“There’s no quiz today,” David Kaiser promised at the start of his KSJ seminar on February 28. With a title slide like “Testing Quantum Theory With the Cosmos,” he must have known his topic was intimidating. But Kaiser, the MIT physicist and science historian, is well versed at making quantum weirdness engaging for nonphysicists.
In addition to writing books like “How the Hippies Saved Physics” (2011), which explores the interwoven history of quantum theory and 1970s counterculture, Kaiser has written for publications like The New York Times and The New Yorker. In his presentation he spoke on the curious phenomenon of quantum entanglement.
Entanglement is but one item in a long list of quantum mechanical eccentricities. In the quantum world, certainty goes out the window, and objects can apparently exist in two opposite states at once (think Schrödinger’s cat, dead and alive). If this rubs you the wrong way, you’re in good company. “Quantum theory seemed an insult to many people,” Kaiser told the KSJ fellows. Prominent among them was Albert Einstein, who famously scoffed that God doesn’t play dice.
Einstein was similarly skeptical of entanglement, which describes two particles whose fates are inextricably intertwined. At any given time, even when they’re light-years apart, the likelihood of observing Particle A in a certain state depends on the current state of Particle B.
To illustrate this oddity, Kaiser invoked the analogy of twins Alice and Bob, who grow up together and eventually separate for college: Alice goes to Cambridge, Massachusetts, and Bob to Cambridge, England. In Massachusetts, Alice visits a restaurant where the waiter always offers her — with no clear pattern — a dessert choice between two baked goods or two frozen treats. She always has a 50-50 shot of choosing one baked good over the other; same with the frozen dessert. When Alice reunites with Bob at Christmas, she learns that he, too, has been visiting a restaurant where he’s been offered this same set of choices. And when the twins compare their history of dessert orders, their selections are correlated far more often than mere chance should allow.
Many people, like Einstein, have found entanglement unnerving. When asked why he thinks it’s such a disconcerting notion — because it contradicts everyday experience? because it apparently limits free will? — Kaiser replied that it’s probably both. Furthermore, because of the accepted cosmic light-travel speed limit, “playing around with instantaneousness makes many physicists concerned.” Also, it “just sounds spooky, like ESP or mind reading.”
But over several decades, experiments have come down on the side of entanglement time and time again. In these experiments, some source of entangled particles spits out particle pairs (analogous to Alice and Bob) toward measuring devices (American and British waiters) that can be set to observe different particle properties (a choice between baked goods or a choice between frozen goods). When researchers compare the particle characteristics measured by each detector (dessert orders), they match up enough to suggest the particles truly are entangled.
To be sure that something as counterintuitive as entanglement is real — especially when rising industries like quantum computing and encryption depend on it — physicists try to brainstorm and debunk alternative theories for explaining the data. “You really have to think like a conspiracy theorist,” Kaiser said.
He and his colleagues recently entered the entanglement-testing fray, pulling the drawstrings on what is called the “freedom of choice” loophole — a “conspiracy theory” arguing that when scientists have total control over their experimental setup, they can’t be sure that their choices don’t introduce “hidden variables” that produce entanglement-like results.
Thus, Kaiser’s team chose to “offshore some of the [experimental] decision making to the universe itself,” he said, by hooking up their particle detectors to telescopes trained on stars. Just before the entangled particles reached the measurement devices, each detector chose which property to measure based on the current color of the star it was observing. The researchers still got data that bore the hallmarks of entanglement. Sorry, Einstein.
“David is a really unique person to hear speak because he’s both a physicist and a writer,” said KSJ fellow Maura O’Connor, “so he has this rare ability to translate the science behind his research into big ideas that expand the interest and understanding of his audience.”
For his part, Kaiser thrives on inviting other people to ponder the enigma of entanglement. He loves that these awe-inspiring — and, yes, disquieting — ideas can be introduced in an hourlong seminar “without smacking people in the face with a bunch of mathematics.” Indeed, Kaiser dashed straight from KSJ to yet another quantum presentation — at the MIT Museum.