String theory is exciting stuff: the idea that the universe we perceive with all its particles and forces is merely a manifestation of a tiny strange world of vibrating strings curled up in dimensions too small for our current experiments to reach. It’s easy to see why it has captured the public’s imagination, and why string theory evangelists like Brian Greene and Steven Hawking are well-known names despite the fact that string theory is exceedingly mathematical and complicated—and I say this as someone who studies general relativity!
You can probably already sense the “however” that’s coming. String theory has been around for over 30 years, but it has yet to produce a single measurable, testable prediction. Despite a lot of brilliant physicists’ efforts, despite the beauty and elegance of the ideas, string theory really cannot even be considered a theory in the usual sense. A theory to a scientist is a framework of ideas that makes testable predictions about the world; string theory (beyond the basic concepts: existence of strings, number of dimensions, etc.) comes in too many varieties to count, fails to produce even the same results that successful particle physics theories have, and hasn’t yet said anything new that we can look for. Part of the problem is the scale of the theory: strings, if they exist, are curled up so small that it would take a lot of energy to uncurl them—many times more energy than our most powerful particle colliders can manage. But that’s not the sole problem: it would be nice if string theory would have testable effects on scales we can test, even if the strings themselves are not reachable.
None of this is to say that string theory is false or wrong. The problem is both better and worse than that: string theory is neither testable (because it doesn’t make testable predictions) nor falsifiable (you can’t rule it out for the same reason).* So it could very well be right! But it’s not really a theory yet: it’s more a proto-theory, one that might yet make major strides towards fulfilling its vast promise as a true Theory of Everything.
Apart from the theoretical physics aspects, as someone who cares deeply about public science education and outreach I have very mixed feelings about string theory. It’s undeniably cool, and its best evangelists are very good at inspiring interest not just in strings but in science in general. I hesitated to even write this post for that reason: I don’t want to burst the bubble of people who find inspiration in string theory (not that really I have that much power, but you know what I mean). On the other hand, it feels like maybe string theory has such good PR that it’s bound to disappoint as years continue to go by without any form of confirmation.
To many scientists, string theory is the way forward for theoretical physics, but that’s obviously not true—many alternatives exist, though inevitably not many look promising. Perhaps we as a community aren’t doing the public a very good service letting string theory hog the limelight. Ultimately the judge is evidence, and until string theory gives us something to compare to that evidence, it’s very premature to promote it as the solution to all the problems in modern physics.
* I should note that string theory is falsifiable in certain domains: since it relies on relativity and quantum theory for its basic structure, if either of these major paradigms fail, string theory will also. (If supersymmetry (SUSY) is shown false, string theory will also collapse, but SUSY is also difficult to rule out completely.) However, a lot more than string theory is at stake in that case!
Galileo's Pendulum 
8 responses to “The Problem of String Theory”
String theory seems to me to be a fancy way of saying that there is a *geometric* basis for the Universe, that is, that all we see is a consequence of various types of entities (the strings) that are mathematical objects. I therefore think that a key test of string theory is whether or not it is capable of calculating the key *dimensionless* numbers of our Universe from a mathematical formula of some sort. In particular, if I saw an announcement that some theoretician had succeeded in deriving the ‘fine structure constant’ from string theory, I would immediately become a believer. We know this number (‘alpha’, ~1/137) to high precision, so it seems to me that this would be a rigorous test for the theory. Do you agree? If so, do you think that there is any reasonable hope for this theoretical result?
Everything you say is correct, but string theory is hardly unique in this regard. All modern theoretical physics is geometric! And of course no modern theory (string theory or otherwise) has succeeded in deriving the fundamental constants from first principles, so in that regard string theory is no worse off than any other theory.
I don’t think string theory will progress at all until its practitioners abandon the “string theory landscape” idea, where every conceivable vacuum state for the universe is included somewhere and we have to use anthropic arguments to distinguish between them. With no possibility for specific predictions, there’s no way to derive the value of the fine structure constant or any other physical parameter—they all have to be put in by hand.
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