Science

Via JenLuc Piquant’s twitter feed, here’s one time I’m not going to stick up for my colleagues in the social sciences: a misguided attempt to cast the search for empirical support as “physics envy.” It’s a New York Times Op-Ed by Kevin Clarke and David Primo, political scientists at the University of Rochester.

There is something rightly labeled “physics envy,” and it is a temptation justly to be resisted: the preference for reducing everything to simple and clean quantitative models whether or not they provide accurate representations of the phenomena under study. The great thing about physics is that we study systems that are so simple that it’s quite useful to invoke highly idealized models, from which fairly accurate quantitative predictions can be extracted. The messy real world of the social sciences doesn’t always give us that luxury. The envy becomes pernicious when we attack a social-science problem by picking a few simple assumptions, and then acting like those assumptions are reality just because the model is so pretty.

However, that’s not what Clarke and Primo are warning against. Their aim is at something altogether different: the idea that theories should be tested empirically! They write,

Many social scientists contend that science has a method, and if you want to be scientific, you should adopt it. The method requires you to devise a theoretical model, deduce a testable hypothesis from the model and then test the hypothesis against the world…

But we believe that this way of thinking is badly mistaken and detrimental to social research. For the sake of everyone who stands to gain from a better knowledge of politics, economics and society, the social sciences need to overcome their inferiority complex, reject hypothetico-deductivism and embrace the fact that they are mature disciplines with no need to emulate other sciences…

Unfortunately, the belief that every theory must have its empirical support (and vice versa) now constrains the kinds of social science projects that are undertaken, alters the trajectory of academic careers and drives graduate training. Rather than attempt to imitate the hard sciences, social scientists would be better off doing what they do best: thinking deeply about what prompts human beings to behave the way they do.

Sorry, but “thinking deeply” doesn’t cut it. People are not especially logical creatures, and we’re just not smart enough to gain true knowledge about the world by the power of reason alone. That’s why empiricism was invented in the first place, and why it’s been so spectacularly successful over the last few centuries.

Clarke and Primo seem to confuse “the need for empirical testing” with “the need for every model proposed to be backed up by data before it gets published.” If they had stuck to rejecting the latter narrow idea, they would have had a decent case. Certainly we physicists don’t require that every model be supported by data before it is published — otherwise my CV (and those of most of my friends) would be a lot shorter! But we all agree that the ultimate test of an idea is a confrontation with data, even if a theory might be too immature for that confrontation to take place just yet.

Zooming around the LHC, colliding at unprecedentedly high energies: 8 trillion electron volts total, in comparison with last year’s 7 TeV. The ultimate goal is to reach an amazing 14 TeV, although that won’t happen soon — the plan is to shut down for quite a while after the end of this year’s run, tighten the gaskets and so forth, and then resume the march to higher and higher energies.

This year’s run is all about luminosity, i.e. getting as many collisions in the can as they can. Last year they reached about 5 inverse femtobarns, while this year they’re shooting for 15 inverse femtobarns. Yes, those are the goofiest units in all of physics. Think of it this way: imagine the protons entering a detector are shooting at a tiny target with some fixed size, measured in units of area. Then we can measure the luminosity by counting the number of protons passing through that area in a fixed moment of time: i.e., the number of protons per square centimeter per second. That’s at any one moment; if we integrate up over the course of a year, the “per second” disappears and leaves us with the total number of protons that have passed through the target area, i.e. a certain number of protons per square centimeter. But that number would be enormously huge, so rather than using square centimeters, particle physicists like to use “barns,” defined as 10^-24 cm². (Broad side of a barn, get it?) But even measuring the luminosity in inverse barns would be really big, so they go for inverse femtobarns (1 fb = 10^-39 cm²). Long story short: 10 inverse femtobarns is equivalent to 10⁴⁰ protons passing through a 1 cm target area. (That’s much larger than the number of collisions — to get the number of collisions for any particular process, you need to multiply by the cross-section for that process, which is often quite tiny. That’s why particle physics is hard! Still, there will be a buttload of collisions.)

Anyway, I’m pretty sure the LHC is back to colliding protons after this year’s winter shutdown, and they’re smashing together at 8 TeV. But to be honest my only hard evidence is from Twitter, where the ATLAS collaboration has tweeted this image.

