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)
4 Answers
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.
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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:
https://www.youtube.com/watch?v=l8gQ5GNk16s&ab_channel=Fermilab https://www.youtube.com/watch?v=RQv5CVELG3U&ab_channel=SabineHossenfelder
Hope it helps!
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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.
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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.
Experiment: https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/single_photon_paper.pdf
Theory only: https://arxiv.org/abs/1110.4309
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