SCHRÖDINGER’S BIRD

WHY AN­I­MALS DON’T READ MAPS

Guru Magazine - - Contents - KYLE PAS­TOR

If birds drove cars, they wouldn’t ar­gue about the GPS di­rec­tions it seems. Mi­grat­ing birds rarely get lost, but how they know which way to fly has re­mained a mys­tery. Un­til now…

When you hear the term ‘quan­tum physics’ there are a few things that might come to mind: the Large Hadron Col­lider, un­in­tel­li­gi­ble equa­tions and Star Trek gob­bledy­gook. Those of us who are more au fait with the realm of teeny-tiny science will know about par­ti­cles tun­nel­ing through a wall, the un­cer­tainty prin­ci­ple and an ill-fated cat in a box. An­i­mals al­most cer­tainly aren’t on any­one’s list. But that’s all about to change. Kyle Pas­tor ex­plains that science’s most poorly un­der­stood spe­cial­ism may have solved a one hun­dred year old bi­o­log­i­cal mys­tery. Quan­tum science is about things that are ex­tremely small, with quan­tum me­chan­ics ex­plor­ing ob­jects at the scale of pro­tons and elec­trons. Be­cause we are talk­ing about things at such a small scale, it is dif­fi­cult to ap­pre­ci­ate the ef­fects of quan­tum me­chan­ics (such as ‘ tun­nel­ing’) in the ev­ery­day world. But this could be about to change. In the past, sci­en­tists have only been able to see quan­tum ef­fects when things are very cold. But we live in a warm en­vi­ron­ment, so all the molecules that make up the world around us jig­gle around so much that they wash out the de­tails that quan­tum me­chan­ics give us. How­ever, new re­search has re­vealed that bi­o­log­i­cal sys­tems like plants and birds use th­ese hid­den quan­tum ef­fects to their ad­van­tage. Birds are much big­ger than an atom (and are warm and squishy), so they would seem to be among the least likely places to see quan­tum-ness. But this is where the Euro­pean robin comes in… As many of you will know, birds are able to nav­i­gate the globe with amaz­ing ac­cu­racy. In gen­eral, this is done by sens­ing the Earth’s mag­netic field. For a long time it was be­lieved that there was a small piece of mag­netic ma­te­rial in the beak of birds that gave them the di­rec­tion of the mag­netic field’s ‘po­lar­ity’ – much like how a nor­mal com­pass works. But it is now be­lieved that this is not the whole story and, in fact, the small amounts of me­tal in the beak may of­ten have noth­ing to do with nav­i­ga­tion. In a re­port pub­lished in a re­cent is­sue of

Na­ture, Na­ture, re­searchers ex­am­ined how birds in the lab­o­ra­tory re­act to changes in the mag­netic field, and found th­ese birds to in fact be blind to the mag­netic po­lar­ity (which way is North or South). They de­ter­mined this by us­ing mag­netic fields set up at dif­fer­ent strengths and dif­fer­ent an­gles, which showed that birds sensed the ‘in­cli­na­tion’ – or an­gle – of the mag­netic field com­pared to the hor­i­zon­tal in­stead of sens­ing the po­lar­ity. If you take a com­pass and hold it flat, its nee­dle will tell you the di­rec­tion of mag­netic North. The com­pass is show­ing you the po­lar­ity of the mag­netic field. Take that same com­pass and flip it 90 de­grees side­ways. The di­rec­tion the nee­dle now shows is called the an­gle dip or in­cli­na­tion of the mag­netic field. In the North­ern Hemi­sphere it points down, and in the south it points up (see im­age on the next page). This, of course, is very use­ful for nav­i­ga­tion. Now, back to our robin. With the pass­ing of the sea­sons, the robin uses the Earth’s mag­netic field to find its way be­tween Africa and North­ern Europe in a search for the best weather. The robin’s ‘in­cli­na­tion com­pass’, which it uses to nav­i­gate th­ese jour­neys, has been dis­cov­ered not in the beak but in the eye. The sci­en­tists found that the bird lost its mag­netic sense when fly­ing in the dark – mean­ing that the mag­netic sense must need light to ac­ti­vate it, and so must

be some­where in the eye. This find­ing makes per­fect sense: it is prob­a­bly giv­ing robins the abil­ity to ac­tu­ally see the Earth’s mag­netic field. Now that’s an in­cred­i­ble abil­ity to have. Lo­cated at the back of the eye, chem­i­cal com­pounds called cryp­tochromes re­spond to mag­netic force by per­form­ing a quan­tum reaction called a ‘pair-rad­i­cal reaction’. This reaction ‘mea­sures’ the in­cli­na­tion of the mag­netic field from any given lo­ca­tion. But how? Here comes the tricky stuff… Imag­ine you want to com­pare a bunch of weights on a bal­ance beam by plac­ing them on each side of the bal­ance. If you do this, the bal­ance beam will tip in the di­rec­tion of the heav­i­est weights. By look­ing at how much it is tilt­ing – by mea­sur­ing the an­gle of the tilt – you can fig­ure out how un­bal­anced it is. When light ex­cites the cryp­tochrome com­pounds in the eye, they can ex­ist in two states, the sin­glet state (S) and triplet state (T); th­ese states are a bit like weights on op­po­site sides of the bal­ance beam. If the S and T states are equal in num­ber, we have the equiv­a­lent of a bal­anced beam. This is the sit­u­a­tion if there is no mag­netic field. But when the bird ex­pe­ri­ences a mag­netic field with an in­cli­na­tion, the bal­ance is tipped and the states ‘pile up’ on one side more than the other. The robin’s brain in­ter­prets this im­bal­ance in S and T states as a change in the mag­netic field – just like we would mea­sure the an­gle of an un­bal­anced beam. Voilà, we have a bi­o­log­i­cal quan­tum com­pass! At the mo­ment sci­en­tists still need some more ev­i­dence to con­firm that this is a quan­tum ef­fect and not any­thing else. They plan to do this by us­ing ‘Quan­tum Con­trol’, a set of tech­niques that can rule out any­thing that may not be quan­tum. Put sim­ply, they will see how the birds re­act in a mix of fluc­tu­at­ing mag­netic fields. Care­ful anal­y­sis of what the birds do will con­firm whether the quan­tum re­ac­tions truly are in con­trol. It’s all pretty deep stuff, and there are some links be­low about quan­tum bi­ol­ogy to give a bit more of an in-depth ex­pla­na­tion. And even if it is tricky to un­der­stand, ‘Quan­tum Con­trol’ is a fan­tas­tic name. It should for­ever be used as the name for all su­per-vil­lain death ma­chines. Or comic book heroes’ su­per­pow­ers. Per­haps the ti­tle for the next Bond movie? Now who said physics was bor­ing?

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