Science

This is an idea that has been bouncing around for a while, but is now apparently seen in experiments: real-world photosynthesis taking advantage of quantum mechanics. (Story in Wired, via @symmetrymag. Here’s the Nature paper on which it’s all based.)

The idea is both simple and awesome: you want to transport energy through an “antenna protein” in a plant cell to the “reaction-center proteins” where it is chemically converted into something useful for the rest of the plant. Obviously you’d like to transport that energy in the most efficient way possible, but you’re in a warm and wet environment where losses are to be expected. But the plants somehow manage the nearly impossible, of sending the energy with nearly perfect efficiency through the judicious use of quantum mechanics.

We can think about this in terms of Feynman’s way of talking about quantum mechanics: rather than a particle taking a unique path between two points, as in classical mechanics, a quantum particle takes every possible path, with simple paths getting a bit more weight than complicated ones. In the case of the protein, different paths for the energy might be more or less efficient at any particular moment, but this bit of quantum trickery allows the energy to find the best possible route at any one time. Imagine at rush hour, if your car could take every possible route from your home to the office, and the time it officially took would be whatever turned out to be the shortest path. How awesome would that be?

The reason you can’t do that is that your car is a giant macroscopic object that can’t really be in two places at once, even though the world is governed by quantum mechanics at a deep level. And the reason for that is decoherence — even if you tried to put your car into a superposition of “take the freeway” and “take the local roads,” it is constantly interacting with the outside world, which “collapses the wave function” and keeps your car looking extremely classical.

Proteins in plants aren’t as big as cars, but they’re still made of a very large number of atoms, and they’re constantly bumping into other molecules around them. That’s why it’s amazing that they can actually maintain quantum coherence long enough to pull off this energy-transport trick. Previous studies had hinted at the possibility, but only by cooling the proteins down and shielding them from external jiggling. This new work happens at room temperature in the context of marine algae, so it seems to indicate that it can happen in real environments.

One step closer to building my teleportation machine. Get to work, quantum engineers!

This weekend at Caltech we had a small but very fun conference: the “Physics of the Universe Summit,” or POTUS for short. (The acronym is just an accident, I’m assured.) The subject matter was pretty conventional — particle physics, the LHC, dark matter — but the organization was a little more free-flowing and responsive than the usual parade of dusty talks.

One of the motivating ideas that was mentioned more than once was the famous list of important problems proposed by David Hilbert in 1900. These were Hilbert’s personal idea of what math problems were important but solvable over the next 100 years, and his ideas turned out to be relatively influential within twentieth-century mathematics. Our conference, 110 years later and in physics rather than math, was encouraged to think along similarly grandiose lines.

And indeed people had done exactly that, especially ten years ago when the century turned: see representative lists here and here. I asked the organizers if anyone was taking a swing at it this time, and was answered in the negative. I was scheduled to give one of the closing summaries, and this sounded more interesting than what I actually had planned, so naturally I had to step up.

Here are the slides from my presentation, where you can find some elaboration on my choices.

And here’s the actual list:
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24 Questions for Elementary PhysicsRead More »

… not yet released, but we’ll find out in just a bit. 2:00 p.m. Pacific time, to be exact.

Last week we mongered the rumor that the CDMS experiment was going to announce an exciting new result soon — and that time is now. (My guess remains: some interesting data that falls well short of “we’ve discovered dark matter!”) If you’d like to watch the talks online, here you go:

• Kavli Institute for Particle Astrophysics and Cosmology
• Fermilab

Further bulletins as events warrant.

All the physics blogosphere is abuzz about rumors that the CDMS experiment might have collected evidence for the direct detection of dark matter, and is going to announce their results on December 18. The original source was Resonaances, where you can read the basic story; see also New Scientist. It’s to the point where it’s more suspicious if we don’t mention it than if we do, so here you are.

Not too much point in speculating — we’ll find out next week! There was some misplaced excitement about a Nature paper, but it is true that CDMS has scheduled simultaneous talks at CERN, Fermilab, SLAC, and elsewhere. Steinn did the citizen-journalist detective work and dug up the abstract for Priscilla Cushman’s talk at CERN:

I will present new results from the recent blind analysis of 612-kg days (before cuts) of data using the CDMS germanium detectors at Soudan. CDMS uses ionization and athermal phonon signals to discriminate between candidate (nuclear recoil) and background (electron recoil) events in Ge crystals cooled to ~ 50 mK. Timing, yield and position information allows us to tune our expected background leakage into the signal region to 0.5 events. I will report on what we saw when we “opened the box”, whether we have seen WIMPs or not, and implications for future dark matter direct experiments.

