Archive for October, 2011

Faster-than-light neutrino results explained?

Thursday, October 20th, 2011

Note: The original version of this post was completely, embarrassingly wrong. I replaced it with a new version that says pretty much the opposite. Then Louis3 in the comments pointed out that I had misunderstood the van Elburg preprint yet again, but, if I’m not mistaken, that misinterpretation on my part doesn’t fundamentally change the argument. I hope I’ve got it right now, but given my track record on this I wouldn’t blame you for being skeptical!

If you’re reading this, you almost certainly know about the recent announcement by the OPERA group of experimental results showing that neutrinos travel slightly faster than light. I didn’t write about the original result here, because I didn’t have anything original to say. I pretty much agreed with the consensus among physicists: Probably something wrong with the experiments, extraordinary claims require extraordinary evidence, Bayesian priors, wait for replication, etc.

Recently, there’s been some buzz about a preprint being circulated by Ronald van Elburg claiming to have found an error in the OPERA analysis that would explain everything. If you don’t want to slog through the preprint itself (which is short but has equations), this blog post does a good job summarizing it.

van Elburg’s claim is that the OPERA people have incorrectly calculated the time of flight of a light signal between the source and detector in the experiment. (This is a hypothetical light signal, used for reference — no actual light signal went from one place to the other.) He goes through a complicated special-relativity calculation involving switching back and forth between an Earth-fixed (“baseline”) reference frame and a reference frame attached to a GPS satellite. I don’t understand why he thinks this complicated procedure is necessary : the final result is a relationship between baseline-frame quantities, and I don’t see why you can’t just calculate it entirely in the baseline frame. But more importantly, his procedure contains an error in the application of special relativity. When this error is corrected, the discrepancy he claims to have found goes away.

As a mea culpa for getting this completely wrong initially (and also for the benefit of the students in a course I’m teaching now), I’ve written up a critique of the van Elburg preprint, in which I try to explain the error in detail. I find it cumbersome to include equations in blog posts (maybe I just haven’t installed the right tools to do it), so I’ve put the critique in a separate PDF document. I’ll just summarize the main points briefly here.

van Elburg calculates the time of flight between source and detector in the following complicated way:

  1. He relates the satellite-frame source-detector distance to the baseline-frame distance via Lorentz contraction.
  2. He calculates the flight time in the satellite frame (correctly accounting for the fact that the detector is moving in this frame — which is what he claims OPERA didn’t do).
  3. He transforms back to the baseline frame.

At the very least, this is unnecessarily complicated. The whole point of special relativity is that you can work in whatever inertial frame you want, so why jump back and forth this way, rather than just doing the calculation in the Earth frame? In fact, I originally (incorrectly) thought that he’d done the calculation correctly but in an unnecessarily cumbersome way. It turns out that it’s worse than that, though: his calculation is just plain wrong.

The main error is in his equation (5), specifically when he writes

This is supposed to relate the time of flight in the satellite frame to the time of light in the Earth frame. But the time-dilation rule doesn’t apply in this situation. It’s only correct to calculate time dilation in this simple way (multiply by gamma) if you’re talking about events that are at the same place in one of the two reference frames. The standard example is two birthdays of one of the two twins in the twin paradox. When you’re considering two birthdays of the rocket-borne twin, you’re considering two events that are at the same place in the rocket frame, and the multiply-by-gamma rule is fine.

But in this case the time intervals under consideration are times of flight. That means that they’re time intervals between one event at one place (radio wave leaves the source) and another event at another place (radio wave arrives at detector). To properly relate time intervals of this sort in two different frames, you need the full machinery of the Lorentz transformation. If you use that full machinery to convert from satellite frame to Earth frame, you find that the time of flight comes out just the way you’d expect it to if you’d done the whole calculation in the Earth frame to begin with. (Of course it had to be that way — that’s the whole point of the principle of relativity.)

Now if the OPERA people had done their analysis the way van Elburg does (jumping back and forth with wild abandon between Earth and satellite frames), and if when they were in the satellite frame they had calculated a time of flight without accounting for the detector’s motion, then they would have been making an error of essentially the sort van Elburg describes. But as far as I can tell there’s no credible evidence, either in this preprint or in the OPERA paper, that they did the analysis this way at all, let alone that they made this error.

So this explanation of the OPERA results is a non-starter. Sorry for originally stating otherwise.

The passive voice can be used

Wednesday, October 19th, 2011

I’m trying to be pretty rigorous in evaluating my students’ writing, but one thing I’m not telling them is to avoid the passive voice. I think that the avoid-the-passive rule, despite its popularity among writing teachers and in usage guides, is pretty much a superstition. It’s a marginally more worthwhile rule than other superstitions such as avoiding split infinitives, but only marginally.

Lots of people disagree with me about this, as I found when I participated in a big discussion of this on my brother’s Facebook wall recently. (I also seem to end up discussing the Oxford comma with surprising frequency on Facebook. No doubt once this information gets out I’ll be deluged with friend requests.) So I was glad to see this spirited defense of the passive by linguist Geoffrey Pullum recently.

Pullum also wrote a blistering takedown of Strunk and White a while back. I had mixed feelings about that one, but I think he’s got things write in the passive-voice piece.

