Mathew Iredale

"Anyone who is not shocked by quantum theory has not understood it."
Niels Bohr

Few areas of scientific research have had (and continue to have) such a profound effect on philosophy as quantum theory. This is largely due to the fact that, as the science writer John Gribbin states:

In the world of quantum mechanics, the laws of physics that are familiar from the everyday world no longer work. Instead, events are governed by probabilities.

Unlike Bohr, Albert Einstein was not just shocked by some of the implications of quantum theory, but appalled. He famously objected to quantum indeterminacy with the simple statement "God does not play dice." Equally upset was one of the architects of quantum theory, Erwin Schrödinger, who devised a simple thought experiment to make plain the absurdity of one of these implications.

He imagined a closed room, or box, in which was contained a live cat and "a hellish contraption" which consisted of a phial of cyanide and a tiny amount of radioactive substance. The amount is so tiny that maybe within an hour one of the atoms decays, but equally probably none of them decays. If an atom decays then a Geiger counter triggers and via a relay activates a little hammer which breaks the container of cyanide, killing the cat.

In the everyday world, there is a fifty-fifty chance that the cat will be killed, and without looking inside the box we can say, quite happily, that the cat inside is either dead or alive. But, according to quantum theory, neither of the two possibilities open to the radioactive substance, and therefore to the cat, has any reality unless it is observed. The atomic decay has neither happened nor not happened, the cat has neither been killed nor not killed, until we look inside the box to see what has happened. Theorists who accept the standard interpretation of quantum mechanics say that the cat exists in some indeterminate state – a "superposition of states" – neither dead nor alive, until an observer actually looks into the box to see whether the cat is dead or alive.

Unfortunately for Schrödinger, not only did this imagined experiment not cause physicists to realise the absurdity of certain features of quantum theory, for the majority it became an example par excellence of the extraordinary and remarkable implications of the theory. Rather than being an embarrassment to quantum theory, a superposition of states became a defining feature of it.

Those who took Schrödinger’s thought experiment in the spirit in which it was intended could take some solace from the fact that the absurd situation in which a cat can be both alive and dead at the same time cannot actually be re-created in the lab.

The reason for this is quantum decoherence – the phenomenon by which a superposition collapses into one state or another. The rate of decoherence of a physical system depends upon its size. Whilst atom-sized entities may exist in a superposition of states, larger entities, especially those the size of cats, which are made up of billions of atoms, are fixed to a single, definite state.

Consequently, those sympathetic to Einstein’s position could claim that although quantum weirdness may apply to the sub-atomic world, in the day to day world of cats, books, people, and so on, to all intents and purposes God does not play dice.

However, now even this (somewhat desperate) defence of common sense is in danger of being overturned. The physicists William Marshall, Christoph Simon, Roger Penrose and Dik Bouw-meester have recently designed an experiment whereby one may evade decoherence to achieve a quantum superposition of states in an object around the size of a red blood cell (specifically, a mirror 10 microns, or 1/100th of a millimetre, across). Not huge by the standards of a cat, but enormous by the standards of an atom. According to Marshall et al, it would be "approximately nine orders of magnitude more massive than any superposition observed to date."

In essence, the hypothetical experiment relies upon the interaction of a single photon of light with a tiny mirror mounted on a tiny arm. The radiation pressure of the photon is sufficient to displace the tiny mirror. Creating a superposition of the photon causes the system to evolve into a superposition of states corresponding to two distinct locations of the mirror.

This is not the first proposal outlining how to create and observe a macroscopic superposition but it is the first that is in reach of current technology. Indeed, work is already underway to build the necessary components and so it may just be a matter of time before a version of Schrodinger’s hellish contraption becomes observable reality. The implications of this are not lost on its creators who state without exaggeration, "It is remarkable that a tabletop experiment has the potential to test quantum mechanics in an entirely new regime."

All the more remarkable given that it evolved from a hypothetical experiment designed to expose the absurdity of quantum theory. Schrödinger would be shocked indeed.

Suggested reading

In Search of Schrödinger’s Cat, John Gribbin (Bantam Books)

Towards Quantum Superpositions of a Mirror, William Marshall et al. http://arxiv.org/pdf/quant-ph/020001