[Max Raker]

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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!

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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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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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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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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.