The cat that wouldn't die

[Celerity]

The weird paradox of Schrödinger’s cat has found a lasting popularity. What does it mean for the future of quantum physics?

https://aeon.co/essays/no-schrodingers-cat-is-not-alive-and-dead-at-the-same-time





In 1935, the Austrian physicist Erwin Schrödinger published a rather critical three-part review of what he called the ‘present situation’ in the relatively new theory of quantum mechanics. For the most part, Schrödinger’s review, written in German, is dry and technical, and not the kind of thing that would detain anyone outside the narrow academic world of quantum physics. But in one short paragraph, written with his tongue firmly in his cheek, he gave flight to a fancy that, 90 years later, continues to resonate in popular culture. The paragraph concerned Schrödinger’s eponymous cat. How did an obscure argument about a mathematically complex and rather baffling theory of physics become embedded in public consciousness as an extraordinary exploration of the human psyche? This essay tells the story. Here’s what Schrödinger wrote (in the English translation by John D Trimmer):



Although this was only a ‘thought’ experiment, the paradox of Schrödinger’s cat was destined to join Pavlov’s dog in science’s bestiary of the bizarre. To understand the point Schrödinger was making, we need to do a little unpacking. The nature of Schrödinger’s ‘diabolical device’ is not actually important to his argument. Its purpose is simply to amplify an atomic-scale event – the decay of a radioactive atom – and bring it up to the more familiar scale of a living cat, trapped inside a steel box. The theory that describes objects and events taking place at the scale of atoms and subatomic particles like electrons is quantum mechanics. But in this theory, atoms and subatomic particles are described not as tiny, self-contained objects moving through space. They are instead described in terms of quantum wavefunctions, which capture an utterly weird aspect of their observed behaviour. Under certain circumstances, these particles may also behave like waves.

These contrasting behaviours could not be starker, or more seemingly incompatible. Particles have mass. By their nature, they are ‘here’: they are localised in space and remain localised as they move from here to there. Throw many particles into a small space and, like marbles, they will collide, bouncing off each other in different directions. Waves, on the other hand, are spread out through space – they are ‘non-local’. Squeeze them through a narrow slit and, like waves in the sea passing through a gap in a harbour wall, they will spread out beyond. Physicists call this diffraction. Push a bunch of different waves together and they will merge to form what physicists call a superposition. The peaks and troughs of all the different waves add together. Where peak meets peak, the result is a larger peak. Where trough meets trough, the result is a deeper trough. Where peak meets trough, they are both reduced and, if they happened to be of equal height and depth, they will completely cancel each other out. Physicists call this interference.



By 1935, the mathematical formulation of quantum mechanics was relatively mature, and acknowledged by most physicists as complete. But the theory does not say where all the quantum weirdness is supposed to stop. When applied to the radioactive atom, quantum theory says that, after an hour, the wavefunction of the atom is represented as an equal mix – a superposition – of decayed and undecayed atoms. We could choose to stop there, but we know that the interaction between the atom and the diabolical device should also be described by quantum mechanics, at least in its early stages. If we choose to take the theory literally, then extending its equations to include the entire device means that the wavefunction evolves into a superposition of decayed atom and triggered device, and undecayed atom and untriggered device. In his review, Schrödinger coined the term ‘entanglement’ to describe this situation. The radioactive atom becomes entangled with the device.

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