Two thought experiments -

  1. Modulated double slit

A double slit experiment in which one slit is repeatedly opened and closed. We expect a non-interference pattern when closed and interference pattern when open.

Q: how is the pattern switch delayed over time relative to the slit switch? Presumably its related to the transit time of the particle from slit to screen.

  1. Delayed modulated detection

Consider a setup like the famous delayed quantum eraser of Kim et al:


Instead of using the green beam splitters, the detectors D3 and D4 are placed directly in the beam paths from the prism PS. There should be no interference at D0 because the dectectors tell us which way the photon comes through the double slit. However now remove D3,D4 quickly so that the beams can continue into space uninterrupted, then we should have interference at D0.

Q: D3 D4 Detection is much delayed compared to D0. If D3 D4 detection is removed suddenly, we might conclude that interference at D0 is restored at an earlier recorded time. But this seems crazy since we can then effect the past from a future decision. From a relativity perspective there is a frame where the detections are all simultaneous. What is wrong here? Possibly assumptions about switching detection.

  • $\begingroup$ "However now modulate the beams so they do not pass to the detectors" is not a precise statement. Presumably you are imagining a scenario in which the experimenter actively chooses to erase or measure the which-path information. This could be done by replacing BS_a and BS_b by mirrors, and then removing these mirrors (or not) to erase the which path information (or not). Right now your question is too ambiguous. $\endgroup$
    – fulis
    Commented Mar 8, 2023 at 13:52
  • $\begingroup$ To clarify: I mean either let the beam continue into space, or block it with a detector. I can even imagine physically moving or removing a detector for the sake of the thought experiment. The point is to avoid further complication for quantum erasure. $\endgroup$
    – ddddmmmm
    Commented Mar 9, 2023 at 13:20

1 Answer 1


Dirac said "every photon interferes with itself" .... Feynman said "every photon determines its own path" .... what modern science says is that "the EM field determines the path of photons". The 3 statements above are all saying the same thing.

So how the heck can photon determine its own path? The most likely explanation is that the EM field is doing it through real photons (energy) as well as virtual photons (forces).

The EM field would be active even before the photon becomes real ... i.e. the excited electron is already interacting with the EM field virtually before the real photon even exists.

Thus a photons path is predetermined by the EM field .... it is on its way with the EM field sensing the entire apparatus; it might interfere ... it might not depending on the setup. Yes you can change the path of a photon from its initial route ... but you have to work very quickly (at the speed of light).


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