birds of a feather

ScientistjenlucLet me clarify, for the benefit of any concerned readers, that my post over the weekend bidding a fond farewell was not a departure from the blogosphere, but from my tenure as Journalist in Residence at the Kavli Institute for Theoretical Physics (KITP), the pretty peach-colored building in the photograph. It was a terrific experience, although not 100% comfortable — which I consider a good thing, because if one is not pushed beyond one's comfort zone once in awhile, one never makes any significant developmental progress. Not only was much of the subject matter unfamiliar (and often incomprehensible) to me, but I was compelled to crystallize my various random thoughts and approaches to science communication into a workshop-type format that would appeal to theoretical physicists (or at least some of them). Did I succeed? Sometimes. The only flat-out failure was my attempt to use PowerPoint Karaoke to jump-start a discussion about communicating across disciplinary boundaries. Talk about a deflating experience. In retrospect, I think I "framed" it incorrectly for my target audience. Next time, it will take place in a local bar and feature copious amounts of alcohol. That seems to have worked very well for the PowerPoint Karaoke event organized by this group of Australians from McCann Sydney.

Anyway, after my final workshop (a post on that is forthcoming later this week), I jumped into my shiny red Prius and navigated my way one last time from Santa Barbara to Los Angeles, just like a homing pigeon seeking to reunite with its avian equivalent of a Spousal Unit. I relied on past experience and my trusty GPS display to find my way home, but apparently, birds use the earth's magnetic field to help them navigate. According to a recent entry on the physics arXiv blog maintained by the mysterious "KFC," "A growing body of evidence points to the possibility that a weak magnetic field can influence the outcome of a certain type of chemical reaction in bird retinas involving radical ion pairs." In fact, it's possible to confuse the navigational abilities of birds by zapping them with magnetic fields that, apparently, disrupt this reaction.

KFC explains that while this proposed mechanism has substantial experimental evidence, to date, it's been a little incomplete theoretically. The ion recombination effect that gives rise to a preferred chemical reaction happens far too quickly to allow for any influence from earth's magnetic field — and yet, the experiments indicate that this field does play a vital role. Hmmm. In a recent paper posted to the arXiv, Iannis Kominis at the University of Crete has outlined an intriguing idea about how to resolve the paradox, namely, by evoking another one: arguably one of the most famous paradoxes in quantum physics, known as the quantum Zeno effect. Per KFC, "It states that the act of observing a quantum system can alter its evolution in a way that maintains the state for longer than expected." A more colloquial phrasing would say, "A watched quantum pot never boils."

Say what? There are quantum teapots? Well, no, not literally. But it's a useful analogy if one takes a bit of extra time to bone up on the broader context. And that means hopping into the Way-Back Machine for a brief visit to ancient Greece. Zeno was a Greek philosopher who logically constructed an argument to prove the (clearly) nonsensical assertion that motion is impossible. (Philosophers often like to play devil's advocate and argue for the impossible.) Zeno envisioned an archer shooting an arrow from his bow. Imagine Legolas Greenleaf from The Lord of the Rings doing just that. Legolastrailernewbow Assuming he shoots directly in front of him — it's tradition in physics to hypothesize idealized situations — the arrow will travel in a straight line indefinitely until it is stopped by an opposing force,  ideally, by piercing the heart of an evil Orc.

Zeno asked what would happen if you divided the distance the arrow must travel to its target into an infinite number of increasingly smaller increments, halving the distance every step of the way. He argued that this would mean the arrow would get closer and closer to its Orc-target but would never be able to reach the creature's heart. All motion would seem to stop. This sort of thing doesn't happen in the macroscopic world of our daily experience, of course: eventually Legolas' arrow will find its mark, and the Orc will perish. (Good riddance!) Zeno's abstract argument rests on the notion that the progression will continue for infinity, but in physical reality there is always some kind of limit. An endless series can still have a finite sum. There's lots of ways to describe the notion of a limit — it's a key concept in modern calculus — but just from a practical standpoint, the arrow has a fixed length (at least over the distance it travels). The distance the arrow must travel would eventually be subdivided to the point where the increments would be smaller than the arrow itself. And at that point, the arrow would hit its mark.

But the quantum world is a much weirder place, governed not by exact absolutes but by probabilities and uncertainty. On the subatomic level, something akin to Zeno's paradox actually happens. Physicists have argued for decades over the nature of a measurement or observation and its implications for quantum mechanics, ever since Werner Heisenberg first proposed his Uncertainty Principle. That's the one that says we can never know the precise momentum (or the precise velocity) associated with a particle, or we can know its exact location, but we can't know both at the same time. The very act of making the measurement changes the state of the atom.

It sounds like magic, but it's really not; it's the result of an actual physical force. We measure and observe atoms via electromagnetism, i.e., light of varying wavelengths. But how much we can see depends on the wavelength (and energy) of the light — a photon's energy is inversely proportional to its wavelength, so the shorter the wavelength of light, the higher the energy of its constituent photons. And the smaller the object we wish to observe, the higher the energy of light we must use in order to get the resolution we need to see that object. An atom is really, really tiny. To locate its precise position, we'd need to hit it with a photon of such high energy that significant amounts of that energy would be transferred to the atom itself, thereby altering it (changing its speed or direction). Basically, we know where the atom was, not where it now is, because our ham-fisted "observation" has knocked it out of its prior position.

