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#41
11-09-2007, 09:27 AM
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Re: Do I Misunderstand The Double Slit Experiment
Hi David, There's nothing that I can add to this thread that hasnt already been said. I just wanted to mention two excellent books that have helped me understand what's going on here:
1) "The Elegant Universe" written by Brian Green 2) "The Fabric of the Cosmos" also by Brian Green Both books cover the infamous double slit experiment, and "The Fabric of the Cosmos" talks about other interesting experiments in quantum physics such as the Beam-splitter experiment and the Delayed-choice quantum eraser experiment. These books are easy to read becuz Green assumes from the outset that the reader knows nothing about quantum physics. 
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#42
11-09-2007, 10:31 AM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
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. [/ QUOTE ] True. The pattern vanishes when you disturb the photon(s). [ QUOTE ] 2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles. [/ QUOTE ] Depends an what you actually mean here. Once the photon hits the camera film, its gone, and you cant make it magically reapper inorder to make an interference pattern or a point particle pattern. Basically its the same a detection it when it passes through the slits, if you do that the pattern vanishes. [ QUOTE ] 3. If the movie camera has no film the pattern will be waves. [/ QUOTE ] If you dont look at them when passing (disturb) then will make an interference pattern. [ QUOTE ] 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! [/ QUOTE ] Not resolved as of yet. Shahriar Afshar did an experiment that does this. And according to his experiment you can actually look (after the fact) which slot it passed and you still get the interference pattern. However this experiment is not conclusive, and might actually point to something completly different going on (new physics).
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#43
11-09-2007, 12:26 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
[ QUOTE ] 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! [/ QUOTE ] Not resolved as of yet. Shahriar Afshar did an experiment that does this. And according to his experiment you can actually look (after the fact) which slot it passed and you still get the interference pattern. However this experiment is not conclusive, and might actually point to something completly different going on (new physics). [/ QUOTE ] About Scenaro #4: As has already been mentioned in this thread by many posters. In order for the camera to observe which slit the photons moves through it has to use other photons to see that photon. This process disturbs the photons in question and gives us "which path" information which leads to the partical pattern on the back screen. So if the camera "disrupts" the passing photons. It doesnt matter if humans arent looking at the screen. It will always be a particle pattern. Furthermore, even if we lose the film before we look at the screen, a particle pattern will still be there. Bottom line is this: Once the photons hit the screen its all over. There's nothing we can do to change the pattern on that screen. How bout this: What if we use some kind of tagging device to see which slit each photon goes through and then "erase" the mark imprinted by the tagging device just before the photons hit the screen. Will there still be a particle pattern on the screen or an interference pattern revealing a wave? The answer is we will see an interference pattern. this in a nut shell is the quantum eraser experiment. One of the keys to understanding the quantum eraser experiment is the nature in which we "tag" the photons to know which slit each one passes through. We have to be able to "tag" each photon in such a way that "which path" information can be erased before each photon hits the screen. For instance, if we placed a photon detector in front of each slit, the detector's readout would establish with certainty whether the photon went through the left slit or through the right slit and there would now be no way to erase this information so there would be no way to recover the interference pattern. So the quantum eraser experiment is directly dependent on exactly how we figure out which slit each photon goes through. Here's a passage from Brian Green's book "The Fabric of the Cosmos" that explains this idea: "The tagging devices are different (from a photon detector) because they provide only the potential for which-path information to be determined-and potentialities are just the kinds of things that can be erased." BTW, everything Ive talked about basically comes from pages 192-194 of Green's book. To sum things up again, Sklansky's scenario 4) cannot happen becuz the photons have already hit the screen, and since the camera used other photons to see which slit each passing photon went through, thus eastablishing which path information, the pattern on the screen will always be a particle pattern whether we lose the film or not. These are the conclusions I have come to based on my limited understanding of quantum mechanics.
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#44
11-09-2007, 12:42 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
To sum things up again, Sklansky's scenario 4) cannot happen becuz the photons have already hit the screen, and since the camera used other photons to see which slit each passing photon went through, thus eastablishing which path information, the pattern on the screen will always be a particle pattern whether we lose the film or not. These are the conclusions I have come to based on my limited understanding of quantum mechanics. [/ QUOTE ] Not exactly. I could be wrong, but I'm pretty sure that in vacuum photons do not interact (or if they do because of some weird vacuum fluctuation / quantum field theory reasons that I don't understand, it certainly doesn't have a big cross section.) So the issue isn't that you're measuring one photon with another, but that any photons that get to the camera represent ones that did not hit the screen, and so you don't know anything about what's going on with the things that are hitting the screen. Looking at the Wiki article (and I should read the paper at some point), the way that real quantum eraser experiments are done usually involves parametric down conversion, which is a nonlinear process in a medium where an incoming photon of frequency 2f gets changed into two entangled photons of frequency f. One of these photons gets sent to the "screen," and the other ends up going into an interferometer, where through some cleverness in design you can extract which way information for some fraction of the photons that make it through. So the trick is that you turn your single photon into two photons, and then use their entanglement so that measurements on one photon tell you something about the other.
