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    ‘From Data to Quanta’ defends Niels Bohr’s view of quantum mechanics

    From Data to QuantaSlobodan PerovićUniv. of Chicago, $45

    Ever since Max Planck introduced the idea of the quantum to the world, physicists have argued about whether reality is more like sand or water.

    Planck’s famous 1900 discovery that energy is grainy — at least when absorbed or emitted — moved him to label those smallest bits of energy grains “quanta.” But he believed that once emitted, as in light from a fire, those grains merged into smooth, continuous waves, just as water seems a smooth liquid to human perception. Einstein, on the other hand, insisted that light quanta traveled through space on their own, behaving like particles later called photons.

    By the mid-1920s, both the wave and particle views of light had gained experimental support, with the additional paradox that electrons — supposedly particles — could sometimes disguise themselves as waves.

    Into this arena of controversy stepped the famed Danish physicist Niels Bohr, the pioneer of exploring the architecture of the atom. Bohr announced that resolving the wave-particle paradox required a new view of reality, in which both notions shared a role in explaining experimental phenomena. In experiments designed to observe waves, waves you would find, whether electrons or light. In experiments designed to detect particles, you’d see particles. But in no experiment could you demonstrate both at once. Bohr called this viewpoint the principle of complementarity, and it successfully guided the pursuit of quantum mechanics during the following decades.

    More recently, as philosopher Slobodan Perović recounts in From Data to Quanta, Bohr’s success has been questioned by some physicists and philosophers and even popular science writers (SN: 1/19/19, p. 26). Complementarity has been derided as an incoherent application of vague philosophy expressed in incomprehensible language. But as Perović’s investigations reveal, such criticisms are rarely rooted in any deep understanding of Bohr’s methods. Rather than Bohr’s philosophy contaminating his science, Perović argues, it is his opponents’ philosophical prejudices that have led to misstatements, misunderstandings and misrepresentations of Bohr’s physics. And Bohr can’t be understood by attempting to understand his philosophy, Perović asserts, because philosophy did not guide him — experiments did.

    In fact, Bohr’s drive to understand the wave-particle paradox was fueled by a deep devotion to comprehending the experimental evidence in its totality. It was the same approach the younger Bohr took when developing his model of the atom in 1913 (SN: 7/13/13, p. 20). Various experiments suggested properties of the atom that seemed irreconcilable. But Bohr forged those experimental clues into a “master hypothesis” that produced a thoroughly novel understanding of the atom and its structure.

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    Perović describes how Bohr’s process began with lower-level hypotheses stemming from features directly given by experiment. Spectral lines — different specific colors of light emitted by atoms — led to basic hypotheses that some vibratory process, of an atom itself or its constituents, produced electromagnetic radiation exhibiting precise patterns. Intermediate hypotheses about the structure of the atom did not explain such lines, though. And then Ernest Rutherford, on the basis of experiments in his lab, inferred that an atom was mostly empty space. It contained a dense, tiny central nucleus encompassing most of the mass, while lightweight electrons orbited at a distance. But that hypothesis didn’t mesh with the precise patterns of spectral lines. And such an atom would be unstable, persisting for less than a millisecond. From all these disparate experiment-based hypotheses, Bohr applied Planck’s quantum idea to construct a master hypothesis. He reconciled the spectral lines and Rutherford’s nuclear atom with a new atomic model, in which electrons maintained stability of the atom but jumped from one orbit to another, emitting specific patterns of spectral lines in the process.

    As Perović demonstrates, Bohr followed a similar course in arriving at complementarity. While numerous experiments showed that light was a wave, by the early 1920s other experiments established that X-rays, highly energetic light, collided with electrons just as though both were particles (momentum and energy were conserved in the collisions just as the particle view required). Bohr’s master hypothesis, complementarity, seemed the only way forward.

    Throughout the book, Perović relates how Bohr has been misinterpreted, his views misleadingly conflated with those of others (like John von Neumann and Werner Heisenberg), and his philosophy incorrectly portrayed as antirealist — suggesting that only observations brought reality into existence. Bohr never said any such thing, and in fact cautioned against using language so loosely.

    Perović’s account offers a thorough survey of other historical investigations into Bohr’s work and draws liberally from Bohr’s own writings. It’s a nuanced and insightful presentation of the interplay of experiment and theory in the scientific process. This book is not easy reading, though. It’s not the place to seek clear explanations of quantum physics and Bohr’s interpretation of it. Perović opts for scholarly thoroughness and careful reasoning with a propensity for long sentences. But then again, Bohr’s writings were no breeze, either. In fact, a major complaint against Bohr has been expressed by authors who say his writings are very difficult to understand. It’s unfortunate that so many seem to think that because they can’t understand Bohr, he must have been wrong. Perović’s book provides a useful antidote to that attitude.

