A century after the birth of quantum mechanics, its puzzles are pushing physicists to redefine reality—with themselves at the center
Standing in a garden on the remote German island of Helgoland one day in June, two theoretical physicists quibble over who—or what—constructs reality. Carlo Rovelli, based at Aix-Marseille University, insists he is real with respect to a stone on the ground. He may cast a shadow on the stone, for instance, projecting his existence onto their relationship. Chris Fuchs of the University of Massachusetts Boston retorts that it’s preposterous to imagine the stone possessing any worldview, seeing as it is a stone. Although allied in their belief that reality is subjective rather than absolute, they both leave the impromptu debate unsatisfied, disagreeing about whether they agree.
Such is the state of theoretical quantum mechanics, scientists’ deepest description of the atomic world. The theory was developed 100 years ago on Helgoland, where a 23-year-old Werner Heisenberg retreated to escape a bout of hay fever—and to reimagine what an atom looks like. The leading picture at the time featured electrons hopping in discrete, or quantized, leaps of energy between fixed orbits around the nucleus. It explained the behavior of hydrogen but failed for bigger atoms. On blustery walks and cold swims in the North Sea, Heisenberg abandoned the simplistic orbital picture, instead developing a new mathematical language that would work for any atom. Later in 1925, Erwin Schrödinger conjured up a complementary lens—his eponymous wave equation—which describes the positions of electrons in probabilistic terms.
Within a few years, their calculations would reveal a disturbingly fuzzy picture of reality, one in which certain properties are inherently unknowable and others take on different values depending on how they’re measured. “What we observe is not nature itself, but nature exposed to our method of questioning,” Heisenberg wrote after winning the 1932 Nobel Prize in Physics.
This year, hundreds of physicists convened on Helgoland to commemorate the birth of quantum mechanics. It has certainly earned its keep over the past century, not only by predicting experimental outcomes with immaculate precision, but also by enabling technologies such as lasers, transistors, and atomic clocks. Yet even today, scientists struggle to interpret what the theory implies about nature. Central to the confusion is how the act of measurement pins down the indeterminate behavior of atoms. The standard framing has an unsettling anthropocentric flavor, suggesting humans play some special role in shaping the universe. Now, bolstered by a string of recent experiments, theorists such as Fuchs and Rovelli are leaning into the discomfort, emphasizing how observers do indeed create the world they inhabit. What’s at stake is nothing less than reality itself.
“We don’t need to fix quantum mechanics to make it compatible with what we observe; we need to recognize that there are alternative ways of looking at the world,” says Alyssa Ney, a philosopher of physics at the Ludwig Maximilian University of Munich. Quantum theory compels physicists to “make room for different notions of what it means to be real.”
THE MYSTERIOUS ROLE of observation in quantum mechanics is highlighted by the classic “double slit” experiment, which has been performed for decades with photons, electrons, and atoms. If you shoot a stream of individual atoms toward a blank wall through a barrier that has two slits, a squiggly interference pattern will appear on the wall. The pattern means each atom must have traveled like a wave, passing through both slits simultaneously in a “superposition” of all of its possible quantum states. However, if the experiment is repeated with a detector at each slit to track individual atoms as they pass through the barrier, the pattern on the wall will instead feature only two broad stripes, formed by individual atoms that took one path or the other. The detector somehow seems to cause the atoms to behave like particles rather than waves.
Heisenberg and his mentors, including Niels Bohr at the University of Copenhagen, took a practical view of the matter. They accepted the dual nature of atoms and that measurement “collapses” waves into particles. But this perspective, sometimes called the Copenhagen interpretation, punts on questions about how exactly the waves collapse and what constitutes a measurement.
Others, including Albert Einstein, believed the atoms must surely have some baked-in rules about how to behave, regardless of measurement. That skepticism led to experiments beginning in the 1970s called Bell tests, named after Northern Irish physicist John Bell, that probed for these so-called hidden variables. The Bell tests rely on another enigmatic phenomenon prescribed by quantum mechanics: the ability to “entangle” two atoms in such a way that their quantum properties are intrinsically linked. When those properties are measured after entanglement, they’re always correlated no matter how far apart the atoms are—a connection Einstein called “spooky action at a distance.”
