The Demon in the Machine

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In his forthcoming book, The Demon in the Machine physicist Paul Davies looks at how the essential transferral and exchange of information underpins so much of how we understand the world. Whether in the realm of biology (genetics) or technology (computers), research into complex webs of information is bringing us ever closer to unlocking the mysteries of life.

In this short introductory extract the author describes how birds have evolved different instinctive tools to navigate with unfailing accuracy – and how scientists are now applying quantum physics to better understand this extraordinary phenomenon.

Spooky birds

Scientists have discovered that birds use a variety of methods to find their way around, taking account of the orientation of the sun and stars as well as local visual and olfactory cues. But this can’t be the whole story because some birds can navigate successfully at night and in cloudy conditions. Special interest has focused on the Earth’s magnetic field, which is independent of the weather. Experiments with homing pigeons in the early 1970s showed that attaching a mag­net to the bird interfered with its ability to orient correctly. But how, exactly, does a bird sense the Earth’s magnetic field, given that it is extremely weak?* A number of physicists claim that it is quantum physics that en­ables the bird to navigate, by allowing it to see the field. Evidently, there has to be some sort of compass inside the creature, coupled to its brain so it can perform ​in‑​flight corrections. Tracking that com­pass down hasn’t been easy, but in the past few years a plausible candidate has emerged, and it depends on quantum ​mechanics – in​ fact, on one of its oddest features.

All fundamental particles of matter possess a property called ‘spin’. The idea of spinning bodies is of course familiar and simple ​enough – ​ the Earth itself spins. Imagine an electron as a scaled​-​down Earth, shrunk to a point in fact, but retaining its spin. Unlike planets, every electron has exactly the same amount of spin, as it does electric charge and mass; it is a basic property they have in common. Of course, electrons go round inside atoms too, and in that manner their speed and direction may vary, depending on which atom and which energy level (orbit) they occupy. But the fixed spin I am talking about is intrinsic to the electron, and the full designation is, unsurprisingly, ‘intrinsic spin’.

What has this got to do with birds? Well, electrons also possess electric charge (they are the archetypal electrically charged particle, which is why they are called electrons). As Michael Faraday discov­ered in 1831, a moving electric charge creates a magnetic field. Even if an electron isn’t moving from place to place, it is still spinning, and this spin creates a magnetic field around it: all electrons are tiny com­passes. So, given that electrons are magnetic as well as electric, they will respond to an external magnetic field much as a compass needle does. That is, the electron will feel a force from the external field that will try to twist it so the poles oppose (north–south)​. There is, how­ever, a complication. Unlike a compass needle, an electron is spinning. When an external force acts on a spinning body, it doesn’t just swing round and line up, it ​gyrates – a​ process called ‘precession’. That is, the spin axis itself rotates about the line of the applied force. Readers familiar with inclined spinning tops (which precess about the vertical due to the Earth’s gravity) will know what I mean…

This is an abridged extract from chapter 5 of The Demon in the Machine, published by Allen Lane – the author will be speaking at Second Home next Thursday 7th 7-8.30pm