It’s quite a trick to picture a theory even weirder than quantum mechanics. Yet many physicists think the best way to make sense of quantum mechanics is to imagine what might have been. Within a vast radiation of conceivable theories, they look for principles that single out the quantum. In so doing, they aim to do for quantum mechanics what astronomers have done for Pluto. Once considered the planetary ugly duckling, Pluto proved to be the leading member of the Kuiper Belt that rings our solar system. Likewise, quantum mechanics begins to look less mysterious when you consider the larger class of postclassical theories of which it is one.
This approach makes up for an earlier imaginative failure in physics. Those who came up with quantum mechanics in the 1920s had no grand vision of what nature might be capable of. They were just trying to piece together a bunch of weird things they had found. Piece together they did, and then they spent the rest of their lives arguing what it all amounted to.
The mathematician John von Neumann was the first to take a stab at situating quantum mechanics in a broader context. In the 1930s he imagined alternative systems of logic, of which quantum mechanics might be one. Unfortunately he managed only to make the theory even more abstruse than it already is. In the mid-’90s physicists took up the search again. In my book I mention Sandu Popescu of the University of Bristol and Daniel Rohrlich at Ben-Gurion University, who envisioned “superquantum” theories with extra powers. On the principle that too much of a good thing is bad, they suggested that superquantum computers and communications would run into some subtle inconsistency, thus providing an explanation of why quantum physics is as it is.
In 2001 Lucien Hardy at the Perimeter Institute surveyed an even wider landscape of conceivable probabilistic theories, defined in very concrete terms of what an experimenter might do and see in the lab. Five principles were enough to pick quantum mechanics out of the crowd, he found. The crucial one was continuity: that the world shouldn’t undergo abrupt jumps. Far from being uncommonly bizarre, quantum mechanics is the simplest possible probabilistic theory with this property of continuity. Promising though Hardy’s approach was, physicists debated whether he had hit upon the right principles.
In the latest contribution to this project, Jonathan Richens and John Selby, grad students at Imperial College London, and Sabri Al-Safi, a Lecturer in Mathematics at Nottingham Trent University, argue this week in Physical Review Letters that the most peculiar and distinctive feature of quantum theory, entanglement, is not in fact specific to quantum theory, but must be part of any theory that goes beyond ordinary classical physics. I’m grateful to fellow physics writer Philip Ball for tweeting about this paper; he also has a nice writeup this week in Quanta of other ways that Hardy and others have developed his pioneering work
Quantum entanglement is a bit like romantic entanglement: a relationship that is interesting but awkward. Two independent things can establish a bond so that they remain correlated even when there is no tangible link—no wire, no force, no communication of any sort—between them. Crucially, they remain correlated despite the randomness of quantum physics. Each of the things in the relationship behaves unpredictably and, by the laws of chance, should sometimes fall out of lockstep with its partner. Yet it doesn’t.
“I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought,” wrote Erwin Schrödinger when he coined the term “entanglement” in 1935. And when it came to entanglements, Schrödinger knew whereof he spoke. He had fled Nazi Germany the previous year and moved to Oxford, where he caused a huge scandal, having brought both his wife and his (pregnant) mistress.
The new paper makes no headway on the puzzle of how particles maintain their uncanny synchronicity, except to say that they somehow must. The essence of the argument is really quite simple. Any theory that goes beyond classical physics endows particles with extra properties that are not directly accessible to us. Classical categories are unable to capture them in their full richness. Our measurement apparatus and indeed our minds are Procrustean beds: whenever we measure or otherwise describe a particle, we force the particle to conform to some category that is ill-suited to it, and we should not be surprised that the outcome is uncertain. If you prepare several particles in the same way and measure their energies, you will, in general, obtain different values.
Yet despite the inadequacy of our classical categories, these postclassical particles nonetheless obey classical rules such as the conservation of energy. Postclassical particles are thus like wizards who are obliged to conceal their magical abilities to pass among Muggles. To do that, their innate properties must be highly constrained. And that is what entanglement ensures.
Richens gives a minimalist example of a counterfactual theory: quantum mechanics minus entanglement. Particles are set up to lack spooky linkages, and their dynamics is restricted so as to maintain their mutual autonomy. The theory still predicts all the uncertainty, randomness, and other quantum phenomena that physicists know and love. But the lack of entanglement sinks it. In order to keep the particles unentangled, the restricted dynamics either must be irreversible in time or must prevent particles from ever interacting, contradicting what we see.
“We are trying to assert what ‘weird’ properties the universe must have, given that it is not classical but any theory must be able to account for the classical behaviors we see around us,” Richens explains. Evidently entanglement is among those “weird” properties. “Weird” is, of course, a human judgment. In a sense, it is we humans who are weird for lacking entanglement even though the particles that make us up (and that make up any allowed counterfactual world) do.
The physicists I asked for comment are as split as a Schrödinger’s cat. Rob Spekkens at the Perimeter Institute is thumbs-up. “The real interest of this result lies in the surprise factor,” he says. He is struck, in particular, that postclassical physics is self-cloaking. The reason the everyday world seems classical to us is that one postclassical concept (entanglement) offsets others (such as uncertainty). On the other hand, he wonders whether entanglement plays this role because of the specific way that Richens and his colleagues think our observed classical realm emerges from the deeper physics. “There might be postquantum theories that have a nontrivial classical limit without having entanglement,” he suggests.
Giulio Chiribella at the University of Hong Kong is less impressed. He says that physicists established long ago that entanglement is ubiquitous in the quantum world: nearly any physical interaction generates it. The same will hold in any postclassical theory, for the reason that entanglement is needed to ensure reversibility in time. “What this result is really telling us is that if we want physics to be both reversible and nonclassical, then we must have entanglement,” he says. Chiribella says he feels the paper downplays the importance of reversibility.
Whatever the outcome, Richens and his colleagues have hastened the day when quantum mechanics will no longer be deemed mysterious—a day that physicists must feel some ambivalence about, because they also revel in a theory that so thoroughly bends our minds.
Photo: Experimental setup at Ronald Hanson’s lab at the Delft University of Technology, which in 2015 conducted one of the most definitive tests ever of quantum nonlocal correlations. Photo by Frank Auperle.