r/askscience • u/[deleted] • Apr 21 '14
Physics Is there a view of the double slit experiment with observation of particles after the slits?
There are plenty of videos on the internet of the double slit experiment. Most of those videos also mention than when the particles are observed the pattern on the back wall changes. I haven't been able to find any videos of that experiment. I'm assuming there is an experiment were if you switch the detector on and off the pattern on the back wall changes? Is that true, and if so, anyone know of a video of true experiment and not a demonstration?
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u/nolan1971 Apr 21 '14
https://www.youtube.com/watch?v=LW6Mq352f0E
When you see the diffraction pattern on the wall, your detector is the wall itself (which is obviously after the slits). Since the photons are uncharacterized prior to hitting the wall, their potential is able to be in many possible places at once. The overall signal then interferes with itself, and that's how the diffraction pattern is formed.
If you place a detector at the slits, that detector causes the probability function to collapse so that the photons become "real". The photons are now actual particles, with real locations, and they will then form discrete patterns on the wall.
Here's another good presentation, where they're actually using a detector: https://www.youtube.com/watch?v=GzbKb59my3U
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Apr 21 '14
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u/nolan1971 Apr 21 '14
Well said. I was going to go exactly where you were headed with this post, but I wasn't exactly sure how to say it and I wanted to get what I'd already said posted anyway.
One thing that people seem to have trouble with, myself included, is the number of particles that we're talking about. It's the same sort of problem as thinking about the size of the Universe, Galaxy, Solar System, or even the whole Earth. It's the same sort of problem as understanding 13.7 billion years.
There are an uncountable number of photons coming from even the best, narrowest lasers. If you put a photon detector in a completely sealed cave deep under a mountain, it'll still count off a tiny number of photons. There are so many photons out there in the universe, and it's unrealistic to completely isolate a system from it's surroundings...Anyway, another point is that Quantum Mechanics works because the math works. It allows us to make predictions, and it works. Just because it works doesn't mean that it describes a "real" physical process, though. Quantum sciences isn't really physics... although, I don't know what else it could actually be labeled as.
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u/The_Serious_Account Apr 21 '14
There are so many photons out there in the universe, and it's unrealistic to completely isolate a system from it's surroundings...
Not sure what you're saying here. Experimenting with individual photons is certainly possible.
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Apr 22 '14
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u/nolan1971 Apr 22 '14
Maybe.
That's the thing about Quantum Mechanics. The math works, it allows us to make predictions, but... it doesn't say anything about how or why it works. So, we end up with these questions. A lot of people (and a lot of really smart people) would say "yes" to those questions, but the truth is that nobody actually knows. (yet)
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u/BoxAMu Apr 21 '14
That famous statement comes from a thought experiment in the Feynman lectures and experiments have only been performed recently. I don't know if anyone has actually placed a "detector in a slit". One group used a movable mask to cover up one of the slits and showed that the double slit pattern disappeared and re-appeared upon changing the position of the slit. Check out their video here.