This is a tricky one to explain simply.
Back in the early part of the 20th century, the scientists of the day struggled to make sense of quantum mechanics. Science had always been based around a deterministic reality. Something definitely exists at a particular point in space and if something else interacts with that, it will behave in a predictable way.
If there’s a pool ball on a table and you cue the white ball into it then the target ball will move in a direction that can be calculated precisely via fairly simple mathematics. I have my doubts this happens when I play pool but the principle here is that the balls exist at all times and behave deterministically based on the forces applied to them.
It’s the world we experience. It’s our instinctive understanding about reality.
Sigh, the quantum world is very different. It’s hard to get your head around those differences because they’re so contrary to what we experience in the normal (so-called ‘classical’) world.
Think about the sea. There are waves and when two waves come together we get a bigger wave and then when the peak of a big wave meets the trough of another, they cancel out to give us what you might call flat water. The sea is a turmoil of various waves, their peaks and troughs of differing sizes interacting to produce peaks and troughs of even more differing sizes.
So when one wave hits another you end up with two waves superposed together. They exist in what’s called a superposition. Both waves are essentially still there but they’re now intrinsically linked together.
Left alone, this is what the prevailing interpretation of quantum mechanics says reality is like. The waves aren’t water of course; they are waves of probability and I’ll leave you to think about what that really means but with a warning that thinking too hard about it can lead to institutionalisation and a nice cup of tea and some pills from a nurse at 3PM.
For the purposes of this article I ask you to just accept that reality exists as superposition of probability waves until we need to use a particular bit of reality in some useful way. The peaks and troughs of these probability waves tell us how likely something is to happen.
Now it would be fairly useless if my kettle only existed as a probability. If it was at a peak I might be lucky and get a cup of tea but if it was in a trough I might have to stay thirsty. It would be no good if my kettle continued to exist in a superposition of being both here or there. If the previously mentioned waves had combined in certain ways it may or may not even exist (peak or trough).
But we all know that kettles usually exist quite nicely, they stay where we put them and make nice cups of Earl Grey on demand. So if reality is actually quantum, how is my kettle such a stable object?
What we’re talking about here is an isolated quantum system. The combination — the superposition — of these waves means my kettle both exists and does not exist whilst that quantum system remains isolated.
This is how the quantum world is until an observation is made. The term ‘observation’ is open to interpretation but it does not necessarily mean a conscious human actually has to look at it. Another part of nature may ‘observe’ something when it needs to interact with it. The isolated, superposed system then has to serve up something we might call a bit more ‘real’ for it to interact with.
So we come to the subject of cats and boxes, which many cat owners will know is a sore topic before we even get started.
Just before we put the cat in the box (having presumably surreptitiously got the box out the night before to prevent the cat from bolting), let’s just say something about radioactive decay.
Individual atoms can radioactively decay. An atomic nucleus will just lose some energy and emit some radiation (photons of light perhaps). Collectively the process is reasonably predictable and we can talk about things like half-lives and such, but the decay of an individual atom is entirely random. It may or may not happen and there’s absolutely no way to predict it. It’s a quantum process.
So Schrödinger takes his cat and puts it in the box. Obviously the cat will splay out all four legs to try and avoid this and Schrödinger may have a few sticking plasters on his hands at the end of the process. But let’s say he manages to get the cat in the box.
Now very angry with his cat, he places a vial of poison in the box that can be released based on the radioactive decay of a single atom, which as I’ve said is completely random. Then he shuts the lid on the growling cat.
The presumption is we’ve now created an isolated quantum system, the state of which is dependent on the random quantum event of a decaying atom. We haven’t really created an isolated quantum system in these circumstances but remember this is just a thought experiment, so let’s assume we have.
As previously mentioned, isolated quantum systems exist in a superposition of probability waves. This particular one is dependent upon the random quantum process of a decaying atom. Until an observation is made, that atom has both decayed and not decayed because the peaks and troughs of the waves have merged. Unhappy cat, isolated in the same box, will also have its waves merged with those of the atom. So the atom is in a superposition of being decayed and undecayed and thus the cat is in the superposition of being both alive and dead at the same time.
Only by opening the box and making an observation will we find out the state of things. The important thing to note here is it’s not just that we don’t know the state of things because it’s all inside a closed box, the wave nature of quantum reality means the cat really is in a superposition. That is the way the quantum world works. Cast your mind back a few paragraphs, the cat really is both alive and dead in every scientific sense.
As ever with things quantum, this article contains a number of simplifications.
Apparently Schrödinger did actually have a cat called Milton, although obviously this is just a thought experiment and no real cats were placed in superpositions, as if they’d cooperate anyway.
Although I happen to believe cats can voluntarily place themselves in superpositions. My cat hates the vacuum cleaner and, whilst I don’t live in a big place, I have no idea where she goes when I vacuum. I’ve tried to find her but never can. She must be here though, so in a sense she’s in a superposition of both being here and not being here at the same time.