It would seem unlikely to me that CDMS would be able to announce a cut-and-dried discovery of dark matter; that would require collecting an awful lot of data. (But what do I know?) It’s more plausible that they would see some kind of provocative signal, but without quite enough significance to be definitive. With many different competing experiments, several of which have been working for quite some time now, it seems like the kind of result that you would gradually sneak up on, rather than dramatically capture in one fell swoop. Or maybe they’re just updating us on their best limits, and some rumor-mongering has spiraled a bit out of control. We’ll see.

The Hubble Space Telescope has come out with a new ultra deep field image — this one in the near-infrared, taken with the Wide Field Camera 3 (WFC3) that was installed on the latest servicing mission. Colors are fake (obviously, unless you have infrared vision), but the spirit of the colors has been preserved — red regions are redder in real life, etc. Click on the image for a higher-resolution version (about 1.6 MB). Even higher resolutions available at HubbleSite.

Not too different, to the naked eye, from previous incarnations of the ultra deep field. That’s okay. I can sit and stare at these images for hours. Every one of those blobs is a galaxy! Holy crap.

This year we give thanks for one of the bedrock principles of classical mechanics: conservation of momentum. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, and the Spin-Statistics Theorem.) There are analogous notions once we include relativity or quantum mechanics, but for our present purposes the version that Galileo and Newton would have recognized is good enough: in any interaction between bodies, the total momentum (mass times velocity of each body, added together vectorially) remains conserved.

Now, you might feel somewhat disappointed, thinking that conservation of momentum is important, sure, but not really cool and interesting enough to merit its own Thanksgiving post. How wrong you are!

First, conservation of momentum isn’t just an important physical principle, it played a crucial role in the development of the idea of reductionism, which has dominated physics ever since. Aristotle would have told us that to keep an object moving, you have to keep pushing it. That sounds wrong to anyone who has taken a physics course, but the thing is — it’s completely true! At least, in our real everyday world, where Aristotle and many other people choose to live. Push a cup of coffee across the table, and you’ll notice that when you stop pushing the cup comes to a stop. Galileo comes along and says sure, but we can go further if we instead imagine doing the same experiment in an ideal environment that is completely free of friction and air resistance — and in that case, the cup would keep moving along a straight line. This has the virtue of also being true, but the drawback of not relating directly to the world we experience. But that drawback is worth accepting, because this backward step opens an amazing vista of progress. If we start our thinking in an ideal world without friction, we can assemble all the rules of Newtonian mechanics, and then put the effects of air resistance back in later. That’s the birth of modern physics — appreciating that by simplifying our problems to ideal circumstances, and understanding the rules obeyed by individual components under these circumstances, we can work our way up to the glorious messiness of the world we actually see.

The second cool thing about conservation of momentum is that it was not Galileo who came up with the idea. As with many grand concepts, it’s hard to pin down who really deserves credit, but in the case of momentum the best candidate is Persian philosopher Ibn Sina (often Latinized as Avicenna). Ibn Sina lived at the turn of the last millenium, and was one of those annoying polymaths who was good at everything — he’s most famous for his contributions to medicine, astronomy, and philosophy, but also dabbled in physics, chemistry, poetry, mathematics, and psychology. Along the way he introduced the idea of “inclination” or “impetus.” Now, Ibn Sina (like anyone else in the year 1000) had some wrong ideas about mechanics and motion, and historians of science argue over whether his notion of inclination really matches our contemporary idea of momentum. But he defined it as “weight times velocity,” and — most importantly — understood that it would be conserved in the absence of air resistance. Sounds like momentum to me.

Finally, conservation of momentum is important because it has sweeping implications for the way the world works at a deep level, implications that many people still have trouble accepting. Back in Aristotle’s time, the natural state of a coffee cup, like anything else, was to be at rest. But we look around us and see all sorts of things moving around. So clearly these motions require an explanation of some sort — something that keeps them moving. Despite the later triumphs of Newtonian mechanics, that way of thinking still seems very natural to us, and leads us to a certain outlook on the ideas of “cause and effect.” Things don’t just happen (this way of thinking goes), they happen for some reason. And we can take this line of reasoning all the way back to a purported First Cause or Prime Mover. But the lesson of conservation of momentum — and indeed, of all of modern physics — is exactly the opposite. Things don’t move because something is pushing them; they move because they just are, and can continue to do so forever. The fundamental relation between different events is not one of cause and effect; it’s one of inviolable patterns, in which no particular events are distinguished as “causes” or “effects.” And this viewpoint, as well, can be traced all the way back to grand questions of the universe — why is there something rather than nothing? There doesn’t need to be an answer to this question of the form “Because X made it so” — the answer can simply be “Because that’s the way it is.”

So thanks, conservation of momentum. The next time I find myself on a perfectly frictionless surface in the absence of any air resistance, I’ll be thinking of you.