And what exactly is it?
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Imagine you have a 5×5 grid. There are 25 possible positions on this grid, from (1, 1) to (5, 5). Our intuition tells us that something that exists on this grid has to exist at one of those positions, and importantly that it *does not* exist at any of the other positions. If the object you’re looking for as at (2, 4) you can say that there’s a 100% probability of finding it at that location and a 0% probability of finding it at any of the other locations.
But we find through observation that quantum particles betray our intuition. Instead of existing 100% at one location they instead consist of a probability distribution across all possible locations. Perhaps the object you’re looking for has a very high probability of being at (2, 4) but it will be less than 100%, and its probability of being in any of the other locations will be greater than 0%. The mathematical formula that describes the object’s range of possibilities across all of the locations is called its *wave function*.
But, of course, things on a macroscopic scale have to exist in specific locations because we interact with them at specific locations. So how do we bridge the probabilistic quantum world and the deterministic, macroscopic world we observe in everyday life? When a quantum particle (or quantum system) interacts with an *observer*, the wave function *collapses* and the observer experiences that quantum object at one specific location: it begins to behave in accordance with our intuition.
I’ll note here that “observer” is something of a misleading term. It really just means anything that needs the particle to have a definite value regardless of whether that thing is conscious or actively trying to observe the particle. It might be easier to just think of it as “the quantum system interacts with something else.”
So how then does a quantum object go from a smear of probabilities to something that has a definite value? What exactly *happens* when the wave function collapses, and what has actually changed about the quantum object? **We have no god damn idea.** This is kind of one of the big mysteries in physics right now. Our description of quantum mechanics is consistent with the behaviors we can observe but it doesn’t make any intuitive sense. We can’t really picture what it is. It’s sort of particle-like, sort of wave-like, and not entirely consistent with anything we can directly observe in the macroscopic world.
Well, in fairness, we have several ideas: the Copenhagen Interpretation, the Many Worlds Hypothesis, and others. We just can’t prove that any one of those is correct.
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