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#45
11-09-2007, 01:28 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
[ QUOTE ] [ QUOTE ] 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! [/ QUOTE ] Not resolved as of yet. Shahriar Afshar did an experiment that does this. And according to his experiment you can actually look (after the fact) which slot it passed and you still get the interference pattern. However this experiment is not conclusive, and might actually point to something completly different going on (new physics). [/ QUOTE ] About Scenaro #4: As has already been mentioned in this thread by many posters. In order for the camera to observe which slit the photons moves through it has to use other photons to see that photon. This process disturbs the photons in question and gives us "which path" information which leads to the partical pattern on the back screen. So if the camera "disrupts" the passing photons. It doesnt matter if humans arent looking at the screen. It will always be a particle pattern. Furthermore, even if we lose the film before we look at the screen, a particle pattern will still be there. Bottom line is this: Once the photons hit the screen its all over. There's nothing we can do to change the pattern on that screen. How bout this: What if we use some kind of tagging device to see which slit each photon goes through and then "erase" the mark imprinted by the tagging device just before the photons hit the screen. Will there still be a particle pattern on the screen or an interference pattern revealing a wave? The answer is we will see an interference pattern. this in a nut shell is the quantum eraser experiment. One of the keys to understanding the quantum eraser experiment is the nature in which we "tag" the photons to know which slit each one passes through. We have to be able to "tag" each photon in such a way that "which path" information can be erased before each photon hits the screen. For instance, if we placed a photon detector in front of each slit, the detector's readout would establish with certainty whether the photon went through the left slit or through the right slit and there would now be no way to erase this information so there would be no way to recover the interference pattern. So the quantum eraser experiment is directly dependent on exactly how we figure out which slit each photon goes through. Here's a passage from Brian Green's book "The Fabric of the Cosmos" that explains this idea: "The tagging devices are different (from a photon detector) because they provide only the potential for which-path information to be determined-and potentialities are just the kinds of things that can be erased." BTW, everything Ive talked about basically comes from pages 192-194 of Green's book. To sum things up again, Sklansky's scenario 4) cannot happen becuz the photons have already hit the screen, and since the camera used other photons to see which slit each passing photon went through, thus eastablishing which path information, the pattern on the screen will always be a particle pattern whether we lose the film or not. These are the conclusions I have come to based on my limited understanding of quantum mechanics. [/ QUOTE ] I know I know. But the experiment that Shahriar Afshar performed, actually "looked" without looking. You will have to look it up yourself to understand it. But in principle he actually did this: - Send photons through slits - Look what slit the photon came through (without actually looking). (hint, this is the clever part) - Observe if they make a point particle pattern or an interference pattern. And he still got an interference pattern.
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#46
11-09-2007, 01:28 PM
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Re: Do I Misunderstand The Double Slit Experiment
"Not exactly. I could be wrong, but I'm pretty sure that in vacuum photons do not interact (or if they do because of some weird vacuum fluctuation / quantum field theory reasons that I don't understand, it certainly doesn't have a big cross section.) So the issue isn't that you're measuring one photon with another, but that any photons that get to the camera represent ones that did not hit the screen, and so you don't know anything about what's going on with the things that are hitting the screen."
I never looked at it this way before. My understanding of quantum mechanics is not deep enough to know if what youre saying is true or not. I will assume for now that what you said is true though. However I do think it would be better for didactic purposes to assume that the camera is using other photons to observe the photons passing through the slits even if this is impossible. "Looking at the Wiki article (and I should read the paper at some point), the way that real quantum eraser experiments are done usually involves parametric down conversion, which is a nonlinear process in a medium where an incoming photon of frequency 2f gets changed into two entangled photons of frequency f. One of these photons gets sent to the "screen," and the other ends up going into an interferometer, where through some cleverness in design you can extract which way information for some fraction of the photons that make it through. So the trick is that you turn your single photon into two photons, and then use their entanglement so that measurements on one photon tell you something about the other." What youre talking about here is called the "Delayed-Choice Quantum Eraser" experiment. This is an extention of the beam-splitter experiment. Since we are talking about a double slit experiment in this thread, a simple version of the quantum eraser experiment is all that is necessary to convey my point. Both the eraser experiment you mentioned and the one I delineated are valid in explaining this counterintuitive phenomena. Again the main point I was trying to illustrate is that once "which path" information on the photon has been established, and these photons have already hit the screen, nothing in the future can change the particle pattern on the screen for these photons, so Sklansky's scenario 4) cannot happen. Again this is based on what little I know about quantum mechanics.
