how do photons and electrons know they’re being observed?
what counts as ‘observation’?
how do we even know they behave differently under observation if we can never see how they act when not being watched?
why do they behave differently under observation in the first place?
In: Physics
“Observed” is a misnomer. To observe something we have to see it. To see it we need light. Light doesn’t show us light nor does it show us electrons because it doesn’t bounced off them.
So for this we use electrons to literally collide with the particle, take data from it, and impart that into a sensor which is why electron pictures are black and white, no light color data.
So imagine you are on the side of a road and the only way to “see” a speeding car is to hit it with another car. If you didn’t the car would go by but you would have no data. If you do then the car is now swerving all over and changing lanes, but you have data.
This can be seen in people too oddly enough. If you watch someone long enough and they realize it they will change their behavior. Especially on camera. (This is a psychological effect not a physical one but you get the idea)
The “observation” part is weird, basically all that means is an electron/photon is in a situation where it has to “decide” *exactly* where to be. Usually, it’s a wave-like “probability smear” flying around, but sometimes (usually in experiments, hence “observation”) it’ll hit a screen for example that will detect it, and then the probability smear collapses and it “chooses” one spot on the screen to light up. We know this because of the [double slit experiment](https://en.m.wikipedia.org/wiki/Double-slit_experiment).
“why do they behave differently under observation in the first place?”
I believe this is one of the unsolved mysteries of quantum mechanism. I think the current explanation for that is wave-particle duality. You treat electron/photons as a probability wave. When you observe it, it collapse the wave function and behave like a single particle. Why this occurs? I don’t think we know the answer to that.
to see something is to interact with it. the only way we see anything is if light hits it and bounces back at our eyeballs. the light hitting it is the interaction that changes the behavior. we just don’t notice it at large scales because the momentum & energy light imparts is very small compared to everyday objects.
Almost all the answers here are just completely wrong. The wave/particle duality has nothing to do with bouncing photons off of something to “see” it, and the (shocking) results of the double slit experiment is precisely what demonstrated that.
Also saying “observe” isn’t a misnomer at all, it’s 100% correct: in this case it refers to measuring the output of the experiment. Just because colloquially it’s taken to mean electromagnetic energy in the visible spectrum being processed by our eyeballs or something to mimic it (a camera), that’s not what “observation” refers to when explaining quantum mechanics.
In fact the answer is pretty hard to distill down to ELI5 because it requires not just knowledge, but highly unintuitive knowledge for those of us living in a world that isn’t obviously driven by quantum mechanics. To try and give some kind of an answer though: at quantum scales, “things” are better thought of as a set of probabilities; that is, they exist only as a likelihood of being in a particular set of “states”, rather than existing in one state or another.
When you observe of a quantum state, it’s said that the “wave function collapses” (for a really hard to follow explanation of this see https://en.wikipedia.org/wiki/Wave_function_collapse), but can best be thought of (and this is where stuff gets weird) as the thing “choosing” a state; that is, one of the probable outcomes literally instantly becomes the outcome and the thing becomes real as we understand it. Critically, it wasn’t “real as we understand it” before that – it had the potential to be a set of real things, and only took the form of one of them when it was measured – observed.
This is why the double slit experiment was such a watershed moment for quantum mechanics. We were able to demonstrate – at a macroscopic scale, our scale – this behavior. We could “make” photons manifest as waves or particles *depending on how we measured it*. This is so important I need to write it again: the exact same experiment will have different results depending on how it’s measured. You can prove that light – photons – are both waves, *and* you can prove they’re particles, depending on how you measure the result.
The key to understanding it all (to the extent a layperson like us can) is the fact that quantum things exist as probabilities prior to being measured. In the quantum world, *the act of measuring something influences it*, because measuring it collapses the waveform, eliminates all other probabilities, and results in one state – the state we end up observing as the result of the experiment.
The Science Asylum did a great video about this called “Wave-Particle Duality and Other Quantum Myths”. Check it out.
To observe you need to take out something or look how it takes anything. At macro sizes this is irrelevant, but at nano and lower scales it can chabge greatly the properties.
Observation involves interacting with the thing you are observing. In the double slit experiment it’s not possible to measure which slit the electron went through without affecting it in a way which changes how it appears in any subsequent measurement.
If the explanations of wave function collapse don’t make you deeply suspicious you haven’t really understood them. Don’t forget that the system doing the observing, including you, is made of quantum stuff. What’s really happening is that the quantum uncertainties of the thing being measured get mixed in with the thing doing the meaduring. The maths of that mixing tell us that the probability of the combined system being in a single state, rather than a multiple states simultaneously, is essentially 1 for systems of more than a handful of parts. This is why building quantum computers is really hard, as you have to try and stop the parts mixing and destroying the quantumness.
> what counts as ‘observation’?
Essentially, when something happens where the definite state of the quantum system is relevant (ie it affects things outside the system), the wave function collapses such that the system has a definite state. [Replace “system” with particle and “state” with position if my generalization confused you]
> how do we even know they behave differently under observation if we can never see how they act when not being watched?
If they behaved the same way they do when we observe them (or as classical/macroscopic objects do) all the time, we could make predictions about how they would move, etc. But those predictions are consistently wrong. Instead, the predictions made based on the wavefunction accurately predict how they behave.
To use an analogy that is a bit of a stretch: Imagine you draw a card from a deck of cards but don’t look at it. Classically, that card is defined as one specific card from the deck with 1/52 probability of it being each specific card. Quantum mechanically, your card is *actually* a combination 52 states with each state making up 1/52nd of the wavefunction. When you look at the card, that combination of states “collapses” into a single observed state.
What’s the difference between the two situations I described? Nothing in this example. But if you start doing tricky things to the card depending on its suit, number, etc, you can create situations where there is a meaningful difference between the card having a definite (but unknown) suit/number and it having a superposition of all suits/numbers. It is really not intuitive because it is completely different from the way the world as we experience it works.
> why do they behave differently under observation in the first place?
You have just stated one of the biggest unanswered questions in quantum physics.
[https://en.wikipedia.org/wiki/Measurement_problem](https://en.wikipedia.org/wiki/Measurement_problem)
I know this is mostly science fiction, but i love the parallel universe theory. In this theory, every single quantum state “splits” the universe into a series of probable universes, resulting in an infinity of parallel universes.
The double slit experiment is then explained like this: when the two wave cones interact, we are literally seeing multiple parallel universes at the sane time, interacting with each other.
But if we try to measure precisely where the particle is, we “position” ourselves on one specific, distinct parallel universe (collapsing the wave) as our selected universe becomes distinct from the other probables ones. We have “chosen” a universe amongst the infinity of them. Another us may exist in another universe seeing the particle somewhere else, etc.
Isn’t it a beautiful theory?
Not sure if any of it could be true, maybe someone with more actual knowledge of quantum theory could tell us…
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