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Ok so 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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up vote 7 down vote accepted

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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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 :-/ – Adam Sulik Apr 4 '13 at 3:09
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 Though I don't think there is any universally accepted definition of measurment in QM.See my edited amswer. – user774025 Apr 4 '13 at 3:40

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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I am still trying to wrap my head around this experiment. A photon has properties such as frequency and intensity and momentum and direction, even when the observer instrument has "collapsed" the wave function. So a free photon is definitely always a wave (not solid). The detector's measurements tell us that both the "collapsed" photon and the "wave" photon are identical. Same frequency, same intensity, same momentum. The only thing that changes is its direction of travel. Moreover, the placement of the detector whether in front or back or at the slits causes the wave to "collapse". It this some trick of time dilation or projection? Not really.

What all of this means is that the detector is not collapsing the photon itself. Rather, it is collapsing a "space field" that existed at the moment the photon was released. The photon rides this preexisting wave to determine its movement through space. This preexisting wave (space field) was centered at the point when the photon was originally released. This wave is, in my opinion, a space density pulse (expanding and contracting). The photon is simply following its space field's tiny differences in density as it travels until it strikes the detector. When the observer "collapses" the wave function, what we really see is that the photon did not follow its space field to determine its trajectory, but went in a straight line. The photon is uniquely tuned to this space field but is be aware of other photon's space field as they pass by. All photons of the same frequency and intensity are identical. See how lasers work video on youtube. This space field center instantly relocates to the photon's new location when the photon's energy is joined with a larger particle. I believe that all subatomic particles have a space field. I also believe that all subatomic particles have a twin somewhere in the universe and their spins are opposite. See spooky action at a distance. There are more things about this "space field" idea I am still thinking about. Hopefully it will explain many unsolved problems in physics and give us a better understanding of what space/time/mass/gravity are and unite Quantum Theory with General Relativity.

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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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