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#47
11-09-2007, 01:30 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
I know I know. But the experiment that Shahriar Afshar performed, actually "looked" without looking. You will have to look it up yourself to understand it. But in principle he actually did this: - Send photons through slits - Look what slit the photon came through (without actually looking). (hint, this is the clever part) - Observe if they make a point particle pattern or an interference pattern. And he still got an interference pattern. [/ QUOTE ] I will look this up. Thank you for this information Muppet.
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#48
11-09-2007, 01:38 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
[ QUOTE ] [ QUOTE ] [ QUOTE ] 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! [/ QUOTE ] Not resolved as of yet. Shahriar Afshar did an experiment that does this. And according to his experiment you can actually look (after the fact) which slot it passed and you still get the interference pattern. However this experiment is not conclusive, and might actually point to something completly different going on (new physics). [/ QUOTE ] About Scenaro #4: As has already been mentioned in this thread by many posters. In order for the camera to observe which slit the photons moves through it has to use other photons to see that photon. This process disturbs the photons in question and gives us "which path" information which leads to the partical pattern on the back screen. So if the camera "disrupts" the passing photons. It doesnt matter if humans arent looking at the screen. It will always be a particle pattern. Furthermore, even if we lose the film before we look at the screen, a particle pattern will still be there. Bottom line is this: Once the photons hit the screen its all over. There's nothing we can do to change the pattern on that screen. How bout this: What if we use some kind of tagging device to see which slit each photon goes through and then "erase" the mark imprinted by the tagging device just before the photons hit the screen. Will there still be a particle pattern on the screen or an interference pattern revealing a wave? The answer is we will see an interference pattern. this in a nut shell is the quantum eraser experiment. One of the keys to understanding the quantum eraser experiment is the nature in which we "tag" the photons to know which slit each one passes through. We have to be able to "tag" each photon in such a way that "which path" information can be erased before each photon hits the screen. For instance, if we placed a photon detector in front of each slit, the detector's readout would establish with certainty whether the photon went through the left slit or through the right slit and there would now be no way to erase this information so there would be no way to recover the interference pattern. So the quantum eraser experiment is directly dependent on exactly how we figure out which slit each photon goes through. Here's a passage from Brian Green's book "The Fabric of the Cosmos" that explains this idea: "The tagging devices are different (from a photon detector) because they provide only the potential for which-path information to be determined-and potentialities are just the kinds of things that can be erased." BTW, everything Ive talked about basically comes from pages 192-194 of Green's book. To sum things up again, Sklansky's scenario 4) cannot happen becuz the photons have already hit the screen, and since the camera used other photons to see which slit each passing photon went through, thus eastablishing which path information, the pattern on the screen will always be a particle pattern whether we lose the film or not. These are the conclusions I have come to based on my limited understanding of quantum mechanics. [/ QUOTE ] I know I know. But the experiment that Shahriar Afshar performed, actually "looked" without looking. You will have to look it up yourself to understand it. But in principle he actually did this: - Send photons through slits - Look what slit the photon came through (without actually looking). (hint, this is the clever part) - Observe if they make a point particle pattern or an interference pattern. And he still got an interference pattern. [/ QUOTE ] For this thread we probably shouldn't mention the Afshar experiment. Most people I've talked to have serious doubts about the setup and conclusion. Even if the Afshar experiment is correct I still think 4 would be wrong since it could be used to send information faster than C.
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#49
11-09-2007, 01:39 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
[ QUOTE ] I know I know. But the experiment that Shahriar Afshar performed, actually "looked" without looking. You will have to look it up yourself to understand it. But in principle he actually did this: - Send photons through slits - Look what slit the photon came through (without actually looking). (hint, this is the clever part) - Observe if they make a point particle pattern or an interference pattern. And he still got an interference pattern. [/ QUOTE ] I will look this up. Thank you for this information Muppet. [/ QUOTE ] Good idea [img]/images/graemlins/smile.gif[/img] But its somewhat of a mess, as it stands they are still discussing if what he did is actually right or not. You would think its simple to resolve, but apparently its not. Personally I think all of QM is an enormous mess, its a good theory, but there is definitely something missing. And its most likely a very good approximation of some other more fundamental theory. So as it stand, resolving this wont be easy.
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#50
11-09-2007, 01:42 PM
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Re: Do I Misunderstand The Double Slit Experiment
[ QUOTE ]
For this thread we probably shouldn't mention the Afshar experiment. Most people I've talked to have serious doubts about the setup and conclusion. Even if the Afshar experiment is correct I still think 4 would be wrong since it could be used to send information faster than C. [/ QUOTE ] I'm actually inclined to agree [img]/images/graemlins/ooo.gif[/img] I only really brought it up, because as it is you cant say for sure that its invalid or for that matter that it is valid. And sending information faster than C is totally ok, as long as its not classical information (spooky action at a distance already says that information can travel faster than C - unless you wanna bring hidden variables into this, and I think your more likely to start worshiping green tea pots).
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Linear Mode
