It’s Nobel season again, one of my favorite times of year. The winners of this year’s prizes will be announced this week, giving us a chance to recognize some of the greatest achievements in science. The awards will be handed out on the anniversary of Alfred Nobel’s death, December 10, (which happens to also be my Birthday)
Some of the most interesting history thought-experiments consider the question of the inevitability of important world events. Was the South destined to lose the US Civil war due to its inherent disadvantages, or could some strategic blunder have turned the tide? The question of contingent history has a parallel in biological evolution. The difference is that science is now beginning to give us the tools to draw actual conclusions about what would happen if we could “rewind” the clock of evolution and play it again. We can start to identify the contingent calf-paths in our ancestry, and distinguish them from the traits, like flying and vision, which have evolved separately multiple times different lineages. The similarity between marsupials and placentals is commonly cited as strong evidence that not everything is by accident. However, the amplifying effects of the “rich-get-richer” is a hallmark of network behavior. Just as the choice, possibly based on minor considerations existing centuries ago, of whether a tiny settlement might be built on bank of the river or the other can dictate the location of a modern metropolis, small, accidental changes may get locked-in by evolution.
We now have the opportunity to watch history play out again, either by doing it ourselves and growing 20 years of bacterial generations, as in the Lenski lab, or by resurrecting ancient genes from 500 million years ago and inserting them into modern specimens. The picture that is forming from these experiments is, as in history, a mix or randomness and inevitability that shows the importance of individual mutation events that can “potentialize” or “actualize” a given adaptation. There is likely to be a crucial, revolutionary advance hit upon by evolution that, by itself, does not totally provide for the trait, but is close enough so that some among the population of its descendants are, under the right external conditions, strongly favored to have a large advantage when they perfect the trait. So the response to our question should be “how far back do you want to rewind?”
That the Gettysburg Address is among the most famous speeches in the American consciousness is one of history’s great ironies. If you actually read the words, Lincoln discusses the powerlessness of words, and that no one will care about what said that day:
“…we can not dedicate, we can not consecrate, we can not hallow this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here.”
Perhaps this is why he spoke for only 2 minutes, while others droned on for several hours at the same dedication. In any case, the role of an observer, who notes but does not alter the situation, can to my mind today as I was teaching physics lab. In my class, overwhelmingly pre-med students, 12 of 15 were female. This might seem surprising for subject matter that had been seen as the Provence of men, but I was not so surprised. I just started reading The End of Men: And the Rise of Women by Hanna Rosin. She is aware that the purpose of her book is not to set forth an argument, but rather to mark a momentous shift in the course of the human experience. For the first time since ever, females are overtaking their chromosomal mismatched counterparts. Rosin points out that the rise of automation has dealt a heavy blow to traditionally “male” occupations, like construction and manufacturing. When given a level playing field, where brawn is not an issue, women are thriving and men are getting left behind. At Nova Southeastern University, where I teach, about 70% of the student body is of the female persuasion, and nationally the figure is around 2/3. In this new connected world, where a machine (or third-world laborer) can do most taxing and menial tasks, the value of education has never been greater. Women are seeing the larger divergence in life- paths between unskilled workers and trained professionals and choosing their education levels accordingly.
