Here’s a fun thing that has been zipping around the internets this week: a collection of “back of the envelope problems” put together by Edward Purcell. Hours of fun reading if you’re the kind of person who likes to spend their leisure time doing word problems (and I mean that in the best possible way).

One of Purcell’s problems is labeled “Electromagnetic energy in your eyeball,” and it concludes with a provocative (and true) observation. The problem asks the reader to calculate the total energy in all the photons that are inside your eyes at any one moment. Roughly speaking — which is the point, since we’re doing back-of-the-envelope problems — these photons come from one of two sources: the visible light from the outside world that enters your pupil, and the infrared light that is emitted as blackbody radiation from your eye itself, since you are an object at body temperature. Purcell suggests that you compare the amount of energy from each source.

And the answer is: there is much more electromagnetic energy in your eye at any one moment from the infrared radiation you’re emitting yourself, than the pittance of visible light you get from the outside world. Between 100,000 and a million times as much. Which raises a question we may never have thought to ask: why does it get dark when we close our eyes? The amount of electromagnetic radiation hitting our retinas hardly changes!

Purcell’s last sentence gives the answer: “Only quantum mechanics can explain why that makes it dark!”

We see light when photons of an appropriate wavelength reach the photoreceptor cells in the retinas of our eyes. The energy from the photon is converted into chemical energy via phototransduction, which sets an electrochemical signal to the visual cortex. (Presumably unnecessary disclaimer: everything I know about vision I learned from Wikipedia.) In particular, the photons are absorbed by a chemical called retinal, which isomerizes from the 11-cis state to the all-trans state. (That last bit was a blatant cut and paste.)

Here’s the part I do understand: isomerization is a matter of nudging a chemical from one structural form to another, without actually changing the chemical formula. Molecules have energy levels, just like electrons in atoms, and in order to effect the change in the retinal via photoexcitation, a photon has to have enough energy to cause a transition between the isomers. That’s a matter of quantum mechanics, full stop. Molecules can’t take on just any old energy; the allowed energies are quantized. As a result, it doesn’t matter that the infrared light inside your eyeball has much more energy than the visible light from the outside world; the energy comes in the form of individual photons, none of which has enough energy to get the reaction going. It’s very analogous to the photoelectric effect in metals, for which Einstein won his Nobel prize.

We often say that quantum mechanics applies to the world of the very small, and involves mind-bending alterations of our everyday reality. Which is true as far as it goes, but the more simple truth is that quantum mechanics applies to absolutely everything. It underlies how the everyday world works, from the stability of matter to the darkness when you close your eyes.

Last month we mentioned a paper on the arxiv that made a provocative claim: evidence from the dynamics of stars above the galactic disk indicates that there is essentially no dark matter in the vicinity of the Sun. I am not an expert on galactic dynamics, but nevertheless I and others were immediately skeptical, especially since there is overwhelming evidence for the existence of dark matter from other measurements. Skeptics, of course, happily piled on. But this isn’t an area where one opinion or the other matters very much — better data and better analysis is what matters.

Now we have a better analysis, from people who are experts: Jo Bovy and Scott Tremaine have a paper in which they examine the claim closely. They find it wanting. This was pointed out here in a comment by Ben; Jester and Peter Coles also have useful blog posts up about it.

Short version: the original authors made assumptions about the distribution of velocities of the stars they were looking at, and those assumptions are known to be wrong. Using a better model (i.e., one more compatible with known data), Bovy and Tremaine show that the observations are perfectly consistent with the conventionally-assumed dark matter density. The good news is that they are actually able to use this technique to get a more precise measurement of that density than was previously available. It’s a rare scientific lemon that can’t be turned into at least a little bit of lemonade.

I’m not sure why people get so emotional about dark matter. The original paper here by Bidin et al. was accompanied by a dramatic press release from the European Southern Observatory. I am known as a “dark matter supporter,” but I have no personal investment; I think it would be much cooler if something crazy were going on with gravity. But that’s not what the data indicate. It’s just some new particle we haven’t yet made in the lab, hardly the end of the world.

On local TV last night, I somehow got reporter Dave Malkoff to take a stab at explaining quantum field theory: the world is made of fields, but we only notice the ripples within them, which we see as particles. Something about Angelina Jolie in there at the end as well.

