The point of the most famous thought-experiment in quantum physics is that the quantum world is different from our familiar one. Imagine, suggested the Austrian physicist Erwin Schrödinger, that we seal a cat inside a box. The cat’s fate is linked to the quantum world through a poison that will be released only if a single radioactive atom decays. Quantum mechanics says that the atom must exist in a peculiar state called ‘superposition’ until it is observed, a state in which it has both decayed and not decayed. Furthermore, because the cat’s survival depends on what the atom does, it would appear that the cat must also exist as a superposition of a live and a dead cat until somebody opens the box and observes it. After all, the cat’s life depends on the state of the atom, and the state of the atom has not yet been decided.
Yet nobody really believes that a cat can be simultaneously dead and alive. There is a profound difference between fundamental particles, such as atoms, which do weird quantum stuff (existing in two states at once, occupying two positions at once, tunnelling through impenetrable barriers etc) and familiar classical objects, such as cats, that apparently do none of these things. Why don’t they? Simply put, because the weird quantum stuff is very fragile.
Quantum mechanics insists that all particles are also waves. But if you want to see strange quantum effects, the waves all have to line up, so that the peaks and troughs coincide. Physicists call this property coherence: it’s rather like musical notes being in tune. If the waves don’t line up, the peaks and troughs cancel each other out, destroying coherence, and you won’t see anything odd. When you’re dealing only with a single particle’s wave, on the other hand, it’s easy to keep it ‘in tune’ – it has to line up only with itself. But lining up the waves of hundreds, millions or trillions of particles is pretty much impossible. And so the weirdness gets cancelled out inside big objects. That’s why there doesn’t seem to be anything very indeterminate about a cat.
Nevertheless, wrote Schrödinger in What Is Life? (1944), some of life’s most fundamental building blocks must, like unobserved radioactive atoms, be quantum entities able to perform counterintuitive tricks. Indeed, he went on to propose that life is different from the inanimate world precisely because it inhabits a borderland between the quantum and classical world: a region we might call the quantum edge.
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