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    A diamondlike structure gives some starfish skeletons their strength

    Some starfish made of a brittle material fortify themselves with architectural antics.

    Beneath a starfish’s skin lies a skeleton made of pebbly growths, called ossicles, which mostly consist of the mineral calcite. Calcite is usually fragile, and even more so when it is porous. But the hole-riddled ossicles of the knobby starfish (Protoreaster nodosus) are strengthened through an unexpected internal arrangement, researchers report in the Feb. 11 Science.

    “When we first saw the structure, we were really amazed,” says Ling Li, a materials scientist at Virginia Tech in Blacksburg. It looks like it’s been 3-D printed, he says.

    Li and colleagues used an electron microscope to zoom in on ossicles from several dozen dead knobby starfish. At a scale of 50 micrometers, about half the width of a human hair, the seemingly featureless body of each ossicle gives way to a meshlike pattern that mirrors how carbon atoms are arranged in a diamond.

    Zooming in on the bumpy growths called ossicles (seen in this electron microscope image) that make up a knobby starfish’s skeleton reveals a meshlike structure similar to the arrangement of carbon atoms in diamond. This arrangement strengthens the ossicles, which are mostly made of calcite, a relatively weak mineral.Ling Li/Virginia Tech

    But the diamondlike lattice alone doesn’t fully explain how the ossicles stay strong.

    Within that lattice, the atoms that make up the calcite have their own pattern, which resembles a series of stacked hexagons. That pattern affects the strength of the calcite too. In general, a mineral’s strength isn’t uniform in all directions. So pushing on calcite in some directions is more likely to break it than force from other directions. In the ossicles, the atomic pattern and the diamondlike lattice align in a way that compensates for calcite’s intrinsic weakness.

    It’s a mystery how the animals make the diamondlike lattice. Li’s team is studying live knobby starfish, surveying the chemistry of how ossicles form. Understanding how the starfish build their ossicles may provide insights for creating stronger porous materials, including some ceramics.

    We can learn a lot from a creature like a starfish that we may think is primitive, Li says.

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    The quantum ‘boomerang’ effect has been seen for the first time

    Some quantum particles gotta get right back to where they started from.

    Physicists have confirmed a theoretically predicted phenomenon called the quantum boomerang effect. An experiment reveals that, after being given a nudge, particles in certain materials return to their starting points, on average, researchers report in a paper accepted in Physical Review X.

    Particles can boomerang if they’re in a material that has lots of disorder. Instead of a pristine material made up of orderly arranged atoms, the material must have many defects, such as atoms that are missing or misaligned, or other types of atoms sprinkled throughout.

    In 1958, physicist Philip Anderson realized that with enough disorder, electrons in a material become localized: They get stuck in place, unable to travel very far from where they started. The pinned-down electrons prevent the material from conducting electricity, thereby turning what might otherwise be a metal into an insulator. That localization is also necessary for the boomerang effect.

    To picture the boomerang in action, physicist David Weld of the University of California, Santa Barbara imagines shrinking himself down and slipping inside a disordered material. If he tries to fling away an electron, he says, “it will not only turn around and come straight back to me, it’ll come right back to me and stop.” (Actually, he says, in this sense the electron is “more like a dog than a boomerang.” The boomerang will keep going past you if you don’t catch it, but a well-trained dog will sit by your side.)

    Weld and colleagues demonstrated this effect using ultracold lithium atoms as stand-ins for the electrons. Instead of looking for atoms returning to their original position, the team studied the analogous situation for momentum, because that was relatively straightforward to create in the lab. The atoms were initially stationary, but after being given kicks from lasers to give them momenta, the atoms returned, on average, to their original standstill states, making a momentum boomerang.

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    The team also determined what’s needed to break the boomerang. To work, the boomerang effect requires time-reversal symmetry, meaning that the particles should behave the same when time runs forward as they would on rewind. By changing the timing of the first kick from the lasers so that the kicking pattern was off-kilter, the researchers broke time-reversal symmetry, and the boomerang effect disappeared, as predicted.

    “I was so happy,” says Patrizia Vignolo, a coauthor of the study. “It was perfect agreement” with their theoretical calculations, says Vignolo, a theoretical physicist at Université Côte d’Azur based in Valbonne, France.

