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 ]

[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.

[Schweitzer]

This is another example of a paradox that I have been wondering about for awhile.

Apparent Paradox:

I run the double-slit experiment but do not observe the slit the electrons travel through. Someone a light-minute away with a powerful telescope flips a coin to decide whether to observe them. Will I see an interference pattern or not?

If I see an interference pattern then the observer should be able to see which slit each electron went through and find a contradiction.

If I do not see interference then I know that the observer will not observe the experiment one minute from now and in effect know the future result of his coin toss. I have observed an effect before the cause.

I am about to earn a B.S. in Physics and yet I don't really understand what will happen in this situation.

[David Sklansky]

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

[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?

[David Steele]

[ QUOTE ]
so spooky that it gives one pause about whether some supernatural entity could have something to do with it.

[/ QUOTE ]

What would the entity do?

[Phil153]

There's an excellent chance that you do.

[Borodog]

[ QUOTE ]
[ 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.

[/ QUOTE ]

Both are correct.