how do particles know when they are being observed?
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Physicists do not agree (or “nobody really knows” if you prefer a more ELI5 answer).
Trying to simplify an extremely complicated subject without misleading you:
What is “actually” going on in quantum mechanics is not something everyone agrees on. Even whether “actually going on” means anything or is useful to talk about is not agreed. Some of the best minds disagree or say that this is still a hard problem, though others don’t.
One way of thinking about it is that if you observe what physicists call a “quantum system”, which might your single particle, something happens to the system. People who think this way might say “the wavefunction collapses” or something like that. This is how Paul Dirac explains it in his textbook on quantum mechanics. Observation does something.
Another way of thinking about it is that observation of a system does not necessarily cause something to happen in the system. I am saying “necessarily” because of course everyone agrees that it is quite easy to observe a system in a way which does affect it.
For example, many people who follow the views of Hugh Everett III would say that when you observe a system you (the observer) and the system you observe become entangled. Nothing actually collapses. The resulting, rather complicated system, may then change in ways that looks a bit collapse-like, but there’s no collapse really happening.
There are other ways of thinking again that aren’t really easy to categorise into either “collapse” or “no-collapse” theories like I have above. I am just trying to give a flavour of the sorts of differences of opinion there might be.
A criticism of the Dirac view is exactly how does the collapse happen – a more sophisticated way of asking roughly your question. In technical terms, what causes the projection postulate to work? In the Everett view the challenge is to explain who the answer you get look just like the probability distributions you would get if the Dirac view was right (warning: there are lots of smart people who think some variant on Everett’s views are self-evidently right, there are also smart people who don’t think Everettians have made their case).
So, the reason why you are getting different answers in this thread is partly because there is no fundamental consensus.
Feynman (who thought clearly about all this in my view) said that the whole mystery is in the two slit experiment. Why, when something looks like it is a nice wave diffracting through two slits, do you only see single dots (one at a time) at the end, distributed in just the way they would be with a wave?
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