    Even though Anderson made his discovery about localized particles more than 60 years ago, the quantum boomerang effect is a recent newcomer to physics. “Nobody thought about it, apparently, probably because it’s very counterintuitive,” says physicist Dominique Delande of CNRS and Kastler Brossel Laboratory in Paris, who predicted the effect with colleagues in 2019.

    The weird effect is the result of quantum physics. Quantum particles act like waves, with ripples that can add and subtract in complicated ways (SN: 5/3/19). Those waves combine to enhance the trajectory that returns a particle to its origin and cancel out paths that go off in other directions. “This is a pure quantum effect,” Delande says, “so it has no equivalent in classical physics.” More

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    Quantum particles can feel the influence of gravitational fields they never touch

    If you’re superstitious, a black cat in your path is bad luck, even if you keep your distance. Likewise, in quantum physics, particles can feel the influence of magnetic fields that they never come into direct contact with. Now scientists have shown that this eerie quantum effect holds not just for magnetic fields, but for gravity too — and it’s no superstition.

    Usually, to feel the influence of a magnetic field, a particle would have to pass through it. But in 1959, physicists Yakir Aharonov and David Bohm predicted that, in a specific scenario, the conventional wisdom would fail. A magnetic field contained within a cylindrical region can affect particles — electrons, in their example — that never enter the cylinder. In this scenario, the electrons don’t have well-defined locations, but are in “superpositions,” quantum states described by the odds of a particle materializing in two different places. Each fractured particle simultaneously takes two different paths around the magnetic cylinder. Despite never touching the electrons, and hence exerting no force on them, the magnetic field shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed (SN: 3/1/86).

    In the new experiment, the same uncanny physics is at play for gravitational fields, physicists report in the Jan. 14 Science. “Every time I look at this experiment, I’m like, ‘It’s amazing that nature is that way,’” says physicist Mark Kasevich of Stanford University.

    Kasevich and colleagues launched rubidium atoms inside a 10-meter-tall vacuum chamber, hit them with lasers to put them in quantum superpositions tracing two different paths, and watched how the atoms fell. Notably, the particles weren’t in a gravitational field–free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, laying bare the eerie Aharonov-Bohm influence.

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    The study not only reveals a famed physics effect in a new context, but also showcases the potential to study subtle effects in gravitational systems. For example, researchers aim to use this type of technique to better measure Newton’s gravitational constant, G, which reveals the strength of gravity, and is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).

    A phenomenon called interference is key to this experiment. In quantum physics, atoms and other particles behave like waves that can add and subtract, just as two swells merging in the ocean make a larger wave. At the end of the atoms’ flight, the scientists recombined the atoms’ two paths so their waves would interfere, then measured where the atoms arrived. The arrival locations are highly sensitive to tweaks that alter where the peaks and troughs of the waves land, known as phase shifts.

    At the top of the vacuum chamber, the researchers placed a hunk of tungsten with a mass of 1.25 kilograms. To isolate the Aharonov-Bohm effect, the scientists performed the same experiment with and without this mass, and for two different sets of launched atoms, one which flew close to the mass, and the other lower. Each of those two sets of atoms were split into superpositions, with one path traveling closer to the mass than the other, separated by about 25 centimeters. Other sets of atoms, with superpositions split across smaller distances, rounded out the crew. Comparing how the various sets of atoms interfered, both with and without the tungsten mass, teased out a phase shift that was not due to the gravitational force. Instead, that tweak was from time dilation, a feature of Einstein’s theory of gravity, general relativity, which causes time to pass more slowly close to a massive object.

    The two theories that underlie this experiment, general relativity and quantum mechanics, don’t work well together. Scientists don’t know how to combine them to describe reality. So, for physicists, says Guglielmo Tino of the University of Florence, who was not involved with the new study, “probing gravity with a quantum sensor, I think it’s really one of … the most important challenges at the moment.” More

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    A century of quantum mechanics questions the fundamental nature of reality

    Scientists are like prospectors, excavating the natural world seeking gems of knowledge about physical reality. And in the century just past, scientists have dug deep enough to discover that reality’s foundations do not mirror the world of everyday appearances. At its roots, reality is described by the mysterious set of mathematical rules known as quantum mechanics.

