how do particles know when they are being observed?
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In order to observe something with your eyes you need photons to bounce off stuff. Particles when bumped with photons move away
Because a photon (particle of light) smashes into them. That’s how we observe them, by measuring the reflection of light off them. Same way we see anything except that at a quantum (tiny) level, the particles being observed are so small that observing them changes there position.
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?
The particle can be anywhere until it *has* to be somewhere.
Rather than the observation revealing where the particle is, it determines places the particle can’t be (or causality would break.)
The particle is everywhere that it could be, until that everywhere is only one place.
Moreover, how do we see what happens when we don’t observe, without still technically observing?
Because you are doing it wrong. Quit staring right at them like a pervert. Learn to use reflective surfaces and your peripheral vision if you are going to check out particles.
OP, every other answer in this thread is wrong at the time of this comment, because they erroneously imply that the particle is perturbed by the act of measuring, which changes the value from what it was previously. Quantum uncertainty is a fundamental physical limit on the accuracy with which a quantity can be know, and even with the best non-intrusive measurement equipment there would still be this uncertainty.
You’ve probably heard before that particles have wave-like properties. In crude summary, what this means is that at the quantum level, the location of a particle is defined by a spread of probabilities called a wavefunction. It is not in one place, waiting for us to detect it at a specific location within this probabilistic range. It has no 100% precise fixed location with hard boundaries. Because it is a wave, not a particle.
When an interaction occurs (physical, chemical, etc.), a particle is forced to pick a specific state in order for the outcome to be calculated. This is called observation, and does not need to necessarily be conscious. These interactions are occurring away from human sight constantly, where particles defined by probability are briefly forced to “fall into” a fixed state by the world around them. This is called a collapse in the wave function, and is usually what people refer to when they talk about “particles behaving differently when observed”.
I know this is overly long for an ELI5. But TL;DR: The particle is not being “tapped” or “knocked” or “shifted” such that its state is changed. Rather it is being forced to “fall into” a fixed value amongst many superimposed probabilities, in order to participate in an external interaction.
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.
They don’t. “Observed” in this context really refers to being measured, or forced to pick a state. It has nothing to do with someone actually looking at them.
For example imagine if a particle can take two paths, A or B, and show up at a detector afterwards. If absolutely nothing changes between them taking either path then the particle acts like it took both paths. But once something changes depending on if it takes one path or the other, like a counter ticks up if it takes path A, then it acts like it takes one path or the other.
How does it “know” to change its behavior this way? Presumably something about the measurement, interacting with the particle in some way, causes this change. It “collapses the waveform” such that instead of acting like a wave which can pass through both paths it acts like a particle that can only go through one or the other.
Particles do not need to know when they are being observed and the observer does not need to be sentient either. In order to measure the properties of a particle, it requires some interaction, typically electromagnetic radiation, i.e. light, radio, x-ray that sort of thing.
This interaction inevitably leads to changes in the quantum state of the particle in one way or another depending on what property is being measured.
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