Let There Be Light, Part I

I'm going to break from tradition today and do something a little different. Instead of the usual kind of puzzle, I'm going to pose a particularly intriguing kind of puzzle: an unsolved mystery. I'd like to give the Grey Labyrinth a little change of pace, and maybe expose a few people to things they don't normally think about.

Now, when many people think of unsolved mysteries, they think of crop circles, the Bermuda Triangle, and the Loch Ness monster. But most of these things are really not that mysterious when examined very carefully and scientifically. They tend to be really normal things exaggerated by human's powerful imaginations.

But today we're going to look at a REAL unsolved mystery... (enter Robert Stack)

And here it is:

OK, the first mystery is, "What is it?"

No, it's not the opening credits from The Matrix, but good guess.

It's actually an image generated by firing a laser through a pair of thin slits onto a light sensitive detector.

And this may not impress you, but trust me, this grainy image is the window onto what could be called the one of the most baffling questions known to man.

You may know that light, and that's all lasers are, is composed of tiny particles of energy called photons. Whenever you flip on a light, countless photons radiate out into the room from the lightbulb, bounce off objects, and some of these eventually collide with cells in your eyes. This is how we see stuff.

Individual photons don't have quite enough energy to be visible to humans unaided. If you were in a completely dark room, it would take a couple dozen of them hitting the same part of your eye before you would notice (some animals, incidentally, can see individual photons).

In the image above, photons were fired from a laser through two narrow slits onto a flat surface producing the image. The tiny dots represent individual photon strikes. Incidentally, the same pattern emerges no matter how fast the photons are fired. If you fired one photon from the laser every day, after a few years the same pattern would appear.

The image to the right shows how the experiment is performed (the size of the slits A & B have been exaggerated). This isn't a complicated experiment- the only sophisticated part is the laser. You could use a laser pointer and see the same effect. The banded image would still be visible, although your eye couldn't detect the individual photon strikes.

The $64,000 question is, what causes the photons to cluster in those bright bands?

You may already have heard the answer to this. Sometimes light behaves like a wave. Waves- like sound waves or even the waves on an ocean- can create interference patterns: points where troughs and crests cancel each other out. If we assume that light is a wave, then the banded pattern is exactly what we would expect to see. Furthermore, this is supported by other experiments we could perform by changing the number of slits.

This is extremely odd, because to produce the banded interference pattern shown, a single photon would have to pass through both slits at the same time, like a wave would. Even more bizarre, additional experiments can be conducted that would show that the photon is spread out in a wave all the time- except when we are looking at it. Any time we try to detect photons, they reliably seem to be in one place.

If in the above experiment we placed detectors at slits A and B, no photon would ever be observed going through both A and B. In fact, it seems to be the case that light is always a wave, spread out through space, until it is observed, when it suddenly is a particle.

So how does a photon know when somebody is looking at it?

Continue on to Part II...