String theory has come under attack because some say it can never be tested; the strings are supposed to be smaller than any particle ever detected, after all. But Arkani-Hamed says the Large Hadron Collider could lead to the direct observation of strings, or at least indirect evidence of their existence.
A recent New York Times article ends with another Arkani-Hamed quote about what to expect at the LHC:
He pointed out that because of the dice-throwing nature of quantum physics, there was some probability of almost anything happening. There is some minuscule probability, he said, “the Large Hadron Collider might make dragons that might eat us up.”
Obviously I’m being unfair to put these two quotes together, but they both raise a basic question about the philosophy of science. When can we legitimately say that a theory is testable and makes a scientific prediction? The most straightforward examples of scientific predictions are cases where we have high confidence that a certain experimental result has to happen if a theory is right: such a theory satisfies Popper’s falsifiability criterion. But many theoretical ideas are not so tightly constrained, and compatible with a range of possibilities. This range generally comes with some notion of probability: certain experimental results are more likely to come out of the given theory, others less likely. This may allow you to gain confidence in a theory even if it is not falsifiable, by seeing things that the theory says are more likely, not seeing the things it says are unlikely. The problem with the idea that the LHC is going to test string theory by seeing strings is that according to the standard framework of string theory, this is just very unlikely. Saying that an experiment is going to test your theory when it is extremely unlikely that it will provide any evidence for it or against it is highly misleading. You’re always free to say “this experiment is unlikely to test my theory, but who knows, I may get incredibly lucky and something unexpected will come out of it that will vindicate me”. But that’s not really a “test” of your theory, that’s wishful thinking.
There’s a new article in New Scientist closely related to this by Robert Matthews entitled Do we need to change the definition of science?. It’s about claims being made that multiverse studies show that we need to re-examine conventional ideas about what is science and what isn’t. I’m quoted saying the sort of thing that you might expect:
I never would have believed that serious scientists would consider making the kinds of pseudoscientific claims now being made…
an outrageous way of refusing to admit failure…
The basic problem with the multiverse is not only that it makes no falsifiable predictions, but that all proposals for extracting predictions from it involve massive amounts of wishful thinking.
Max Tegmark argues against a straw man:
Some people say that the multiverse concept isn’t falsifiable because it’s unobservable – but that’s a fallacy
noting that just because some implications of a theory aren’t directly observable doesn’t mean the theory is untestable. If a theory passes many convincing tests involving things we can observe, and the theoretical structure is tight enough, then we have good evidence about what is likely to be going on with phenomena we can’t observe. This is certainly true: if the string theory landscape made lots of testable predictions so that we had good reason to believe in it, and the same structure implied a multiverse, that would be good reason to believe in the multiverse. The problem is that the landscape makes no predictions and we have no reason to believe in it. It’s not a real testable scientific theory, rather an untestable endpoint of a failed theory. As such it implies nothing one way or another about the existence of a multiverse.
Matthews quotes various people arguing for a “Bayesian” view of science, that what is going on is that experimental observations probabilistically provide evidence for and against theories, with the falsifiability case of probability zero or one not usually occurring. This may be a good way of thinking about how science actually works. But by this criterion, string theory unification and the multiverse remain pseudo-scientific, as no one has been able to come up with proposed experimental tests that have a significant chance of providing such evidence for or against these theories.