Philosophy of Science
October 26, 2009

If you want a great twenty-minute introduction to this field, here’s the best one I’ve encountered:

I took a class on philosophy of science several years ago, and really enjoyed it. I think we actually discussed Deutsch’s variational theory of science in class, but unfortunately I’ve misplaced my notes and only remember that this stuff seems vaguely familiar. But I definitely do agree with his general introduction and the idea that science is theory-laden instead of positivist, even if I doubt that his specific definition of science is correct in the details. And he actually makes it seem like this stuff matters, which is quite an accomplishment!

Finally, it doesn’t come up at all in the talk, but Deutsch is also apparently one of the progenitors of the many-worlds interpretation of quantum mechanics, which makes the most sense of any interpretation I’ve seen. More on that some other time, perhaps.


Climate Policy Lecture 4
October 11, 2009

This week’s lecture was delivered by an atmospheric scientist who specialized in the study of convective vortices (hurricanes, tornadoes, supercell thunderstorms, etc). He was obviously an expert in this field, but unfortunately, he seemed not to have thought very deeply about broader issues of climate science, and particularly climate policy. He breezed (ha) quickly through the idea that global warming causes more hurricanes and makes them more intense, an idea that makes theoretical sense but I thought still lacked solid observational support because El Nino has suppressed Atlantic hurricane formation or something. I would have liked to hear more on this.


He also presented a plot similar to the one above, showing global mean temperature over geologic time (the time axis is logarithmic) that highlights all the variability over different timescales. It’s a really fascinating record, and I would also have liked to know more about this, but then he admitted he hadn’t thought much about this plot, and then demonstrated that by offering essentially no interpretation of it.

Finally, he gave the first simple explanation I’ve ever heard for why there are multiple equilibria in the climate, but it’s not really interesting enough to share here. The next two weeks there aren’t any lectures, so I’ll have to find something else to write about. (revised on Oct. 12 to make it nicer)

Bose-Einstein Condensates
April 26, 2009

I’ve been working on a political science post that’s taken a lot more thought than I thought, so in the meantime here’s some info on Bose-Einstein condensation. First, let me remind (or teach) you that all elementary particles are divided into bosons and fermions based on their quantum statistical properties. While protons, electrons, and neutrons are all fermions, they can be combined to make composites that act like bosons; the simplest example is the element Helium. When a bunch of bosons are compressed to sufficient density (usually by cooling them to nearly absolute zero) they form a Bose-Einstein Condensate (BEC). This is a state of matter distinct from solid, liquid, gas, or plasma; here’s its Wikipedia page. The physics of Bose-Einstein condensation is pretty interesting, but it’s covered in detail in any worthwhile statistical physics book and, in my opinion, to focus on the equations distracts from the truly ridiculous chemical and physical properties of this state of matter.

To start with, a BEC is completely frictionless and aviscous. Not low-friction like a ball bearing, or low-viscosity like water; rather, a BEC is literally unaffected by both friction and viscosity. This means that if you have a glass half-full of a BEC, the BEC will, in real time, climb up the walls of the glass and escape in a manner analogous to capillary action. It also means that if you stick a spoon in a bowl of BEC and start it spinning, the resulting vortex would last forever. A zero-friction material is really remarkable and should hopefully be filling your mind with exciting potential applications. The closest technology to which I can compare it is magnetic levitation, which is itself incredibly useful but still subject to friction in the form of air resistance.

However, a BEC can’t easily be poured into a bowl and stirred like a liquid can. condensate1 This is partially because of experimental constraints: the required density is high enough that we can currently only make microscopic BECs and once they warm up slightly past absolute zero they revert back to normal matter. It’s also partially because of that word ‘condensate’ – when BECs form they ‘condense’ down nearly to point sources, bound in volume and velocity space only by the Uncertainty Principle. So they don’t look very exciting; here’s a nice artist’s conception of a Rubidium BEC. 

Finally, there’s also an obscure effect related to BECs that, as far as I can tell, has not yet been satisfactorily explained. This is known as the ‘bosenova’ (pun intended, apparently), and relates to a 2001 experiment wherein a varying magnetic field was applied to a microscopic BEC and caused it to collapse in on itself and then explode, in similar fashion to a Type II Supernova but for completely different (and still mysterious) physical reasons.  This effect is totally baffling to me and I have no idea what its physical significance is, but it has some historical notoriety because of speculation that the collapse of something as dense as a BEC would produce a microscopic black hole, and we all know that a microscopic black hole could SWALLOW THE ENTIRE EARTH; obviously, the official Large Hadron Collider Safety Blog has felt compelled to discuss this issue in some detail, and they’re not worried.

For more information on BECs, I have two links. First, above is a nice YouTube video of superfluid Helium that shows some of the effects I discussed. Superfluid helium actually exists along the transition between a BEC and a liquid, so it exhibits some features of each, but it’s much easier to make and play with than a true BEC. I like to think of it as the liquid/BEC transition’s analogue of the solid/liquid hybrid cornstarch and water mixture. Secondly, this webpage provides a great nonmathematical, elementary-physics-level introduction to BECs, replete with little Java apps and references for more information.

