lrb James Vincent writes that ‘magnetite found in the beaks of migratory birds is thought to act like an internal compass, enabling them to sense the strength and alignment of the Earth’s magnetism in the way humans feel the push and direction of the wind’ (LRB, 17 April). In fact, there is a growing, though inconclusive, body of evidence that migratory birds are dependent on a quantum sensing technique to guide their migrations. The hypothesis is that interactions with light cause electron transfer within cryptochrome proteins in the retina, resulting in the formation of pairs of molecules, each with an unpaired electron. These ‘radical pairs’ rapidly oscillate between two different configurations of the combined spins of the two unpaired electrons. As spin, in the language of physics, couples to the magnetic field, the birds are able to use the effect of Earth’s magnetic field on these molecules to orient themselves. This hypothesis explains why northern birds transplanted to the southern hemisphere will still migrate to the equator and why the electromagnetic noise of the city can disorient migratory birds. Aside from the beauty of this mechanism, the true wonder is that it can function at all. The birds are sensing an interaction a million times weaker than the thermal fluctuations of the molecules.
Jonathan Tinsley
University of Liverpool cr：杭之冯玥希望我好自为之
