Double slit experiment: What if the detector is removed quickly?

  • #1
Aj_
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Hi all!
I'm a physics enthusiast and I've been reading/watching a lot of stuff about the double slit experiments. I was watching a youtube video about the delayed quantum eraser experiment and it was really interesting and got me thinking. So here's my question.

What if the detector is placed, a photon is emitted, and before the photon reaches the slits or the detector itself(depending on whichever is closer to the photon source), the detector is removed far away so that no measurement can be made and this is repeated over time for each photon. What would be the output of this experiment?

Will it prevent the interference pattern from appearing? Has an experiment like this ever been done? Perhaps there is a link?

The reason I'm asking this is because I've read that a photon doesn't experience time since it is moving at light-speed. So, perhaps for the photon, it wouldn't matter at what time the detector existed in front of the screen and would this cause the wave function to collapse?
 

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  • #2
PeterDonis
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Hi @Aj_ and welcome to PF!

If the detector is removed, nothing is detected, so it makes no sense to ask whether an interference pattern is detected or not. You have to have the detector there to get a result from the experiment at all. So your question makes no sense to me.

I've read that a photon doesn't experience time since it is moving at light-speed
This is not correct. The correct statement is that the concept of "experienced time" makes no sense for a photon. That statement does not allow you to draw the inferences you are trying to draw. Which is good since those inferences don't make sense anyway; see below.

Perhaps for the photon, it wouldn't matter at what time the detector existed in front of the screen
It should be obvious to you that this can't be right. Suppose I run the experiment today, and then wait ten years and put a detector screen there. The photon from the experiment has long since flown off and is now ten light-years from Earth (we'll suppose I did the experiment out in space near the Earth, so the photon wouldn't hit anything else). How could it possibly hit the detector?
 
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  • #3
Aj_
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Hi! Thanks for the swift reply.

It should be obvious to you that this can't be right. Suppose I run the experiment today, and then wait ten years and put a detector screen there. The photon from the experiment has long since flown off and is now ten light-years from Earth (we'll suppose I did the experiment out in space near the Earth, so the photon wouldn't hit anything else). How could it possibly hit the detector?
Let me add some more information about why I am asking this question.

I was watching this youtube video by a channel called PBS Space Time titled "How the Quantum Eraser Rewrites the Past" (I don't know if I can add a link since I am new here) . At 5:41 he explains how the "delayed choice quantum eraser" causes the interference pattern to disappear even though the entangled particle was measured by the slit-detector (A and B in the video) only after the other one had reached the screen. This got me thinking: What if the slit-detectors were to be removed quickly from their position once the pattern has formed on the screen, so that no measurements are made? Wouldn't this be somewhat equivalent to what I was describing?

To clarify, I'll rephrase: Suppose the detector is placed in front of the slits, in such a way that it can detect which slit the photon came through. Then a particle is emitted, maybe from such a far distance such that it will take, say, 5 seconds to pass through the slits and reach the detector. At second 4, when the particles have crossed the slit but haven't reached the detector, you remove the detector, so no real detection/measurement takes place. Will this cause the wave function to collapse?

I don't know if the video is oversimplifying the matter. Perhaps there are many missing details there?
 
  • #4
PeterDonis
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Suppose the detector is placed in front of the slits, in such a way that it can detect which slit the photon came through.
This language is confusing, since "detector" is normally used to describe the thing that is after the slits and records where the particles land after they have passed through the slits.

Then a particle is emitted, maybe from such a far distance such that it will take, say, 5 seconds to pass through the slits and reach the detector.
The particle does not have a definite speed unless you measure it, so if you don't measure it, you can't say for sure where it is while it is inside the apparatus.

At second 4, when the particles have crossed the slit but haven't reached the detector, you remove the detector, so no real detection/measurement takes place.
If the particles have passed the slits, the detector will already have recorded a result. Removing it does not change the result it recorded. Nor will it change anything that happens afterwards.

I don't know if the video is oversimplifying the matter. Perhaps there are many missing details there?
I haven't seen the video, but the delayed choice quantum eraser is a fairly advanced experiment and I would not trust any pop science presentation of it. If you search these forums, you should find a number of threads discussing this experiment, which should also contains some links to actual papers by experimental and theoretical physicists.
 
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  • #5
Aj_
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I haven't seen the video, but the delayed choice quantum eraser is a fairly advanced experiment and I would not trust any pop science presentation of it. If you search these forums, you should find a number of threads discussing this experiment, which should also contains some links to actual papers by experimental and theoretical physicists.

That's true I guess. I'll search the forums. It kinda hard to find exactly what I am searching for though.

Anyway, I've embedded the video. Perhaps it will be easier for you to explain the doubt I have about this video than the derived question I have. I'd appreciate it if you could take a look.
From 4:54 to 6:03 He explains the first part of the experiment.
From 5:42 onwards, he says how one of the entangled particles reach the detectors A and B only after the other one has reached the screen. So my question is basically this: What would happen if the detectors A, B were removed immediately after the currently emitted particle hits the screen, such that no measurement is made? This is repeated for every particle that is emitted. I suppose in the end you will get an interference pattern since none of the particles were ever measured.

Thanks a lot for your time.
 
  • #6
PeterDonis
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It kinda hard to find exactly what I am searching for though.
"Delayed Choice Quantum Eraser"
 
  • #7
Aj_
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"Delayed Choice Quantum Eraser"
Haha, I know that. I meant a question that is very similar to mine. Anyway, I think I found some more information on this elsewhere. Seems like the youtube video was over-simplifying stuff. Thanks again for your help.
 
  • #8
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Hi all!
I'm a physics enthusiast and I've been reading/watching a lot of stuff about the double slit experiments. I was watching a youtube video about the delayed quantum eraser experiment and it was really interesting and got me thinking. So here's my question.

What if the detector is placed, a photon is emitted, and before the photon reaches the slits or the detector itself(depending on whichever is closer to the photon source), the detector is removed far away so that no measurement can be made and this is repeated over time for each photon. What would be the output of this experiment?

Will it prevent the interference pattern from appearing? Has an experiment like this ever been done? Perhaps there is a link?

The reason I'm asking this is because I've read that a photon doesn't experience time since it is moving at light-speed. So, perhaps for the photon, it wouldn't matter at what time the detector existed in front of the screen and would this cause the wave function to collapse?
If you understand the Bohm Interpretation, which eliminates all notions of this strange wave collapse, everything will make sense even the delayed choice which is nothing more than the Quantum Potential acting instaneously at a distance. In the Bohm Interpretation, everything is real. The wave is real and the "particle", as we recognize it, is a wave perturbation.

To answer your question, if there is no detector, the Quantum Potential remains real and unaffected. No need to go any further.
 
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  • #9
Aj_
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@richrf Thanks. I'll read on it.
 

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