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In the double slit experiment in which single photons are fired at a pair of slits one at a time forming an interference pattern at the detector, what happens when one or both slits are closed after each photo is predicted to have passed the slits but before it’s detected?

Jinglesting
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  • The experiment cannot be done/ Photons move with the velocity of light, and as far as experimental errors go one does not have the time to close a slit before detection. – anna v Apr 28 '22 at 18:46
  • @annav surely the emission of the photon can be localized in time and the slit(s) closed when the photon is almost certain to gone through the slit(s) but has almost certainly not yet been detected. The slits and the detector can be arbitrarily far apart. – Roger Wood Apr 28 '22 at 19:56
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    If you don't know which slit a photon went through, you get an interference pattern, and if you do know which slit a photon went through, you don't get an interference pattern. After many clever experiments, it has been demonstrated that you get this result regardless of how you design the experiment. – David White Apr 28 '22 at 20:02
  • As we don’t know for sure when the photon actually passes the slits given we haven’t any detection apparatus at the slit opening were not sure if we’re closing before or after it’s passed. If the path of the photon is only determined at a detection event on the screen then could closing one slit, even after the photon is expected to have passed the slits, but before it’s detected at the screen, eliminate the pattern? – Jinglesting Apr 29 '22 at 22:40
  • @Jinglesting You can still make the statement that if you close the slit after the photon passes, then the photon time will continue on to the detection screen. – Bill Alsept May 01 '22 at 22:31

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You could periodically cover one slit and from the timing information deduce whether a photon passed the system of two or one slits. Then you will find that the two types of photons build two different patterns, a one slit diffraction and a two slit combined diffraction and interference pattern. You will then trivially know that the first type went through the uncovered slit, but have no which way information about the second kind.

my2cts
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  • It wouldn’t matter because a two slit interference pattern is already the combination of two single slit interference patterns and a single slit interference pattern is the combination of two edge interference patterns. They all come from the single edges, no matter what. – Bill Alsept Apr 30 '22 at 16:14
  • This assumes you’re closing a slit before the photons have passed. The question is about what happens if it’s done after but before the wave function collapses – Jinglesting May 01 '22 at 11:50
  • No it does not. – my2cts May 01 '22 at 13:13
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It makes sense to break down the experiment into simpler phenomena.

As I am sure you have been taught, these characteristic stripe-shaped intensity distributions of light are also formed behind a single slit. And they also form behind a single edge. You can convince yourself of this with another experiment at your institute.

The individually emitted photons also appear on the observation screen (the CCD chip) as individual points. Only after a longer observation phase will the statistical representation of the impinging points produce a stripe-shaped intensity distribution. The individual photons form dots on the screen, not waves and thus no interference.

It follows directly from the above that covering the edges after the passage of a photon has no influence at all on the impact of the photon. Rather, it makes sense to include the interaction between the edge(s) in the consideration.

HolgerFiedler
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  • I think wave function explanations are like flat Earthers, there’s nothing you can do to convince them that photons are real particles actually doing something. – Bill Alsept Apr 30 '22 at 16:17
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    @BillAlsept If a photon is purely a particle why does it produce a diffraction pattern and more puzzling would be why is the double slit pattern not simply the sum of two single slit patterns? – Jinglesting May 01 '22 at 12:05
  • @Jinglesting A single photon cannot produce a diffraction pattern and If by purely you mean solid then that’s not true. A photon has a frequency because it’s oscillating as it travels along at the speed of light. A 500nm photon has a frequency of more than 600 trillion oscillations per second. A single edge produces an interference pattern based on this. A double slit has four edges that produce that same pattern and they all overlay each other to produce the double slit pattern. – Bill Alsept May 01 '22 at 15:05
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The experiment can be done if the distance is far enough but one photon will not give you any kind of pattern. What you will find is the photons can be diverted left or right at the slit before hitting the screen. Closing the slit after has no effect on photons that have already passed.

Bill Alsept
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  • Can the anonymous down voter at least speak their mind? What is wrong with my answer? – Bill Alsept May 01 '22 at 05:33
  • I didn’t downvote but the idea would be to fire many photons one at a time. Also how can you be sure that it won’t have an effect given the wave function won’t collapse until a detection event at the screen? – Jinglesting May 01 '22 at 11:43
  • @Jinglesting photons are individual particles and after they pass through a slit they continue on their way to the detection screen. Closing a slit after would have no effect on their trajectory. There’s no reason or reasoning that can explain how that could happen. – Bill Alsept May 01 '22 at 15:11
  • @Jinglesting thanks for the response. – Bill Alsept May 01 '22 at 15:14