Over the weekend, someone said to me, breathlessly, that they’d read that “Results from the Large Hadron Collider [LHC] have blown string theory out of the water.”
Good Heavens! I replied. Who fed you that line of rubbish?!
Well, I’m not sure how this silliness got started, but it’s completely wrong. Just in case some of you or your friends have heard the same thing, let me explain why it’s wrong.
First, a distinction — one that is rarely made, especially by the more rabid bloggers, both those who are string lovers and those that are string haters. [Both types mystify me.] String theory has several applications, and you need to keep them straight. Let me mention two.
- Application number 1: this is the one you’ve heard about. String theory is a candidate (and only a candidate) for a “theory of everything” — a silly term, if you ask me, for what it really means is “a theory of all of nature’s particles, forces and space-time”. It’s not a theory of genetics or a theory of cooking or a theory of how to write a good blog post. But it’s still a pretty cool thing. This is the theory (i.e. a set of consistent equations and methods that describes relativistic quantum strings) that’s supposed to explain quantum gravity and all of particle physics, and if it succeeded, that would be fantastic.
- Application number 2: String theory can serve as a tool. You can use its mathematics, and/or the physical insights that you can gain by thinking about and calculating how strings behave, to solve or partially solve problems in other subjects. (Here’s an example.) These subjects include quantum field theory and advanced mathematics, and if you work in these areas, you may really not care much about application number 1. Even if application number 1 were ruled out by data, we’d still continue to use string theory as a tool. Consider this: if you grew up learning that a hammer was a religious idol to be worshipped, and later you decided you didn’t believe that anymore, would you throw out all your hammers? No. They’re still useful even if you don’t worship them.
BUT: today we are talking about Application Number 1: string theory as a candidate theory of all particles, etc.
Now what’s so silly about this notion that the LHC has ruled out string theory is that the whole reason a lot of people hate string theory is that it doesn’t make any testable predictions! So obviously you can’t rule it out with current experiments… that would require testable predictions!
Caution! Do not interpret the above bold-faced statement to mean that string theory as a theory of all particles etc. makes no predictions at all. String theory certainly makes predictions. For instance (taking string theory in its most vanilla form), string theory predicts that all particles are actually tiny strings. And if you were to take one electron with a motion-energy of a million million million GeV or more, and take another electron with similar energy but moving in the opposite direction, and slam them into each other, you would find the scattering is governed by the formulas for how relativistic quantum strings bounce off each other. Specifically,
- The probability that they would scatter at large angle is extremely small.
- The probability that they scatter at a small angle has a bell-shaped distribution centered around zero angle; the width of the bell shrinks as the energy increases.
- The overall scattering rate increases with energy.
- And since all particles (not just electrons) are strings, if string theory is right, the same results should hold for scattering of any two particles, not just electrons.
These features are very different from what you’d expect if particles had no detectable size or shape. Also, the formulas are more precise than my words, so these are things you could check in detail, not just qualitatively.
There’s only one problem: although these predictions would allow definitive test of the theory, the tests can’t be carried out with current or currently imaginable technology. In current experiments, the scattering energy is far too low to detect whether the particles of nature are tiny strings, as small as (vanilla) string theory would suggest. The pattern of scattering we see when two electrons scatter is what you’d expect if particles had no intrinsic size or shape. In short, we can’t actually build a particle accelerator capable of testing these predictions; our technology is about a thousand million million times below where it would need to be, so this isn’t going to happen soon. Nor does nature provide a natural laboratory; cosmic rays aren’t nearly energetic enough. The problem is practical, not one of principle.
So why would anyone even think for a moment that string theory was ruled out by the LHC? It apparently comes from the following line of argument.
- String theory predicts supersymmetry.
- Supersymmetry has been ruled out by the LHCb experiment at the LHC, so string theory is ruled out too.
Both of these statements are wrong, in multiple ways.
First, does string theory predict supersymmetry? Even if supersymmetry is present in nature, there is nothing in string theory that predicts that supersymmetry lies hidden at the energies that can be accessed by the LHC; i.e., even if there exist superpartner particles, they may be far, far too heavy for the LHC to find. Furthermore, although supersymmetry is in some subtle ways built deeply into string theory (as we currently understand it), that doesn’t mean that it shows up in a simple way: it is not necessarily the case that there will be a recognizable superpartner particle for every particle, as traditional supersymmetry predicts.
The only situation in which we expect supersymmetry to show up at the LHC is if supersymmetry is solving the naturalness problem of the Standard Model, and thus stabilizing the hierarchy between the extreme weakness of gravity and the strengths of the other forces. For short, let’s call this “natural supersymmetry”. If supersymmetry exists in nature but has nothing to do with the naturalness problem, we don’t have any reason to expect to find any sign of it at the LHC.
Second, did LHCb rule out supersymmetry? Well, I just told you that’s impossible, because if supersymmetry exists but natural supersymmetry does not, then superpartners could be far, far, far out of reach of the LHC. But did LHCb rule out natural supersymmetry? No; I’ve said this twice before, and I’ll just send you to the links (here and here). What has actually come close to ruling out natural supersymmetry (but we’re not quite there yet) are the results of the searches for superpartner particles done at ATLAS and CMS. LHCb is just trying to steal their thunder, but their claims in the press are bombastic at best and inaccurate at worst.
Still, even if LHCb, ATLAS and CMS together ruled out natural supersymmetry definitively (and that won’t happen til 2016, most likely), that would not rule out supersymmetry (which might exist but not have anything to do with the naturalness problem of the Standard Model) or string theory (which might not even exhibit supersymmetry in any usual way).
So forget it. The LHC will not rule out string theory; it won’t even come close. The machine is powerful, but not even remotely powerful enough to do this. It’s like asking an ameoba to climb Mount Everest.
[Note Added for experts regarding making predictions in string theory: In response to complaints by commenter Peter Woit (see discussion below in the comments) let me reiterate the importance of my remark that I was taking string theory in its vanilla form. Keep in mind that quantum field theory in general doesn't predict how protons scatter or whether they even exist --- you have to choose a specific quantum field theory, or at least a specific class of such theories, before you can do that. Similarly, string theory in general doesn't predict how particles scatter. But if you choose a specific vacuum of string theory, or a specific class of such vacua, then you can make predictions. (Not that we understand string theory well enough yet to understand the different vacua in detail --- another practical problem.) The `vanilla form' of string theory is a class of vacua in which the particles of nature are tiny strings that have no very strong forces acting on them. Tell me we're in such a vacuum: then I can make predictions. In other vacua, I'd make other predictions. In any vacuum, predictions can be made, though the answers depend, to a greater or lesser degree, on which vacuum you are in. Presumably, even if string theory is right, only an interplay between experiment and theory can tell us which vacuum we might be in, just as such an interplay was necessary to tell us which specific quantum field theory to use in particle physics.]