# Double slit measurement problem

1. Aug 12, 2010

### marky3

In the two slit experiment there is an interference pattern generated whenever there are no detectors at the slits. This must mean that the wavefunction associated with the particles have not decohered. But the wavefunction must have interacted in some way with the barrier in order to produce the superposition state reflecting the presense of the two slits. But i thought that decoherence takes place whenever a wavefunction interacts with an object that is entangled with the environment, as presumably the barrier is. Why is it not happening in this case? If it isn't simply the interaction of the wavefunction with an object that is entangled with the environment that causes decoherence then what does cause it?

2. Aug 12, 2010

### alxm

The wave function only 'collapses' (or decoheres if you prefer the more modern viewpoint) if an interaction occurs such that you can (in principle, not in practice) determine the property in question. The wave's interaction with the wall is not such that you can determine from it which slit the wave passed through.

3. Aug 13, 2010

### marky3

If this collapse only happens whenever the interaction enables you to determine which slit the particle went through then what physical mechanism is at play. Is this one of the aspects of quantum mechanics that is not properly understood or am i missing something. It is not obvious to me how the system 'knows' whether this sort of information is being made available to a potential observer. Also this same principle would presumably apply to all cases of wavefunction collapse/decoherence. This would seem to emphasize the oddity of the system always knowing that it is giving information away. It is not obvious that a physical mechanism is involved.

4. Aug 13, 2010

### zonde

Let's say that to determine which slit particle passed through you have to tune detector so that it is in coherence with wavefunction passing trough one slit and is in decoherence with wavefunction passing trough other slit. But in this case two wavefunctions are in decoherence between themselves as well.
On the other hand to observe interference two wavefunctions should be in coherence between themselves but in this case it is impossible to tune your detector so that it is in coherence with only one of two wavefunctions.