One obvious question that might now be asked is: if BECs occur when bosons are compressed to high density, what happens when fermions are compressed to high density? The answer is something wonderful called degenerate matter, which I will set aside for later.

Hardy’s Paradox
March 13, 2009

I just read about a crazy quantum mechanical thought experiment from 1992 called Hardy’s Paradox, wherein electrons and positrons can essentially undergo incomplete annihilation. As far as I can tell, it’s not quite a true paradox: it’s a prediction of quantum mechanics that is perfectly mathematically consistent, it just happens to completely defy our physical intuition. I’ll provide my attempt at a summary of the paradox below, and describe a ridiculous experiment that has just been published which offers a direct observation of the effect. If you want a more thorough explanation of Hardy’s Paradox, I refer you to “The Mystery of the Quantum Cakes”, an article (available free here, for some reason) aimed at an audience of science teachers.

The Electrostatic Two-Step
February 22, 2009


I went to a talk a few weeks ago by this guy, about the spontaneous electrostatic charging of particles. Here’s the talk summary:

The electrostatic charging that occurs when two surfaces rub (“triboelectric” or “contact” charging) is one of the most well known phenomena in science. Everyone has noticed it at some point when they walk across a rug and then get a shock when they touch a doorknob, and everyone is familiar with the elementary science demonstration of rubbing a balloon on their head and then seeing how it charges the balloon and the hair.

This electrostatic charging plays an important role in atmospheric and space sciences; e.g., [lightning] is caused by charged ice particles, particle charging alters the flows of wind blown sand and snow, and charged dust causes problems for space missions to the moon and other planets. Electrostatic charging also has consequences in industry, both beneficial and harmful; e.g., the operation of photocopiers and laser printers is based on charged toner particles, and explosions at gas pumps can be caused by sparks due to the charge that builds up on a person that has rubbed against a car seat while exiting the car.

Despite the widespread importance of triboelectric charging, there is no scientific explanation of how the charging occurs. Our work aims to improve this understanding.

In the talk, Professor Lacks presented a very straightforward and simple model for how a bunch of chemically identical electrically neutral dust particles can spontaneously build up huge amounts of charge through collisions. The basic requirement is a range of dust grain sizes, with each grain having electrons on its surface trapped in high-energy molecular states. The grains are therefore electrically neutral, but not quite in equilibrium: they want to still have the same amount of electrons, but to have them in lower-energy states.

The way to reach this equilibrium is through collisions. If two grains collide in the right way, they can transfer surface electrons back and forth, so the higher-energy electron on each grain can move to the other grain and settle in a lower-energy state there. One might expect this process to maintain a neutral charge on average, since each grain is both gaining and losing an electron in every collision. The trick is that, once a few collisions have occurred, the smaller grains will have given away most of their higher-energy electrons, so when they collide they no longer have anything to give, but they can still receive an electron from a larger grain. Thus, in the aggregate, the model predicts a tendency for smaller grains to spontaneously accrue negative charge, and for larger grains to accrue positive charge. This matches with observations, such as electric fields in dust devils and in volcanic plumes. Nice work, Lacks et al.!

Of course, their model describes only the first step to reaching equilibrium. The second step is for enough of this charge to build up to ionize the air around the dust, which allows for sparking and/or lightning. After the lightning, the charge is uniformly distributed again, and hopefully the electrons are in lower-energy states than when they started. It’s a rather shocking way of achieving equilibrium, but it works.

Darwin Day
February 12, 2009


Today is Darwin Day and the 200th anniversary of Darwin’s birth. I’m no biologist, but I do think the theory of evolution is pretty dandy, and so I’m happy to celebrate it a bit. It is somewhat troubling, however, that people associate Darwin and evolution so closely, so now you hear it called it “Darwinism” or “Neo-Darwinism”. Historically, he was pretty important, but not that important; my impression is that these terms have been primarily popularized by creationists who want to suggest that evolution is just as much a religion as creationism. After all, if both theories are on the same epistemological footing, then who wouldn’t prefer to be created in God’s image?

So, today seems like a very good day to link to this article by Jerry Coyne in the New Republic. It’s actually a book review, but it serves as a pretty exhaustive summary and critique of Intelligent Design and the ongoing battles between atheism and religion, written mostly for an audience of agnostics. And for the later parts of the review, when Coyne overreaches slightly in his argument, Ross Douthat has provided a useful counterargument from the religious perspective.

If you just read one article about evolution today, I’d suggest the one above. But I want to include slightly more of a tribute to the theory. First, here’s an excellent piece of science writing that explains natural selection and places Darwin’s contribution in the proper context. It also makes an unnecessary swipe about which, frankly, I don’t know what to say, namely that economic studies show that rejection of evolution in favor of creationism is correlated with “what might be described as the intensity of the struggle for existence”, i.e. the selection pressure. And I also want to point out that evolution has broader applications than just within biology, and one cool example is genetic algorithms, which use the principle of natural selection to solve maximization problems throughout science and mathematics. A recent contribution of these algorithms can be seen in chemistry, where they led to the creation of a new form of Boron. Just like everything else powerful, however, these algorithms also have downsides.

