# Double slit experiment outcome changing by observing the slits

my question is as I believe quite simple since I'm new to physics. However here it is: if we take a double slit and constantly shoot helium atoms on it with a constant speed one by one we will see a certain interference pattern on a properly set up screen behind the double slit (scenario 1). If we now set up a detector that tells us which of the slits each atom is passing the pattern on the screen will change (scenario 2), so far so good. But if we were to set up the same detector but without looking at what it tells us and the information wouldn't be saved what would happen? So the machine detects where each atom is passing but nobody looks at the information and it goes basically instantly lost, would we see the pattern from scenario 1 or 2? (Btw I'm not a native English speaker so I apologize if I used unprofessional terms at some point)

• Hint: in your scenario 2 that you seem to accept, will the interaction between the detector and the atom change in any way depending of whether a human learns about the new state of the detector after the interaction has taken place? Commented Mar 22, 2023 at 16:53

I hope the following explanation isn't too advanced for you. The gist is - in real quantum mechanics you don't need to philosophize about what is and is not a measurement. An interaction with a detector ruins the particles' ability to make an interference pattern.

Interaction with a detector introduces an uncontrolled random phase on the wavefunction which makes the interference pattern disappear. A more sophisticated understanding of quantum mechanics recognizes that there is no distinct moment that can be called a "measurement." Instead, when you have your particles interact with something like a detector, "coherence" (defined as having a well-defined, reproducible phase between two parts of a wave function) is lost, and although the particle remains in a superposition, the two halves of the wave function dont consistently add and subtract from eachother the way they did when they were making the interference pattern. And the result of the experiment is the same as if you had just sent particles through either particular slit one at a time and averaged the results.

• "n real quantum mechanics you don't need to philosophize about what is and is not a measurement".I disagree.The measurement problem is a real problem because it affects the interpretation of reality. Commented Mar 22, 2023 at 18:22
• I recommend this lecture series - it's where I get most of my understanding of loss of coherence and how (particularly for answering a question like this) it's a more useful way of understanding "measurement" ocw.mit.edu/courses/… To some extent there remains a philosophical question that goes something like "what is a conscious entity and when does it decide what part of the wave function to be aware of" but for the purpose of this question there is no such issue. Commented Mar 22, 2023 at 18:26
• PBS spacetime seems to have propagated this misunderstanding that the detector adds a random phase. That's not necessary (see physics.stackexchange.com/questions/204100/…, e.g.). Also, if it did add a random phase, then we would expect to see a smeared-out interference pattern, and not two bright lines like we actually do.
– A_P
Commented Dec 2, 2023 at 2:47
• @A_P Not sure about pbs spacetime, but I linked a lecture series from a nobel prize winner where this idea is presented (although this was a long time ago so I no longer remember where in the lecture). But your second sentence is completely wrong. If the distribution of landing positions through each single slit do not overlap, there will be no interference pattern. Just look at the wiki for the double slit experiment where it compares the single slit result to the double. You do not get two separate bright lines when you add the detector en.wikipedia.org/wiki/Double-slit_experiment Commented Dec 2, 2023 at 22:21
• @AXensen Do you mean my third sentence? Yes, you're right that if there are actually detectors there, then there will be no interference pattern at the screen whether or not they add a phase shift. What I mean to say is that if they only add a phase shift (and don't entangle with the electron) then you will not reproduce the two bands; you will merely smear out the interference pattern. Are you certain that this Nobel laureate really claims that the decoherence here is caused by adding a random phase? Because it's really not necessary, as my link shows (and as all QC students learn).
– A_P
Commented Dec 4, 2023 at 2:30

The type of experiment you are thinking are known as Quantum Erasure experiments:

https://en.wikipedia.org/wiki/Quantum_eraser_experiment

The short answer is, yes, you can create an experiment where each photon goes through slits and gets tagged on which slit it passed. If you look at the tag, the interference pattern disappears. However, you can "untag" the photon and you will restore the interference pattern.

In my experience, though, you need to look at the details of each experiment: you can't make a general conclusion. If you really look at the setup of each experiment, you find that it is less "surprising" than the abstract made it look like.

You can see these videos for a better discussion:

Hope it helps!

This question can be answered without reference to human interaction, see the reference below for the experimental demonstration using photons.

The general rule for double slit interference is: There will be interference UNLESS there is the possibility, in principle, for determining which-slit information. It does not matter whether you know which slit the particle goes through, it is enough to eliminate the interference that you could have obtained this information.

In the cited experiment, a polarizer is placed in front of each of the 2 slits. When the polarizers are oriented parallel, the traditional interference pattern appears (there is interference). When the polarizers are crossed (orthogonal), there is no interference pattern. (Note that you can vary the angle between the polarizers from 0 to 90 degrees and get a mixture of more or less of the interference pattern.)

In this scenario with slit filters crossed (90 degrees apart), it would be possible to further filter the particles hitting the detection screen to determine which slit the particle went through. Therefore no interference occurs. It matters not that you don't actually obtain this information, it is enough that you could have. Note that in this experiment, you don't need to consider that the human does or doesn't look at the results.

Theory only: https://arxiv.org/abs/1110.4309

In general you do not need to make the conscious observation to destroy the pattern, interaction with the light will be enough of an interaction to alter the particle path after the slits ... the slits are no longer part of the wave function.

Interference patterns whether they be photons themselves, electrons or even the Buckyballs ..... are all a result of forces that favour the energy (photons) or masses (electrons, particles) to move to certain areas (bright spots) and not other areas (dark lines). The responsible force is the EM (electromagnetic force) which we say is governed by the EM field which is everywhere.

For photons and electrons the EM field is already active even before the photon or electron leaves the atom, i.e the excited electron in the atom is fully interacting with the EM field over distances. For buckyballs the EM field interaction is more subtle.

• The easiest double slit experiment to set up is with light - not charged particles. Here they do it successfully with buckyballs (C60) from an oven at 900K documents.epfl.ch/groups/i/ip/ipg/www/2016-2017/…. But moreover, I think this answer is just wrong - why do you think it matters whether we use voltages or gravity or just velocity? Commented Mar 23, 2023 at 8:32
• That's a good question. Another article also proves your point. arxiv.org/pdf/1402.1867.pdf . In your referenced paper there is an ionization step (!), this reference is cleaner in that the observation involves no ionization. I'll make some changes above. Commented Mar 23, 2023 at 16:27
• @AndrewChristensen see comment above, forget to add name. Commented Mar 24, 2023 at 15:27
• I only brought up the massless particles to make the broader point that this answer is totally wrong about what causes interference. The dark lines in interference are not in any way due to forces pushing the particles out of those regions. It is because of two halves of the wavefunction cancelling out because they have opposite phase. This is particularly clear in the case of photons (no forces whatsoever act on photons). I'll link to one source explaining the effect pressbooks.online.ucf.edu/phy2053bc/chapter/… Commented Mar 24, 2023 at 16:11
• @AndrewChristensen If you can agree that the wave functions are virtual then I think we are in agreement. I'm a Feynman fan, every photon's path is pre-determined ... and this can only be due to the EM field. The EM field is full of virtual as well as real photons .... and the EM field guides everything (photons, electrons, buckyballs). Commented Mar 24, 2023 at 19:43