Ted Bunn’s Blog

The guys at Car Talk got a math-physics based puzzler wrong this week.  You can hear or read the question and their answer at their Web site.

The puzzler in question is labeled week of Jan. 17 as of now, but it should really be the week of Jan. 15 — in fact, the original question aired here in Richmond on the 14th. Anyway, it’s the one about how to tell when a cylindrical gas tank is 1/4 full.

Here’s what I wrote to them:

Sorry, but you got the answer to last week’s puzzler (i.e., the puzzler from Jan. 14, 2011) wrong. You assumed that the center of mass of an object has the property that there are equal amounts of mass on both sides, but that’s not true.

To convince yourself of this, think of the following example: take a 100-pound weight, and a 200-pound weight, and join them together with a long bar. The weight of the bar is small, say 1 pound. Now where’s the center of mass of this funny asymmetrical dumbbell? It’s somewhere between the two masses, about 2/3 of the way along the bar. But there’s certainly not the same amount of weight on the two sides of that point: there’s over 200 pounds on one side, and just over 100 pounds on the other.

In the case of a semicircle, the difference isn’t as dramatic as that, but there still is a difference. The center of mass is about 42.4% of the way out from the center (4/3pi, if you must know). The correct answer (i.e., the point where half the mass is on one side and half the mass is on the other) is only 40.3% of the way out.

Unfortunately, I don’t know a good way to answer the original question: I can’t think of a way to get the 40.3% value without some annoying calculus.

Update: Ray admits he got it wrong.

 I learned (from the Cosmic Variance blog) about a Web site with interesting discussions of a wide variety of questions in physics. It reminded me of some parts of my misspent youth.

Back in the 1990s, I spent a lot of time reading and posting articles on the various physics and astronomy Usenet newsgroups. For those who don’t know, newsgroups are forums in which people can discuss a huge variety of topics. They allowed freewheeling electronic communication back in the days before blogs, and even before the invention of the Web. Last time I checked, Usenet newsgroups still existed, but with all the other options out there nowadays, they don’t play the same role they used to.

Newsgroups had participants with a wide variety of backgrounds and interests. Participants in the physics newsgroups included graduate and undergraduate students, professional scientists, and tons and tons of interested laypeople. The freedom of anybody to participate was both a blessing and a curse. It made for a lot of variety, of course, but it also meant that discussions could easily be hijacked by crackpots. Some of the crackpots were highly entertaining, but eventually most serious people would get frustrated with all the noise and go away.

One solution to this problem was moderated newsgroups. I was one of the moderators of the group sci.physics.research for quite a while. One of us moderators would have to approve each post before it could appear in the newsgroup. The big problem with this, of course, was that it was labor-intensive.

OK, that’s it for the history lesson. What reminded me of all this is the Web site Physics Stack Exchange, which aims to produce a similar sort of forum for discussion of physics questions. The site has moderators, but they don’t approve each post manually as we did. Rather, the participants in the group vote answers up or down, so that the ones that are deemed most useful rise to the top. There’s a complicated set of rules whereby only people who have earned the right (through useful participation in the past) are allowed to vote.

Once I started looking at it, I couldn’t resist posting some answers of my own.

There’s a lot of good stuff there. Check it out, and participate if you’re interested!

Benjamin Bederson, physics professor emeritus at NYU, has a letter in today’s NY Times magazine:

As a physicist myself, I read with great interest Jonah Lehrer's article about Geoffrey West, who is interested in developing a general theory of cities from first principles. Physicists €” myself included €” are intrigued by the idea that application of rigorous laws in the world of natural science can inspire similar applications in other areas, including social ones.

It may be true that methods from the natural sciences can be fruitfully applied in the social sciences, but as a matter of diplomacy I’d rather physicists refrained from “we’re rigorous and you’re not” language.  Whether or not it’s true, I don’t think it’s helpful.

But that’s not what bothers me about this letter.  He goes on to say

This quest has been going on for a long time: for example, the efforts to apply the Heisenberg uncertainty principle to human behavior.

This would be fine, if he went on to clarify that “efforts to apply the Heisenberg uncertainty principle to human behavior” are utterly stupid and pointless.  In fact, he seems to think precisely the opposite, namely that such efforts are a good example of the use of natural-science ideas in the social sciences.

The Heisenberg uncertainty principle is often used as a metaphor, sometimes to convey the banal notion that it’s often hard to measure stuff, and sometimes to convey the slightly more interesting idea that measurements affect the system being measured.  I suppose that what Bederson means by “apply[ing] the Heisenberg uncertainty principle to human behavior” is simply that when you survey people the act of surveying them has an effect on them.  That’s true, but it’s much less interesting than the actual Heisenberg uncertainty principle.   It’s also such an obvious idea that it’s downright insulting to suggest that social scientists needed physicists’ help to figure it out.

Edward Tufte is auctioning off a bunch of rare old manuscripts from his library at Christies in New York.  There’s a Galileo first edition going for a mere $5000-$7000, which sounds like a steal to me (not that I know anything about this stuff.)  I love old manuscripts like this.  I were in the area, I’d love to go.

