Quantum mechanics is generally considered to be the weirdest of sciences. But, it was thought, this is kind of OK because it was only needed to account for the behaviour of the tiniest objects – electrons, protons, photons etc – and not to the big stuff that we can see. But, one of the founders of quantum mechanics, Erwin Schrödinger, pointed out that things were not so simple. Schrödinger imagined sealing a cat inside a box. The cat’s fate was linked to the quantum world through a vial of poison that will only be released if a single radioactive atom decays. Quantum mechanics insists that the microscopic atom must exist in a peculiar state called superposition until it is observed, a state in which it has both decayed and not decayed at the same time. But because the cat’s survival depends on what the atom does, then, according to quantum mechanics, the very macroscopic cat must also exist as a superposition of a live and dead cat until somebody opens the box and observes it.
Of course nobody really believes that a cat can be simultaneously dead and alive. Schrödinger’s thought experiment was to point out that it isn’t so easy to compartmentalise the world into quantum and non-quantum regions. And although no sinister scientist has ever set up the cat experiment, in reality, the fate of lots of macroscopic objects will sometimes depend on the dynamics of individual particles or small numbers of particles that are subject to weird quantum laws, particularly when those macroscopic objects are alive.
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