It’s well known that all of our evidence for dark matter (and dark energy too, but that’s not the subject here) at the present time is indirect: it comes from observing the gravitational influence of the hypothetical stuff, not from detecting it “directly” (i.e., using some interaction other than gravitational). So it’s natural to ask whether we can do away with dark matter by positing some modification of the behavior of gravity; I’ve certainly wondered that myself.

And it may very well turn out that the behavior of gravity on large scales does not precisely match the prediction of ordinary general relativity. Nevertheless, I think that by now we’ve accumulated enough data to conclude that the universe cannot be explained solely by modifying gravity; there is ample evidence of gravitational forces pointing in directions where there isn’t any (ordinary) “stuff” to create them, leading us to accept the existence of some form of dark matter. About a year ago I put up a post that explained this point of view, and took aim in particular at the popular framework known as MOND.

This led to some good discussion in the comments, and also to a behind-the-scenes email exchange between Rainer Plaga, Stacy McGaugh, and me. It’s a bit of old news, but I thought there would still be some interest in our discussion, so (with permission) I’m posting our emails here. Seeing how the sausage is made, as it were. It’s a bit of a long read, sorry about that.

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We are very close to the end of the semester here at Penn, and the last couple of weeks have been the usual flurry of activity as teaching comes to an end, exam period begins, and a few late semester/early summer meetings all take place at the same time.

A week or so ago, I spent a couple of days back at Syracuse University, where I was a faculty member for quite a few years. I was there primarily to participate in a special event that preceded the East Coast Gravity Meeting being held there on the following weekend. The event was a celebration – GoldbergFest – of the career of Josh Goldberg, a good personal friend, and an eminent relativist at Syracuse, who has been an emeritus professor there for many years now.

Josh began as a graduate student at Syracuse in the early 1950′s, working on conservation laws in General Relativity (GR) and on canonical quantization. At the time Syracuse had one of the few well-known relativity groups in the world, led by Peter Bergmann, and an impressive group of young people were trained under him, and later under Josh, as students and postdocs; people like Ted Newman, Ray Sachs, Art Komar, Roger Penrose, and many others. I’m certainly no expert on the precise history of the Syracuse group, but fortunately, as part of a special issue of General Relativity and Gravitation dedicated to Josh, to which I was honored to also contribute, Ted Newman describes it wonderfully. The Fest was a lovely event. I enjoyed the other speakers’ talks – John Stachel, Rafael Sorkin and Peter Saulson, and Ted Newman’s hilarious and touching after dinner speech, and the reminiscences of the other people at the dinner made for what I hope Josh thought was a wonderful day.

Over the next two days quite a few of our students and postdocs from Penn gave talks at the East Coast Gravity Meeting, and I was delighted to hear that our very own Godfrey Miller won the award for the best student presentation.

Returning To Penn, I just about had enough time to finish putting together the take-home final exam for my graduate General Relativity course, before heading off to NYU on Wednesday with our whole group for our semesterly joint meeting. We were joined, as usual, by a nice crowd from Columbia and Case Western for a day of talks and discussion. I always find these meetings to be incredibly useful scientifically, because the group is so interactive, boisterous and interested in the material, while being such warm and friendly hosts. It makes for an enjoyable day every time. Beyond the obvious exchange of ideas, these meetings also provide an opportunity for our students to get used to giving talks on their work. This time my student – Garrett Goon – and one of my colleague Justin Khoury’s students – Austin Joyce – gave our student talks, leading to some healthy discussion Wess-Zumino terms in new field theories and conformal cosmology, respectively. Both did a terrific job, although they’re becoming old pros at this point, rather than beginning students in need of practice.

To close out last week, Greg Gabadadze from NYU came down on Friday so that we could try to finish up some details in a project that is close to completion, before we start dispersing for various summer conferences. I’ll discuss these soon, I expect.

Today my final exam will be turned in and grading starts, an old friend is delivering a seminar in our group, and Sean’s student Kim Boddy arrives for a week so that the three of us can try to finish up a paper. The end of the semester always seems to go this way. While all these things are fun, life becomes excessively hectic for two weeks, and then travel begins.

Some of you may have been following a tiny brouhaha (“kerfuffle” is so overused, don’t you think?) that has sprung up around the question of why the universe exists. You can’t say we think small around here.