A Meditation on Power
January 9, 2009

Image Credit Fred Bruenjes (

Image Credit Fred Bruenjes (

Electrical power, that is. The “clean coal” companies want a few billion dollars of the Obama stimulus money to “invest” in more coal power plants for our country, which they claim will be “cleaner” than current coal plants and thus provide cheap power without hurting the environment. Nirmal had a great post a few weeks ago highlighting some of the deception behind the “clean” part of “clean coal”, which I recalled today when I read there was just another such ash spill in Alabama. Also, coal power is by far the biggest contributor to global warming – even more than those fearsome SUVs!

We shouldn’t build any more coal power plants at all. What should we do instead? Personally, I’m a big fan of solar power. Photovoltaic solar panels don’t emit any carbon or other pollution once they’re built (more on that some other time). Solar power is completely sustainable for as long as the sun keeps burning (about 5 billion more years). And, with current technology, it can easily provide all of the power America needs. I find that last statement surprises a lot of people, so I’ll walk through a simple calculation to back it up. This is one of my favorite arguments for solar power, and some of you may have heard it before, but for those who haven’t, it’s really worth following it through one time. And I guess I should warn that it contains some numbers and very basic math. Feel free to challenge assumptions in comments if you’d like.


Climate 2008
December 25, 2008

Just a quick post about the climate. It’s still in serious trouble, if you’re wondering. Here’s a really effective plot to emphasize the threat, taken from Hansen et al. 2008. The left 80% of the plot shows the past 400,000 years of climate history measured from an ice core sample; the green line is the amount of greenhouse gas in the atmosphere and the red line is the mean temperature at the ice core location. The correlation between the two is pretty obvious, and plenty of science establishes the causation as well. In the right-most 20% they have plotted the greenhouse gas (green) and mean temperature (purple) for the past 120 years, along the same vertical scale. Now, it takes about 1000 years for the temperature to catch up to the greenhouse gas, but clearly we’re in for some interesting times. *


And now for a paragraph dealing with climate change denial. The gold standard for this stuff is, a climate change blog for the public written by a bunch of climate scientists in their free time. They’ve got two excellent posts up right now. The first one deals with the estimated mean temperature for 2008, which you’ll start seeing headlines about next week or so. The second is more philosophical, and muses more on the nature of science and consensus in placing climate change denial in a broader context, using the old debate between Lamarckianism and natural selection as an example.

Finally, in terms of dealing with climate change, there is a global summit in Copenhagen scheduled for December 2009. The Economist has a good preview of the summit and the likely outcome:

A substantive deal in Copenhagen therefore looks unlikely; but the world’s leaders are not likely to give up trying to save the planet there and then. Perhaps the likeliest outcome in Copenhagen in 2009 is a repetition of what happened in Kyoto in 2000—a big bust-up, another meeting called and a deal done the following year.”

*While the green line is the important line in the long run, in the short term, temperature is driven instead by the black line, which accounts for other forcings on the global temperature, such as volcanic activity, solar cycles, and dusty air pollution.


EDIT: Oh yeah, the obvious other thing to add is the recommendation from the abstract of that paper I cited: “If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less that.”

The Omnidirectional Dielectric Mirror
December 22, 2008


I read an article on CNN last week about the recent invention of a “perfect mirror”. The article, naturally, is short on science so I found some of the original papers on the subject (here, and here, if you’re interested) and a press release from MIT to provide some context. From what I understand of it, this looks like a very significant advance.

I would recommend reading the press release first – it explains that there are two types of mirrors. Metallic mirrors (e.g., car mirrors) reflect light omnidirectionally over a wide range of wavelengths, but they absorb a few percent of the light they reflect, so they don’t work well for reflecting really intense light like lasers. Dielectric mirrors can reflect light nearly perfectly (so they can reflect laser light), but usually only one specific wavelength of light and only from one specific direction.

In 1998, Yoel Fink (then a graduate student at MIT and now a professor), figured out how to stack dielectric mirrors so they can reflect light omnidirectionally and over a wide range of wavelengths. The trick seems to be simply choosing the right combination of properties of the layers of mirror so that the math works out to produce a significant wavelength region of omnidirectional reflectivity. And everybody had just assumed this was impossible, so nobody had really looked for it before.

I should point out the evolution of the applications for this mirror. The research was funded by DARPA, the Defense Advanced Research Projects Agency, which is an awesome government program that funds far-fetched future technology and makes it reality (the Internet used to be DARPAnet, for example). This research was intended for military use with laser (!) weapons, to provide portable laser weapons for our troops or to reflect enemy laser fire (to protect a satellite or missile, say). In the 1998 press release, the scientists suggested putting the mirrors in paint for insulating buildings. And now, the most significant application seems to be medical: the mirrors can be used to make fiber-optic laser guides, so surgeons are now free to use their powerful lasers like ultra-sharp scalpels. They’ve even introduced laser pens! Nice work, Fink et al. 1998. And just goes to show how useful it is to fund pure research.