The Bell tests examine correlations between multiple quantum properties—such as the position of one atom and the velocity of another. And they demonstrate that the observed amount of correlation exceeds what’s possible if both atoms merely had preconfigured positions and velocities. Assuming there’s no way for atoms to send signals faster than light, the only logical conclusion is that the atoms truly are in a superposition of all possible states up until the moment of measurement—implying, as Israeli physicist Asher Peres famously put it, “Unperformed experiments have no results.”
The result is a perplexing puzzle about what is real, which can be brought into stark relief using a simple thought experiment proposed by Hungarian American physicist Eugene Wigner in 1961. Wigner stands outside a sealed lab, in which his friend Jimena measures a quantum property of her atom. To Wigner, who is oblivious to Jimena’s measurement, the quantum state of the atom—and even Jimena herself—is undefined, in a superposition of possibilities. Quantum theory says Wigner should even be able to verify this superposition by treating Jimena and her lab as one entangled quantum system and performing an interference experiment. Meanwhile, inside the lab, Jimena has already measured and determined her atom’s state. The two friends seem to experience different realities.“We’re applying the same theory in two different ways and getting contradictory results, it appears,” says Nick Ormrod, a postdoctoral researcher at the Perimeter Institute who studies quantum foundations.
SINCE THEN, dozens of quantum interpretations have sprouted in an attempt to resolve this paradox, which the Copenhagen philosophy simply dodged. Some do so by invoking physical processes in the act of measurement—such as gravity or consciousness—that cause the collapse of waves into particles. Others cling to the idea that hidden variables underlie the spooky connections, opting to give up the assumption that nothing can travel faster than light. These approaches suggest Wigner is simply ignorant, and Wigner’s friend experiences the real truth—either by definitively collapsing the atom’s superposition or by revealing its predetermined nature. Experiments have yet to vindicate either idea. More recently, a third camp has gained support: addressing the measurement problem by rethinking reality entirely.
One who took eagerly to the challenge was Rovelli, whose insubordinate youth had primed him to challenge conventional wisdom. Studying physics in Bologna, Italy, in the 1970s, during a wave of political and cultural revolutions, Rovelli and his comrades lived on a commune and disavowed traditional societal values. “We were the explorers of a new way of living,” he says. “The hope was a world without weapons, without countries, without school, without family, without fixed couples.” This exercise of shedding deeply held beliefs eventually bled into his work in theoretical physics. Aided by psychedelic trips and forays into Eastern philosophy, he was willing to sacrifice orthodox intuitions about the nature of reality.
In 1996, Rovelli laid out a new interpretation of quantum theory that does away with any universal notion of existence. It draws on the “many worlds” idea, first proposed by U.S. physicist Hugh Everett in 1957 to explain the multitude of possible outcomes of a measurement. Each time a measurement is made, Everett proposed, reality splinters off into multiple branches—one in which you had cereal for breakfast and another in which you ate eggs, for example—but the branches have no ability to interact. Critics dismiss the idea as ungrounded and fantastical, pointing out the lack of an explanation of how or when this branching happens, as well as the unfathomably large number of worlds beyond our reach. But the boldness of the idea gave Rovelli the courage to get creative.
Like Everett’s approach, Rovelli’s relational quantum mechanics rejects the idea that one coherent story describes the universe. But rather than positing multiple branches of reality, Rovelli argues the world can only be understood from a particular point of view, which need not align precisely with any other perspective. Objects and events do not exist in and of themselves—they manifest through relationships with other physical systems. “This implies that I cannot give an absolute, universal, objective description of what is going on,” Rovelli says. In the case of Wigner’s friend, Rovelli argues it’s a mistake to assume Jimena’s reality should count for Wigner in any way.
Fuchs came to similar conclusions by dreaming big in a different direction. Growing up poor in the small town of Cuero, Texas, Fuchs was drawn to physics as an avid Star Trek fan but quickly became disheartened by the slim prospects of interstellar space travel. He told his best friend, “If the laws of physics won’t allow us to get to the stars, the laws of physics must be wrong.”
In high school in the 1980s, he attended a Texas Boys State summer camp at the University of Texas (UT) at Austin. He hated the predawn calisthenics and the right-wing politics, but for one moment, fortune smiled on his sci-fi fantasies. During his free time, he tracked down his scientific hero John Archibald Wheeler, a pioneering theorist of black holes, cosmic wormholes, and other strange children of Einstein’s theory of general relativity. In Wheeler’s vacant office, Fuchs found a manuscript draft with a passage that read, “There is no law except the law that there is no law.” Fuchs saw this as his ticket to the stars: “Ca-ching,” he says.