First Lawrence Krauss came out with a new book, A Universe From Nothing: Why There Is Something Rather Than Nothing (based in part on a popular YouTube lecture), which addresses this question from the point of view of a modern cosmologist. Then David Albert, speaking as a modern philosopher of science, came out with quite a negative review of the book in the New York Times. And discussion has gone back and forth since then: here’s Jerry Coyne (mostly siding with Albert), the Rutgers Philosophy of Cosmology blog (with interesting voices in the comments), a long interview with Krauss in the Atlantic, comments by Massimo Pigliucci, and another response by Krauss on the Scientific American site.

I’ve been meaning to chime in, for personal as well as scientific reasons. I do work on the origin of the universe, after all, and both Lawrence and David are friends of the blog (and of me): Lawrence was our first guest-blogger, and David and I did Bloggingheads dialogues here and here.

Executive summary

This is going to be kind of long, so here’s the upshot. Very roughly, there are two different kinds of questions lurking around the issue of “Why is there something rather than nothing?” One question is, within some framework of physical laws that is flexible enough to allow for the possible existence of either “stuff” or “no stuff” (where “stuff” might include space and time itself), why does the actual manifestation of reality seem to feature all this stuff? The other is, why do we have this particular framework of physical law, or even something called “physical law” at all? Lawrence (again, roughly) addresses the first question, and David cares about the second, and both sides expend a lot of energy insisting that their question is the “right” one rather than just admitting they are different questions. Nothing about modern physics explains why we have these laws rather than some totally different laws, although physicists sometimes talk that way — a mistake they might be able to avoid if they took philosophers more seriously. Then the discussion quickly degrades into name-calling and point-missing, which is unfortunate because these are smart people who agree about 95% of the interesting issues, and the chance for productive engagement diminishes considerably with each installment.

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Jorge Cham visits CERN, and comes back with tales of particles and mass.

A flowchart I put together for The Particle at the End of the Universe. Feel free to spread around, with appropriate attribution.

Sorry for the tiny writing, there are a lot of particles! Click to embiggen and get a legible version.

Science keeps advancing, in fits and starts. It was a good week for intriguing results from experiments.

The first bit of news, which has been the subject of the most internet buzz, is a new paper by Chilean astronomers C. Moni Bidin, G. Carraro, R. A. Mendez, and R. Smith, which claims that there’s no evidence for dark matter in the dynamics of stars near the Sun. If this were true, it would imply something funny going on with the distribution of nearby dark matter, which could have significant implications for direct searches here on Earth (see below). It wouldn’t really be much of a threat to the idea of dark matter itself, since there’s plenty of evidence for dark matter elsewhere. But it might mean that the distribution in the Milky Way was very different from the kinds of models we like to use, for example by being much lumpier.

We just heard a great physics colloquium here at Caltech by Katie Freese, who talked about this result very briefly. Her opinion matched those of the skeptics in Ron Cowen’s article linked above: this paper makes a lot of assumptions, some of the a bit dubious, and we would need to see something much more solid before we become convinced. The biggest issue is that they don’t actually measure the DM distribution near the Sun; they try to measure it in a region between 1500 and 4000 parsecs below the galactic plane (which is actually pretty far away), and then fit to a model and extrapolate to what we should have nearby. (more…)

Sorry for a post title that will attract the crazies. Carl Zimmer has a story in the New York Times that discusses a growing unease with the practice of science among scientists themselves.

In tomorrow’s New York Times, I’ve got a long story about a growing sense among scientists that science itself is getting dysfunctional. For them, the clearest sign of this dysfunction is the growing rate of retractions of scientific papers, either due to errors or due to misconduct. But retractions represent just the most obvious symptom of deep institutional problems with how science is done these days–how projects get funded, how scientists find jobs, and how they keep labs up and running.

However… essentially all the examples are from biologically-oriented fields. I’ll confess that Carl asked me if there is a similar feeling among physicists, and after some thought I decide that there really isn’t. There are certainly fumbles (faster-than-light neutrinos, anyone?) and scandals (Jan Hendrik Schön being the most obvious), but I don’t have any feeling that the problem is growing in a noticeable way. Biology and physics are fundamentally different, especially because of the tremendous pressure within medical sciences when it comes to any results that might turn out to be medically useful. Cosmologists certainly don’t have to worry about that.

But maybe this is a distorted view from within my personal bubble? Happy to hear informed opinion to the contrary. The relevant kind of informed opinion would actually involve a comparison of the situation today with the situation at some previous time, not just a litany of things you think are dysfunctional about the present day.