# Does Double Slit Detection Rate Change when Path Is Known vs. Unknown?

Is the overall rate of particle detections you will obtain in a classic double slit experiment dependent on whether the experiment is set up to cause an interference pattern or not?

Another way of asking this is whether you can interpret the interference pattern as the ABSENCE of particles you would have ostensibly detected had their paths been defined?

The motive behind this is that in quantum eraser experiments, the initial detection pattern always appears as random noise; it's only when you filter the results based on the "erasure" of the which-path information for the entangled twins that the pattern emerges from the noise. While often presented as proof of retrocausality, one could (and IMO should) interpret the results merely as a data gathering and filtering exercise.

But of course in the classic double slit scenario, there is no correlating and filtering after the fact; the pattern emerges then and there. But does that pattern arise because the particles we wound up detecting behaved differently due to the presence or absence of path detectors, or - as in the quantum eraser - because we failed (intentionally or not) to detect the ones that would have otherwise landed between the bands? And if the detection rate is unaffected by path knowledge in the classic experiment, does this suggest a different physical mechanism is at play vs a quantum eraser?

• maybe this classroom single photon at a time double slit experiment will help sps.ch/artikel/progresses/… Nov 20, 2022 at 18:15
• "This shows that when each photon is forced to follow a specific path (right), its probability to be detected is higher than when it is left with a choice of two possible paths (left).". So then the answer to my question is yes, the detection RATE - and not just the geometric distribution - changes depending on whether the path is defined? This seems to put the double slit in a much different light, and frankly less mysterious one. Nov 21, 2022 at 0:17
• Quantum mechanics mathematics was invented in order to calculate and fit these "rates", as probability distributions, Nov 21, 2022 at 4:33

Is the overall rate of particle detections you will obtain in a classic double slit experiment dependent on whether the experiment is set up to cause an interference pattern or not? Another way of asking this is whether you can interpret the interference pattern as the ABSENCE of particles you would have ostensibly detected had their paths been defined?

No, regardless of whether the setup is designed to get which-way information (thereby destroying the interference pattern) or not, the same number of photons will hit the screen*. While the total number of photons is the same, however, their distributions in the interference case is of course different from the non-interference case.

While often presented as proof of retrocausality, one could (and IMO should) interpret the results merely as a data gathering and filtering exercise.

I agree with this, and at some point I was also fooled by explanations of the delayed choice double slit which didn't make this clear.

But does that pattern arise because the particles we wound up detecting behaved differently due to the presence or absence of path detectors, or - as in the quantum eraser - because we failed (intentionally or not) to detect the ones that would have otherwise landed between the bands?

The former is correct here and the latter is not.

And if the detection rate is unaffected by path knowledge in the classic experiment, does this suggest a different physical mechanism is at play vs a quantum eraser?

In the quantum eraser the detection rate at each point is still the same regardless of any path knowledge obtained afterwards. So I would see it as the same physical mechanism, unless you had a different idea in mind? In addition to this same physical mechanism, the delayed choice quantum eraser has a filtering effect, but I understand that as a separate effect which results from the entangled photons that are created.

• Thank you, this makes a lot more sense now. The quantum eraser feels more like sleight of hand. The "classic" double slit is a lot more interesting because if the particle detection rate is the same regardless whether there's path detection in place, then they really are behaving differently due to the mere presence of the detectors. To me the distinction is, are we detecting different things because we changed the detection regimen, or are the particles actually BEHAVING differently because we changed the detection regimen. I believed it was the latter, and you confirmed this. Nov 22, 2022 at 21:50
• Thanks, glad I could help clear things up. Yes, it is certainly the interaction between the particles and the which-way detector which puts them in a state that does not allow interference to occur. So the particles changed their state, and not just the detector. Nov 22, 2022 at 21:52
• @PeterMoore We take about REAL photons all the time but there are virtual photons (these are the force carriers) than are seldom mentioned. An excited electron in the source is interacting with the EM field virtually (i.e forces) even before the real photon is created. Virtual fields/forces interact with the entire setup: slits/screen/source itself and and other devices like detectors, polarizers, crystals, beam splitters, etc ..... most probable paths are where the energy eventually flows (real photons) ...... per Feynman path integral the probable paths can be calculated/predicted. Nov 23, 2022 at 14:49
• Since I'm asking specifically about detection count - an objectively observable quantity - I don't see how virtual photons are relevant to this question. Perhaps I was less clear here than in my other question - physics.stackexchange.com/questions/737883/… Nov 23, 2022 at 18:06