Dead and Alive

Quantum physics opens up an entire new world of possibilities. You can tell, because this must be like the fifth post about it and we’ve barely covered a reasonable proportion of this field of physics.

In A (Qu)Bit More, Harvey briefly outlined the famous Schrödinger’s cat thought experiment. A hypothetical cat is enclosed in a box along with a radioactive source and a vial of poison. The radioactive source has a 50% chance of decaying and a 50% chance of not decaying. When it decays, the poison is released, killing the cat. If it doesn’t decay, the cat is still alive, and we don’t have to hear from PETA.

So when you open the box, you have a 50-50 chance of finding the cat alive or dead. But what state was the cat in just before you opened the box to check? The answer seems simple: it was either alive or dead but we just didn’t know which one it was.

Quantum physics says this is false though. Before you open the box, the cat is actually in a superposition of both states, i.e. it is both alive AND dead. You might think this is all physics jargon – why should we believe this?

We can demonstrate superpositions using Young’s double slit experiment. This was an experiment performed by Thomas Young in 1801 to demonstrate the wave nature of light, and it was this experiment that convinced the scientific community of the wave nature of light for the next century or so.

A beam of light is shone through two slits. Here the light acts as a wave, and whenever a wave passes through a gap, it will diffract (spread out). Because there are two slits, two separate diffracting waves are formed.

We have a set of rules for describing what happens when two waves meet. When the crest of a wave meets the crest of another wave, they add up – this is constructive interference. When the crest of a wave meets the trough of another wave, they cancel out – this is destructive interference. The resultant wave is smaller, or even non-existent.

constructive-and-destructive-interference
Image by mtgriffiths – Get Revising.

This is one of the simplest examples of superposition – the superposition of two waves.

On the screen at the end, we see a pattern of alternating bright and dark fringes, called an interference pattern. The bright fringes are where the waves added up, and the dark fringes are where the waves cancelled out.

double-slit-experiment
Image by Genesis Science Mission.

This pattern is almost certainly a consequence of the interference of the two waves, because if you did the same experiment but with only one slit, you get a blur. (There is sometimes a weak pattern of alternating bright and dark fringes, but that’s a result of the diffraction of waves around the edges of the slit that can produce two separate waves.)

However, a superposition of waves isn’t a quantum superposition, so it’s not great evidence for a cat being both alive and dead. Quantum superpositions have far weirder results, including photons that predict the future. But we’ll get there next week.

Yanhao

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