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I understand that in the single-electron-at-a-time double slit experiment, if a detector is placed before the slit, the interference pattern vanishes.

Suppose I left the detector on, but put a bag over its screen (I can't tell what state the electron is before it passes the slits), does the interference pattern come back?

If so, does that mean the electron "knows" I'm not "looking" and proceeds to interfere with itself?

Edit: I had photons passing through the slit initially. I've updated it to electrons to reflect a more common setup.

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In the following, I have replaced "photon" going through slits with "electron", and the measuring device with a "photon". This is the traditional Heisenberg setup.

The interaction of the photon and the electron entangles the photon and the electron, so that the electron cannot make the interference pattern. It has nothing to do with looking at the screen. The looking at the screen only serves to let you know what the result of the observation was.

The entanglement of measuring devices and particles is what causes collapse relative to a detector state, independent of interpretation. The only question is when the collapse turns into a definite outcome, so that the detector state becomes a definite thing, as opposed to an indefinite superposition.

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Which electron are you referring to? – Manishearth♦ Mar 26 '12 at 7:43
@Manishearth "electron" in Ron's answesr is "the particle which interferes"= "photon" in OP's example, while Ron's "photon" is the "stuff the particle interacts with" = "atoms in the detector plate" in OP's terminology. A bit consufing, I must admit :) – Slaviks Mar 26 '12 at 13:51
@Slaviks aah, I get it now, thanks. I guess a one-electron-at-a-time ydse is more common than its photon counterpart.. – Manishearth♦ Mar 26 '12 at 14:05
Let me update the question so an electron is the particle, and a photon is the detector. – Justas Mar 26 '12 at 18:52
Ron, sounds like what you're saying is that the wavefunction collapse takes place when two superpositioned particles interact with each other (ie. the electron before it passes through the slit and the photon from the detector). Is that right? – Justas Mar 26 '12 at 19:01
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No. The electron doesn't care at all whether you look at the screen or don't. It's a common misunderstanding from taking the notion of "observer" too seriously. In quantum mechanics, the "observer" is anything that causes decoherence. In your example, that's the detector. Putting a bag over the screen doesn't change anything about it.

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It doesn't require "decoherence" to get rid of the interference, entanglement with a single photon, even fully coherent entanglement, is enough. The photon can come back and undo the half-done measurement. THe decoherence only happens when the entanglement is with a macroscopic object. – Ron Maimon Mar 27 '12 at 18:52

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