IN THE BEGINNING WAS THE oBSERVER:

[Hungarian-American physicist Eugene Wigner] was frustrated by the paradoxes arising from the vagaries of quantum mechanics--the theory governing the microscopic realm that suggests, among many other counterintuitive things, that until a quantum system is observed, it does not necessarily have definite properties. Take his fellow physicist Erwin Schrödinger's famous thought experiment in which a cat is trapped in a box with poison that will be released if a radioactive atom decays. Radioactivity is a quantum process, so before the box is opened, the story goes, the atom has both decayed and not decayed, leaving the unfortunate cat in limbo--a so-called superposition between life and death. But does the cat experience being in superposition?

Wigner sharpened the paradox by imagining a (human) friend of his shut in a lab, measuring a quantum system. He argued it was absurd to say his friend exists in a superposition of having seen and not seen a decay unless and until Wigner opens the lab door. "The 'Wigner's friend' thought experiment shows that things can become very weird if the observer is also observed," says Nora Tischler, a quantum physicist at Griffith University in Australia. [...]

Until quantum physics came along in the 1920s, physicists expected their theories to be deterministic, generating predictions for the outcome of experiments with certainty. But quantum theory appears to be inherently probabilistic. The textbook version--sometimes called the Copenhagen interpretation--says that until a system's properties are measured, they can encompass myriad values. This superposition only collapses into a single state when the system is observed, and physicists can never precisely predict what that state will be. Wigner held the then popular view that consciousness somehow triggers a superposition to collapse. Thus, his hypothetical friend would discern a definite outcome when she or he made a measurement--and Wigner would never see her or him in superposition.

This view has since fallen out of favor. "People in the foundations of quantum mechanics rapidly dismiss Wigner's view as spooky and ill-defined because it makes observers special," says David Chalmers, a philosopher and cognitive scientist at New York University. Today most physicists concur that inanimate objects can knock quantum systems out of superposition through a process known as decoherence. Certainly, researchers attempting to manipulate complex quantum superpositions in the lab can find their hard work destroyed by speedy air particles colliding with their systems. So they carry out their tests at ultracold temperatures and try to isolate their apparatuses from vibrations.

Several competing quantum interpretations have sprung up over the decades that employ less mystical mechanisms, such as decoherence, to explain how superpositions break down without invoking consciousness. Other interpretations hold the even more radical position that there is no collapse at all. Each has its own weird and wonderful take on Wigner's test. The most exotic is the "many worlds" view, which says that whenever you make a quantum measurement, reality fractures, creating parallel universes to accommodate every possible outcome. Thus, Wigner's friend would split into two copies and, "with good enough supertechnology," he could indeed measure that person to be in superposition from outside the lab, says quantum physicist and many-worlds fan Lev Vaidman of Tel Aviv University.

The alternative "Bohmian" theory (named for physicist David Bohm) says that at the fundamental level, quantum systems do have definite properties; we just do not know enough about those systems to precisely predict their behavior. In that case, the friend has a single experience, but Wigner may still measure that individual to be in a superposition because of his own ignorance. In contrast, a relative newcomer on the block called the QBism interpretation embraces the probabilistic element of quantum theory wholeheartedly (QBism, pronounced "cubism," is actually short for quantum Bayesianism, a reference to 18th-century mathematician Thomas Bayes's work on probability.) QBists argue that a person can only use quantum mechanics to calculate how to calibrate his or her beliefs about what he or she will measure in an experiment. "Measurement outcomes must be regarded as personal to the agent who makes the measurement," says Ruediger Schack of Royal Holloway, University of London, who is one of QBism's founders. According to QBism's tenets, quantum theory cannot tell you anything about the underlying state of reality, nor can Wigner use it to speculate on his friend's experiences.

Another intriguing interpretation, called retrocausality, allows events in the future to influence the past. "In a retrocausal account, Wigner's friend absolutely does experience something," says Ken Wharton, a physicist at San Jose State University, who is an advocate for this time-twisting view. But that "something" the friend experiences at the point of measurement can depend upon Wigner's choice of how to observe that person later.

The trouble is that each interpretation is equally good--or bad--at predicting the outcome of quantum tests, so choosing between them comes down to taste. "No one knows what the solution is," Steinberg says. "We don't even know if the list of potential solutions we have is exhaustive."

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