Here's a picture of the atom before quantum mechanics. I'm sure you've seen illustrations like this before -- the electrons orbit the nucleus like planets around the sun:
|The atom, before quantum mechanics|
But that's not what the atom really looks like. In the quantum mechanical picture, it looks more like this:
|The atom of quantum mechanics|
Much of the weirdness of quantum mechanics is related to this problem of randomness and measurement. How can the location of the electron really be undetermined until you measure it, and how does your act of measurement force the electron into a definite location?
Maybe quantum mechanics is really just very complicated and only appears random, so if we dig deep enough we'll find a nonrandom theory buried underneath. For example, a game of roulette appears random, but if you had good enough information about the motion of the wheel and the ball, you could predict the correct roulette number every time. This was Einstein's hope ("God does not play dice with the universe"). But all of our experiments seem to refute this idea.
The conventional interpretation, called the "Copenhagen interpretation," says that the electron in the atom starts out in a superposition of all possible locations, and observing it causes the wave to "collapse" down to a single location. Many people find this unsatisfying, since it means that your act of observing the electron is what causes it to assume a single position.
A more bizarre idea is the "many-worlds interpretation." In this theory, the wave that corresponds to the electron never collapses down to a single location. Instead, every time you measure the location of the electron, the universe itself splits into different branches, with each branch corresponding to a different possible location. And you split as well. It's as though you came to a fork in the road and decided to take... both paths! Of course, you are fully conscious of only the one universe that you happen to find yourself in, but there is another "you' who ended up in the other universe. The many-worlds interpretation means that all possible outcomes of every decision and every experiment are realized somewhere, in some reality. When I studied quantum mechanics in college, our professor presented this idea and then said, "Believe it or not, there are grown men who believe this."
As you might suspect, the many-worlds hypothesis is a gold mine for science fiction. The classic many-worlds story is "All the Myriad Ways," by Larry Niven. Niven's story takes aim at an obvious problem with the many-worlds interpretation: if every decision I make, and its opposite, are both taken in some branch of reality, what difference does it make what I decide to do? In Niven's story, scientists are able to confirm the many-worlds interpretation by actually travelling between the different realities, but when they fully understand the implications, it leads to despair.
An even more impressive effort is Paul Melko's "Ten Sigmas," which is available online. I won't spoil the plot for you, since it's short and you can read it yourself, but Melko imagines a human being who, through some freak of nature, is able to experience all of the realities of the many worlds. It's a virtuoso effort. If you like it, I recommend his novel about parallel universes, The Walls of the Universe.
Next time I'll look at an even weirder offshoot of the many-worlds hypothesis: the idea of quantum immortality.