Wavefunction collapse with a single photon?

[Xilor]

Hello, I was wondering what exactly happens when you observe part of the wavefunction of a particle, does this always cause collapse? Or only when the probability distribution decides that the particle is indeed there?

What I mean is, is an observation in the form of photons interacting with the wave more like a wall that cancels out part of the wave if the particle isn't there, or does it always cause full collapse?


If its always full collapse, why isn't the wall with the slits in it causing collapses? Why don't random photons in the area cause collapses (such as those from blackbodyradiation of the instruments)?

If it just blocks out parts of the wave, isn't it possible to try to observe one of the slits in such a way that not the entire wave is blocked out, still causing an interference pattern but an altered one? If that is possible, how can you ever fully block out a slit? It seems impossible to observe the entire slit area at once since photons are quantized.

 

[azntoon]

Observing part of the wavefunction of a particle always causes collapse, regardless of whether or not the probability distribution decides that the particle is there. This is because the act of observation itself changes the wavefunction by introducing a new factor into it which changes its shape. The photons interacting with the wave act like a wall that blocks out part of the wave, thus causing collapse. This is why the wall with the slits in it causes collapses, as photons from the instrument interact with the wave and cause collapse. Random photons in the area may also cause collapse if they are energetic enough to interact with the wave. It is possible to try to observe one of the slits in such a way that not the entire wave is blocked out, still causing an interference pattern but an altered one. However, it is impossible to fully block out a slit as photons are quantized and therefore can only interact with the wave in discrete amounts.