Do I Misunderstand The Double Slit Experiment

[David Sklansky]

What I think I know about it comes from a couple of forty year old books. And what I remember reading is so spooky that it gives one pause about whether some supernatural entity could have something to do with it. But every time I mention it the scientists pooh pooh me. Which makes me think that I am thinking things about the experiment, and related exoeriments that aren't true.

So without going into detail, are the following things true?

1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

3. If the movie camera has no film the pattern will be waves.

4. IF THE CAMERA HAS FILM AND WE DON'T LOOK AT THE SCREEN UNTIL AFTER WE LOOK AT THE PICTURES, AND ON THE WAY TO THE DRUGSTORE WE FALL AND RUIN THE FILM, WE WILL SEE WAVES ON THE SCREEN. In other words the photons "know" that we will not be able to see them go through the slits, even though our inability to do that is because of an event in the future!

Am I wrong?

[gumpzilla]

Well, if individual photons are coming through the slits, if they make it to the camera, that means they scattered off something and didn't get to the screen. So to be pedantic, they shouldn't tell you anything about the interference pattern, meaning you'll see interference behaviors.

There are ways in which you can do experiments similar to your case 4. They tend to go by the name "delayed choice quantum eraser," and it's been a while since I've read about them, so I can't say offhand if your impressions are exactly correct. It's a pretty subtle issue, obviously.

EDIT: From the Wikipedia on delayed choice quantum eraser,

[ QUOTE ]
In the double slit experiment, a photon passes through one of two slits in a double slit apparatus, and then registers on a detector. The detector shows where the photon hit it, like an image projected on a screen. If many photons individually pass through the double slit apparatus, and nothing observes which slit a given photon travels through, an interference pattern emerges on the detector. The interference pattern indicates that the light beam is in fact made up of waves. However, if someone observes which of the two slits each photon passes through, a different result will be obtained. In this case, each photon hits the detector after going through only one slit and a single concentration of hits in the middle of the detection field. This result is consistent with light behaving as individual particles, like tiny bullets. It is counterintuitive that a different outcome results based on whether or not the photon is observed after it goes through the slit but before it hits the detector.

In a quantum eraser experiment, one arranges to detect which one of the slits the photon passes through, but also construct the experiment in such a way that this information can be "erased" after the fact. It turns out that if one observes which slit the photon passes through, the "no interference" or particle behavior will result, which is what quantum mechanics predicts, but if the quantum information is "erased" regarding which slit the photon passed through, a wavelike interference pattern can be observed.

However, Kim, et al. have shown that it is possible to delay the choice to erase the quantum information until after the photon has actually hit the target. But, again, if the information is "erased," an interference pattern can be recovered in a certain subset of the photons which reach the detector, even if the information is erased after the photons have hit the detector

[/ QUOTE ]

[DougShrapnel]

From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

[Borodog]

[ QUOTE ]
From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

[/ QUOTE ]

Hmm. This is news to me. Also, it doesn't quite make sense. How could you look at a subset of photons that were seen at a particular detector if you have destroyed the information from the detectors? [img]/images/graemlins/confused.gif[/img]

[DougShrapnel]

[ QUOTE ]
[ QUOTE ]
From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

[/ QUOTE ]

Hmm. This is news to me. Also, it doesn't quite make sense. How could you look at a subset of photons that were seen at a particular detector if you have destroyed the information from the detectors? [img]/images/graemlins/confused.gif[/img]

[/ QUOTE ]It's all there in the wiki article they use a beem spliter on entagled particles. And test only a subset. I'm not an expert on this stuff, so grain of salt.

[Borodog]

[ QUOTE ]
What I think I know about it comes from a couple of forty year old books. And what I remember reading is so spooky that it gives one pause about whether some supernatural entity could have something to do with it. But every time I mention it the scientists pooh pooh me. Which makes me think that I am thinking things about the experiment, and related exoeriments that aren't true.

So without going into detail, are the following things true?

1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

3. If the movie camera has no film the pattern will be waves.

4. IF THE CAMERA HAS FILM AND WE DON'T LOOK AT THE SCREEN UNTIL AFTER WE LOOK AT THE PICTURES, AND ON THE WAY TO THE DRUGSTORE WE FALL AND RUIN THE FILM, WE WILL SEE WAVES ON THE SCREEN. In other words the photons "know" that we will not be able to see them go through the slits, even though our inability to do that is because of an event in the future!

Am I wrong?

[/ QUOTE ]

This is essentially correct, barring the fact that you can't just use a "movie camera". After all, you can see the interference pattern while *looking* at the beam hitting the slits. You have to actually be able to *detect* which slit an individual photon passes through, which requires interacting with it, and localizing it, which is what destroys the interference pattern. But my understanding is that if you do as you say, that is, not observe the screen and wipe out the data of which slit each photon went through, and then observe the screen, you will observe an interference pattern.

Edit: Er, what gump said.

[David Sklansky]

Hey Not Ready come back! Did you know about this stuff?

[NotReady]

[ QUOTE ]

Hey Not Ready come back! Did you know about this stuff?


[/ QUOTE ]

Very flattering but you have more than one expert explaining it in this thread. My only take on it is that we change the quantum world by observing it. If you try to grab something very light that's floating in water the object will move because of the action of your hand causing waves which move the object. I think something like that is what happens when we try to "grab" the quantum world by observation. Just a total layman's view.

[Schweitzer]

[ QUOTE ]
This is essentially correct, barring the fact that you can't just use a "movie camera". After all, you can see the interference pattern while *looking* at the beam hitting the slits. You have to actually be able to *detect* which slit an individual photon passes through, which requires interacting with it, and localizing it, which is what destroys the interference pattern. But my understanding is that if you do as you say, that is, not observe the screen and wipe out the data of which slit each photon went through, and then observe the screen, you will observe an interference pattern.

Edit: Er, what gump said.


[/ QUOTE ]

So what you are saying is if light hits the particles they become localized and no interfence pattern is observed whether or not the light is observed?
I was under the impression that if light interacts with the particles, but the light is not observed, that an interfence pattern would be observed.

[dknightx]

so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

At this point in time, Person A returns, does what he see differ than what he saw earlier?