# Schrödinger’s Cat is a thought experiment in which a cat is said to be both alive and dead

One cat, in a box. Placed next to the cat is a vial of poison together with a radioactive atomic nucleus. If the nucleus decays it will emit a particle that will release the poison and kill the cat. Close the box.

The decay of the nucleus can only be described in terms of probability. Without measuring it (by opening the box and taking a peak), we cannot say that the particle has been emitted or not. There is only a probability that it has, or hasn’t. There is also a probability that it has been emitted and it hasn’t been emitted, at the same time. This duality of events is known as a quantum superposition. Before we look into the box, all possibilities exist at the same time. When we open the box, we will see just one outcome.

So before we open the box, Schrödinger’s Cat is alive, dead and both alive and dead at the same time.

So why can’t we open the box and see the cat in its superposition of being both alive and dead? The theoretical answer is whilst we are not looking into the box, there exists only probabilities. The probability that the nucleus has emitted a particle, the probability that it hasn’t and the probability that it has and hasn’t at the same time. This is known as a probability distribution. By looking into the box, we force the probability distribution to collapse into one definitive outcome.

But there is still a probability that the outcome will be the superposition, that the particle has been released, and has not been released. Albeit a very, very small probability. If this was the case when we opened the box, would we then see the cat in both states? Well no. There is an additional theory called decoherence.

Decoherence occurs when a sub atomic, or quantum, system, such as a particle in a superposition, gets entangled with the much larger, and much more complex, macroscopic environment.

It’s as if the tiny superposition gets lost in amongst all of the other possible superpositions of the much larger system around it.

This is why we don’t see everyday objects, like cats, displaying the properties of superposition.

**By Graham Foster**

Graham is a graphic designer with a passion for science

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