    Conceived at the turn of the 20th century and then emerging in its full form in the mid-1920s, quantum mechanics is the math that explains matter. It’s the theory for describing the physics of the microworld, where atoms and molecules interact to generate the world of human experience. And it’s at the heart of everything that made the century just past so dramatically unlike the century preceding it. From cell phones to supercomputers, DVDs to pdfs, quantum physics fueled the present-day electronics-based economy, transforming commerce, communication and entertainment.

    But quantum theory taught scientists much more than how to make computer chips. It taught that reality isn’t what it seems.

    “The fundamental nature of reality could be radically different from our familiar world of objects moving around in space and interacting with each other,” physicist Sean Carroll suggested in a recent tweet. “We shouldn’t fool ourselves into mistaking the world as we experience it for the world as it really is.”

    In a technical paper backing up his tweet, Carroll notes that quantum theory consists of equations that describe mathematical entities roaming through an abstract realm of possible natural events. It’s plausible, Carroll argues, that this quantum realm of mathematical possibilities represents the true, fundamental nature of reality. If so, all the physical phenomena we perceive are just a “higher-level emergent description” of what’s really going on.

    “Emergent” events in ordinary space are real in their own way, just not fundamental, Carroll allows. Belief that the “spatial arena” is fundamental “is more a matter of convenience and convention than one of principle,” he says.

    Carroll’s perspective is not the only way of viewing the meaning of quantum math, he acknowledges, and it is not fully shared by most physicists. But everybody does agree that quantum physics has drastically remodeled humankind’s understanding of nature. In fact, a fair reading of history suggests that quantum theory is the most dramatic shift in science’s conception of reality since the ancient Greeks deposed mythological explanations of natural phenomena in favor of logic and reason. After all, quantum physics itself seems to defy logic and reason.

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    It doesn’t, of course. Quantum theory represents the ultimate outcome of superior logical reasoning, arriving at truths that could never be discovered merely by observing the visible world.

    It turns out that in the microworld — beyond the reach of the senses — phenomena play a game with fantastical rules. Matter’s basic particles are not tiny rocks, but more like ghostly waves that maintain multiple possible futures until forced to assume the subatomic equivalent of substance. As a result, quantum math does not describe a relentless cause-and-effect sequence of events as Newtonian science had insisted. Instead science morphs from dictator to oddsmaker; quantum math tells only probabilities for different possible outcomes. Some uncertainty always remains.

    Quantum mechanics says that whether an electron behaves as particle or wave depends on how it is observed.Max Löffler

    The quantum revolution

    The discovery of quantum uncertainty was what first impressed the world with the depth of the quantum revolution. German physicist Werner Heisenberg, in 1927, astounded the scientific community with the revelation that deterministic cause-and-effect physics failed when applied to atoms. It was impossible, Heisenberg deduced, to measure both the location and velocity of a subatomic particle at the same time. If you measured one precisely, some uncertainty remained for the other.

    “A particle may have an exact place or an exact speed, but it can not have both,” as Science News Letter, the predecessor of Science News, reported in 1929. “Crudely stated, the new theory holds that chance rules the physical world.” Heisenberg’s uncertainty principle “is destined to revolutionize the ideas of the universe held by scientists and laymen to an even greater extent than Einstein’s relativity.”

    Werner Heisenberg, shown in 1936, declared with his uncertainty principle that a particle’s position and velocity couldn’t both be precisely measured at the same time.AIP Emilio Segrè Visual Archives

    Heisenberg’s breakthrough was the culmination of a series of quantum surprises. First came German physicist Max Planck’s discovery, in 1900, that light and other forms of radiation could be absorbed or emitted only in discrete packets, which Planck called quanta. A few years later Albert Einstein argued that light also traveled through space as packets, or particles, later called photons. Many physicists dismissed such early quantum clues as inconsequential. But in 1913, the Danish physicist Niels Bohr used quantum theory to explain the structure of the atom. Soon the world realized that reality needed reexamining.

    By 1921, awareness of the quantum revolution had begun to expand beyond the confines of physics conferences. In that year, Science News Bulletin, the first iteration of Science News, distributed what was “believed to be the first popular explanation” of the quantum theory of radiation, provided by American physical chemist William D. Harkins. He proclaimed that the quantum theory “is of much more practical importance” than the theory of relativity.

    “Since it concerns itself with the relations between matter and radiation,” Harkins wrote, quantum theory “is of fundamental significance in connection with almost all processes which we know.” Electricity, chemical reactions and how matter responds to heat all require quantum-theoretic explanations.