The webcomic xkcd has many gems, but one my all-time favorites pokes fun at how teachers have a pechant for over-emphasizing the wrong-ness of a “misconception.” In an usually admirable but misguided effort to move students from a naive way of thinking, “overzealous” instructors salt the Earth so that no further thinking can occur in the “wrong” tracts. However, sometimes the most seemingly incorrect views reappear when least expected. One of the famous scientific controversies middle school biology students are taught regards the theory of Lamarckism, which essentially states that evolution occurs because offspring inherit changes that occurred to their ancestors. The classic example is of a giraffe, which is said to acquire a long neck from stretching for the leaves at the top of trees. According to Lamarck, this useful trait (long neck) is then inherited by its children. Ask any elementary school teacher about the “heritability of acquired characteristics” today, and you are overwhelming likely to hear stories involving mice getting their tails hacked off, but going on to give birth to normally-tailed offspring. Students get drilled into them the concept that only germ cells can carry heritable information, and that what happens to a organism during its lifetime has no bearing on the “book of life” it gifts to its offspring. But what if that book can have annotations? The growing field of epigenetics is showing almost daily that just a copy of the “instruction manual” is not enough to get through the business of living. Genes can be turned on or off, in way we are only beginning to fully appreciate. For example, all the cells in your body, whether nerve, muscle, lymphocyte or whatever, carry around a full copy of your DNA sequence. The assignment to become a certain cell type requires changes that “mark-up” the genes needed or not needed, depending on the jobs. This may be done with chemical tags; for example, adding a methyl group to a nucleotide base to change how its gene it encodes is expressed. We are finding that some changes, like the difference between worker bees as nurses or foragers, are reversible, or even heritable from your parents experiences.
While it may be important to replace naive conceptions of giraffes with more sophisticated ones, teachers often do a disservice to their students by “overselling” certain concepts. Because nature is (and must be) more complex than that.
Even life has to obey the second law of thermodynamics, which states that the amount of Entropy, essentially, the degree of disorder, can only increase in a closed system. Increases in entropy in some region must be balanced by increases in other region, or else by the conversion of useful energy into “heat,” that is no longer able to do further work. Therefore, every “ordering” process, including the self-replication of an bacterium can be assigned a minimum amount of energy required for that process. Although the sun is constantly radiating energy down on Earth, collecting and using it is not a simple task. Life has found ways to economize and reproduce copies without consuming much more than the theoretical limit.
The New York Times science section recently profiled the Royal Society, the “world’s oldest continuous scientific society.” I am very interested in the history of science, but I seem to perceive a dichotomous attitude eminating from scientists about their own story. One the one hand, it can be very illuminating to see how, and specifically, the order discoveries were made. Often, this is mirrored, consciously or not, by teachers whose lessons recapitulates this sequence. On the other hand, scientists might rather forget the wrong turns taken before reaching the currently accepted answer. Once you know the solution to the puzzle, why waste any more effort on incorrect guesses? There is, however, another possible reason. The natural sciences try to remove the human element, at least to the extent this is possible. Almost by definition, the laws of nature studied should be unaffected by the presence, or lack thereof, of humans to observe them. And they should certainly be immune to the vicissitudes of then-current events. However, as pointed out by the books The Age of Wonder, and The Clockwork Universe, show the interaction, both ways, of science and history. The Royal Society did more than anyone to promote its motto Nullius in Verba, in a world in thrall to received wisdom. Discoveries backed by the Society, like exotic island tribes and new planets, sparked a sense of amazement in the general population. Science, like everything, is subject to the whims of history. Chemist Antoine Lavoisier’s work was cut short by the guillotine. Texas might have celebrated the discovery of the Higgs Boson long ago, but for budget cuts. When I had the opportunity to visit the British Museum, I was duly awed by the amazing collection of artifacts, but it is hard to doubt the importance of Empire, and associated colonialism, that allowed such a collection to exist.
Growing up in Ohio, and working as a postdoc in Kansas, I got used to the possibility of having an unplanned University holiday called a snow day. Having moved to Florida, I expected the frequency of snow days to be sharply curtailed. However, today we, along with the Republican National Convention, are “enjoying” an day off thanks to Tropical Storm/Hurricane Isaac.
Speaking of absences, I thought I’d talk about at article in Nature that caught my attention last week. It highlights some of the weirdness of quantum mechanics, and shows how metaphors in science can end up becoming surprisingly real. In short, researchers in the article were able to construct a layered system of two conductive planes separated by an insulating layer so thin that it was comparable to the spacing between electrons in the conducting layers. If a current is driven in one of the conductive layers, electrons flowing near the barrier would would “drag” holes in the other layer.
