Spiritual Computers & Quantum Cats

The Background

Physicists have this neat thing called the 'dual slit experiment'. Briefly, you shine a light at an opaque screen with two slits in it, and let the light that shines through the slits land on some kind of screen. In practice, they use a laser for the light because (1) it has a single wavelength, (2) it's really bright, and (3) lasers are cool, aren't they? The slits are very narrow and very close together, and you can use a photographic plate for the screen if you want a permanent record of your experiment.

Why would they do this? Well, back in the mists of time there was this debate about whether light was made of waves or of particles. If it were particles, you'd expect the dual slit experiment to show just two marks, one for each slit where the light shines through onto the screen (or film). If light were made of waves, you'd get an interference pattern, meaning really, two bright spots directly in line with the two slits, but then lots of bands or fringes in between and on either side, due to the way the waves from the two slits sometimes add up and sometimes cancel out. (Hold your thumb and forefinger really close to one another, and really close to your eye, with something like a bright sky in the background. Watch carefully - just before they touch, you can see interference fringes)

Well it turns out they figured out that light is both particle and wave at the same time. In the basic set up, the dual-slit experiment shows the wave nature of light, because uncountable zillions of photons interact, and the wave nature dominates, as indicated by the interference pattern you usually see. Some smart guy who used to be a patent clerk figured out that it was also particles ('photons'), by the way the photoelectric effect works, but that's another story.

Anyway, apparently it is possible to generate single photons of light. Someone had the bright idea to feed single photons into the dual slit experiment and see what would happen. If only one photon at a time passed through the apparatus, the reasoning goes, there ought to be no way for the interference fringes to occur, because how can one photon do anything but pass through one slit or the other?

The surprise was that, somehow, single photons DO interfere with .. themselves, apparently. This was sufficiently weird that whoever it was decided to find out where the photon was really going and put some kind of detector into the apparatus which would tell which slit the photon went through. That only made things weirder: when it was possible to observe which way the photon went, suddenly it started doing what was expected in the first place; acting like a particle.

Which leads to ..

The Punchline

This sort of thing seems to happen all over the place in the quantum world. Observations affect the outcome. Why? Is there something magical about knowing something about a system, that influences the system?

What I would like to see tried is this: is it sufficient for a computer to "know" the outcome of an experiment? Modify the apparatus so that the information about which way the photon went goes into a computer, so that the information about the system is inside the computer, but - and this is important! - so that the computer destroys the information before we, the human observers can interpret it. In effect, we just have a more complicated apparatus that doesn't say which way the photon went. Assuming that the which-way detector is symmetrical and everything, would this setup yield interference patterns for single photons? If it did, that would show that the universe has some perverse grudge against us finding out too much about what's really going on. On the other hand, if it didn't, would that mean that the computer qualifies as an "observer"?

OK, I've been a bit glib with the setup here, but seriously, there does seem to be something specially weird about how reality bends when observed, so the question I'm trying to state here is, "what qualifies as an observer"?

A Meta Posting

Here are half a dozen topics on which I keep planning to offer my two cents, but can't seem to find the time to do a proper job of: (1) why "stereo" headphone jacks are evil; (2) a wimshurst generator from cardboard, dummy CDs, and hot glue; (3) my Cessna 182 R/C building project; (4) a cardboard and hot glue "galvo" for laser projection on the cheap; (5) an evolutionary programming tic-tac-toe playing system; (6) on the weirdness of living in the surface space of a 4D hypersphere, with resulting odd effects on things like, oh, gravity; (7) on putting a computer in the box with Schroedinger's Cat; (8) places to bike in RI/southeast MA w/o getting run over.

Well, OK, more than six. Stay tuned. Maybe I will tackle these soon. Only lately I seem to be spending most of my 'free' time learning to get my Sig Kadet Sr off and back onto the ground.

-J

Summer Vacation?

Woops - haven't posted in a while. That's mainly b/c I took the entire week of the 4th off, and concentrated on getting some serious stick time on my Sig Kadet Sr, which for those that don't recognize it, is an R/C trainer with an 80" wingspan. It has an OS 50sx (2-stroke) glow engine, which pulls nicely even in 10mph breezes. Managed my first take-off, kind of sloppy, but it worked! The next two take-off attempts didn't go so well, but the wind had switched around by then, learning runway techniques in a cross breeze is not something I'd recommend.

More soon ..