Double slit: Human vs Machine Observer

[DParlevliet]

Coherent superposition is required for the interference pattern to emerge, which requires the particle to maintain a coherent phase and frequency and have a distributed position. If you take one of these away then, the photon can't contribute to an interference pattern.

After some thinking I am less satisfied with this.
- Phase is right (when uncorrellated, not fixed). With one photon there will be interference (there are positions on the detector where the photon will never arrive). But with multiple photons it is not possible to build up a measurable pattern.
- Frequency: Suppose a detector exists which emits a new photon with lower frequency but fixed phase. Then according classical wave still interference would be possible. But transferring higher to lower frequency with fixed phase is essential not possible. So the basic reason here is also uncorrellated phase.
- Distributed position: as mentioned before, after leaving the detector the photon is a wave again, so a distributed position. There is no difference with the photon before the detector.

 

[craigi]

After some thinking I am less satisfied with this.
- Phase is right (when uncorrellated, not fixed). With one photon there will be interference (there are positions on the detector where the photon will never arrive). But with multiple photons it is not possible to build up a measurable pattern.
- Frequency: Suppose a detector exists which emits a new photon with lower frequency but fixed phase. Then according classical wave still interference would be possible. But transferring higher to lower frequency with fixed phase is essential not possible. So the basic reason here is also uncorrellated phase.
- Distributed position: as mentioned before, after leaving the detector the photon is a wave again, so a distributed position. There is no difference with the photon before the detector.

Regarding frequency, as I've mentioned before, photons always intefere, but to see an interference pattern the interference must be from photons that have properties that are coherent across multiple paths. A coherent frequency shift could still result in an interference pattern. For example, if at each slit the frequency is shifted by an equal amount. A shift of a different amount on each path is unlikely to result in interference, exceptions to this would be very small shifts ie. wavelength shifts that are much smaller than than the scale of the experimental apparatus or shifts that involve exact or nearly exact wavelength shifts.

Regarding position detection. If a photon has been detected going through one slit, it can't have gone through the other. There is no wave from the other slit to interfere with, so no inteference is possible with this path, hence this photon can't contribute to an interference pattern. The important thing to remember here is the wave represents the probabilty of finding the particle at a location. If it is found at one location, it can't be at another. Once its position is known the wave spreads out again in future time, in fact the more accurately the position is known the wider the spread of the wave. This is the same as the process which causes the wave to spread out after passing through a slit.

 

[DParlevliet]

Position detector: I agree, as I also proposed in my answer #48. With detectors there is no interference pattern because the wave of the new emitted photon in the detector cannot reach the second slit and the old wave is gone.
Therefore my remark about phase is also not applicable anymore.