The Sciences

Everything You Ever Wanted to Know About Quantum Mechanics, But Were Afraid to Ask

Cosmic VarianceBy Sean CarrollJul 7, 2008 5:07 PM

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Sorry, not in this post, but upcoming. I'm scheduled to do another episode of Bloggingheads.tv with David Albert, and we've decided to spend the whole hour talking about quantum mechanics. Start with the basics, try to explain this crazy theory and some of its outlandish consequences in ways that anyone can understand, and then dig into some of the mysteries of measurement, superposition, and reality. So -- what do you want to know? What are the really interesting questions about QM that we should be talking about? One thing I don't think we science-explainers get as clear as we could is the idea of the Wave Function of the Universe. It sounds scary and/or pretentious -- an older colleague of mine at MIT once said "I'm too young to talk about the wave function of the universe." But it's a crucial fact of quantum mechanics (arguably the crucial fact) that, unlike in classical mechanics, when you consider two electrons you don't just have a separate state for each electron. You have a single wave function that describes the two-electron system. And that's true for any number of particles -- when you consider a bigger system, you don't "add more wavefunctions," you beef up your single wave function so that it describes more particles. There is only ever one wave function, and you can call it "of the universe" if you like. Deep, man. Here is another thing: in quantum mechanics, you can "add two states together," or "take their average." (Hilbert space is a vector space with an inner product.) In classical mechanics, you can't. (Phase space is not a vector space at all.) How big a deal is that? Is there some nice way we can explain what that means in terms your grandmother could understand, even if your grandmother is not a physicist or a mathematician? (See also Dave Bacon's discussion of teaching quantum mechanics as a particular version of probability theory. There are many different ways of answering the question "What is quantum mechanics?")

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