Do I Misunderstand The Double Slit Experiment

[Arp220]

Metric gave a good answer. Here is a different take on your questions:

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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.

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Correct, sort of. A better phrasing would be:

If you determine which slit each individual photon goes through, then you will see two bright spots on the screen in front of each slit, as if you shot a stream of particles straight at each slit. If you do not determine which slit each photon goes through, then you'll see an interference pattern on the screen, with a bight blob in the middle, between the slits, accompanied by 'fringes' as if you'd fired one big wave at the slits.

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

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Correct, as long as the movie camera is measuring which slit each individual photon goes through.

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

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I would say this is incorrect. This would be taking the measurement, but not writing the result down [img]/images/graemlins/wink.gif[/img]

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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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This is wrong. You will see particles. If you measure which slit the photons go through then you wont see an interference pattern on the screen. It doesn't matter if you record the result on film or not. Its the action of measurement that is important.

I'll end with something spooky. Lets say you set up the experiment and dont measure which slit the photons go through. You'll see a wave pattern on the screen. Now, if you replace the screen with a very sensitive potodetector that can in principle show where individual photons are arriving... then you'll see individual photons arriving... BUT with many more photons arriving in the bright fringes, and very few arriving in the dark fringes. In other words... you see individual photons, but arriving in the intensity pattern expected from a wave going through the slits and interfering with itself!

Spooky, isnt it?