Preposterous Universe

Friday, April 02, 2004
Quantum hermeneutics

Wow, with a title like that, I should just end the post right now; it's going to be all downhill from there. You could make a lot of money from a book with a title like that. Or did Deepak Chopra already write it?

Okay, I'm delaying the inevitable. I was hoping today to write about quantum gravity, after once and for all explaining the mysteries of quantum mechanics in the previous post. But I carelessly brought up the issue of the interpretation of the theory, which deserves more nuanced discussion. Not that I'm qualified to give it. You can read something about the issues at Michael Nielsen's blog (two posts).

But I would like to at least say some words about what I think the issue is, even if I don't want to make a strong case for any particular resolution. There certainly is an "issue," which may or may not be a "problem."

Every scientific theory comes in two pieces: a formal structure, and an "interpretation" that maps this structure onto what we see. Usually the interpretation is perfectly obvious, and we don't worry about it. But in quantum mechanics, what we see is not what there is, so we need to think more deeply. What is it that really happens when we do a measurement? For example, consider an unstable nucleus. Its wavefunction is a combination of two classical possibilities: the nucleus has already decayed, or it hasn't. But when we observe it, we don't see this superposition of possibilities; we see that it has either decayed or not. What really happens when we look at it? The Copenhagen interpretation says that the wavefunction collapses to the possibility that we have observed, while the many-worlds interpretation says that the observer+nucleus system evolves smoothly to a superposition of "nucleus decayed, observer saw decay" and "nucleus intact, observer did not see decay."

The MW interpretation is nice because everything is smooth evolution obeying the laws of physics (in this case the Schrodinger equation). But it's tricky because, since "I" actually do or do not see the nucleus decay, I need to identify "I" with a certain "branch of the wavefunction," not with the entire wavefunction. This is hard to do, both technically and conceptually. ("What does "I" mean? How does this branching process take place?)

I'm in the camp that says it's fair to call this a philosophy problem, not a physics problem. But it's a perfectly legitimate philosophy problem, not a silly waste of time. Fortunately for physicists, we don't need to know the answer to make progress on the questions we really care about. (Apparently, anyway; statements like that have a way of showing up in future textbooks as evidence of how misguided past generations were.)

Ideas on culture, science, politics.
Sean Carroll

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