But here comes my shameful confession: I’ve tried to like Edward Tufte’s sensibility, and I just can’t do it.  I look at the Minard map of Napoleon’s Russian campaign, which Tufte says “may well be the best statistical graphic ever drawn,”and I just find it confusing and cluttered.  Just because you can layer seven (or however many it is) dimensions of data on one graphic doesn’t mean you should.

Even when he aims at the low-lying fruit of anti-PowerPoint ranting, I can’t really get behind him.  Yes, lots of PowerPoint presentations (including some I’ve perpetrated) are deadly, but I don’t think it’s because of the “cognitive style of PowerPoint.”  It’s because, no matter what tools you use, creating a bad presentation is much easier than creating a good presentation. Take it from me, young folks: even before PowerPoint, most talks were bad.

Nobody had ever won both a Nobel and an Ig Nobel prize, until this year.

Here‘s what Geim won the Ig Nobel for:

[kml_flashembed movie="http://www.youtube.com/v/A1vyB-O5i6E" width="425" height="350" wmode="transparent" /]

For quite a while now, I’ve been part of a collaboration working on the development of QUBIC, a new kind of instrument for measuring the polarization of the microwave background.  For those who want details, we’ve written a paper describing the current status and prospects of the project.  It’s posted on the arxiv, and it’s been submitted for publication in the journal Astronomy and Astrophysics.  Enjoy.

There’s a piece in Discover describing an experimental result that, according to the article, “shows that measurements performed in the future can influence the present.”  This sort of thing should always set off your extreme-skepticism response.  Sure enough, although the experimental result is pretty cool, it absolutely in no way implies backward-in-time causation.  Even by the standards of other silly woo-woo articles on quantum mechanics, this one is pretty bad.

If you read the article carefully, or better yet if you read this more technical and more accurate description, you’ll find that what’s actually been shown is that later measurement results are correlated with earlier ones.  To be specific, suppose you prepare a bunch of particles at one time,  then make a measurement on them at an intermediate time, then finally make another measurement at  a later time.  You then retroactively split the particles into two groups based on the result of that later measurement.  What’s been discovered is that those two groups yield different outcomes on the earlier measurements.

That’s not at all surprising, and it in no way implies backwards-in-time causation.  Suppose we both know that Professor Bertlmann‘s socks never match.  Suppose I look at one of his socks now, and you look at his other sock an hour from now.  If you “measure” that his sock is green, then you can conclude that my measurement must have yielded non-green.  Do you think that your measurement somehow sent influences back in time, causing my measurement to be non-green?  If so, then you should think the same thing about these quantum mechanics results.  If not, not.

As far as I can tell (this isn’t my field), the precise nature of the correlations that result in this experiment are interesting and potentially quite useful in allowing more accurate measurements of various things that people are interested in.  So it’s a good experiment.  It just doesn’t have the amazing philosophical implications imputed to it.

My last post on this subject was prompted by an anti-tenure article that I thought was silly and poorly argued.  As a result, I think it made me seem like a stronger proponent of the current tenure system than I really am.

(In case anyone doesn’t know, let me stipulate a couple of things: 1. I have tenure.  2. As someone who places a high value on security, I like having tenure.)

The traditional argument for tenure is that professors need to be able to advance controversial ideas that attack the power structure without fear of reprisal.  That idea isn’t wholly without merit, but I have to admit that I don’t see it as such a pressing need that we should structure the entire academic employment system around it.

No doubt my perspective is influenced by the fact that the work I do has no political implications.  If I worked in a more controversial field, I’d probably see things differently.

Certainly an alternative to the tenure system would certainly have to have strong protections against ideological firing of faculty members for espousing controversial ideas, but it seems likely to me that infinite job security is an overreaction to this threat.

For most faculty members, I suspect, tenure’s not really about academic freedom.  It’s just a job benefit, like a dental plan or a day-care center.   And like those other benefits, if employers got rid of it they’d have to offer either higher salaries or other benefits, in order to compete for the same pool of potential employees.

I don’t know if anyone’s tried to estimate the economic value of tenure to typical faculty members and hence how much it would cost to eliminate it.  Academic salaries are not high, compared to other jobs with similar levels of training, and I suspect that the job security of tenure is quite valuable to many people who have it.  I doubt the cost of eliminating tenure would be trivial.

The most often mentioned cost of the current tenure system is the dead-weight, lazy, unproductive, incompetent tenured professor.  Such people no doubt exist, although I don’t think that they’re terribly common for reasons I mentioned before.  Still, I think it’d be possible to design a system with strong protections against ideological firing but that still allowed universities to get rid of people for actual incompetence.

To me, the biggest disadvantage of the current tenure system is its rigidity.  People’s lives are complicated, for all sorts of personal and family reasons, and the current system makes it hard for anyone who can’t conveniently follow the specific tenure-track career timeline.