Scientists are often taught to write in the passive in order to de-emphasize the role of the experimenter. You’re supposed to say “The samples were collected” instead of “We collected the samples,” because it’s not supposed to matter who did the collecting. Personally, I think this is another superstition, roughly equal in silliness to the no-passive-voice superstition. Ignore them both, and write whatever sounds best. (In most cases like the above,  I think that the active-voice construction ends up sounding more natural.)

I have heard one cogent argument in favor of teaching the avoid-the-passive rule: even if write-whatever-sounds-better is a superior rule, it’s not one that most inexperienced writers are capable of following. They need firm rules, even if those rules are just heuristics which they’ll later outgrow.

There’s some truth in this, and as long as we’re all clear that avoid-the-passive is a sometimes useful heuristic, as opposed to a firm rule, I have no major objections. But there are so many exceptions to this rule that I’m not convinced it’s all that good even as a heuristic. As Pullum points out, Orwell’s essay warning against the passive is itself 20% passive.

At least in the case of my students, overuse of the passive doesn’t seem like one of the top priorities to address. If I’m looking for heuristics to help them improve their writing, this one wouldn’t be near the top of the list. Here are two better ones that come immediately to mind:

  • Cut out all intensifiers (“very”, “extremely”, etc.), unless you have a good reason for them.
  • If you feel the need to include a qualifier like “As I mentioned earlier,” then the sentence in question probably doesn’t need to be there at all.
(Rules like these can be hard to follow. I initially wrote “a very good reason” in the first one, for instance.)

Addenda: Libby rightly points out “In other words” as a marker for the sort of thing I’m talking about in the second “rule.” A couple more I’d add to the list:

  • Don’t use fancy words for their own sake, especially if you’re in any doubt about the word’s precise meaning. Plain, familiar words are just fine.
  • Read your work aloud. Often, a sentence that look OK on the page sounds unnatural when you hear it.
  • If you’ve got a really long paragraph (at a rough guess, greater than about 200 words), chances are that you’ve muddled together several different ideas, each of which deserves its own paragraph.
One final point, emphasized by Pullum: An additional problem with teaching the avoid-the-passive rule is that most people don’t find grammar intuitive and don’t even recognize passive constructions correctly a lot of the time. (This is the place where his takedown of Strunk and White is most compelling. Even they get it wrong most of the time.) The avoid-the-passive rule seems to be meant as a simple proxy for more difficult rules, but it’s not even simple for most people in the target group.

A scientist teaching writing

Tuesday, October 11th, 2011

I’m teaching a first-year seminar this semester, which is quite a different sort of course for me. We’re at the halfway point in the semester, which seems like as good a time as any to reflect a bit on how it’s going.

First, some background. First-year seminars replaced the Core course we used to require of all entering students (a change I strongly supported, by the way). Under the current system, all students have to take a first-year seminar each semester of their first year. These courses cover a wide variety of topics, based on faculty interest and expertise, but they’re all supposed to have certain things in common. Perhaps the most important of these is that all seminars are “writing-intensive.”

My seminar is called “Space is Big.” It’s about how ideas about the size of the Universe have changed over time, focusing on three periods: the Copernican revolution, the early 20th century, and the present.

So what do I have to say at the halfway point?

First, reading and grading essays takes a lot of time. It’s much harder than grading problem sets and exams. This is not a surprise, of course. The most time-consuming part is writing comments on each essay. I find pretty detailed comments are necessary, both to clarify my own thinking about why I’m giving the grade I am and more importantly to give the student guidance for improvement.

There are some teaching duties we scientists have that others don’t (designing labs, for instance). When we feel like complaining about that sort of thing, we should remember how much easier we generally have it when it comes to grading. (Not that we’ll stop complaining. Complaining is one of the great joys of life. It sets us apart from the animals.)

Based on my experience so far, the main problems our students have with their writing involve organization and structure: they’re pretty good at the individual sentence level, but they sometimes have trouble combining those sentences in a coherent way. The most common serious flaw in my students’ essays is the long, rambling paragraph that contains lots of true facts in no discernible order. Other problems include unnecessary repetitiveness and puffed-up, diffuse phrases that add no meaning. (I should add that not all of my students have these problems: some of them write quite well.)

This should be reassuring to my science colleagues, some of whom are convinced that they’re not qualified to teach writing because they don’t know rules of grammar and usage. True, some science professors do get confused about grammar and usage in ways that you wouldn’t expect to see from, say, an English or history professor. (Present company, excluded, of course! I’m a bit of a usage geek, and while I have many flaws, you’re not likely to catch me in a comma splice.) But based on my experience, the main sort of help students need with their writing concern structuring an argument clearly, logically, and concisely, not misplacing apostrophes.

We as scientists are perfectly qualified to teach and evaluate writing in this sense. We spend huge amounts of our time writing and evaluating other people’s writing (papers, grant proposals, etc.). We wouldn’t have gotten anywhere in science without skill in both these areas. That’s not to say that teaching and evaluating writing is easy — for scientists or anyone else. But we can do it.

And by the way, for those who are concerned about gaps in their ability to teach grammar and usage (or other aspects of writing), the University’s Writing Center has good support for faculty and students.

This course is far more work than a “normal” course of the sort I’m used to, but on the whole it’s been fun, mostly because I get to read and think about familiar subjects in a new way. I urge my science colleagues not to be scared to try it out.