Re: Where can I find someone to do a new version of the single slit experiment?

I will start by answering the question in the subject line, which is quite easy to answer, and move on to the more philosophical question in a little while.

Back in the day, counting single elementary particles was a tricky business. However, nearly anyone can do it today. I am not saying that a hobbyist will find it cheap, but typically the hardest thing to find was a good source of particles. However, if you stick to particles of light, these are very easy to come by nowadays. You probably have a good source of near-monochromatic (single colour) photons hanging off your keychain in the form of a laser pointer.

Take one laser pointer, a couple of razor blades and a piece of paper and you have an instant single-slit experiment. With a little more thought you have a double-slit experiment. The hard (and expensive) part is actually the detection of single photons. This requires either a photomultiplier tube (PMT) or an avalanche photodiode (APD). Both of which can probably be bought at surplus shops (Toronto Surplus*, for example has PMTs available at $99CAN), but there is a fair amount of associated power supplies and counting electronics which go along with such a setup. However, any school/college should be able to put such a setup together for a modest price. The photomultipliers require some 1000V to make them work. Avalanche photodiodes typically only require 50-100V. WARNING: APDs and PMTs can both be damaged by too much light when a voltage is applied to them - they are very very very sensitive devices.

*www.torontosurplus.com

To work at the single-photon level one would also have to attenuate the laser beam. A red laser beam with an average power of 100 microwatts generates about 300000000000000 photons per second. This would have to be attenuated. A few (or actually a lot of) filters would be required reduce the photon number down to about 1-10 per second. The setup in total probably looks like the following:

In the diagram the photomultiplier tube can move from side to side to take an image of the diffraction pattern (a slit should also be placed in front of the PMT to get a sharp image of the pattern). Such a setup would be able to easily map out the single slit diffraction pattern at the level of a single photon. The current to voltage converter in the diagram could just be a resistor.

Now, to the second point of the question which requires two switches, one of which functions to generate a particle (photon), and another, which does nothing. The easiest way to do this would actually be to put a mechanical shutter in front of the laser beam which is only opened by one of the switches. a solenoid with a lump of opaque material on the end would probably suffice here. Tada! Instant(ish) single slit experiment.

Okay so those were the technical details, so what about the "philosophy"? Actually, this is where I get stumped since I think the questioner has confused why the interference pattern occurs at all. Let us just go over the idea of a single slit experiment:

The idea is (approaching the problem from already knowing the answer) that the diffraction pattern exists, despite the fact that only one photon goes through the slit at a time. Even if the photon rate is dropped to one per year, a diffraction pattern will build up (eventually). If we just had one photon there would be no pattern - just a single click at one position (in the case of the moving photomultiplier). The reason for requiring a number of photons is just so that the difference can be seen better between an area of space through which photons preferentially travel due to constructive interference compared to one which "sees" less photons due to destructive interference. The outcome works for one photon just as well as it works for a bunch - it just that to see the pattern you have to average over an ensemble.

Since I can't quite follow the reasoning of the proof of whether the wavefunction is real or not, let me just give you my perspective on this. Physics is a mixture of mathematical tools and intuition (more like gut feeling). We use our intuition to think up mathematical expressions to describe stuff and then make up experiments to test those expressions. It just so happens that quantum mechanics is less intuitive than most other things. This is mostly because it happens (typically) on very small scales which we don't get to experience in everyday life. This is why it has taken so long to understand what is going on (more than a century now). However, just because it is counter-intuitive doesn't make a wavefunction in quantum mechanics any less real than, say, momentum or moment of inertia in mechanics. If I ask you to describe moment of inertia, I bet you would rather write down an equation than describe this property of an object in words. Quantum mechanics is really not so mysterious or esoteric as some might make out - it really is just like the rest of physics.

More information on single slit experiments can be found at:

http://www.ph ys.hawaii.edu/~teb/optics/java/slitdiffr/

http://phys.educ.k su.edu/vqm/html/singleslit.html

http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/sinslit.html

http://www.surendranath.org/Applets/Optics/Slits/SingleSlit/Sngl SltApplet.html