In the paper, Wheeler also argued that observers shape the universe through measurement. Fuchs would cling to that idea of agency. After studying under Wheeler at UT, Fuchs came to view quantum mechanics through the lens of those who employ it—treating the theory as a user’s manual for placing smart bets on measurement outcomes.
The result was QBism—originally short for Quantum Bayesianism, a reference to a philosophy of statistics that argues probabilities are not fixed, but should be updated as new information arises. QBism extends this approach to quantum probabilities. The core premise, which Fuchs and colleagues first laid out in 2001, is that the state of a quantum object reflects the subjective degree of belief that an agent has about its properties. “It’s a state of mind,” Fuchs says. The outlook makes it easier to permit different observers to have conflicting descriptions of the same system, as these simply reflect perceived probabilities based on the information each one gains from measurements.
At the conference on Helgoland, Rovelli and Fuchs put their competing visions of reality head to head. For both, quantum theory describes properties of a physical system as they relate to something else. To Rovelli, that something else can be any physical object—even a stone. The stone contains information about the world around it—from the sunlight that reflects off its surface to the creatures fossilized inside it millions of years ago, he says. But Fuchs argues such information has significance only for a gambling agent who can put it to use.
Rovelli finds the distinction trivial. “QBism and relational quantum mechanics are very, very similar,” he says. “In fact, for me, they are the same.” But Fuchs insists the two theories have fundamentally different aspirations: QBism aims not to describe what nature is doing, but rather to offer a guidebook that coaches agents to make smart decisions when predicting measurement outcomes. “Now imagine giving the book to a rock … silly, right?” Fuchs says.
He thinks the misunderstanding stems from vague arguments on both sides that are “still at the poetic stage.” But he’s confident that continuing to mathematically hone their points will lead to new insights—an exercise he says is already paying off. Following the QBist prescription, Fuchs has managed to reformulate equations in quantum mechanics so they can be expressed completely in terms of probabilities. Yale University physicist Michel Devoret—recipient of this year’s Nobel Prize in Physics—says the reworked equations have helped him design new experiments. “No matter the angle you choose, quantum mechanics forces you to bend your mind,” Devoret says.
SUPPORT FOR Fuchs’s and Rovelli’s ideas is growing—and branching into various offshoots—amid a wave of recent work on extensions of the Wigner’s friend thought experiments. Their goal is to draw out the tension between absolute events and quantum theory, no matter how it is applied. Essentially, they go as follows: Imagine a copy of Wigner and Jimena—let’s call them Wagner and Jamuna. The atoms in Jimena and Jamuna’s labs are entangled, so their quantum states are always correlated. From their respective labs, Jimena and Jamuna measure one quantum property of their atoms. From outside the labs, Wigner and Wagner can probe other properties of the entangled atoms by performing their own interference experiments on the entire labs.
In 2018, researchers outlined situations in which the four friends all make measurements of the systems—and cannot agree on the outcomes. Over the following years, multiple teams developed theorems to sharpen this inconsistency, suggesting the observed properties cannot take on definite values. “There are a bunch of really clever tricks designed to elevate [Bell tests] from an argument about hidden variables to an argument about the actual experiences of actual observers,” Ormrod says.
In one analysis, Eric Cavalcanti, a physicist at Griffith University, used these mental games to prove that to avoid contradictions, quantum theory requires you to give up one of two things: either local agency—the idea that actions cannot influence events back in time or faster than the speed of light—or the absoluteness of observed events. Relinquishing the former would undermine all of Einstein’s work on gravity, so he’s more willing to sacrifice the latter. His takeaway builds off the adage from Peres: “Even performed measurements do not have absolute results,” Cavalcanti says.
To Fuchs and Rovelli, these efforts only confirm what they’ve been arguing for years. “They’re breaking a window which was already open,” Rovelli says. He points out how interpretations that reject absoluteness are gaining popularity particularly among young researchers, as evidenced by a recent Nature poll of more than 1100 scientists about their preferred quantum interpretation. Taken together, relational and information-based frameworks (including QBism) were favored by 21% of the respondents. But significantly more Ph.D. students and early-career researchers voted for the radical theories than did advanced-career and retired scientists. Even philosophers of physics such as Ney, who had previously written off relational quantum mechanics and QBism as inconsistent and antirealist, respectively, are warming up to the ideas.
