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If we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave function by observation. Here is my question. If we setup the camera to record like above but NEVER EVER EVER look at the result of what was recorded. Does the wave function still collapse. If so then perhaps its the camera causing it. If not then it is truly based upon the observer.

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    $\begingroup$ The problem is that even a camera is not fully passive observer. For a camera to record something, this something must be lit or must somehow hit the camera sensor, and such kind of interaction is strong enough to make the idea of passive observation impossible. $\endgroup$
    – JustAMartin
    Commented Jul 10, 2018 at 7:54
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    $\begingroup$ Possible duplicate of Double Slit Experiment. What effect does the detector actually cause? $\endgroup$
    – my2cts
    Commented Sep 1, 2019 at 6:06
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    $\begingroup$ @my2cts Or maybe that question should be closed a duplicate of this one... if either question really needs to be closed. There are some good answers here. $\endgroup$
    – PM 2Ring
    Commented Sep 1, 2019 at 6:44
  • $\begingroup$ Actually I was looking for another duplicate where I explained how making the two paths distinguishable destroys the interference. $\endgroup$
    – my2cts
    Commented Sep 1, 2019 at 7:31
  • $\begingroup$ @my2cts Although this question is old, I am adding this: Passive observation is possible as you describe, making the paths distinguishable. That can be done by placing polarizers over each slit. Oriented parallel: Interference (fig.8); oriented perpendicular, NO interference (fig.9). Either way, there are polarizers but one orientation leads to distinguishability. It does matter that that "which slit" information is never recorded. See sciencedemonstrations.fas.harvard.edu/files/… $\endgroup$
    – DrChinese
    Commented May 12 at 23:41

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If you place a camera you will not see any interference pattern. So, the answer is yes. The camera will cause the wavefunction to "collapse". But I don't like the term "wavefunction collapse", because wavefunction is not really any physical object. What the camera will basically do is cause an abrupt change in the state of the particle.

Here is the defintion of measurement from Landau's book

By measurement, in quantum mechanics, we understand any process of interaction between classical and quantum objects, occurring apart from and independently of any observer. The importance of the concept of measurement in quantum mechanics was elucidated by N. Bohr. We have defined "apparatus" as a physical object which is governed, with sufficient accuracy, by classical mechanics. Such, for instance, is a body of large enough mass. However, it must not be supposed that apparatus is necessarily macroscopic. Under certain conditions, the part of apparatus may also be taken by an object which is microscopic, since the idea of "with sufficient accuracy" depends on the actual problem proposed.

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    $\begingroup$ Thanks for the answer! how about some other things, such as what if the camera is left off? What if it is on, but the lense cap is on? and so on. Has much work been done playing around like that to see what does and what does not cause the abrupt change in the state of the particle? I've looked but not found much :-/ $\endgroup$
    – Adam Sulik
    Commented Apr 4, 2013 at 3:09
  • $\begingroup$ I think your question is what isdefinition of measurement in quantum mechanics or equivalently when does a "wavefunction" collapse happen? For this you can read this article en.wikipedia.org/wiki/Measurement_in_quantum_mechanics Though I don't think there is any universally accepted definition of measurment in QM.See my edited amswer. $\endgroup$
    – user774025
    Commented Apr 4, 2013 at 3:40
  • $\begingroup$ Although this answer is old, I am adding this: Passive observation is possible, making the paths distinguishable. That can be done by placing polarizers over each slit. Oriented parallel: Interference (fig.8); oriented perpendicular, NO interference (fig.9). Either way, there are polarizers but one orientation leads to path distinguishability without performing a "measurement". It does matter that that "which slit" information is never recorded. See sciencedemonstrations.fas.harvard.edu/files/… $\endgroup$
    – DrChinese
    Commented May 12 at 23:45
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This has been explained on this site several times before. You can think of the interference pattern a the squared sum of a left and a right part of the wave function, $\psi_L$ and $\psi_R$. These two parts are not orthogonal so they interfere. A detector that can distinguish left and right can only do so by being entangled with this wave function. The new wave function parts $\psi_L'$ and $\psi_R'$ are then orthogonal so the interference is absent.

Any wave function collapse then it happens only on your desk by ignoring the contribution of the detector to the wave function. There is no mysterious effect of observation or consciousness.

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If we setup the camera to record like above but NEVER EVER EVER look at the result of what was recorded. Does the wave function still collapse?

The answer is that we just don't know. We can tell that the wave function has collapsed (in Copenhagen terms) only when we humans look at the system -- in the canonical experiment that means looking at the landing pattern to see if we have fringes or clumps. And even though in your example we are, on purpose, not looking at the "which path" information in the camera, it's not at all clear that what makes that camera information be "which path" information is not conscious observation.

But the problem is actually much deeper than that. It doesn't matter what if any "role" -- in the sense of a mechanism of interaction -- conscious observation plays in inducing collapse in the system. What matters is, saying anything about the system first requires a conscious observation. In science (as opposed to metaphysics, say, or mathematics) that -- our observations -- is the basis of what we say stuff about when we say stuff. So conscious observation is always in there, getting in the way, and leaving us uncertain - in fact in scientific terms, utterly clueless to be precise -- as to what part, if any, it's playing.

