Quantum mechanics (QM) is more than a set of equations. It's also an interpretation: a mapping from math to the physical world. The laws of QM say that objects are described by weird probability wavefunctions which change in a smooth way as time passes. The usual view of QM is that tiny particles follow the rules --- but only until somebody big makes a measurement or an observation. At that point, the croupier says "No more bets", the roulette wheel slows to a stop, and the ball is found lying in a single slot. Red or black (or green), odd or even, high or low, there's no ambiguity: it is what it is.
That approach (the "Copenhagen Interpretation") makes the quantum world of fuzzy waves match up with the classical world of everyday life. It predicts the results of laboratory experiments with astounding accuracy. But it's profoundly unæsthetic and confusing. Why should QM stop working and wavefunctions suddenly collapse just because a macroscopic observer peeks in? And what is an observer, anyway? Can a machine make an observation, or does it take consciousness to clobber a wavefunction? How much consciousness? The legendary Schrödinger's Cat story highlights the paradoxes. Connect a kitten to a QM system so that the creature is killed with a probability tied to the evolution of a wavefunction. Does the kitty take on the utterly nonclassical attributes of the quantum world? Does it exist in a superposition of ghostly states until a human looks at it?
There's a radical alternative, a perspective that cuts through these knotty problems and still gives the same good answers to lab experiments. It's called the Many-Worlds Interpretation (or "Everett-Wheeler" after two physicists who developed it). From the Many-Worlds viewpoint, there is no classical universe --- everything is quantum mechanics, all the way from the tiniest particles to the largest clusters of galaxies. It's like the old model of the flat Earth as a disk resting on the back of a gigantic turtle, which in turn stands on another turtle, which stands on another, etc. "It's turtles all the way down!"
In the Many-Worlds Interpretation people have wavefunctions too, and our wavefunctions become correlated with one another as we interact. The same holds for electrons, atoms, molecules, light, and everything else. The cosmos is one big interlocking set of pieces, continuously and intimately connected. When you make a measurement of something, the correlations fit together so that my observations thereafter will line up with what you saw, to an extraordinarily high probability. No wavefunctions collapse; all possible events actually happen, simultaneously.
If we picture the course of history as a branching tree, with forks where decisions have been made, then all the branches are alive at once. These are the "many worlds". New worlds spring forth every moment, in vast profusion, as particles move and interact. QM equations keep track of the relationships among wavefunctions and prevent paradoxes or arguments among observers.
What does the Many-Worlds Interpretation mean for us personally? Everything that can happen, will happen, or has already happened. Big things (like people) still seem "classical"; no measurements see alternate universes. Schrödinger's cat is both dead and alive, and we all agree on which --- because we ourselves are in the same mix of states. Everything is bound together through complex numbers and probability amplitudes, linked in a single harmonious dance.
There's no need for mysticism; this is just the way the world works. It's simple, consistent, and beautiful. Check the equations, and get used to it!
Sunday, October 24, 1999 at 19:12:06 (EDT) = Datetag19991024