists can do, however, is offer a better explanation of the evidence that already exists. That is the main driving force of string theory: How to keep quantum mechanics and general relativity from contradicting each other; and how to keep the Standard Model of particle theory from contradicting itself. String theory is an incomplete effort to do that, but it is also “the only game in town”. It is the only known attempt at quantum gravity that isn’t in the hospital and close to flatlined. And, resolving the Standard Model of particle physics would be extra icing on the cake.That is, string theory needs no experimental evidence because there is no known observational conflict between quantum mechanics and general relativity. And while string theory is not consistent with any known physics, the competitors are not either, and are not as fashionable. Just like theoretical quantum computation, which also has very little known relevance to experiment.
Of course, any kibbutzer can talk about the problems of “theory without experiment”. But the truth is that the best theorists can get pretty far with just parsimony and logical consistency. Both general relativity and cosmological inflation — both of which are cousins of quantum gravity and string theory — were correctly developed before any help came from experiment. In the case of inflation, 20 years before good experimental confirmation.
Indeed, quantum computation has the same intellectual feature. Theoretical quantum computation gets a little help from experiment, but really hardly any. It is for the most part a compelling extrapolation of existing physics.
As a historical argument, it is not true that general relativity was "correctly developed before any help came from experiment." First, special relativity arose directly from experiment, as expressed at the time:
The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. – Hermann Minkowski, 1908Second, general relativity also follows from those same experiments, in spite of the mathematical difficulty. Lubos Motl explains:
While it's true that special relativity is a limit of general relativity obtained for gravitational fields going to zero, the actual "hierarchy of power" is the opposite: general relativity is just one application of special relativity – the incorporation of the gravitational field in a special-relativity-invariant way. While general relativity is arguably the prettiest (and geometrically most non-trivial) classical application of the rules of special relativity, in principle it is on par with Yang-Mills theory or any other (special) relativistic field theory.Third, Einstein was driven to explain Mercury's orbit anomaly, following Poincare's relativistic partial explanation. So the development of general relativity was driven by experimental evidence, just like all the other great scientific theories.
Cosmological inflation has not really been confirmed, and remains just a loose collection of ideas for how the early universe might have expanded. There has been no Nobel prize for it.
I am a stickler for the history of relativity because it is always being misrepresented to justify bad science.