14    Nonlocality

What are we scientists doing? Before quantum mechanics came along we would have replied that we are studying the properties of the world. But if there is anything that we have learned from Bell’s Theorem and the experiments that test it, it is that the microworld does not necessarily have certain properties.

To be specific, let us return to the EPR scenario in which Alice and Bob’s detectors are parallel. In this configuration they always get opposite results. But why? We used to think that it was because the two particles heading toward those detectors had spins pointing in opposite directions. But as we saw in chapter 9 that simple picture does not work. Furthermore, as Bell’s Theorem and the experiments that test it have shown, if we make the locality assumption no picture will work that attributes definite properties to those particles. So once again—what makes Bob’s detector get different results than Alice’s?

It used to be a trivial question. Suddenly it is not so trivial.

If the answer to that question does not involve the properties of the particle heading toward the detector, then it must involve something else. What else? There is only one possibility. This possibility has to do, not with particles, but with measurements. It is that in some strange way Bob’s result is connected to a result—the result of Alice’s measurement. We are forced to conclude that the very fact that Alice’s detector gets one result influences Bob’s to get the other.

We must cease thinking about the particles heading toward detectors, and start thinking about something else—about the behavior of these detectors. We must realize that these behaviors are connected—invariably connected, perfectly connected. Our discovery is that Alice’s and Bob’s detectors always behave in ways that are synchronized, and that they do so even if there are no wires leading from one to the other, even if there are no radio transmissions from one to the other, and even if they are thousands of miles apart. We must understand that the world is utterly connected.

Physicists term this connection “nonlocality.” Things happening far away are linked to things happening right here.

In proving his result, Bell was careful to analyze theories in which nonlocality had no place. It is only local theories that his theorem and the experiments that test it have ruled out. If, on the other hand, we recognize that the world actually is nonlocal, then Bell’s Theorem has no validity. In such a case, particles in the quantum world can possess perfectly definite properties.

(A theory along these lines was long ago developed by the physicist David Bohm—indeed, it was by thinking about this theory that Bell was led to his discovery. Within Bohm’s picture quantum particles have perfectly definite attributes. But even so, his world is utterly unlike the normal world of daily experience, for it is profoundly nonlocal.)

Quite aside from Bohm’s theory, nonlocality denotes an intimate connection between widely separated events. At first glance this linkage might not seem so very strange. Perhaps it reminds us of the utterly connected nature of everyday life, in which we stay in touch with friends through Facebook, follow events in China through CNN, and buy avocados grown in Mexico. But quantum nonlocality is not like all this. It is not like anything we have ever encountered before.

On the one hand, the nonlocal influence must be able to exert itself across gigantic gulfs of space. This is because Alice and Bob get opposite results even if they are very far away from one another. Nonlocal connections grow no weaker with distance. Even were Alice located on some distant planet in a faraway galaxy, this invisible agency must be able to exert its controlling sway.

Furthermore, it must do so instantaneously. Our daily connections, whether by telephone, internet, or the like, travel at the speed of light or slower—but this influence must travel faster than light. For suppose the two electrons in our experiment were set forth on their journeys from a point half-way between Alice and Bob—and then Alice were to take one small step forward. She would receive her electron a fraction of a second before Bob. So the influence we are postulating must travel from her to him in that fraction of a second. Indeed, Alice and Bob could be located at enormous distances from one another. Alice’s home might lie in a galaxy a million light years distant, so that a ray of light from her to Bob would require a million years to arrive—but her influence would still get there in no time flat.

Indeed, we are forced to postulate that our mysterious influence travels at a literally infinite velocity. So this strange new phenomenon has nothing to do with the “telephone calls” between particles that Alain Aspect’s experiment had dealt with (chapter 11). It is another matter altogether.

And yet, according to Einstein’s theory of relativity the very concept of “no time flat” has no meaning—because while two events may happen at the same instant to one observer, they do not for another. Many people believe that we are facing here a major conflict between the two great discoveries of twentieth century physics, relativity and quantum theory.

And finally, it is not at all clear who is doing the influencing. If Alice receives her particle first we might be willing to say that the result of her measurement caused the result of Bob’s. But if she takes a few steps back then Bob would be the first to register a measurement. Is it now Bob’s detector that is calling the shots? And finally, what would we say if Alice and Bob receive their particles at the very same instant? Then what is influencing what?

The lesson we must take from this is that we cannot think of one result “causing” the other. We must think only of a synchronization between the results. Of a correlation between the behaviors of the detectors.

Doesn’t this correlation violate Einstein’s principle that nothing can travel faster than light? After all—something that travels at an infinite velocity certainly seems to be achieving this remarkable feat. It does not—for three reasons. In the first case, Einstein’s principle applies to objects (spaceships and the like) but our postulated influence is not an object. In the second case, Einstein’s principle applies to causes—to physical processes that exert a causative effect. But the influence we are postulating is not a cause in anything like the ordinary sense of the term. There is nothing Alice can do to make Bob’s detector do anything. She cannot cause his detector to obtain a certain result—because she cannot cause her detector to obtain its result. It is not Alice who influences Bob’s detector: it is the result she obtained. And finally, Einstein’s principle applies to information—to messages that we send one another. But even though Alice and Bob might have a prior agreement that, say, receipt of an electron with spin up means “sell all your stock in Facebook,” the fact that Alice cannot control the result Bob’s detector gets means that she cannot control the message he gets … and an uncontrollable message is no message at all.

Perhaps strangest of all is that Alice and Bob might not even know that their electrons are connected in this strange fashion. If Alice studies only her particle, nothing she can do will alert her to the fact that it is associated with Bob’s. The same applies to Bob. Both experimenters believe themselves to be studying isolated, individual particles. Only if they were to get together and compare notes would they realize that the electrons they were studying were actually connected. The same applies to any pair of particles. Perhaps the electrons in your prefrontal cortex are intimately linked with those in the brain of that person across the room—a person you have never met before. Or perhaps they are linked with electrons in the body of some alien creature living on a world in a distant galaxy of which we are entirely unaware.

The message of nonlocality is that the world is utterly connected. The fall of a tree in Chile might be linked with the rising of a plume of dust on Mars. The fall of a sparrow in Norway might be linked with the birth of a baby next door.