Ergo, Heisenberg concluded that the mere act of observation can determine the outcome of a quantum experiment. But experimental measurements are made in single, fixed, brief moments in time. What if it were possible to continuously observe an experiment? And at what point does observation become continuous? Scientists actually know the answer to both questions. Back in 1977, researchers discovered that a radioactive atom would never decay if it were "observed continuously." And the critical transition point is one measurement every four-thousandths of a second. 

We have that precise figure thanks to the work of scientists at the National Institute of Standards and Technology (NIST) in Colorado. In 1989, they trapped 5000 charged beryllium atoms in a magnetic field and then tried to "boil" them by zapping them with a radio frequency field to raise their temperature. They expected the atoms to absorb the extra influx of energy and jump to higher ("hotter") energies. But this only happened if they didn't make any further measurements in the interim. The more often, they tried to measure the energy state of the atoms, the fewer of those atoms would reach the higher energy level. And at the rate of one measurement every four-thousandth of a second, no atoms at all jumped to the higher energy state. They just wouldn't heat up. It still happened even when the scientists used an automated measuring device.

Why does this happen? Blame it on uncertainty: the act of measurement interferes with the atoms' ability to absorb extra energy. The Spousal Unit once penned a classic blog post about this topic, employing quantum puppies to discuss the notion of quantum interrogation, which explains things beautifully even if the cuteness of the puppies tends to overpower all else. 

I like to think of it in the more concrete terms of Legolas' arrow. Let's imagine that this arrow is imbued with some elfin magical property by which it can grow longer over short intervals of time. That's a pretty decent analogy for what's happening to the uncertainty associated with two atomic energy states. At some point the uncertainty becomes large enough to bridge the two energy states — akin to lengthening Legolas' arrow to the point where it can reach an Orc's heart — the atom shifts to the higher energy state (and the arrow downs the evil Orc). The "uncertain arrow" then collapses back down to its original length and the whole process starts over again.

But every time we make a measurement of an atom's energy, or the length of Legolas' magic arrow — and no, that is not a euphemism! Get your minds out of the gutter and back onto the curb with the rest of us! — we reduce uncertainty, so it can't increase. Every time someone tries to measure Legolas' magic arrow, it becomes just a little bit shorter (oh, stop it!), to the point where it's never long enough to reach the Orc's heart. That's what happens to the energy states of atoms in the quantum Zeno effect. Uncertainty gets smaller with every measurement, because each measurement yields new information about the atoms, reducing the "fuzziness" of their energy states. Make those measurements often enough, and uncertainty never becomes sufficiently large to enable to atom to heat up. So a "watched" quantum pot never boils.

I know — it's really weird, and utterly counter-intuitive. That's quantum physics for you. By now you're probably wondering what the hell any of this has to do with birds and their navigation skills — assuming folks have even read this far. But according to Kominis in Crete, it is indeed relevant! Let's recap: scientists think that a weak magnetic field (like that of the earth) influences "the outcome of a certain type of chemical reaction in bird retinas involving radical ion pairs," but the sticking point is that the ion recombination happens too quickly for earth's magnetic field to have an actual impact. And yet it really does seem to influence the avian navigational process.

Per KFC, Kominis knew that it's "possible to slow down the rate at which molecules convert from ortho to para isomers when they are constantly involved in collisions." Something similar, he believes, happens in birds, namely, "The presence of a geomagnetic field extends the lifetime" of that recombination process, thereby giving the magnetic field more time to influence the outcome of the recombination. This really could turn out to be an extraordinary insight, since it means that birds have a built-in quantum sensor — roughly akin to a GPS chip, perhaps, or at least a compass — that determines their macroscopic behavior (i.e., navigation). It would also explain why birds occasionally are afflicted by a 30-degree "heading error," and why these built-in "compasses" only seem to be sensitive to a certain type of magnetic field strength.

Kominis even speculates that a similar mechanism might play a role in photosynthesis. It could be a brave new world out there, indeed, if it turns out that quantum effects can impact macroscale behavior. As KFC rightly notes, in his trademark style: The quantum consciousness people are going to be all over this like freshmen at a sorority party." Let the arguments begin!

3 thoughts on “birds of a feather”

  1. As a rule, I get particularly cautious when physicists make claims about biology (and, for what it’s worth, that’s what my day job involves doing right now). Often enough, they (we) are idealizing the wrong part of the system, ignoring what biologists already know, making predictions which have already been falsified, etc.

  2. Not to say that’s what is happening here, of course — it’s just important to keep occupational hazards in mind. 😉
    This is a priori more plausible than many another application of physics to biological systems, since magnetism is intrinsically a quantum thing anyway. The details of the issue are written up in Volume III of the Feynman Lectures, but this blog comment window might be too narrow to contain them.

  3. Dang! You beat me to a post on the subject! I thought I’d share my own classical analogy to the quantum Zeno effect: my cat Zoe.
    Like many cats, Zoe gets irritated when a bathroom door is closed, presumably blocking her from her territory. Once I’ve shut the door, there’s a lag of a few seconds and then she rapidly approaches the bathroom door, ending up pressed against it. I’ve discovered, however, that if I swing the door open soon after closing it, I can interrupt Zoe’s progress. If I open and shut the door with a rapid frequency (yep, I act weird around cats), I can keep Zoe from moving towards the door at all (until she gets bored with me and wanders off somewhere else.) By repeated ‘measurements’ of Zoe’s progress, I suppress her natural instinct to scratch on the outside of the door.

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