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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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3 Answers 3

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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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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 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. –  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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