Bird migration is a spectacular event. Every year, birds fly for thousands of kilometres, across continents and hemispheres, finding their way from their summer homes to their winter ones and back again.
But how exactly do they navigate this journey?
As ornithologist Scott Weidensaul explains in this extract from A World on the Wing, it could be that birds use a phenomenon from quantum physics to find their way.
- A World on the Wing is one of our top science books for March 2021
It’s a pretty weird [idea], but then, so are most things in the quantum realm. Here is the framework, as it’s currently understood.
A migrating bird, flapping through the night sky, glances up at the stars. A photon, having left one of those stars millions or even billions of years earlier, enters the bird’s eye and strikes a molecule of a form of cryptochrome, almost certainly a specific variant known as cryptochrome 1a, or Cry1a.
This encounter takes place in the retina, probably within a set of specialised vision cells known as double-cones, whose function had heretofore been a mystery. The photon knocks free one of the Cry1a’s electrons, kicking that electron into a neighbouring Cry1a; because they now each have an odd number of electrons, the two molecules are known as a radical pair, and are linked – entangled, in the jargon of quantum mechanics.
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They are also magnetic, because the electrons have a property known as spin (which isn’t really spin, in the way you’d picture a top; it’s actually a state known as spin angular momentum, but never mind – let’s not go down too many quantum rabbit holes right now).
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Such entangled particles are joined regardless of distance, defying classical physics and common sense. They have become, in effect, one thing; if you measure the properties of one – even were they separated by millions of light-years – you could infer the properties of the other.
Einstein, whose own work helped spawn the concept, famously rebelled against this idea of entanglement, which he dismissed in the 1930s as “spooky action at a distance.” Yet experiments have borne it out.
In the eye of a migrating bird, the effect of countless radical pairs probably creates a dim shape or smudge – visible as the bird moves its head, but not opaque enough to interfere with normal vision – that shifts with the bird’s position relative to the ground and to the inclination of the magnetic field lines arcing out of the planet.
But if you’ve heard about entanglement, it’s likely because of the seriously strange uses to which it’s being put. In 2017, for example, Chinese scientists used entanglement to “teleport” two entangled photons – or, at least, the information in those photons – from an orbiting satellite to ground stations more than 700 miles apart.
And in 2020, the same Chinese team said they used quantum entanglement to transmit an unbreakable encryption code to a satellite. These are a long way from beaming up a Star Trek character, but they were hailed as the first steps to an unhackable quantum internet, and possibly even faster-than-light communication.
Ironically, entanglement itself may not be essential to the process that allows birds to see a magnetic field. This strange branch of quantum theory may be, in the words of two leading researchers, “something one gets ‘for free’ in a cryptochrome,” not essential for the molecule to act as a magnetic compass.