Equipment required to run the two-slot physics experiment.

You know, the experiment that established quantum uncertainty and the puzzle of wave/particle duality. With one slot, light acts like particles, with two slots lights acts like a wave. Raising the question: how does the light particle know whether or not the second slot is open?

Anyway, just curious to know what level of equipment and expertise is required to actually perform the two-slot experiment live, in real-time. How expensive is the equipment, how heavy is it, how difficult is it to operate, how dangerous is it, etc.

Considering it as a demonstration for a small group.

Wait... are you an earth-creature?

Never mind.

(this looks good):
http://www.altair.org/TwoSlit.html
to see the wave nature. As far as deeper looks, you're on your own, it doesn't look good. From the source:

Whats more, things we used to feel certain were just particles, such as electrons and even atomic nuclei of metals, can act this way. Stranger yet, we now have the technology to track individual particles and watch what path they take when they encounter the two slits. Amazingly, they now act as ordinary particles, and the interferenca pattern vanishes. Quantum wave interference only happens when "nobody is watching" them pass thru the slits. As soon as we stop tracking the path of the particles, the quantum wave interference reappears. How do the particles "know" when its okay to turn into waves?

The <altair.org> link describes how to create the two-slot-open wave pattern, but doesn't discuss how to close one slot to create (as I understand it) not a wave pattern but a bar. I guess you could just put a tape over the slit to close it.

I threw up in my mouth a little at the altair.org page's subtitle, "Physics cannot explain how the two-slit experiment works - but it does!"

That is utter bullshit - the statement only comes close to being true if you define "physics" to exclude wave mechanics.

But if you're happy to work with photons as your particles, the 2-slit experiment is not difficult or dangerous at all unless you make it so by unnecessarily using a dangerously powerful laser.

:P

I think what the writer used the wrong word, but was getting at is that physics doesn't provide a good interpretation for WHY it works, that's why we have the many universes interpretation and so much other weirdness. And I don't blame scientists. Im not even an amateur physicist, but I am fascinated by this experiment, and the more I understand it the more it twists my head around. Its true as you say that there are models that explain "how" it works, but they define any kind of basic common sense intuitive explanation you can get with so much of the rest of science.

Physics can explain the wave behavior in terms of wave mechanics. Trivial.
Physics can explain the particle-like behavior with simple mechanics.
What physics cannot "explain" (but only describe) is how the same "object" can be both a wave and a particle. All so-called explanations are hand-waving. (Disclaimer: I only have an MS in physics, so I won't claim to be an expert, but I've had many experts tell me that anyone who says they understand it, really understand it, is full of crap.) Mostly, people who have a good grasp of the basic of freshman physics can confidently tell themselves they understand it, but they're only kidding themselves. At the deepest level it makes no sense at all!

it's still acting like a wave, you can see wave properties even with one slit. The key is the reletive sizes of the wavelength and the key measurements of the system, which are 1) the width of each slit and 2) the distance between the slits if there are more than one.

There's really not a lot of danger involved, other than shooting the laser in someones eye.

As far as what you need, it kind of depends on what you want the group to do. If you want them to be able to make measurements and do calculations with them, then you'll need more equipment than i you just want to demonstrate the effect. The minimum equipment would be a two slit mask, which can be found online I would imagine. Any sort of laser should do, though you get different levels of visibility from different lasers.

If you want to truely macguiver something together, it's possible to observe single slit diffraction with just a laser pointer and a pair of fingernail clippers. Shooot the laser between the mouth of the clippers and carefully squeeze down on the clippers. As the gap narrows, you should see the spot of laser light stretch out into a line just before it vanishes. In the line you should be able to see several diffraction minima. This is sort of too small scale to be done as a demo, but easy enough that everyone could do it for themselves

What's more important than the exeriment though, is the exposition leading up to it. For the results to mean something, people need to know what to expect to see from both of the models. If light were acting truely like a particle, then behind a single slit mask, you'd expect to see a single bright spot, behind a two slit mask, you'd expect to see 2 bright spots, etc. With a wave model though, it is possible to get more bright spots than slits, due to interferance and diffraction effects.

I thought single-slot result looked like shooting paintballs through a narrow window, without any wave characteristics. That's what I've gathered from numerous popular science prose explanations.

Anybody got a link to actual photos of resulting patterns of single/double slot runs?

Though the one slit thing is probably right. Regarding the strangest part of the experiment, what I heard is that two slits gets wave like behaviour. If you put a photon detector by one of the slits to "watch" the photon going by, it collapses the wave like behaviour and then it acts like paintballs going through two slits. IF you fire one photon at a time same result. If you fire one photon, record it going through but erase it before it hits the back wall, it goes back to wave behaviour. So the behaviour is changed by whether or not you record particle like behaviour. That's why its so confusing. THAT's the part I would like to test because it sounds like BS to me. But I haven't the equipment to fire and detect one proton....

