Eli5: what is a superposition?

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Eli5: what is a superposition?

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Pretty much the belief that if you don’t do something, like cross your fingers when you say something bad, that it’ll happen to you

It is the idea that something, namely a particle, exists in multiple states (or positions)simultaneously until observed or measured.

It’s hard to quantify because we are humans, not particles. But think about this. Have you ever seen someone or something sleep so still that for a brief moment you thought they were dead, not sleeping? Well for that brief moment, the person is both sleeping AND dead. It is only once you observe them that you confirm if they are sleeping OR dead. Without that observation, the person exists in a state of superposition whereby they are BOTH sleeping and dead.

Superposition as a concept is actually surprisingly simple, considering we mostly hear about it in things that aren’t very simple at all.

Basically, it’s the idea that you can just add up all the effects on the system to figure out it’s final state.

For example, if you and I have a shared savings account and I put $5 into and you put $15 into it the total amount of savings has now gone up by $20. That $20 increase is a superposition of our two changes.

This is used all over the place in physics. Since stuff tends to just…affect other stuff.

The simplest illustration of it is the famous double-slit experiment. This involves shining a laser at an object with two tiny slits in it, very close together. The light waves passing through each slit interfere with each other and form a characteristic pattern, consisting of a series of dots that receive a lot of light, with the gaps between the dots getting very little light. The weird part is that this pattern still shows up if you just fire one individual photon at the slits at a time – it’s more likely to hit the places where those dots were, and less likely to hit the gaps between the dots. Effectively the photon behaves as if it passes through both the slits and interferes with itself.

All kinds of variations of this experiment have been carried out over the decades, including ones in which the laser is replaced with a stream of electrons, atoms, or even fairly large molecules, and they all show the same behaviour.

The only coherent way to understand experiments like this is to model fundamental particles as if, instead of being in a single state, they are in a “superposition” of different states simultaneously. For example, at a given time, an electron does not have a single location – it is effectively smeared out across a region of space. This all sounds vague, but it has a concrete mathematical description and has been tested with all kinds of experiments. The only big remaining mystery is the so-called “measurement problem”, which is essentially the question of how exactly our large-scale world, in which things seem to have defininte locations, is built up out of quantum systems in which everything is in a superposition of different states. There are various proposed answers, but it has turned out to be very difficult to do experiments to probe this question.

u/Caucasiafro is referring to the [superposition principle](https://en.wikipedia.org/wiki/Superposition_principle), which is a property of a lot of systems that you can calculate the effect of multiple stimuli by adding up what their individual effects would have been.

u/Dane_Bramage is referring to [quantum superposition](https://en.wikipedia.org/wiki/Quantum_superposition). It has some mathematical similarities to the principle above, but as Dane’s comment says, a quantum superposition itself is a statement about the current existence of a particle, not to a stimulus/response process. There is a superposition principle for these quantum states (the way the whole system responds in certain interactions can be seen as the sum of individual responses for the different states it is in), which is why the name was carried over from one to the other.