    As for atoms, traditional physics asserts that atoms and their parts can move about “in a large number of different ways,” Harkins stated. But quantum theory maintains that “of all the states of motion (or ways of moving) prescribed by the older theory, only a certain number actually do occur.” Therefore, events previously believed “to occur as continuous processes, actually do occur in steps.”

    Quantum theory “is of fundamental significance in connection with almost all processes which we know.”William Harkins

    But in 1921 quantum physics remained embryonic. Some of its implications had been discerned, but its full form remained undeveloped in detail. It was Heisenberg, in 1925, who first transformed the puzzling jumble of clues into a coherent mathematical picture. His decisive advance was developing a way to represent the energies of electrons in atoms using matrix algebra. With aid from German physicists Max Born and Pascual Jordan, Heisenberg’s math became known as matrix mechanics. Shortly thereafter, Austrian physicist Erwin Schrödinger developed a competing equation for electron energies, viewing the supposed particles as waves described by a mathematical wave function. Schrödinger’s “wave mechanics” turned out to be mathematically equivalent to Heisenberg’s particle-based approach, and “quantum mechanics” became the general term for the math describing all subatomic systems.

    Still, some confusion remained. It wasn’t clear how an approach picturing electrons as particles could be equivalent to one supposing electrons to be waves. Bohr, by then regarded as the foremost of the world’s atomic physicists, pondered the question deeply and by 1927 arrived at a novel viewpoint he called complementarity.

    Bohr argued that the particle and wave views were complementary; both were necessary for a full description of subatomic phenomena. Whether a “particle” — say, an electron — exhibited its wave or particle nature depended on the experimental setup observing it. An apparatus designed to find a particle would find a particle; an apparatus geared to detect wave behavior would find a wave.

    At about the same time, Heisenberg derived his uncertainty principle. Just as wave and particle could not be observed in the same experiment, position and velocity could not both be precisely measured at the same time. As physicist Wolfgang Pauli commented, “Now it becomes day in quantum theory.”

    But the quantum adventure was really just beginning.

    In the many worlds interpretation of quantum mechanics, all possible realities exist, but humans perceive just one.Max Löffler

    A great debate

    Many physicists, Einstein among them, deplored the implications of Heisenberg’s uncertainty principle. Its introduction in 1927 eliminated the possibility of precisely predicting the outcomes of atomic observations. As Born had shown, you could merely predict the probabilities for the various possible outcomes, using calculations informed by the wave function that Schrödinger had introduced. Einstein famously retorted that he could not believe that God would play dice with the universe. Even worse, in Einstein’s view, the wave-particle duality described by Bohr implied that a physicist could affect reality by deciding what kind of measurement to make. Surely, Einstein believed, reality existed independently of human observations.

    On that point, Bohr engaged Einstein in a series of discussions that came to be known as the Bohr-Einstein debate, a continuing dialog that came to a head in 1935. In that year, Einstein, with collaborators Nathan Rosen and Boris Podolsky, described a thought experiment supposedly showing that quantum mechanics could not be a complete theory of reality.

    In a brief summary in Science News Letter in May 1935, Podolsky explained that a complete theory must include a mathematical “counterpart for every element of the physical world.” In other words, there should be a quantum wave function for the properties of every physical system. Yet if two physical systems, each described by a wave function, interact and then fly apart, “quantum mechanics … does not enable us to calculate the wave function of each physical system after the separation.” (In technical terms, the two systems become “entangled,” a term coined by Schrödinger.) So quantum math cannot describe all elements of reality and is therefore incomplete.

    Bohr soon responded, as reported in Science News Letter in August 1935. He declared that Einstein and colleagues’ criterion for physical reality was ambiguous in quantum systems. Einstein, Podolsky and Rosen assumed that a system (say an electron) possessed definite values for certain properties (such as its momentum) before those values were measured. Quantum mechanics, Bohr explained, preserved different possible values for a particle’s properties until one of them was measured. You could not assume the existence of an “element of reality” without specifying an experiment to measure it.

    Niels Bohr and Albert Einstein disagreed over the nature of reality.Photograph by Paul Ehrenfest, courtesy of AIP Emilio Segrè Visual Archives, Gamow Collection

    Einstein did not relent. He acknowledged that the uncertainty principle was correct with respect to what was observable in nature, but insisted that some invisible aspect of reality nevertheless determined the course of physical events. In the early 1950s physicist David Bohm developed such a theory of “hidden variables” that restored determinism to quantum physics, but made no predictions that differed from the standard quantum mechanics math. Einstein was not impressed with Bohm’s effort. “That way seems too cheap to me,” Einstein wrote to Born, a lifelong friend.