Meanwhile, results are still coming out from last year’s run. Sadly, they’re doing a great job at constraining possible new physics, but no convincing discoveries as yet. Here’s a recent result from LHCb, the experiment that looks at decays of mesons containing b quarks. This plot is from David Straub, from a talk at Moriond, based on this paper.

Horizontal axis is the fraction of time (the branching ratio) bottom/strange mesons decay into two muons, while the vertical axis is the fraction of time bottom/down mesons do the same thing. These numbers have specific predictions within the good old Standard Model, but it’s very easy for new physics such as supersymmetry to enhance the numbers quite a bit. LHCb has put an upper limit on both quantities, which rules out all the gray area of the plot, leaving only the colorful part at the bottom left. The colors correspond to possible predictions in different versions of supersymmetry. As you see, it would have been very easy to have detected a substantial deviation from the Standard Model by now, but no such luck. This doesn’t mean some other version of supersymmetry isn’t right, just that we’ll have to try harder. No question that a proper update of our likelihood functions will have to decrease the chance that we expect fo find SUSY at the LHC compared to what we would have thought a few years ago, however. This is why the march to higher energies will be so important.

If you want to ask some detailed questions about the accelerator and the experiment, the CMS and ATLAS collaborations are having a Google+ hangout this Wednesday that you are welcome to join. It starts at 7 am Los Angeles time, so I’m unlikely to make it, but let us know if anyone here participates.

David Tong, a theoretical physicist at Cambridge, is excited about solitons. And he wants to share that excitement with you, and he’s willing to climb in a bathtub to do it.

It’s a fun video, produced by the Institute of Physics. David’s interest is really in the issue of quark confinement in QCD, one of the Clay Millenium Prize problems. But we get there by thinking about bubbles and vortices and smoke rings. Worth a look.

We all knew that when the OPERA experiment announced preliminary evidence that neutrinos were traveling faster than the speed of light, the result was so hard to swallow that independent confirmation from other experiments would be necessary before too many people jumped on the bandwagon. In the meantime, a number of theoretical papers pointed out difficulties in accepting the result at face value (probably the cleanest by Cohen and Glashow). And just last month OPERA itself announced that they had located a couple of possible systematic errors in their experiment, without actually backing off the original result. But lets just say things haven’t been looking good.

Now we have what might be the nail in the coffin: another experiment, ICARUS, at the same laboratory in Gran Sasso in Italy, has reported an independent measurement of the neutrino time-of-flight from CERN. (The CERN twitter feed points to an frustratingly vague press release; more useful info from Tommaso Dorigo.) Answer: spot on the speed of light. They even have a paper on the arxiv, from which we get this lovely plot:

Colloquially, we would say “game over, man.” The new measurements sit spot on the speed of light (zero on the plot), and are inconsistent with OPERA. (Actually neutrinos have tiny masses and therefore move just a bit slower than light, but it’s close enough as to be invisible in this plot.) Note that ICARUS had previously “refuted” OPERA, but in a much more indirect way, by checking that the neutrinos hadn’t lost any energy along the way. This new result is a straight-up check of the original claim, and it falls short.

As Tommaso points out, the precision of the ICARUS result is comparable to that of OPERA, so if you live in a mental space free of theoretical priors you could assign 50/50 weight to each one. Those of us in the real world should be ready to accept that the speed of light isn’t just a good idea: it’s the law.

I know you’re all following the Minute Physics videos (that we talked about here), but just in case my knowledge is somehow fallible you really should start following them. After taking care of why stones are round, and why there is no pink light, Henry Reich is now explaining the fundamental nature of our everyday world: quantum field theory and the Standard Model. It’s a multi-part series, since some things deserve more than a minute, dammit.

Two parts have been posted so far. The first is just an intro, pointing out something we’ve already heard: the Standard Model of Particle physics describes all the world we experience in our everyday lives.

The second one, just up, tackles quantum field theory and the Pauli exclusion principle, of which we’ve been recently speaking. (Admittedly it’s two minutes long, but these are big topics!)

The world is made of fields, which appear to us as particles when we look at them. Something everyone should know.