Holes are are places were electrons could be, but aren’t, as explained below. The attraction between the negatively charged electron and the positively charged hole creates an bound state called an “exciton.” An exciton acts like an atom, in which electrons orbit a positively charged nucleus, but in this case, the electron and hole are in different materials. The surprising part is that, under certain circumstances, the induced “drag” current MUST be exactly equal and opposite to the driving current. This is a quantum mechanical effect, and has surprising implications.
When I was a graduate student at Ohio State, we studied conductive polymers, which, unlike virtually all organic materials, are conductors of electricity. We spoke a lot about excitons, since these were a very important feature in the story of how charge gets transported across the polymer molecules. And “charge transport” is exactly what electrical current is. The amazing thing is that this charge could either be an electron, or, just as easily, a hole, which is really nothing at all! The best way to understand the concept of a hole is to think of a crowded movie theater, where every seat is filled, except for one. If a person wants to shift seats, he or she has to be adjacent to the empty seat. If you watch from the projector room above, you can describe this situation as people moving, but it might be easier to keep track of the empty seat as it “moves” around. This is also like the “15 puzzle” games where only the tiles next to the empty space can move.
Physicists use a similar idea when talking about electrons, or the absence thereof. In materials, there are states available for the electrons to exist in, called orbitals. The famous Pauli exclusion principle dictates that two electrons cannot exist in exactly the same quantum state. This is what keeps all matter from instantly collapsing when all the electrons try to “fall” to the lowest energy orbital. The lowest energy state for this system occurs when the electrons fill the lowest energy levels first, and then higher levels, and so on, until all the electrons have a spot. Sometimes this is called the “Fermi sea,” since all the orbitals below a certain energy are filled, and all the orbitals above are empty. If energy is added, for example, by shining light on the material, a photon can strike one of the electrons and cause it to be “excited” to a higher energy orbital. This leaves an absence, an orbital that could be filled, but isn’t, in the material. When working out the equations that describe the material, it turns out that this “hole” can be treated just like an positively charged “antielectron.” That it, if the electron “falls” back and fills the hole, both are “annihilated,” and energy is released in the form of another photon. In fact, this is a useful way to think about what happens when matter meets actual antimatter – both are annihilated in a flash of energy.
While it might seem that the concept of a hole is just a cute metaphor by scientists who noticed a similarity in the equations, it turns out that holes are much more “real” than that. As indicated before, electric current is the movement of charges. Normally, as in the metal wires that bring electricity to your house, the charges are moving electrons. But in some materials, like many conducting polymers, it is more likely that the moving charges are holes! Yet the electrical current is just as real. You might say that in this case, the holes aren’t really moving, it’s the electrons moving the other way that gives the perception that the holes are shifting around. This is true enough, but these holes, just like electrons, have angular momentum, or spin. In excitons, they also serve as the positively charged “nucleus” that the electron is bound to. Not bad for empty space!
In the paper, the researchers showed that a drag current can be induced in one layer by pushing charges in the other. This doesn’t sound so impressive, besides the technical feat of making two conductive layers that are so closely spaced, but still separated by an insulator. The real surprise comes in the fact that the induced current is required to be equal in magnitude, and opposite in direction. The reason touches on a well-known but still totally non-intuitive aspect of quantum mechanics. That is, the idea of superposition. Superposition means that systems are not always constrained to exist in one state or another. They can be in some linear-combination of possible states. For example, an electron has a spin that, if measured in any direction, will always be either up or down. But the electron itself might have existed as some combination of up and down. In this case, the electron-hole pair existed as a superposition of a state where the electron was in the upper conductor and the hole in the lower one, and a state where the opposite is true, the electron was in the lower conductor, and the hole in the upper one. This symmetry makes the drive and drag currents equal and opposite. So not only are holes “real” in a sense, they can exist in two places at once, just like electrons(!)
So what does it all mean? It is always important to be clear when using metaphors what features are being accurately described, and which are not. Sometimes, however, we come across a metaphor that becomes more than just a way of thinking about a problem, and you get out more than what you put in. This is the hallmark of a successful scientific model.