    Einstein died in 1955, Bohr in 1962, neither conceding to the other. In any case it seemed like an irresolvable dispute, since experiments would give the same results either way. But in 1964, physicist John Stewart Bell deduced a clever theorem about entangled particles, enabling experiments to probe the possibility of hidden variables. Beginning in the 1970s, and continuing to today, experiment after experiment confirmed the standard quantum mechanical predictions. Einstein’s objection was overruled by the court of nature.

    Still, many physicists expressed discomfort with Bohr’s view (commonly referred to as the Copenhagen interpretation of quantum mechanics). One particularly dramatic challenge came from the physicist Hugh Everett III in 1957. He insisted that an experiment did not create one reality from the many quantum possibilities, but rather identified only one branch of reality. All the other experimental possibilities existed on other branches, all equally real. Humans perceive only their own particular branch, unaware of the others just as they are unaware of the rotation of the Earth. This “many worlds interpretation” was widely ignored at first but became popular decades later, with many adherents today.

    Since Everett’s work, numerous other interpretations of quantum theory have been offered. Some emphasize the “reality” of the wave function, the mathematical expression used for predicting the odds of different possibilities. Others emphasize the role of the math as describing the knowledge about reality accessible to experimenters.

    Some interpretations attempt to reconcile the many worlds view with the fact that humans perceive only one reality. In the 1980s, physicists including H. Dieter Zeh and Wojciech Zurek identified the importance of a quantum system’s interaction with its external environment, a process called quantum decoherence. Some of a particle’s many possible realities rapidly evaporate as it encounters matter and radiation in its vicinity. Soon only one of the possible realities remains consistent with all the environmental interactions, explaining why on the human scale of time and size only one such reality is perceived.

    This insight spawned the “consistent histories” interpretation, pioneered by Robert Griffiths and developed in more elaborate form by Murray Gell-Mann and James Hartle. It is widely known among physicists but has received little wider popularity and has not deterred the pursuit of other interpretations. Scientists continue to grapple with what quantum math means for the very nature of reality.

    Using principles of quantum information theory, a particle’s quantum state can be replicated at a distant location, a feat known as quantum teleportation.Max Löffler

    It from quantum bit

    In the 1990s, the quest for quantum clarity took a new turn with the rise of quantum information theory. Physicist John Archibald Wheeler, a disciple of Bohr, had long emphasized that specific realities emerged from the fog of quantum possibilities by irreversible amplifications — such as an electron definitely establishing its location by leaving a mark after hitting a detector. Wheeler suggested that reality as a whole could be built up from such processes, which he compared to yes or no questions — is the electron here? Answers corresponded to bits of information, the 1s and 0s used by computers. Wheeler coined the slogan “it from bit” to describe the link between existence and information.

    Taking the analogy further, one of Wheeler’s former students, Benjamin Schumacher, devised the notion of a quantum version of the classical bit of information. He introduced the quantum bit, or qubit, at a conference in Dallas in 1992.

    Schumacher’s qubit provided a basis for building computers that could process quantum information. Such “quantum computers” had previously been envisioned, in different ways, by physicists Paul Benioff, Richard Feynman and David Deutsch. In 1994, mathematician Peter Shor showed how a quantum computer manipulating qubits could crack the toughest secret codes, launching a quest to design and build quantum computers capable of that and other clever computing feats. By the early 21st century, rudimentary quantum computers had been built; the latest versions can perform some computing tasks but are not powerful enough yet to make current cryptography methods obsolete. For certain types of problems, though, quantum computing may soon achieve superiority over standard computers.

    Quantum computing’s realization has not resolved the debate over quantum interpretations. Deutsch believed that quantum computers would support the many worlds view. Hardly anyone else agrees, though. And decades of quantum experiments have not provided any support for novel interpretations — all the results comply with the traditional quantum mechanics expectations. Quantum systems preserve different values for certain properties until one is measured, just as Bohr insisted. But nobody is completely satisfied, perhaps because the 20th century’s other pillar of fundamental physics, Einstein’s theory of gravity (general relativity), does not fit in quantum theory’s framework.

    For decades now, the quest for a quantum theory of gravity has fallen short of success, despite many promising ideas. Most recently a new approach suggests that the geometry of spacetime, the source of gravity in Einstein’s theory, may in some way be built from the entanglement of quantum entities. If so, the mysterious behavior of the quantum world defies understanding in terms of ordinary events in space and time because quantum reality creates spacetime, rather than occupying it. If so, human observers witness an artificial, emergent reality that gives the impression of events happening in space and time while the true, inaccessible reality doesn’t have to play by the spacetime rules.

