how do particles know when they are being observed?
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So… This is a really interesting question, and super counterintuitive. In any case the question isn’t how a particle knows it’s being observed, that’s easy. It was hit by a photon or something that was already in a definite state. The weirdness comes in because in some experiments like the double slit experiment it seems like the particle will somehow decide whether or not to act like a wave or a particle before it interacts with the observer, and that messes with our notion of cause and effect, since it seems like information has traveled into the past.
But in ELI5 terms I would say that the particle “knows” it’s being observed, because it doesn’t have a choice about the matter.
Really “knowing” isn’t really the right way to think about it. It reflects our understanding of the world at a big scale where objects always travel definite paths. On a quantum scale a particle always exists along all possible paths. That’s just the laws of physics. There’s no why for it any more than there is a why for the speed of light being constant. It “knows” whether it’s observed or not in the same way that rocks “know” how to fall when you drop them.
So with that understanding a particle that is detected going though a single slot in the double slit experiment still exists on all possible paths. It’s just that there is only one possible path that results in it being detected. A non detected particle exists just the same on all possible paths. But the a non detected particle has two slits it can pass through and so exists on both of those paths in a quantum superposition.
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