I’m always amused at how very reasonable remarks so often generate attacks from unreasonable people. I wrote a perfectly ordinary post about what one does and doesn’t learn from LHCb’s important new measurement at the Large Hadron Collider [LHC] (and in fact I overstated the significance of the result — more on that later), and somehow I touched off a mini-firestorm. Well, that just indicates how essential it is to have calm people expressing sensible points of view. When people become so politicized that they can’t distinguish propaganda from science, that’s not good.
Forget supersymmetry — because none of my remarks have anything to do with this theory in particular, and the theory doesn’t deserve the excessive attention it’s getting. Take any theory: call it Theory X. Extra dimensions; compositeness of quarks and leptons; non-commutative spacetime; grand unification; your-theory-here. The idea behind theory X may be very clever, but as always, there are many variants of theory X, because an idea is almost never precise enough to permit a unique realization. Each variant makes definite predictions, but keep in mind that detailed experimental predictions may very well differ greatly from variant to variant.
Now, here is a logical fact: one of two options is true.
- Option A: One variant of theory X is “correct” (its predictions agree with nature) while all other variants are “wrong” (disagree with nature)
- Option B: All variants of theory X are wrong.
Nature is what it is; there are no other options (and this is not the place for a discussion about this basic scientific assumption, so pace, please, philosophers.). [More precisely about option A: the space of variants is continuous, so the correct statement is that an arbitrary small region in this space is correct; you can put in the correct calculus vocabulary as you like. I'll stick with the imprecise language for brevity.]
For either option, as more and more data is collected, more and more variants of theory X will become “dead” — excluded because of a disagreement with data. Therefore — obviously! — a reduction in the number of live (i.e. unexcluded) models always takes place over time. And this has absolutely no bearing on whether, at the end, all variants of X will be dead, or one (or perhaps several very similar ones) are still alive.
And thus it makes absolutely no sense to describe, as a “blow to theory X” — in particular, to the idea behind theory X — a measurement that excludes (“kills”) even a big fraction, but not virtually all, of the variants of theory X. It’s certainly a blow to those variants; in fact, it is a fatal blow for them. But it does nothing to distinguish between Option A and Option B. It only tells us that if Option A is true, the variant of X that will be alive at the end is not among the ones that have just been killed.
This isn’t rocket science, folks. It’s logic. [Well - As a commenter points out, it's not "logic" in the strictest sense; but it is basic scientific reasoning.] And if we take theory X to be the Standard Model itself, I’ve just described its history.
The counter-argument? “Well, but if we know which variants of theory X are more likely, then if we rule out the likely ones, we can say the theory X is now less likely.”
There’s no point in arguing about this claim, because the premise is typically wrong: we do not typically know which variants of theory X are more likely. Neither general scientific reasoning nor scientific history provide a reliable guide. It’s not necessarily the simplest ones that are more likely; it’s not necessarily the most symmetric ones; it’s not necessarily the ones that most efficiently resolve the problem that the inventors of the idea intended to solve. Go back and read the history of the Standard Model, and tell me with a straight face that theorists 40 years ago would have guessed that nature is the way it is. Actually, many are still alive; you can just ask Steve Weinberg, or Sheldon Glashow, or the Higgs particle inventors, what they thought back then, and whether the variant of the Standard Model that we now find ourselves using was considered likely back in, say, 1974, or 1984. I know from my own experience that it wasn’t considered likely in 1988, when I started graduate school.
Now, a word about the politics. Supersymmetry is kicked around like a football now, and this ridiculous display is an embarrassment to science. The theory, specifically as something we would observe at the LHC, was wildly over-promoted by certain people. And this promotion was strongly resented by certain other people. I’m not interested in this political debate, and I’m not alone among my theory colleagues, in the United States especially. I’m interested in nature. The supersymmetry zealots ought to remember that signs of supersymmetry were quite reasonably expected to show up in the 1980s or 1990s, at the LEP II collider if not before; from many points of view, supersymmetry’s been in increasing question ever since then. The dark matter argument that bolsters it is full of holes; the grand unification argument that bolsters it is compelling but thin, and need not lead to detectable superpartners at the LHC. The naturalness argument (cf. the hierarchy problem) that motivates it has been under some suspicion since the 1998 discovery that the universe’s expansion is accelerating. But meanwhile, the anti-supersymmetry zealots ought to remember that the death (or “hospitalization”?!) of supersymmetry has been declared, by physicists and by the press (and nowadays by bloggers), numerous times already over the past few decades; the latest declaration by the BBC’s reporter is nowhere near the first (see here and here, for instance), and it won’t be the last. Similarly, classic technicolor, which lacks a Higgs-like particle, was declared dead (or hospitalized) many times over my career, at least as far back as the early 1990s, but it really wasn’t until this July, when a particle was discovered resembling a Standard Model Higgs particle, that the evidence against Higgs-particle-less classic technicolor became very strong (and even now there are people who claim a few variants of classic technicolor are still alive.)
In my view, we (scientists) should disregard the politics, and focus on the hard work that actually needs to be done, by experimentalists and by theorists, to make sure we close as many of the remaining loopholes as we can, for as many types of theories as we can. Certain outsiders who aren’t actually doing anything useful will say we’re wasting our time, and some will say that the Standard Model is “obviously” true, but we have to ignore their absurd and irresponsible statements, and defend our right and our obligation to do our job properly. And that job is to test the Standard Model at the LHC in every way possible, prodding it and checking it, excluding as many variants of every reasonable theory X that we can think of, until we have squeezed every available drop of information out of the LHC’s data.