A cat is placed in a steel box. Also in the box, out of the cat’s reach, there is a Geiger counter and a tiny bit of a radioactive substance – a piece so small that there is a 50% chance that in the course of one hour one of its atoms will decay. If this happens, the release of radiation will be detected by the Geiger counter, which will trigger a spring mechanism that causes a hammer to break a small flask containing hydrocyanic acid. The deadly gas will then escape and kill the cat.
So, if the box is opened after an hour, there is a 50:50 chance that the cat will be dead. But things are not as simple as they seem. For modern physics tells us that the behaviour of matter and energy on atomic and subatomic scales – including the radioactive material sharing the box with the cat – is most accurately described in terms of quantum mechanics. And according to the view of the quantum world most widely held by physicists today, the motion and interaction of atoms and subatomic particles are essentially indeterminate until they are measured. In the case of the cat, the atom is in a ‘superposition’ of two possible states – decayed and not decayed – and it remains in this unresolved state until an observation is made. Until this time, the quantum event has an essentially fuzzy or indefinite character that can be described only in terms of the probability of possible outcomes. While such indeterminacy may not seem intolerably odd in the microscopic world, it is harder to stomach when its bizarre consequences visit the world of our everyday experience. And in this case, until the box is opened, it seems that the cat is in some sense both dead and alive!
Schrödinger’s cat The thought experiment outlined above was first devised in 1935 by one of the pioneers of quantum mechanics, the Austrian physicist Erwin Schrödinger. Far from wishing to promote the idea of dead-and-alive cats, his aim was to demonstrate the absurdity of the orthodox understanding of the quantum world. The problem that Schrödinger focused on – the so-called ‘measurement problem’ – is but one of the great oddities thrown up by this most peculiar branch of science. Yet, in spite of its deeply counterintuitive aspects, quantum mechanics has proved to be a massively successful model whose results have been validated experimentally on innumerable occasions. Recognized, along with Einstein’s theories of relativity, as the crowning achievement of 20th-century science, it underpins virtually every aspect of the current practice of physics. It has also had a profound practical impact on technology, with applications ranging from superconductors to super-fast computing.
‘When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images.’
Niels Bohr, undated
How, then, should we reconcile the quantum world with the world of our everyday experience? Does the bizarre behaviour of atoms and subatomic particles oblige us to reappraise our understanding of reality?
From desperation to hope At the beginning of the 20th century, physicists’ understanding of the world had in most respects diverged little from the classical path begun more than 200 years earlier by Isaac Newton. With respect to light, there was near-consensus that its behaviour could best be interpreted in terms of its observed wave-like properties. While this worked well for phenomena such as diffraction and interference, it manifestly failed to explain others, including the absorption and emission of light. It was chiefly in response to such failures that the first steps into the quantum world were taken.
One notable failure of the prevailing classical view was its inability to explain so-called ‘black-body radiation’: the way hot bodies radiate heat, glowing red, yellow and finally white as they grow hotter. It was to address this apparent anomaly that the German physicist Max Planck was driven to what he himself described as an ‘act of desperation’. Essentially as a ‘fix’ to make the equations describing black-body radiation work, Planck made the bold assumption that the radiation (energy) emitted by a hot body is emitted not continuously but in discrete packets, which he called ‘quanta’ (from the Latin meaning ‘amounts’).
While Planck himself did not suppose that his assumption was a reflection of an underlying reality, five years later Einstein successfully applied a similar method to another problem that had proved resistant to solution within classical mechanics: the photoelectric effect – the way a metal surface produces electricity when light is shone on it. Inspired by Planck’s ‘quantization of energy’, Einstein’s solution depended on the crucial assumption that light is made up of discrete entities (i.e. quanta) called photons. Further confirmation of Planck’s hypothesis came in 1913, when the Danish physicist Niels Bohr proposed a new structure of the atom which used quantum principles to explain its stability while absorbing and radiating energy.
Wave–particle duality Light, then, was presenting a puzzle and a challenge. While the classical wave theory demonstrably worked in some areas, the approaches taken by Planck, Einstein and Bohr only succeeded by ascribing particle-like behaviour to light. It was becoming increasingly clear that at the level of elementary particles, it was no longer possible to sustain the sharp classical distinction between waves (radiating through space, carrying energy only) and particles (moving from place to place, carrying mass and energy). So what was light: wave or particle?
The answer that eventually emerged was that, in some strange sense, it was both. Electromagnetic radiation (including visible light) and the elementary particles of which matter is composed exhibit so-called ‘wave–particle duality’. Recognition of this idea – arguably the most fundamental concept in quantum mechanics – was formally made by the French physicist Louis de Broglie in 1924. Just as Einstein had earlier proposed that radiation can display particle-like behaviour, so now de Broglie argued that matter – electrons and other particles – could exhibit wave-like properties.
In an extraordinary flurry of activity in the mid-1920s, a number of (mainly) German physicists succeeded in formulating the mathematical basis of quantum mechanics. In 1925 Werner Heisenberg developed an approach known as matrix mechanics; and in the following year Schrödinger formulated wave mechanics, demonstrating at the same time that his method was mathematically equivalent to Heisenberg’s. In this dramatically new account of the elementary nature of matter, the classical picture of electrons as discrete particles in orbit around a nucleus had been replaced by ghostly halos smeared (like Schrödinger’s cat) across probabilistically defined paths.
the condensed idea
The strange poetry of atoms