Wednesday, June 24, 2015
Tangled Up in Quanta
Quantum entanglement is easy to describe but not easy to understand. It plays a key role in my novel Secret Passages, newly reissued by Discover Books, and is crucial to the theory developed by my protagonist, Manolis Minakis, that allows him to look deep into his own past. (Yes, that last part is fiction. Maybe).
Thornton Glover of Berkeley Lab’s Advanced Light Source gives one of the best one-minute explanations of quantum entanglement I know, so have a look. What he calls red and blue can be any number of quantum properties in a pair of particles prepared together, such as the spin orientation of electrons or the polarization of photons. The states remain superposed, like Schrödinger’s alive-dead cat, until one is measured.
Because the two (or more) particles are entangled, measuring one instantly determines the state of the other, no matter how far apart they are. Einstein called it “spooky action at a distance,” but the effect has been demonstrated over many kilometers by Alain Aspect and other experimentalists using polarized photons.
The act of measurement is known as collapsing the wave function. The wave function includes all possible outcomes of the particle’s superposed states; collapsing it picks just one. Consider a system of entangled electrons, all of them spin up and spin down, not one or the other. They can stand for superposed bits of information in a quantum computer. A few entangled electrons in nitrogen-vacancy centers of a ring-sized diamond, for example, could store more data than a classical supercomputer and, upon the collapse of the wave function, process it instantly.
So what’s the fuss? In fact, for young physicists who were already tackling quantum mechanical calculations in high school or before, there is no fuss; the attitude is “get over it, that’s the way the world works.”
Not all physicists, even young ones, are so blasé. In those for whom philosophy is not a dirty word, quantum entanglement raises the specter of instantaneous communication, and such horrors as causes coming before their effects. An obvious objection is that if two particles can be as far apart as from here to Jupiter, and measuring the state of the one here instantly fixes the state of the one there, then information must be traveling much faster than the speed of light. At the very least it violates relativity theory.
Einstein claimed this showed that quantum mechanics is incomplete. He and his colleagues said extra terms were needed, what became known as hidden (local) variables. First John Bell and later Alain Aspect disproved Einstein’s argument.
Other proposals followed and are still coming. Perhaps the most famous is Hugh Everett’s many worlds interpretation (MWI), which has proved tremendously useful to many science fiction writers, including me; it gives us an infinity of not-quite-identical worlds to play in. MWI does away with collapsing wave functions by supposing that every possible state of every particle exists in its own parallel reality. A single wave function covers them all.
John Wheeler and Richard Feynman theorized that particles actually do reverse in time, an indirect way of avoiding the collapse of the wave function because, it seems to me, it leads to a universe that doesn’t change: the same particles move backward and forward in time, constantly switching temporal direction, constituting everything we experience and know.
Wheeler and Feynman were the inspiration for my favorite theory, John Cramer’s transactional interpretation of quantum mechanics, which posits two waves (estimates of probability) for every quantum event, one emitted forward in time, the other emitted backward, which meet and with a “handshake” settle the true state of the event. This is the basis, as far as real science goes, of Manolis Minakis’s backwards-in-time experiment in Secret Passages.
A recent, ingenious attempt to evade the paradoxes of quantum entanglement and the collapse of the wave function must also be mentioned, the QBism of Christopher Fuchs. Quoted by Amanda Gefter in Quanta Magazine, Fuchs describes physics as a “dynamic interplay between storytelling and equation writing”—right on!—and regards the wave function not as a description of reality but of our personal beliefs about and knowledge of reality at a given moment. Lots to think about here, so I encourage you to read Gefter’s interview with Fuchs.
Meanwhile I’ll conclude by saying that a storyteller has to cheer the prospect of confronting the role human involvement plays in seemingly hands-off physics. Fuchs may be onto something, whether or not his math holds up in the long run. The measurements needed to collapse wave functions have long been a thorn in the side of those inclined to fret. Collapses happen all the time; the world goes on even when we’re not looking. Who’s doing the observing? Einstein put the dilemma succinctly: “Is it enough that a mouse observes that the moon exists?”