    In a crude way this view echoes that of Parmenides, the ancient Greek philosopher who taught that all change is an illusion. Our senses show us the “way of seeming,” Parmenides declared; only logic and reason can reveal “the way of truth.” Parmenides didn’t reach that insight by doing the math, of course (he said it was explained to him by a goddess). But he was a crucial figure in the history of science, initiating the use of rigorous deductive reasoning and relying on it even when it led to conclusions that defied sensory experience.

    Yet as some of the other ancient Greeks realized, the world of the senses does offer clues about the reality we can’t see. “Phenomena are a sight of the unseen,” Anaxagoras said. As Carroll puts it, in modern terms, “the world as we experience it” is certainly related to “the world as it really is.”

    “But the relationship is complicated,” he says, “and it’s real work to figure it out.”

    In fact, it took two millennia of hard work for the Greek revolution in explaining nature to mature into Newtonian science’s mechanistic understanding of reality. Three centuries later quantum physics revolutionized science’s grasp of reality to a comparable extent. Yet the lack of agreement on what it all means suggests that perhaps science needs to dig a little deeper still. More

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    Quantum physics requires imaginary numbers to explain reality

    Imaginary numbers might seem like unicorns and goblins — interesting but irrelevant to reality. 

    But for describing matter at its roots, imaginary numbers turn out to be essential. They seem to be woven into the fabric of quantum mechanics, the math describing the realm of molecules, atoms and subatomic particles. A theory obeying the rules of quantum physics needs imaginary numbers to describe the real world, two new experiments suggest.

    Imaginary numbers result from taking the square root of a negative number. They often pop up in equations as a mathematical tool to make calculations easier. But everything we can actually measure about the world is described by real numbers, the normal, nonimaginary figures we’re used to (SN: 5/8/18). That’s true in quantum physics too. Although imaginary numbers appear in the inner workings of the theory, all possible measurements generate real numbers.

    Quantum theory’s prominent use of complex numbers — sums of imaginary and real numbers — was disconcerting to its founders, including physicist Erwin Schrödinger. “From the early days of quantum theory, complex numbers were treated more as a mathematical convenience than a fundamental building block,” says physicist Jingyun Fan of the Southern University of Science and Technology in Shenzhen, China.

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    Some physicists have attempted to build quantum theory using real numbers only, avoiding the imaginary realm with versions called “real quantum mechanics.” But without an experimental test of such theories, the question remained whether imaginary numbers were truly necessary in quantum physics, or just a useful computational tool.

    A type of experiment known as a Bell test resolved a different quantum quandary, proving that quantum mechanics really requires strange quantum linkages between particles called entanglement (SN: 8/28/15). “We started thinking about whether an experiment of this sort could also refute real quantum mechanics,” says theoretical physicist Miguel Navascués of the Institute for Quantum Optics and Quantum Information Vienna. He and colleagues laid out a plan for an experiment in a paper posted online at in January 2021 and published December 15 in Nature.

    In this plan, researchers would send pairs of entangled particles from two different sources to three different people, named according to conventional physics lingo as Alice, Bob and Charlie. Alice receives one particle, and can measure it using various settings that she chooses. Charlie does the same. Bob receives two particles and performs a special type of measurement to entangle the particles that Alice and Charlie receive. A real quantum theory, with no imaginary numbers, would predict different results than standard quantum physics, allowing the experiment to distinguish which one is correct.

    Fan and colleagues performed such an experiment using photons, or particles of light, they report in a paper to be published in Physical Review Letters. By studying how Alice, Charlie and Bob’s results compare across many measurements, Fan, Navascués and colleagues show that the data could be described only by a quantum theory with complex numbers.

    Another team of physicists conducted an experiment based on the same concept using a quantum computer made with superconductors, materials which conduct electricity without resistance. Those researchers, too, found that quantum physics requires complex numbers, they report in another paper to be published in Physical Review Letters. “We are curious about why complex numbers are necessary and play a fundamental role in quantum mechanics,” says quantum physicist Chao-Yang Lu of the University of Science and Technology of China in Hefei, a coauthor of the study.

