Sunday, July 15, 2007

Somewhere over the rainbow

One of the wackier ideas to have emerged from modern physics is parallel universes. That's right, folks. This universe that we live in may not be the only universe. There may exist an uncountable number of universes, some nearly identical to this one, others wildly different. Even as you read, zillions of new universes may be blossoming into existence.

If these other universes exist, then some of them contain other yous and other mes.

How strange those words look on the page: You and me in the plural. There are no such words in the dictionary. Until recently there have been no such concepts in Western philosophy. But, then again, that other me in that other universe may not be me, by virtue of being there. On the other hand, that parallel me may be more like this me than the me that I will be a moment hence.

Oh dear, this is getting out of control. I'd better back up a bit.

It all started with Schrodinger's cat.

Erwin Schrodinger was one of the founders of quantum physics, a mathematical way of describing the world that has proven fabulously successful. In fact, quantum physics is perhaps the most successful physical theory there ever was, in the ways it satisfactorily describes the world we experience. The trouble is, no one knows why quantum physics works.

Let me try to explain.

When quantum physics is used to describe physical events, the descriptions have a kind of fuzziness. For example, if one tries to pin down the position of an electron, then the electron's velocity necessarily becomes less precisely known -- the Uncertainty Principle. This fuzziness is not just a product of our ignorance, but seems to be built right into the fabric of reality.

This same fuzziness applies to energy and time, and to other physical concepts we use to describe the world. In the quantum world, precise description and prediction is impossible. The best we can do is calculate the probability that when we make an observation a certain thing will be observed.

The cat, for example.

Schrodinger imagined a cat inside a box. Also in the box is a little bit of radioactive material, and a radiation detector that will trigger a trip on a hammer poised above a glass vial containing cyanide. If even a single particle is emitted by the radioactive substance, the hammer will fall, the vial will break, and the cat will die.

With the box closed, let us turn on the detector for just long enough that the probability of a particle being emitted is 50-50. According to quantum physics, the emission of particles by radioactive substances has an intrinsic randomness to it. We can know nothing about the exact time a particle will be emitted, only the probability that a particle will be emitted during an interval. At the atomic level, nature is fuzzy.

So what is in the box after the experiment is run? A dead cat? A live cat? Apparently, there is no way to know without opening the box and looking in. But Schrodinger still held to the classical idea that nature is strictly deterministic, and the uncertainty about the cat's fate struck him as unsatisfactory. If there is nothing in the laws of nature to unambiguously specify the outcome of the observation, he asked, then why should nature chose one outcome rather than another?

But maybe, just maybe, nature doesn't choose between two equally probable outcomes. Exactly 50 years ago, Hugh Everett proposed that both outcomes actually happen -- the so-called Many Worlds interpretation of quantum mechanics. The particle is emitted and it isn't emitted. The detector trips the hammer and the detector does not trip the hammer. Instead of settling for one or the other outcome, the universe splits into two simultaneous, non-interacting universes. In one universe the cat dies; in the other universe the cat lives.

And that's why quantum physics (and nature) appears fuzzy, say the Many World physicists. It's because the universe is constantly branching into myriad near copies of itself. All of these parallel universes fully exist in some hyperspace and hypertime, but we are only aware of the universe we line in. The ensemble of all universes is precisely determined by the laws of nature; our universe appears fuzzy because we see only a part of the whole.

Now if all of this sounds wildly farfetched and almost impossible to understand, well, it's because it is wildly farfetched and almost impossible to understand (which is not to say that some wildly imaginative physicists don't take it seriously). I only mention it because it's baseball season. And -- well, once again, the Boston Red Sox are out in front of the league, but you know what's going to happen.

In this universe (the one we inhabit) they'll win the pennant and lose the Series in the bottom of the last inning of the seventh game. Two men out, a man on base, the Sox leading by one run. A grounder to short. The throw to first. A bounce -- to the left, or to the right -- a bounce so finely tuned that it all depends on a quantum event occurring somewhere back along the line of endlessly-fissioning parallel universes.

The bounce is to the left. The catch is missed. The next man up knocks the ball out of the park. The Sox, not unexpectedly, lose.

But here, dear friends, is a source of solace. In any number of parallel universes the bounce is to the right and the catch is made. In those other universes, other yous and other mes will celebrate a Sox victory. And in some greater heaven that overlooks all of these parallel universes, Schrodinger's cat smiles.

Discuss this essay and more over on the Science Musings Blog.