Re: @user774025's, Landau quote in which measurement is defined as "...occurring apart from and independently of any observer". That runs straight into one of the most fundamental challenges in science, not just QM[1], namely that science Just Is an activity performed by conscious observers. Landau's definition tries to de-couple science from the observer, but in doing that he is no longer talking about science. Science is the act of observation (plus a bunch of other things, of course[2]).

Consider: there is not a single scientific experiment, ever, that did not culminate in an observation by a consciousness[3]. So in science, the answer to the question:

"What happens if we don't look?"

is at very least

"We don't and can't know"

but in fact is probably better put as

"Why are you asking that? Science Just Is looking. I thought we were doing science!?"_

And of course, once we've looked, we have "contaminated" our experiment with a conscious observation, and we cannot tell what effect that has had.

Which is why we don't know and cannot know if the not-looked-at-directly camera-based which-path detector caused collapse until we look at the electrons' landing pattern to see if collapse occurred at all. And by then, although we have confirmed collapse, we've added a new factor -- the looking.

We can never tell what an unlooked-at system looks like without looking at it, at which point it is no longer unlooked-at.

[1] The reason this kind of thing gets attention in QM is because experiments like double-slit served to highlight the problem by showing us a peculiar form of measurement error that is fundamentally different from the everyday kind, such as turning on a light so we can see to count how many cockroaches there are in a dark room. But the problem pre-dates QM, and in fact is fundamental to what science is -- in fact what observation overall is.

[2] For example, in the professional scientist's case: writing up those observations and their opinions about them, presenting them at conferences, kicking doctoral students into doing the same, and playing multi-user Call of Duty because although their current grant money is about to run out, and their doc-students are whining about it, they just can't face the prospect of another mind-numbing, morale-destroying, rather-poke-myself-in-the-eye-with-a-sharp-stick round of writing the next grant proposal.

[3] Even if we take an eliminative view of what consciousness is.

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    $\begingroup$ And what about delayed choice quantum eraser experiment? Still, it doesn't prove anything because it also is not fully passive observation of a single particle. $\endgroup$
    – JustAMartin
    Commented Jul 10, 2018 at 7:57
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The following may help:

Suppose the experiment consists of Bob at hole 2 in the double slit experiment being able to open and close the hole instantly. Let the intensity be so low that on average only one particle at a time is in the apparatus. By closing the hole he ensures the particle must go by route 1 if it is to hit the screen. Now what happens if he manages to re-open hole 2 just before the particle is detected at the screen? From the quantum eraser experiments we know the answer:By repetition of the experiment the pattern built up at the screen is the interference pattern (case a). If the hole were closed at the time the particle is irreversibly detected at the screen then the pattern built up would not show interference at all (case b). The state of the apparatus at the exact moment of the irreversible detection of the particle at the screen determines if the particle is contributing to case a or case b pattern. Note that in this experiment Bob hasn't detected any particles himself but keeps a record of the time at which he opens or closes the hole which can be correlated with the particle arrival times at the screen. we can thus group the screen observations into two groups- those that occurred with the hole open and those with it closed. The first ones show the interference pattern the second do not. What happens if Bobs records are destroyed before the screen results are analyzed into these two groups? We see a mixed pattern of both a and b so interference fringes on top of high background which tends to wash out the fringes. The point is destroying Bobs information doesn't change the results which were observed at the screen. But those results were determined by the now lost information, they don't suddenly change.

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There are occasions where there are no extant answers to a question so no references are available. Either the act of observation is totally passive, and as such cannot alter what is being observed, or it is active, in the sense of having some interaction with the system being observed. In the 1st case, waveform collapse cannot occur. In the second, the waveform collapses because of the 'active' method of observation. There is no other alternative. This is a typical 'paradox' in that a poorly framed question or experiment yields ambiguous or paradoxical answers. Similarly with Olber's and the speed of light.

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It seems to me that the double-slit "mystery" is simply active observation effects on the wavefunction. There is no camera or sensor that does not "vacuum up" electrons or photons to give you a reading. Even the human eyeball or eardrum is not completely passive, they ever-so-slightly affect the light and sound waves of the observed environment.

So there is an interference by the sensor collection that causes waves to act like particles. It seems to me like this could be borne out in experiments. Suppose you use sensors and cameras that aren't actually sensors or cameras but ACT like sensors and cameras. Take the "observation" out of the equation. The results being the same tells us that the double-slit has nothing to do with human observation, but simply the mechanical reaction effects of the dumb tool you are using.

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You COULD get an answer to this if you found a way for the camera to interpret the picture and read the results to you. Then you will have never actually observed the photon. Only the camera will have 'seen' the event.

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The answer is no. Only in the presence of observers can it be unambiguously stated that quantum possibility becomes actuality. But think, according to quantum physics before a measurement or an observation, the observer’s brain that is doing the looking is also quantum possibility. We observe a sub microscopic object like an electron with a measuring apparatus. But in truth, that measuring apparatus cannot truly measure. Why? Because being made of sub microscopic objects like electrons protons and neutrons it to must be an object of possibility, only macro. But by the same token the observer’s brain being made of the same sub microscopic objects must consist of quantum possibilities and yet obviously in any act of observation, the observer’s brain and object of observation actualize together but the observer never sees any brain. Instead, he or she identifies with the brain and experiences being an “i” observing an object. Jon Von Neumann theorem proved that the quantum effects continue all the way to macro measuring apparatuses and computers.

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