The founders of quantum mechanics understood it to be deeply, profoundly weird. Albert Einstein, for one, went to his grave convinced that the theory had to be just a steppingstone to a more complete description of nature, one that would do away with the disturbing quirks of the quantum.
Then in 1964, John Stewart Bell proved a theorem that would test whether quantum theory was obscuring a full description of reality, as Einstein claimed. Experimenters have since used Bell’s theorem to rule out the possibility that beneath all the apparent quantum craziness — the randomness and the spooky action at a distance — is a hidden deterministic reality that obeys the laws of relativity.
Now a new theorem has taken Bell’s work a step further. The theorem makes some reasonable-sounding assumptions about physical reality. It then shows that if a certain experiment were carried out — one that is, to be fair, extravagantly complicated — the expected results according to the rules of quantum theory would force us to reject one of those assumptions.
According to Matthew Leifer, a quantum physicist at Chapman University who did not participate in the research, the new work focuses attention on a class of interpretations of quantum mechanics that until now have managed to escape serious scrutiny from similar “no-go” theorems.
Broadly speaking, these interpretations argue that quantum states reflect our own knowledge of physical reality, rather than being faithful representations of something that exists out in the world. The exemplar of this group of ideas is the Copenhagen interpretation, the textbook version of quantum theory, which is most popularly understood to suggest that particles don’t have definite properties until those properties are measured. Other Copenhagen-like quantum interpretations go even further, characterizing quantum states as subjective to each observer.
“If you’d have said to me a few years ago that you can make a no-go theorem against certain kinds of Copenhagen-ish interpretations that some people really believe in,” said Leifer, “I’d have said, ‘That’s nonsense.’” The latest theorem is, according to Leifer, “assailing the unassailable.”
Bell’s Toll
Bell’s 1964 theorem brought mathematical rigor to debates that had started with Einstein and Niels Bohr, one of the main proponents of the Copenhagen interpretation. Einstein argued for the existence of a deterministic world that lies beneath quantum theory; Bohr argued that quantum theory is complete and that the quantum world is indelibly probabilistic.
Bell’s theorem makes two explicit assumptions. One is that physical influences are “local” — they can’t travel faster than the speed of light. In addition, it assumes (à la Einstein) that there’s a hidden deterministic reality not modeled by the mathematics of quantum mechanics. A third assumption, unstated but implicit, is that experimenters have the freedom to choose their own measurement settings.
Given these assumptions, a Bell test involves two parties, Alice and Bob, who make measurements on numerous pairs of particles, one pair at a time. Each pair is entangled, so that their properties are quantum mechanically linked: If Alice measures the state of her particle, it seemingly instantly affects the state of Bob’s particle, even if the two are miles apart.
Bell’s theorem suggested an ingenious way to set up an experiment. If the correlations between Alice’s and Bob’s measurements are equal to or below a certain value, then Einstein was right: There is a local hidden reality. If the correlations are above this value (as quantum theory would predict), then one of Bell’s assumptions must be wrong, and the dream of a local hidden reality must die.
Physicists spent nearly 50 years performing increasingly exacting Bell tests. By 2015, these experiments had essentially settled the debate. The measured correlations were above the level known as Bell’s inequality, and Bell tests were consistent with the predictions of quantum mechanics. As a consequence, the idea of a local hidden reality was put to rest.
Weak Assumptions, Strong Theory
The new work draws from the tradition started by Bell, but it also relies on a slightly different experimental setup, one originally devised by the physicist Eugene Wigner.
In Wigner’s thought experiment, a person we’ll call Wigner’s friend is inside a lab. The friend measures the state of a particle that’s in a superposition (or quantum mixture) of two states, say 0 and 1. The measurement collapses the particle’s quantum state to either 0 or 1, and the outcome is recorded by the friend.
Wigner himself is outside the lab. From his perspective, the lab and his friend — assuming they are completely isolated from all environmental disturbances — continue to evolve together quantum mechanically. After all, quantum mechanics makes no claims about the size of the system to which the theory applies. In principle, it applies to elementary particles, to the sun and the moon, and to human beings.
If quantum mechanics is universally applicable, Wigner argued, then both the particle and Wigner’s friend are now entangled and in a quantum superposition, even though the friend’s measurement has ostensibly already collapsed the particle’s superposition.
The contradictions raised by Wigner’s setup highlighted fundamental and compelling questions about what qualifies as a collapse-causing measurement and whether collapse is irreversible.
As with Bell’s theorem, the authors of the new work make seemingly obvious but nonetheless rigorous assumptions. The first one states that experimenters have the freedom to choose the type of measurements they want to do. The second says that you can’t send a signal any faster than the speed of light. The third says that outcomes of measurements are absolute, objective facts for all observers.
Note that these “local friendliness” assumptions are weaker than Bell’s. The authors do not presume that there’s some kind of deterministic reality underlying the quantum world. Therefore, if an experiment can be done, and if the experiment works, that means “we’ve actually found out something even more profound about reality than from Bell’s theorem,” said Howard Wiseman, the director of the Center for Quantum Dynamics at Griffith University in Australia and one of the leaders of the new work.
The new theorem also identifies a large set of mathematical inequalities, which include but also extend beyond those formulated by Bell. “It’s possible to violate Bell inequalities but not violate our inequalities,” said team member Nora Tischler, also at Griffith.
