If the wave function is not a real physical object and instead only measures experimental probabilities, then more than one wave function could describe a single physical state, say the position of a photon (just like that red ace could come from either deck). The notion that a slew of different wave functions could describe the same underlying reality falls apart in quantum mechanics, says Pusey. Reality can’t come from two decks. He and his colleagues showed that the probabilistic interpretation becomes a problematic one.
“It leads to so many possibilities that you can prove that quantum mechanics wouldn’t allow it,” says Pusey. “It wouldn’t make sense for one physical state to be compatible with so many different wave functions. The predictions those wave functions make are so different.” The PBR theorem shows that quantum states must therefore correspond uniquely with something that’s real — that is, it proves the wave function actually exists and is not just an abstract measure of probability.
Despite some rave reviews, the PBR result hasn’t changed many minds. “I was a bit disappointed that the people who liked it were the people who already believed the conclusion,” says Pusey. The naysayers instead deny one of PBR’s main assumptions: that there exists an objective reality we can measure in the first place.
A Malleable Universe
The notion of a completely objective reality is the bedrock principle of science, which is the main reason Einstein was so uncomfortable with Bohr’s “nothing exists without observation” take on quantum theory. Yet Christopher Fuchs, a physicist now at the University of Massachusetts, and Ruediger Schack of Royal Holloway University of London disagree. They contend that Bohr was on to something: Our notion of an objective reality needs modification. The physical world cannot be separated from our own efforts to probe it. How could it be otherwise, since we ourselves are embedded in the very world we’re seeking to understand?
They call their way of looking at quantum mechanics QBism, a modified version of a theory they developed with University of New Mexico physicist Carlton Caves called Quantum Bayesianism. QBism combines quantum mechanics with Bayesian probability, a variation on standard probability in which the odds of any given event are revised as one gains more knowledge of the many possible conditions tied to the event. For example, if a patient complains of headaches to a doctor, the initial odds of a diagnosis of brain cancer might be low. As the doctor examines the patient, the odds of a cancer diagnosis may go up or down.
Who knows what will happen? It may be 20 years of work down the drain. We don’t know.
QBism applies similar reasoning to physics experiments: Whenever physicists perform an experiment, they are updating their own subjective knowledge. There is no fixed underlying reality that different observers can independently experience. Just as a doctor must assess each patient individually, so too must a physicist approach the fresh, ever-changing phenomena presented by the quantum world. In QBism, the experimentalist cannot be separated from the experiment — both are immersed in the same living, unpredictable moment.
“If QBism says one radical and important thing about the nature of reality, then observer participancy is it,” says Schack. “Subjects matter. And reality, if QBism is right, cannot be conceived without always including the subject. That’s certainly a bold statement about the real world, about reality. It’s just a feature of reality that is very fundamental.”
Quantum theory, Schack says, offers profound observations about the real world, but the theory itself is not a description of the world. He posits that the right way to think of quantum mechanics is as a set of rules about how to correctly conduct experiments.
“Whether you see a wave or particle depends on what question you ask,” says Schack. “What do physicists do? They choose experiments. You could describe any experiment as a gamble on the outcome. Quantum mechanics is a useful guide to action: It tells you how to put together your experimental apparatus so that it works in the end.”
Schack says he and Fuchs like to use a term they’ve borrowed from the American philosopher William James, who saw reality as being “malleable.” QBism, says Schack, makes the same point. What sort of universe do we inhabit? Is it like a giant machine, with the future evolving from the past according to immutable laws? Or is it inherently interactive? “Why would you want a clockwork universe?” he asks. “QBism gives a much richer universe. It’s a reality in which we matter far more than we ever could in a clockwork universe.”
Back to the Beginning
If QBism is right, if the wave function isn’t real and quantum theory doesn’t give us a direct description of reality, it leaves unanswered the most basic of all questions: What then is the quantum world actually like? What is it made of? Particles? Waves? Something beyond our ability to imagine? For theoretical physicist Valentini, the answer has been there from the earliest days of quantum theory.
In 1927, the French physicist Louis de Broglie, who first proposed that particles could behave like waves, developed an interpretation of quantum mechanics called pilot wave theory, where waves and particles are both equally real. Each particle rides its own wave. The pilot wave is a bizarre thing — it exists in multiple dimensions — but it is a real physical object.
Pilot wave theory explains the strange two-slit experiment: A particle always goes through one slit or the other; at the same time its pilot wave travels through both slits. But there’s no wave-particle paradox because the experimental apparatus and the wave-surfing particle all form one interdependent system described by a pilot wave. Adding or removing a detector from the experiment changes the system’s pilot wave and the pattern on the screen.
Bohr and other physics luminaries rejected de Broglie’s idea, though, in part because it didn’t provide any way to predict the exact paths of particles. In the 1950s, David Bohm, a leading American physicist, did some additional work with de Broglie’s idea, but for the most part pilot wave theory languished until the early 1990s when it hooked Valentini as a grad student.
Valentini has devoted his career to almost single-handedly resurrecting the pilot wave idea. Now his years of work actually have a chance — a small one, he admits — of being vindicated. Of the many interpretations of quantum theory, pilot wave theory is unique in that Valentini has found a way in which it might be experimentally tested. No other interpretation of quantum mechanics can make that claim. Many Worlds, Bohr’s interpretation and others are all experimentally indistinguishable — they reproduce the results of standard quantum theory. But if Valentini is right, certain effects predicted in pilot wave theory may have left an imprint on the cosmic microwave background, the primordial radiation left over from the Big Bang that still pervades all of space.
The temperature of that radiation is almost a perfectly uniform 2.725 degrees Celsius above absolute zero. Detailed observations, however, have found slight variations in the radiation. Standard quantum theory can explain nearly all of these variations, but in 2015, new data released by the European Space Agency’s Planck spacecraft revealed evidence of small anomalies in the background radiation. And that is just the kind of thing Valentini has been looking for. While conventional quantum theory predicts that random quantum fluctuations in the early universe have left celestial imprints, pilot wave theory predicts fluctuations that are less random, leaving slightly different wrinkles in the cosmic microwave background radiation.
“It’s tantalizing,” Valentini says. “We’re carrying out the analysis partly to understand things better and partly to see what the data can tell us about the predictions that we have.” Another two years of data and analysis should settle the question.
Valentini also feels encouraged by the PBR theorem because it lends support to a central tenet of pilot wave theory: The wave function is real. Nevertheless, he realizes the odds of his life’s work being confirmed are slim. “Who knows what will happen?” he says. “It may be 20 years of work down the drain. We don’t know. You have different camps pushing hard for their own interpretation. But really, if we’re going to be honest, as scientists, if a member of the public asks us what is the meaning of our most basic theory of physics, I think we all have to say we don’t know.”