We’ve known about “quantum weirdness” for more than 100 years, but it’s still making headlines. In the summer of 2015, experimental groups in Boulder, Delft, and Vienna announced that they had completed a decades-long quest to demonstrate quantum nonlocality. The possibility of such nonlocal effects first captured the attention of physicists in the 1930s, when Einstein called it, “spooky action at a distance”—indicating that he perceived it as a bug of the nascent theory. But on this particular issue, Einstein couldn’t have been more wrong: nonlocality isn’t a bug of quantum mechanics, it’s a pervasive feature of the physical world.
To understand why the scientific community has been so slow to embrace quantum nonlocality, recall that 19th century physics was built around the ideal of local causality. According to this ideal, for one event to cause another, those two events must be connected by a chain of spatially contiguous events. In other words, for one thing to have an effect on another, the first thing needs to touch something, which touches something else, which touches something else … eventually touching the other thing.
For those of us schooled in classical physics, the notion of local causality might seem central to a rational outlook on the physical world. For example, I don’t take reports of telekinesis seriously—and not because I’ve taken the time to examine all the experiments that have tried to confirm its existence. No, I don’t take reports of telekinesis seriously because it seems irrational to believe in some sort of causality that doesn’t involve things moving through space and time.
But QM appears to conflict with local causality. According to QM, if two particles are in an entangled state, then the outcomes of measurements on the second particle will always be strictly correlated (or anticorrelated) with measurements on the first particle—even when the second particle is far, far away from the first. Quantum mechanics also claims that neither the first nor the second particle has any definite state before the measurements are performed. So what explains the correlations between the measurement outcomes?
It’s tempting to think that quantum mechanics is just wrong when it says that the particles aren’t in any definite state before they are measured. In fact, that’s exactly what Einstein suggested in the famous “EPR” paper with Podolsky and Rosen. However, in the 1960s, John Bell showed that the suggestion of EPR could be put to experimental test. If, as suggested by Einstein, each particle has its own state, then the results of a certain crucial experiment would disagree with the predictions made by quantum mechanics. Thus, in the 1970s and 1980s, the race was on to perform this crucial experiment—an experiment that would establish the existence of quantum nonlocality.
The experiments of the 1970s and 80s came out decisively in favor of quantum nonlocality. However, these experiments left open a couple of loopholes. It was only in 2015 that the ingenious experimenters in Boulder, Delft, and Vienna were able to definitively close these loopholes—propelling quantum nonlocality back into the headlines.
But is it news that quantum mechanics is true? Didn’t we already know this, or at least, wasn’t the presumption strongly in its favor? Yes, the real news here isn’t that quantum mechanics is true. The real news is that we are learning how to harness the resources of a quantum world. In the 1920s and 30s, quantum nonlocality was a subject of philosophical perplexity and debate. In 2015, questions about the meaning of quantum nonlocality are being replaced by questions about what we can do with it. For instance, quantum nonlocality could facilitate information-theoretic and crytographic protocols that far exceed anything that could have been imagined in a world governed by classical physics. And this is the reason why quantum nonlocality is still making headlines.
But don’t get carried away—quantum nonlocality still doesn’t make it rational to believe in telekinesis.