Turtles and Strings: Where Does Science Stop?

The infamous Davies op-ed has been collected together with some responses at edge.org, and one of the responses is by Sean Carroll, who reproduces his response at Cosmic Variance. Sean’s a smart guy, and I basically agree with his argument, but I’m a contrary sort, and want to nitpick one thing about his response.

He builds his response around the question, raised by Davies, “Why do the laws of physics take the form they do?” He considers and discards a few responses, before writing:

The final possibility, which seems to be the right one, is: that’s just how things are. There is a chain of explanations concerning things that happen in the universe, which ultimately reaches to the fundamental laws of nature and stops.

That’s fine, as far as it goes. My nit-pick is that this isn’t helpful. More importantly, it’s not even consistent with other things that Sean has written in the past.

Way, way back in 2005, Sean wrote a passionate argument for string theory, which starts off with:

String theory, with all of its difficulties, is by far the most promising route to one of the most long-lasting and ambitious goals of natural science: a complete understanding of the microscopic laws of nature. In particular, it is by far the most promising way to reconcile gravity and quantum mechanics, the most important unsolved problem in fundamental physics.

He’s made essentially the same argument many times since. We need string theory, he argues (as do many others), because general relativity and quantum mechanics do not play well together. They work on completely different theoretical bases, and string theory may be the best hope of bringing them together into a single theoretical framework.

My question is, if “that’s just the way it is” is sufficient to cut off inquiry into fundamental questions of natural law before the level where Davies wants to operate, why isn’t it also sufficient to cut off inquiry before you get to string theory? Why do general relativity and quantum mechanics have such different bases? That’s just the way it is. Drive home safely.

Sean’s response is very well-written, and presented with great conviction. It is not, however, a useful principle for deciding what questions are open to scientific inquiry. Unless I missed a memo over the holiday weekend, and Sean is now the ultimate arbiter of what is and is not science (and wouldn’t that make Peter Woit and Lubos Motl both froth at the mouth…).

If we’re going to say that there are certain questions about which science can’t say anything meaningful, and thus head off Paul Davies’s arguments about emergent natural laws and consciousness, we need some more useful way to make a determination of what science can and can’t talk about. I wish I had one to offer, but I’m not even willing to offer myself in Sean’s place as the Ultimate Arbiter– I have a book to write.

A tentative step in the right general direction might be to say that for something to be a legitimate subject of scientific inquiry, there has to be a model (or set of models) that makes some useful prediction of observable phenomena from the theory. If there’s no conceivable way to distinguish between models, then it’s not really a scientific subject, and it’s ok to stick with “that’s just how things are,” at least until somebody comes up with a model predicting observable phenomena.

That way lies the swamp of “Is string theory really science?,” though, so maybe we’re better off with letting Sean decide, after all.