Sunday, January 30, 2011

Bird magic

The gist of an article in the latest issue of Wired Science is so utterly amazing that it's hard to imagine that the phenomenon in question could take place as described… in the everyday world of birds.

The starting point is a familiar question: How does a tiny bird such as the European robin find its way down to Africa?

The basic answer—as we've known for several decades—is that the bird is capable of detecting the direction of the Earth's magnetic field lines. More precisely, a bird's eye contains optical cells that react to the local magnetic field in such a way as to provide the tiny creature with a kind of black-and-white picture of the field lines, which it uses as a map or, rather, as a compass. OK, fair enough. In "explaining" things in that fashion, we're merely using ordinary words to describe our observations in a common-sense style. To put it even more succinctly, the robin can apparently "see" the imaginary lines that represent the geomagnetic field. But the big question that remains unanswered is: How can a bird's eye actually "see" a magnetic field line? Well, we humans can see the direction of light rays entering a room, say, through a partly-opened window. So, maybe birds detect the direction of geomagnetic "rays" in much the same way that we react to the presence of light. OK, but how do they actually do this?

It's a recently-proposed answer to that question that takes us into a magical domain of physics that was designated by Albert Einstein (who discovered this phenomenon) as "spooky action at a distance". Today, this mysterious phenomenon, which defies common sense, is known as quantum entanglement. It's such a weird affair that it can't really be apprehended directly… unless you happen to be a European robin bound for Africa. For humans, the only way of coming to grips with this concept is through advanced mathematics. But let me nevertheless propose a kind of fuzzy analogy of the situation. This analogy is in no way rigorous, nor even correct (in fact, it's totally wrong and absurd), but it has the merit of highlighting the weirdness of entanglement.

Suppose you own a pair of twin cats, which are hungry, as indicated by their constant meowing. So, they're waiting for you to feed them.

But they happen to be shut up in adjacent rooms of your house. Now, you're in one room, with one of the cats, but you know that the second cat is located in the adjacent room, because you can hear the meows of both animals. You give the first animal a bowl of cat food, which it gulps down immediately. Now, I should have pointed out that the two cats in my example are not only twins; they're also quantum entangled… whatever that might mean. So, when you open the door in order to step into the adjacent room in order to feed the second cat, you discover with amazement (unless, of course, you've become blasé about quantum phenomena) that the second cat is no longer meowing. What's more, you find that this second cat has apparently had its hunger satisfied by the food you just gave to the first cat! I warned you: quantum entanglement is crazy stuff… so there's no point in trying to "grasp" what might be happening.

Let's get back to the robin's eye, which contains a protein named cryptochrome. In a typical molecule of cryptochrome, pairs of electrons exist in a state of quantum entanglement. When a photon of light hits a pair of entangled electrons in a cryptochrome molecule, the photon's energy affects both particles simultaneously, but one of the entangled electrons gets knocked a tiny distance away from its initial position. In this new position of the second electron, the geomagnetic field line is oriented in a slightly different way to what it is in the case of the first electron. And the bird's eye uses this infinitesimal difference—along with data of the same kind from countless neighboring pairs of entangled electrons being hit similarly by photons—to build up its map of the Earth's magnetic field. Straightforward, no?

Now, if there's anything that's not quite clear in my explanations, please let me know, and I'll do my best to enlighten you. But try to make your questions as precise as possible. Use mathematics, if you like...

Hey. Whatever happened to that lovely little European robin that alighted here just a moment ago? Jeez, I fear it has got eaten (simultaneously) by my pair of entangled cats!

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ADDENDUM
[of an intentionally lighthearted nature, unlike most of what I've just been saying]

QUESTION (to make sure you've been following me): What's the difference between a European robin?

You ask me: Between a European robin... and what? I'm sorry, there's no "and what" at the end of my question, which I'll repeat once again: What's the difference between a European robin?

ANSWER: There is, in fact, no difference whatsoever between a European robin. It has two legs, which are of exactly the same length. Particularly the left leg.

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