Weighing the Importance of Locality

The following is an excerpt from Spooky Action at a Distance: The Phenomenon That Reimagines Space and Time—and What It Means for Black Holes, the Big Bang, and Theories of Everything by George Musser.

When I first learned about nonlocality as a graduate student in the early 1990s, it wasn’t from my quantum-mechanics professor: he didn’t see fit to so much as mention it. Browsing a local bookshop, I picked up a newly published book, The Conscious Universe, which startled me with its claim that “no previous discovery has posed more challenges to our sense of everyday reality” than nonlocality. The phenomenon had the taste of forbidden fruit.

In everyday speech, “locality” is a slightly pretentious word for a neighborhood, town, or other place. But its original meaning, dating to the seventeenth century, is about the very concept of “place.” It means that everything has a place. You can always point to an object and say, “Here it is.” If you can’t, that thing must not really exist. If your teacher asks where your homework is and you say it isn’t anywhere, you have some explaining to do.

The world we experience possesses all the qualities of locality. We have a strong sense of place and of the relations among places. We feel the pain of separation from those we love and the impotence of being too far away from something we want to affect. And yet quantum mechanics and other branches of physics now suggest that, at a deeper level, there may be no such thing as place and no such thing as distance. Physics experiments can bind the fate of two particles together, so that they behave like a pair of magic coins: if you flip them, each will land on heads or tails—but always on the same side as its partner. They act in a coordinated way even though no force passes through the space between them. Those particles might zip off to opposite sides of the universe, and still they act in unison. These particles violate locality. They transcend space.

Evidently nature has struck a peculiar and delicate balance: under most circumstances it obeys locality, and it must obey locality if we are to exist, yet it drops hints of being nonlocal at its foundations. That tension is what I’ll explore in this book. For those who study it, nonlocality is the mother of all physics riddles, implicated in a broad cross section of the mysteries that physicists confront these days: not just the weirdness of quantum particles, but also the fate of black holes, the origin of the cosmos, and the essential unity of nature.

For Albert Einstein, locality was one aspect of a broader philosophical puzzle: Why are we humans able to do science at all? Why is the world such that we can make sense of it? In a famous essay in 1936, Einstein wrote that the most incomprehensible thing about the universe is that it is comprehensible. At first glance, this statement itself seems incomprehensible. The universe is not a conspicuously rational place. It is wild and capricious, full of misdirection and arbitrariness, injustice and misfortune. Much of what happens defies reason (especially when romance or driving is involved). Yet against this backdrop of inexplicable happenings, the world’s rules glow with reassuring regularity. The sun rises in the east. Things fall when you drop them. After the rain comes a rainbow. People go into physics out of a conviction that these are not just gratifying exceptions to the anarchy of life, but glimpses of an underlying order.

Einstein’s point was that physicists really had no right to expect that. The world needn’t have been orderly at all. It didn’t have to abide by laws; under other circumstances, it might have been anarchic all the way down. When a friend wrote to ask Einstein what he’d meant by the comprehensibility remark, he wrote back, “A priori one should expect a chaotic world which cannot be grasped by the mind in any way.”

Spooky Action At A Distance: The Phenomenon That Reimagines Space and Time—and What It Means for Black Holes, the Big Bang, and Theories of Everything

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Although Einstein said comprehensibility was a “miracle” we shall never understand, that didn’t stop him from trying. He spent his entire professional life articulating exactly what it is about the universe that makes it make sense, and his thinking set the course of modern physics. He recognized, for example, that the inner workings of nature are highly symmetrical, looking the same if you view the world from a different angle. Symmetry brings order to the bewildering zoo of particles that physicists have found; entire species of particles are, in a sense, mirror images of one another. But among all the properties of the world that give us hope for understanding it, Einstein kept coming back to locality as the most important.

Locality is a subtle concept that can mean different things to different people. For Einstein, it had two aspects. The first he called “separability,” which says that you can separate any two objects or parts of an object and consider each on its own, at least in principle. You can take your dining chairs and put each one in a different corner of the room. They will not cease to exist or lose any of their features—size, style, cushiness. The entire dining-room set derives its properties from the chairs that make it up; if each chair can seat one person, a set of four chairs can seat four people. The whole is the sum of its parts. The second aspect that Einstein identified is known as “local action,” which says that objects interact only by banging into one another or recruiting some middleman to bridge the gap between them. Whenever a distance separates us from someone, we know we cannot have any effect on that person unless we cross the distance and touch, talk to, punch—somehow, make direct contact with—that person, or send someone or something to do it for us. Modern technology does not evade this principle; it merely recruits new intermediaries. A phone translates sound waves into electrical signals or radio waves that travel through wires or open space and then get translated back into sound on the other end. At every step of the way, something has to make direct contact with something else. If there is even a hairline crack in the wire, the message gets as far as a scream on an airless moon. Simply put, separability defines what objects are, and local action dictates what they do.

Einstein captured these principles in his theory of relativity. Specifically, relativity theory says that no material thing can move faster than light. Without such an ultimate speed limit, objects might move infinitely fast and distance would lose its meaning. All the forces of nature must wend their way laboriously through space, rather than leap across it in a single bound, as physicists used to suppose. Relativity theory thereby provides a measure of isolation among separated objects and ensures their mutual distinctness.