As is typical in quantum theory, not everyone agrees. Many physicists continue to defer to the Copenhagen interpretation (the most common preference in the Nature poll), adopting the traditional “shut up and calculate” approach. And some hold out for alternatives that would leave objective reality intact. Ivette Fuentes, a physicist at the University of Southampton, believes the measurement problem arises because quantum mechanics is incomplete, as it leaves out gravity. “I think these gravitational effects are the culprit,” Fuentes says. She dismisses interpretations that treat the quantum wave function as purely relational or informational as sweeping “the problem under the rug.”
THE HALLMARK OF SUCCESS for any interpretation will be predicting new experimental results. One study in 2019 attempted to conduct a real-world extended Wigner’s friend experiment in the lab using three pairs of entangled photons to represent Wigner and Wagner, Jimena and Jamuna, and their respective quantum experiments. The researchers claimed the outcomes—correlations in the photons’ polarization—were observer-dependent.
Critics questioned some of the study’s assumptions, including whether individual photons can count as observers. But Alessandro Fedrizzi, a physicist at Heriot-Watt University who led the experiment, stands by the conclusions. “Whenever you do something foundational, everyone pops up and … explains from their perspective why you’re wrong,” he says. Cavalcanti is now trying to close loopholes in the setup by using better detectors, more spaced-out photons, and “observer” photons that are coupled to increasingly macroscopic objects.
Still, a precise implementation of an extended Wigner’s friend in the lab requires complete quantum control over an isolated system containing a sufficiently complex observers. To that end, Cavalcanti and colleagues have proposed experiments on large quantum computers as the ultimate realization of the thought experiment. Artificial intelligence agents running on such a computer (which has not yet been built) could act as the two observers, and the underlying qubit architecture would allow the different friends to be isolated, entangled, and operated on as needed. “Doing the experiment with more and more complex observers will make the results more and more convincing and constraining,” Cavalcanti says.
The rejection of absolute events raises a crucial problem, though: If physicists can’t all agree on what took place, “how can we do science?” asks V. Vilasini, a physicist at Inria, France’s national research institute focusing on computer science and applied math. In a paper last year, she and a colleague proposed a framework for reckoning with Wigner’s friend scenarios that are inherently perspectival. It does so by mathematically formalizing the philosophical idea of a “Heisenberg cut”—the exact rift where superposition collapses. The conclusion is that physicists can avoid paradoxes and contradictory statements by refraining from making inferences that rely on different Heisenberg cuts—such as those made by Wigner and Jimena. “You actually don’t have to give up on quantum theory; you just have to use it a bit more carefully,” she says.
The framework also helps explain why, in our everyday world, events seem to be absolute. Our classical perception of definite events on which we all agree is a product of us all living in the same bubble—in which outcomes are independent of the choice of Heisenberg cut. As macroscopic beings, we are incapable of tracking the intrinsic indeterminacy of quantum particles, which are routinely collapsed into definite outcomes by a constant mess of interactions that serve as measurements. “What remains is a world in which the relativity of facts is invisible,” Rovelli says. “So, anything you and I agree on as fact we can call a universal fact.”
Why care about the subjective nature of reality if we can’t access it, then? Markus Müller, a physicist at the Institute for Quantum Optics and Quantum Information Vienna, draws a comparison to general relativity and its time dilation effects, which must be accounted for in technologies such as GPS. “It’s pretty much irrelevant in everyday life,” Müller says, and yet “ultimately, it turned out to be very important to understand.” If nonabsoluteness is indeed a general property of physical theories, we should center physics around experience, he argues. “We belong to be in the picture—that’s what quantum theory tells us.”
Rovelli takes the subtle lesson to heart. Recognizing the subjectivity of truth offers him a new perspective on life. “It takes away a sense of anguish because I don’t access the ultimate reality,” Rovelli says. “What’s real is what’s real relative to us.” Fuchs uses a similarly mystical tone, appreciating that “the universe has this character that what you do within it matters,” he says. His high school best friend recently suggested QBism is a sort of spiritual tradition. “Maybe so,” Fuchs admits. “If quantum mechanics says there’s room for me to change the world … somehow that makes life worth living.”
Continue at: https://www.science.org/content/article/100-years-quantum-mechanics-redefining-reality-us-center
The text above is owned by the site above referred.
Here is only a small part of the article, for more please follow the link
Also see:
Manostaxx – Industrial Management Consulting
