how do particles know when they are being observed?


how do particles know when they are being observed?

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Particles don’t ‘know’ anything. But what is doing the observing affects the environment around itself.

At the quantum level, observation isn’t a passive thing. Rather than just looking at an object like we would in normal life, it’s more like a blind person tapping something with their cane. Predictably, if that object is something like a baseball on a hard floor, tapping it would tell the person where it is, but also change its velocity. The same kind of concept is in play with quantum particles because we have to interact with them somehow to get information.

A particle does not change, you see it does. You have to put it in a state that you can observe, which changes it nature.

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.

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.

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.

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.

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.

Moreover, how do we see what happens when we don’t observe, without still technically observing?

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.

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?

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.

In order to observe something with your eyes you need photons to bounce off stuff. Particles when bumped with photons move away

The short answer is “we don’t really know”. The Quantum Mechanics allows you to predict your observations (and is very good at it), but doesn’t answer the question „what really happens” (assuming this question is meaningful.

Observing a particle means it gets entangled with the observation apparatus. Why does it have to pick a classical state then is an object of interpretations of quantum mechanics. None of these was confirmed yet. Examples of interpretations are:

**Objective collapse** — when the quantum system becomes sufficiently large, something happens (e.g. because of gravity) that causes the system to fall into a classical state (this, however implies that information travels faster than light).
**Many world** — there is no collapse. Everything stays in superposition. Including the observer. There is a version of you that observes every possible outcome of the quantum experiment.
**Superdeterminism** — everything is determined. Including the fact if you decide to make an observation or not.

You can look at visualizations of some QM interpretations here:

They don’t “know”, but in order to observe them, we have to interact with them (often by slamming them into another particle and measuring the energy and such that’s released), and the interaction changes their energy/spin/position.

It’s not like with a microorganism where you can just… look at it through a microscope.

We “observe” an atom by observing energy or matter that has already bumped into that atom. We observe it by lighting it, warming it, or pinching it in some way. The real problem is trying to study one lonely atom, floating in cold space in the dark, without changing its speed or direction or temperature, since small particles are even changed by simple light. It’s like a police officer who pulls you out of traffic, asks for your license, and shines a flashlight in your face. You’re no longer going your original speed and direction, and you’re blushing red. You have been “observed”, and it’s the method of observing you that changes you because you are sensitive to those things. We don’t have a way to hide from atoms and observe them with anything they don’t react to because they react to just everything.

At a really basic level, the only way we can observe particles is by bouncing something off them or touching them somehow (like how we have to bounce photons off an object in order for our eyes to see it). So it’s not that the particles “know” they’re being observed, it’s that in order to observe them, we have to affect them somehow, and affecting them alters their behavior.

In quantum physics “observing” actually means “measuring”

In the Double Slit experiment it’s not enough to just look at the slits. You have to measure which hole it went through. This measurement affects the experiment, as you have to interact with the particle to know where it is.

You think of light as just something we do to see. But light comes in tiny little packets called photons, like marbles. In order to see other tiny particles, we have to shine light on them. The marbles come out of the light source, bounce off the object and back to our eyes. Just like marbles though, when our photon marble hits the object, it changes which way it’s going and how fast.

Sean Carrol’s Mindscape on YouTube is a good resource. He is a proponent of the “Many Worlds” theory in which the universe splits every time a quantum event happens (yes, thats happening an unbelievable number of times per second). But if you have some time kill, he is probably the best person to listen to as his entire career is built on quantum particles being observed and doing something because of it.

The TL;DR of these quantum theories is “fields” theories – that most particles you are talking about “act” differently when the Wave AKA Schrodinger equation gets triggered and collapses into a particle. Electrons are one of the things considered to be a wave function, not a bouncing dot. Things like photons and all sorts of things are fields of energy that, when they get observed, collapse into a particle.

That is important to understand for a few reasons:

– Some people don’t believe in fields theory to the extent that others do. There are people who believe electrons ARE particles that teleport or quantum tunnel or whatever and that they are merely seeing something ***we dont understand yet.*** (not that we understand quantum phenomena to a happy degree yet).

– Some people believe EVERYTHING is one big wave instead of a lot of small waves that move through the universe. That we are all already connected and the entire universe split before you ever looked as there were only so many ways that wave could collapse so it split into all possible version and you are just in the version where it is the way it is now. so ***you are not observing a split, it already happened***. You just feel like you observed it, but it had little to do with you.

There are mature theories out there but my amateur ass can only come up with those two as the fundamental ones. I don’t think there is any kind of wide held belief that particles “know” anything and the “Conscious brain” theory of observation isn’t a widely held one because thats super unscientific. The measuring tool one makes more sense because you are somehow interfering and this is why you’ll read about tools that measure tools and they measure different part at different time of when a particle traveled. There was an article like 2 weeks ago about a particle that would have been on one side or the other of a split and one tool measure one thing and another measured another so they wouldn’t be collapsing it into just the type of thing one or the other could measure. That shit? They just don’t know.

One of the reason’s they have a hard time with this is because of quantum entanglement. They have the rule “nothing is faster than light, including information” and when you flip a quantum state, the other one, supposedly, flips instantly and that violates everything. However, they said if you were on mars and someone on earth flipped it, theyd have to send a message to you at the speed of light to tell you what theirs was to verify that yours was indeed the opposite and you wouldn’t KNOW, FOR SURE, that it worked until you got the information it happened. So when we talk about quantum information and collapse and observation, the information being transmitted by the event needs to be taken into account and that there may be something more fundamental happening we just don’t have the ability to quantify at this point.