The Quantum World

Entranced by the complexity of "entanglement," a lapsed physics major returns to her calling.

I STILL REMEMBER THE DAY I FELL IN LOVE WITH ENTANGLEMENT. IT WAS my junior winter and I was adrift, a lapsed physics major. I had found a haven down in the depths of the Hop at the Paddock Music Library, where I would take whatever work I had that needed to be leavened by music—particularly David Grisman’s superlative mandolin playing—and where the cozy, padded pleather earphones and blond wooden carrels created a simpler, more focused world.

I was taking a philosophy-of-science class to fulfill a distributive requirement and had found its readings heavy going, so I was slightly dreading the seven stapled pages I had set down before me in the carrel that winter evening. But its title, "Quantum Mysteries for Anyone," was welcoming, and it was written by David Mermin, a lowtemperature physicist (i.e., not a philosopher). The first thing that surprised me was that Mermin, unlike most physicists and even more philosophers, had a beautiful writing style that managed to be witty, direct and profound.

About a third of the way into the paper, however, I was no longer merely enjoying a good writers style. Even my simple surroundings—the carrel, the sounds of the mandolin, my body—had dropped away, and all that was left was a mind meeting an idea for the first time. The word "mystery" in the title was no rhetorical device. Here was something fundamental about the world that was tantalizingly inexplicable.

The mystery was implicit in the equations of quantum mechanics—the study of matter and light on their most fundamental level. Both matter and light behave in some ways as if made up of particles (like a beach) and in other ways as if made up of waves (like an ocean). To emphasize the contrast here: When we speak of a particle, we are speaking of something that, at largest, is as much smaller than you as you are smaller than the sun; when we speak of a wave, we are speaking of something that, at largest, covers the universe. Yet both descriptions can apply to the same quantum object. From this paradox stems much of the mindboggle of the quantum theory.

One way to talk about it (which misleads, like all the others) is to simply refer to these fundamental pieces of matter and light as particles—but particles whose behavior is mathematically described by a wave equation.

Entanglement involves two of these particles. They interact. They separate. But the waves that describe them do not separate, no matter how far apart the particles travel. These particles can get arbitrarily far apart without breaking the entanglement, as long as they don't interact too strongly with anything else (they are fickle; they will entangle with almost anything they meet). So from a wave perspective, we have one huge wave, but if we look for particles, we will find two of them, separated by a great distance. This is entanglement: These apparent two act as one—as a single object with shared characteristics—no matter how far apart they travel.

By 2007 a team of physicists had demonstrated the phenomenon between particles separated by 90 miles of equatorial Atlantic Ocean, from one of the Canary Islands to another. One tiny particle seemed to react instantly to a measurement performed on its tiny twin almost 100 miles away.

It was to learn about things like this that I had wanted to be a physics major! Why had something so amazing been so downplayed? For three-quarters of a century it had been lying out in the open in the most basic equations of quantum mechanics—first pointed out by Einstein in the 1930s, named in 1935 by Schrodinger as "the characteristic trait of quantum mechanics," elucidated theoretically by John Bell in 1964 and demonstrated in the lab across a distance of 16 feet byjohn Clauser and Stuart Freedman in 1972. But in 1998 the term did not even appear in the index of any of my textbooks-despite these books being full of examples of it. The few people who wanted to examine entanglement—from Einstein and Schrodinger to Bell and Clauser— had been dismissed by their colleagues as "naive realists," as ignorable as their subject. For the second half of the 20th century the correct answer to the question, "What does the quantum theory mean?" was—as described to me by one physicist—"Shut up and calculate!"

I had been frustrated by how often physics classes seemed completely divorced from the physical world they were supposed to illuminate. My own mathematical slowness was much to blame for this. Like a hopeful classics major driven to a panic by an actual Sappho poem or passage of The Odyssey, I had found that my tin ear for mathematics—the language of physics—often dropped a veil between me and the lessons physics had to teach. Another reason was a climate of textbook writing that great physicists from Einstein to Richard Feynman had railed at: books that prize jargon over clarity, memorization over experience and abstraction over images and specifics. In the student lab we would usually have to interact with the physical world (or, worse, a simulation thereof) through a computer, which can feel a bit like trying to pet a baby rabbit in the dark while wearing ski gloves.

Now I was eager to clamber over these obstacles, internal and external. Back I biked to Wilder Lab and walked up the old familiar outside and inside steps. Standing in the office of Mary Hudson, the physics chair, I asked if I could return to the physics department and do an independent study on entanglement.

Suddenly, my college experience took on a new urgency and excitement. I sat for hours in the office of a new professor, Miles Blencowe, who had kindly agreed to supervise my independent study, while he taught me the quantum mechanics of entanglement. I photocopied dozens of Physical Review papers, and their contents started to make sense. The mathematics of physics began to feel like an exciting tool. I devoured memoirs and biographies of the physicists involved, which told of battles— intellectual and emotional—between all those names I had previously met only as hallowed adjectives (the Bohr atom, the Schrodinger equation, the Heisenberg uncertainty principle).

The physics department gave me an office in the almost-completed annex of Wilder. I have a vivid memory of sitting on its hot black roof on a particularly beautiful day in early spring, as pine boughs waved at the edges of my field of vision, engrossed in a paper by John Clauser and Abner Shimony. A month later, my senior year almost done, Blencowe invited Shimony to give a public lecture at Dartmouth. Afterward we sat with the grandfatherly physicist and his wife in Blencowe's office, listening to a personal history of entanglement.

From both a scientific and a historical angle, I was entranced. The science and the history of entanglement were so interrelated that you barely could talk of one without the other. While reading an apparently dry book or playing with a spiky-looking equation, I felt again and again as if these were windows through which I was catching glimpses of the most fascinating movie I had ever seen, full of vivid characters reacting in unexpected ways to the most profound aspects of the physical world. Sometimes I discovered I was facing a piece of that profundity myself.

Physics classes seemed completely divorced from the physical world they were supposed to illuminate.

LOUISA GILDER is the author of The Age of Entanglement: When Quantum Physics Was Reborn (Knopf, 2008), anaccessible history of quantum physics and thescientists who have contributed to the field. Shespent eight years writing the book and lives inTyringham, Massachusetts.