This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Fans of Brian Greene’s NOVA programs, such as The Fabric of the Cosmos, will no doubt enjoy what amounts to a 90-minute live episode. Kicking off the 2013 World Science Festival in New York City on Wednesday night, the show, “Spooky Action: The Drama of Quantum Mechanics" provides an entertaining and thought-provoking journey through the strange reality of quantum mechanics.
Greene serves as host and narrator, and he gets help from stage actors who deliver famous (and infamous) lines from physics history, such as Albert Einstein saying that god doesn’t play dice with the universe, Niels Bohr responding that Einstein shouldn’t tell god what to do and Werner Heisenberg saying that Erwin Schrodinger’s wave equations were bullshit (at least that is how his German got translated to English onstage).
With the help of a Princeton University colleague, Greene also conducts an onstage double-slit experiment, in which particles passing through two closely spaced slits form over time the classic light-dark interference pattern of waves. But most of the night focused on the intellectual tension brought on by Einstein and Bohr in their famous debate at the Solvay conference in 1930.
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Although people familiar with the weirdness of quantum physics might be disappointed by details left out—Greene glossed over the resolution of Einstein’s “box experiment” that on the surface seems to defy the uncertainty principle, for instance—overall, he does a great job in explaining the nitty gritty. He deserves particular kudos for tackling Bell’s theorem, which he said NOVA producers did not want in his Fabric series, feeing that viewers would switch over to Top Chef.
Bell’s theorem is the critical idea that enabled physicists to determine whether quantum mechanics is indeed fundamentally probabilistic, as Bohr insisted, or whether the probabilities in quantum mechanics simply reflect our lack of knowledge about the system, as Einstein argued. In particular, Bell addressed the Einstein-Podolsky-Rosen paradox, a thought experiment in which two entangled particles move in opposite directions and whose spins are detected separately, as either up or down. Does measuring the spin of the particle at one detector instantaneously set the spin state for the particle at the second detector, or were their spins previously set, like the handedness of a pair of gloves, but hidden to us?
In Greene’s description of Bell’s theorem, each detector can also measure spin along three different axes, but an observer can only set each detector to record one of those axes at a time (and hence the spin based on its relative angle to that axis). Because each detector offers three choices for a measurement of either spin-up or spin-down, there are a total of nine (3 x 3) different outcomes for spins (up or down). If the particles are like pairs of gloves, as Einstein believed, then one detector will record “up” and the other “down” at least 5 out of every 9 measurements, on average. Years later, real-life experiments would show the number to be less than 5/9—in fact, they would show that the detectors recorded opposite spins 50 percent of the time, indicating that indeed, quantum mechanics is probabilistic at its very core.
Quantum mechanics is an accepted theory, but that doesn’t mean theories about its interpretation are set. Back in 1994, I edited an article by David Z Albert on an alternative view from David Bohm. The cover is a representation of Schrodinger’s cat, which quantum mechanics suggests (in a gedanken experiment) is in a superposition of being alive and dead until it is observed. (The cat in the photo shoot, by the way, played Toonces the driving cat from the old Saturday Night Live sketches.)
Physicists and philosophers still puzzle over the true implications of quantum weirdness—after all, why don’t large objects like cats behave like the quantum particles they are made of? What do we make of the most straightforward mathematical interpretation of quantum mechanics, that of the “many worlds” idea that each measurement fractures the universe into different realities? As I watched Greene rip through these conundrums, I kept thinking how lucky we are to live in this weird, wonderful reality.
My colleagues George Musser and John Matson made a great video last year summing up the puzzle of quantum entanglement and Bell’s theorem:
Spooky Action: The Drama of Quantum Mechanics, repeating on Thursday and Friday at 8 PM at the New Victory Theater, are sold out, but a limited number of tickets are available at the door 30 minutes before show time. You can also check out video highlights from Wednesday’s performance.