How does the electron understand that it being observed in the double slit experiment?

I was reading about the double slit experiment that proved the wave and particle nature of electron. I read that electrons give a diffraction pattern when they are not observed (wave nature) and passes through the slits separately like particles when they are observed.

My doubt is, how does the electron understand that it is being observed? What is forcing it to behave as a particle when we make an observation?

• This is called the "Wave function collapse", it is a central issue in quantum mechanics known as the "Measurement Problem"; there are various interpretations as to how/why this happens, Copenhagen, Many Worlds, Bayesian... but none have officially won. Commented Feb 25, 2021 at 4:59
• The electron always travels as a particle with wave properties. The wave properties are visible when you tightly constrain the path (small source, slits , target distance) only allowing the certain paths per Feynman path integral theory. If you disturb the electron after the slits it becomes free to travel many more paths again (but still has wave properties). Commented Jul 3, 2021 at 1:43
• In my opinion, there is an "information conservation law" which hasn't been formulated yet. For EVERY double slit experiment which has ever been performed, including the delayed choice quantum eraser, if you know which slit a particle went through, you get a particle result and if you don't know which slit a particle went through, you get a wave result. Commented Aug 1, 2021 at 14:42
• "My doubt is, how does the electron understand that it is being observed?" -- because observations are not done supernaturally with some power of the mind. It's a physical process that couples (i.e., causes to interact) a measurement device with the electron.
– user87745
Commented Apr 21, 2022 at 21:59

"how does the electron understand that it is being observed?

Your statement is based on the assumption that "being observed“ is a completely passive process. But that is not the case.

Let’s rephrase your question: how does the electron understand that it is being detected. Well, that’s a simple one: because it interacts with the detector! This interaction causes the electron to behave differently compared to the situation when it is not detected.

In contrast to our everyday terminology, observation always requires some form of interaction. “Seeing” the electron yourself seems not to require any other detector than yourself. But again this is not the case: it requires shining a light (shooting photons) on the electron that is bounced of (interact with) the electron to reach your eye. In reality you are not the detector; you are only part of a detector. The other part is the light source and the photons interacting with the electrons.

The electron doesn't 'know' anything- it simply interacts with energy and matter in accordance with the laws of physics. What physicists do is to design their experiments to investigate the nature of those interactions. When an electron is 'observed' in a two-slits experiment, what we mean is that it interacts with the particles that form the detecting screen and we see the results of the interaction. To take an old fashioned example, the detecting screen might just be a photographic film. The electron interacts with the molecules on the coating of the film causing an effect that can be seen when the film is developed. There is nothing magical about an 'observation' or 'measurement' of a particle- those words simply mean that the particle has interacted with other particles in some apparatus to cause a physical effect, and it is the effect we interpret as a measurement.

Electrons are quantum mechanical entities, and interact quantum mechanically with the environment. This means that there are differential wave equations, whose solutions control the probability of how an electron will interact . Probability means that an accumulation of the same boundary condition events should be made, in order tos see an effect.

In the double slit case , the boundary conditions are "electron falling on double slit given distances separation and width of slits". This is controls the boundary conditions that choose the particular wave-function solution. It becomes a different experiment if the electron is disturbed in order to detect which slit it went through, different boundary conditions and thus different wavefunctions .

In different words, in order to detect an electron, an interaction has to happen, all interactions disturb the original boundary conditions, and a different wavfunction will control the track of the electron destroying the coherence needed in order to sum many electrons with the same boundary conditions.

• It is clear that the detection process disturbs the original boundary conditions. But still, why is it that the new boundary conditions always lead to an observation of particle behavior and never wave behavior? I believe the OP is not asking just why electron behaves differently when it is observed, but why behaves differently in a very specific way.
– Javi
Commented Feb 25, 2021 at 6:11
• @Javi I say it in the last sentence. The wavefunctions have phases, and the reason one must have the same boundary conditions, is that each electron's wavefucntion is inphase with the other electrons. That is why in the accumulation an interference patern appears. Once an electron interacts the phase is lost , because it becomes randomized. Look at the one electron at a time.en.wikipedia.org/wiki/… . If it is a different wavefunction there will be no coherent pattern in the accumulation Commented Feb 25, 2021 at 6:30
• @Javi I mean the wavefunction in space coordinates for each electron must be coherent , have a fixed phase difference between the individual electrons so that an interference in the \$Ψ^*Ψ* can be seen by accumulation Commented Feb 25, 2021 at 8:29

One way to "observe" a tiny particle like an electron is by detecting its presence via its electric field. That detection necessarily requires that the electron disturbs some part of the the detection device's electric field if it is to be registered by that detection device. Due to Newton's third law, the electron must be similarly disturbed. Or if you prefer to "see" that electron with a photon, you must necessarily use a photon with a very short wavelength (aka very high energy) because the electron is so small. That high energy photon will also disturb the electron when it reflects off of it, due to conservation of momentum. In other words, the electron does not "understand" that it is being observed ... it is so very tiny that any force that interacts with it such that you can determine its position, will change its behavior, unlike common macroscopic objects which are so very massive that bouncing photons off of them has no discernible effect.

• I like this answer as it explains how measurement disturbs the electron. But it still does not explain why this disturbance yields a particle behavior instead of a wave one. For example, could there be some measurement device that disturbs the electron "the opposite way", yielding a wave behavior? Why not?
– Javi
Commented Feb 25, 2021 at 6:04
• @Javi, I can only tell you that measurement disturbs the interference pattern. Why such disturbance yields a "particle" result rather than a wave result is one of the ongoing mysteries of quantum mechanics. My own personal opinion (and that's all it is) is that there is some law of conservation of information which hasn't yet been quantified. An interference pattern indicates that you don't know which slit the electron went through. A particle pattern indicates that you do know which slit the electron went through. Commented Feb 25, 2021 at 17:17
• While it is true that in many setups the measurement device disturbs the system being measured, this is not always the case (see en.wikipedia.org/wiki/Interaction-free_measurement ) and is not intrinsically related to the problem of quantum measurement. Commented Jul 2, 2021 at 3:39
• @Javi The electron always travels as a particle with wave properties. The wave properties are visible when you tightly constrain the path (small source, slits , target distance) only allowing the certain paths per Feynman path integral theory. If you disturb the electron after the slits it becomes free to travel many more paths again (but still has wave properties). Commented Jul 3, 2021 at 1:44

The wave equations satisfy a boundary equation with the double slit. This results in the incident wave showing an interference pattern. Whatever you do that would measure "where do these low-intensity photons cross" is actually not a double slit pattern anymore, hence you get a single gaussian fringe

We do not know how does it know, we just experimentaly proven that it knows. The experiment is delayed-choice quantum eraser experiment. It basicaly does double slit experiment with a trick to produce two photons out of one AFTER it gone through the slits.

One photon travels to the detector where we observe or do not observe interference. The other photon travels to an array of detectors in which it has 50% chance to go to a detector that tells us which slit photon gone through, and 50% chance to go to a detector which erases that information and we no longer know which slit photon went.

Surpise surprise when we knew which slit photono came through there were no interferance paterns, when we didnt interferance paterns emerged.

Fun fact, the photons that told us which slit original photon came through hit sensors 8ns later than the one which generated paters. So the photon knew what we would know (or what we will not know(which slit it came through)) 8ns later. How? We do not quite know.

Here is video explaining experiment: https://www.youtube.com/watch?v=8ORLN_KwAgs