“What Really Exists”

For some light holiday reading, check out this slightly mistitled article from the NY Times on the still-unsolved mysteries of Quantum Mechanics. It’s always good PR to have Einstein’s name in the title, but really it’s about a theory — Quantum Mechanics — that Einstein didn’t like much. That article spawned an excellent post from Sean Carroll attempting to tease out the Quantum-Mechanical idea that the world is, at bottom, probabilistic. 97 thought-provoking comments and counting, including my own reaction to Sean’s statement that “what we can observe is only a small fraction of what really exists” and pointing to the quantum-mechanical wavefunction as “what really exists”: I don’t think this can be correct, since the wavefunction depends on what we know, not on what is out in the world. The wavefunction is certainly a useful (to again use Sean’s words) construct to enable us to do calculations; but that doesn’t make it real.

Today, Sean has posted this followup, talking about the many-worlds interpretation of Quantum Mechanics (which I admit seems ridiculous to me, although it has many eminent adherents): whenever a quantum-mechanical “decision” happens, the world splits in two, one with each of the possibilities. One world where the canonical Schrodinger’s cat is dead, one where it is, happily, alive.

Now, the problem with all of these discussions is that most of the possible interpretations of Quantum Mechanics are distinctions without difference: they make identical predictions which the fit the well-known and well-tested rules. (Not quite like Intelligent Design vs Evolution: ID makes the same predictions as Evolution by throwing away any predictions that don’t fit the facts a posteriori, whereas the different interpretations of Quantum Mechanics are a priori identical, alas.) Chris Fuchs likens the situation to physics before Einstein came up with Special Relativity — one hundred years ago. All of the facts were in place — Lorentz contractions, Maxwell’s equations of electromagnetism, the Michelson-Morley experiment showing that light didn’t seem to propagate through an ether — but it took Einstein’s genius to notice that this could be explained by two simple principles: the speed of light is constant, and physics is the same in all inertial frames. So, it all comes back to Einstein in the end, especially while it’s still 2005.