Quantum wonders: The Hamlet effect

Physics 5 May 2010
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Boiling, maybe

(Image: OJO Images / Rex Features)

A WATCHED pot never boils.” Armed with common sense and classical physics, you might dispute that statement. Quantum physics would slap you down. Quantum watched pots do refuse to boil – sometimes. At other times, they boil faster. At yet other times, observation pitches them into an existential dilemma whether to boil or not.

This madness is a logical consequence of the Schrödinger equation, the formula concocted by Austrian physicist Erwin Schrödinger in 1926 to describe how quantum objects evolve probabilistically over time.

Imagine, for example, conducting an experiment with an initially undecayed radioactive atom in a box. According to the Schrödinger equation, at any point after you start the experiment the atom exists in a mixture, or “superposition”, of decayed and undecayed states.


Each state has a probability attached that is encapsulated in a mathematical description known as a wave function. Over time, as long as you don’t look, the wave function evolves as the probability of the decayed state slowly increases. As soon as you do look, the atom chooses – in a manner in line with the wave function probabilities – which state it will reveal itself in, and the wave function “collapses” to a single determined state.

This is the picture that gave birth to Schrödinger’s infamous cat. Suppose the radioactive decay of an atom triggers a vial of poison gas to break, and a cat is in the box with the atom and the vial. Is the cat both dead and alive as long as we don’t know whether the decay has occurred? …

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