    But the results don’t rule out all theories that eschew imaginary numbers, notes theoretical physicist Jerry Finkelstein of Lawrence Berkeley National Laboratory in California, who was not involved with the new studies. The study eliminated certain theories based on real numbers, namely those that still follow the conventions of quantum mechanics. It’s still possible to explain the results without imaginary numbers by using a theory that breaks standard quantum rules. But those theories run into other conceptual issues, making them “ugly,” he says. But “if you’re willing to put up with the ugliness, then you can have a real quantum theory.”

    Despite the caveat, other physicists agree that the quandaries raised by the new findings are compelling. “I find it intriguing when you ask questions about why is quantum mechanics the way it is,” says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. Asking whether quantum theory could be simpler or if it contains anything unnecessary, “these are very interesting and thought-provoking questions.” More

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    Physicists have coaxed ultracold atoms into an elusive form of quantum matter

    An elusive form of matter called a quantum spin liquid isn’t a liquid, and it doesn’t spin — but it sure is quantum.

    Predicted nearly 50 years ago, quantum spin liquids have long evaded definitive detection in the laboratory. But now, a lattice of ultracold atoms held in place with lasers has shown hallmarks of the long-sought form of matter, researchers report in the Dec. 3 Science.

    Quantum entanglement goes into overdrive in the newly fashioned material. Even atoms on opposite sides of the lattice share entanglement, or quantum links, meaning that the properties of distant atoms are correlated with one another. “It’s very, very entangled,” says physicist Giulia Semeghini of Harvard University, a coauthor of the new study. “If you pick any two points of your system, they are connected to each other through this huge entanglement.” This strong, long-range entanglement could prove useful for building quantum computers, the researchers say.

    The new material matches predictions for a quantum spin liquid, although its makeup strays a bit from conventional expectations. While the traditional idea of a quantum spin liquid relies on the quantum property of spin, which gives atoms magnetic fields, the new material is based on different atomic quirks.

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    A standard quantum spin liquid should arise among atoms whose spins are in conflict. Spin causes atoms to act as tiny magnets. Normally, at low temperatures, those atoms would align their magnetic poles in a regular pattern. For example, if one atom points up, its neighbors point down. But if atoms are arranged in a triangle, for example, each atom has two neighbors that themselves point in opposite directions. That arrangement leaves the third one with nowhere to turn — it can’t oppose both of its neighbors at once.

    So atoms in quantum spin liquids refuse to choose (SN: 9/21/21). Instead, the atoms wind up in a superposition, a quantum combination of spin up and down, and each atom’s state is linked with those of its compatriots. The atoms are constantly fluctuating and never settle down into an orderly arrangement of spins, similarly to how atoms in a normal liquid are scattered about rather than arranged in a regularly repeating pattern, hence the name.

    Conclusive evidence of quantum spin liquids has been hard to come by in solid materials. In the new study, the researchers took a different tack: They created an artificial material composed of 219 trapped rubidium atoms cooled to a temperature of around 10 microkelvins (about –273.15° Celsius). The array of atoms, known as a programmable quantum simulator, allows scientists to fine-tune how atoms interact to investigate exotic forms of quantum matter.

    In the new experiment, rather than the atoms’ spins being in opposition, a different property created disagreement. The researchers used lasers to put the atoms into Rydberg states, meaning one of an atom’s electrons is bumped to a very high energy level (SN: 8/29/16). If one atom is in a Rydberg state, its neighbors prefer not to be. That setup begets a Rydberg-or-not discord, analogous to the spin-up and -down battle in a traditional quantum spin liquid.

    The scientists confirmed the quantum spin liquid effect by studying the properties of atoms that fell along loops traced through the material. According to quantum math, those atoms should have exhibited certain properties unique to quantum spin liquids. The results matched expectations for a quantum spin liquid and revealed that long-range entanglement was present.

    Notably, the material’s entanglement is topological. That means it is described by a branch of mathematics called topology, in which an object is defined by certain geometrical properties, for example, its number of holes (SN: 10/4/16). Topology can protect information from being destroyed: A bagel that falls off the counter will still have exactly one hole, for example. This information-preserving feature could be a boon to quantum computers, which must grapple with fragile, easily destroyed quantum information that makes calculations subject to mistakes (SN: 6/22/20).

    Whether the material truly qualifies as a quantum spin liquid, despite not being based on spin, depends on your choice of language, says theoretical physicist Christopher Laumann of Boston University, who was not involved with the study. Some physicists use the term “spin” to describe other systems with two possible options, because it has the same mathematics as atomic spins that can point either up or down. “Words have meaning, until they don’t,” he quips. It all depends how you spin them. More