"If you fire one photon, record it going through but erase it before it hits the back wall..."

By "erase", you mean, erase the Photon Detector's record, right? Like the Photon Detector records a 0 for no-photon and 1 for photon. If you somehow change the 1 to 0 before it hits the back wall, then the behavior changes?

Man, I'd love to see that too.

Oh, one more thing. Several folks have suggested using an ordinary laser pointer. I'm seeing how you could run the single-slit experiment with a laser pointer, but I don't see how you could do the double-slit experiment with a laser pointer.

I appreciate all the DU brainpower on my layman's question.

but the slits need to be very close together. This here says tiny pinpoints in aluminum foil only 1mm apart.
http://www.physicsforums.com/showthread.php?t=72564
Presumably the laser pointer is a little more than 1mm wide. So the same laser beam is going through both slits.

As far as the erase thing, here is the science, I haven't looked into details but that general idea is correct:
http://en.wikipedia.org/wiki/Quantum_eraser_experiment

The quantum eraser experiment is a variation of Young's double slit experiment that established that when a photon is acted upon in any way that permits determining which of the slits it has transited then it cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The quantum eraser experiment demonstrates that it is possible to create situations in which a photon that has been "marked" so as to enable determination of which of two slits it transited in order to reach a detection screen can later be "unmarked." A photon that has been so marked cannot interfere with itself. But a photon that has been marked and then unmarked can interfere with itself. When a stream of photons has been marked and then unmarked it can once again produce the fringes characteristic of the Young experient.

So after it is erased (unmarked) it goes back to acting like a wave, being in may places at once so its peaks and troughs cancel each other out making the wave pattern you see with the two slits. So therefore the PHYSICAL act of observing it doesn't collapse the wave behavior, something later. Like the fact that a human mind observes it collapses the wave behavior. (That's my Matrix interpretation) or you can have something ugly like cause and effect working backwards through time...

All you need is a laser, a board with two fine slits a measured distance apart, a flat surface, and something to hold everything together.

The hard part is finding the board with slits, because they need to be a specific distance apart for the specific type of laser you use.

When I first did the two slit experiment in high school lasers were a little hard to come by. So we had to use a gas tube energized with a clumsy high voltage contraption you wouldn't let anywhere near high school students today. Our physics teacher said not to touch the machine when it was on; otherwise we might electrocute ourselves. Teenaged boys love dangerous stuff especially when they break the rules and don't die. (Yes, that thing could give a ferocious zap!)

Now high schools use laser diodes in those rare classes where they actually do hands-on stuff, which sorta takes the fun out of it. But I'm sure the kids shine lasers into their eyes because they are told not to.

Anyways, one slit is easy enough to whip up at home, and you will get a nice pattern with one slit, or even with the edge of a razor blade, so it's not that definitive to say, okay, this is the difference between one slit and two unless you know what to expect. Tracking the path of a single photon, or electron, or any other particle takes some fancy equipment, so you you usually have to settle with huge numbers of photons or electrons and discern their individual behavior from their collective behavior.

Nevertheless you can demonstrate some weird, non-intuitive quantum effects without going to the trouble of making two very fine slits.

I just put together this contraption on the freezer in my garage using a laser level I picked up at Harbor Freight for six bucks, a fine mesh reusable coffee filter, and a bit of junk mail:



This is the pattern I got:



You might think this pattern makes sense, that it's somehow the shadow of the coffee filter screen, but then when you start moving the screen around you'll see that it doesn't behave like a shadow at all...

In the two slit experiment you get something similar. With one slit you get a single bright band with a few lesser bands around it where a few photons interact with the edge of the slit in wavelike manner, but the bulk of them go through the single slit in a particle-like manner. But with two slits you get a mess of brighter bands with an interference pattern that implies many more photons are interacting with the slits as waves than would reasonably be expected from the results of the one slit experiment.

In the coffee filter experiment the photons are passing through multiple holes as if they are waves.

For further research I liked this guys web site, mostly because he has Spock on his home page and claims to be a Marxist:

http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html

You can't just shoot 1 photon with a laser pen.

the interference pattern would not show up. Thus the reason it's a mystery. Don't try to understand it. The honest truth is NOBODY understands it. The best that ANY physicist can do is use his equations to DESCRIBE (but not explain) it. Hence: "Quantum Weirdness".

Look it up just about anywhere. The experiment is totally trivial. I did it myself when I was 12, about the same time I started building my first telescope. (No it does NOT have to use a laser. And ordinary flashlight bulb and a pinhole in black construction paper works as a light source.)

http://www.altair.org/TwoSlit.html
http://forums.randi.org/showthread.php?t=52659
http://answers.yahoo.com/question/index?qid=20090416200632AAFcJVO
http://www.youtube.com/watch?v=DfPeprQ7oGc