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In November 1949 Chien-Shiung Wu and her graduate student, Irving Shaknov, descended to a laboratory below Columbia University's Pupin Hall. They needed antimatter for a new experiment, so they made their own, using a machine called a cyclotron. The machine's multiton magnet was so gigantic that, according to university folklore, a decade earlier administrators had to blast a hole in an exterior wall and recruit the football team to maneuver the block of iron into the building.

The magnetic field produced by a cyclotron accelerates particles to dizzying speeds. In the lab, Wu and Shaknov used it to bombard a sheet of copper with deuterons, generating an unstable isotope, Cu 64, as a source of positronsthe antimatter. When a positron and an electron collide, they annihilate each other, releasing two photons that fly apart in opposite directions. A few years earlier physicist John Wheeler had predicted that when matter and antimatter met, the resulting photons would be orthogonally polarized. Wu and Shaknov were looking for conclusive proof of Wheeler's so-called pair theory.

They weren't the first. An earlier team of experimentalists had a high margin of error, so their results were not sufficiently reliable. A second team came back with results that were too low to match Wheeler's predictions. But Wu was known for her extreme precision and strategic experimental design. The prior year she had proved Enrico Fermi's theory of beta decay after more than a decade of attempts by others.

Wu and Shaknov packed the copper isotope into a tiny capsule, eight millimeters long, and waited for electrons and positrons to collide inside the apparatus. Then they tracked the resulting annihilation radiation at the farthest edges of their experiment, using two photomultiplier tubes, anthracene crystals and a scintillation counter as a gamma-ray detector.

Ultimately they captured significantly more data than their predecessors, and what they saw was astonishing. Their evidence suggested that pairs of photons from particle collisions remained polarized at right angles to each otherconsistentlyas if somehow connected, even at a distance. Their experiment had proved Wheeler's pair theory, and Wu and Shaknov published their findings on New Year's Day in 1950 in a one-page letter to the Physical Review. But it also became the first experiment to document evidence of something weirder: that the properties of entangled particles are always perfectly correlated, no matter how far apart they stray. Entanglement is so strange that Albert Einstein thought it proved where quantum physics went wrong.

In 2022 the Nobel Prize Committee honored experimental work on entanglement by three physicists. John Clauser, Alain Aspect and Anton Zeilinger had each produced increasingly convincing evidence for the phenomenon by improving on their predecessor's experimental design. They ruled out one alternative explanation after another until, finally, entanglement was the only conclusion left standing. Although Wu's 1949 experiment had not been designed to rule out competing explanations, historians agree it was the first to document entangled photons. Yet Wu, who died in 1997, was not mentioned when the 2022 awards were announced. It's not the first time she has been overlooked.

Chien-Shiung Wu was born the same year as the New Republic of China, in a small town in the Yangtze River basin. Her father, Zhong-Yi Wu, was an intellectual, a revolutionary and a feminist. To celebrate his daughter's birth and the end of dynastic rule, Zhong-Yi hosted a party in the spring of 1912 where he announced his daughter's name and his new plan to open the region's first elementary school for girls. At a time when most names for girls suggested a delicate fragrance or beautiful flower, Zhong-Yi's name for his daughter translated to strong hero.

Chien-Shiung grew up in the crosscurrents of Chinese nationalism and the New Culture Movement that criticized traditional Confucian values. In 1936, at age 24, having reached the limit of what China could offer in physics training, she boarded the SS Hoover bound for California. Political movements were calling for science and democracy, along with a generation of scholars who could elevate China's status. Wu was off to pursue a Ph.D. in physics. She would study under pioneers such as Emile Segr, Ernest Lawrence and J. Robert Oppenheimer.

At the University of California, Berkeley, Wu became a star student. Her dissertation research on the fission products of uranium was so sophisticated and sensitive that it was turned over to the military and embargoed until the end of World War II. Yet Wu had trouble finding a job after graduation. For two years she depended on mentors for research appointments. At the time, none of the top 20 research universities in the country had a woman on the physics faculty.

Gender bias was not Wu's only obstacle. A year after her arrival in the U.S., the escalation of World War II cut off communication with China, and discrimination against Asian immigrants had intensified, especially on the West Coast. In 1940 Berkeley's acting comptroller wrote to Wu's supervisor to warn him that Wu's employment would be approved only on a temporary basis; less than a year later he wrote again: Regulations laid down by the Regents meant Miss Wu is not eligible for employment, and immediate steps should be taken to dismiss this employee from your staff. When Oppenheimer left Berkeley in 1942 to lead the Manhattan Project, he brought many of his students along; Wu, despite her acclaim, was not invited.

Eventually Wu moved East for a teaching position at Smith College. The following year she became the first woman hired to the Princeton University physics faculty. Not long after, the Manhattan Project finally recruited her, and she played a quiet, conflicted and crucial role in the development of the atomic bomb. Yet Wu navigated repeated investigations by immigration authorities and threats of deportation for years. When she had left China in 1936, Wu expected to be away for only a short while. In 1945, when the silence between the U.S. and China lifted, China was embroiled in a brutal civil war, and relatives cautioned against returning too soon. By 1949, the year Wu observed evidence of the criterion for entanglement, Mao Zedong had established communism in the People's Republic of China, and McCarthyism was ramping up in the U.S., making travel home nearly impossible. She never saw her family again.

Entanglement emerges from the most rigorous branches of mathematics and physics yet has poetic appeal. Abner Shimony, a philosopher and physicist, called it passion at a distance. Entanglement offers the wild notion that once certain particles or systems interact, they can no longer be described independently of one another. What happens to one, no matter how far it may travel from its entangled partner, instantly affects the other, as decades of evidence now shows. The characteristics of entangled particles are correlated, without any apparent communication, and at any distance. What's more, each member of the entangled pair seems to lack a complete set of definite properties until the moment when one partner is measured. Then, instantly, the entangled pair will be in synceven if the particles have drifted galaxies apart. It's the ultimate star-crossed love.

To grasp entanglement's full strangeness, it helps to understand that when quantum physicists first set out to quantify the position and motion of subatomic particles, the tiny objects could not be pinned down. Sometimes particles seemed localized and distinct. At other times, the particles showed a broad and wavelike behavior, with influence spreading out over large regions of physical space relative to their natural size. Sometimes early 20th-century experimentalists couldn't be sure the particles were even tangible objects at all.

In 1927 physicist Werner Heisenberg called this problem the uncertainty principle. He studied under the founder of quantum mechanics, Niels Bohr, and Bohr had coined the term complementarity to describe the uncanny experimental results that quantum physics produced. For Bohr, one way to think about the entire confusing situation was to presume that certain pairs of observations such as a particle's position and momentum were complementary to one another; complementary characteristics could not be perceived or measured exactly in the subatomic world at the same time. Perhaps those characteristics did not even exist until the very moment of measurement. Things got weirder, though, when the mathematics of quantum mechanics suggested that measuring one particle might instantaneously influence the state of another particle far away. This seemed especially odd if the particles had no measurable attributes in the first place until the two, somehow, telepathically connected.

In 1935 Einstein, Boris Podolsky and Nathan Rosen tried to poke holes in quantum mechanics by pointing out how counterintuitive it seemed. The famous Einstein-Podolsky-Rosen paradox (EPR) pointed directly at entanglement. EPR suggested that there had to be a better explanation for why and how one particle could impact its entangled partner faster than the speed of light. Einstein derisively nicknamed the phenomenon spooky action at a distance. For Einstein and his co-authors, spooky action proved that quantum theory was still incomplete.

Like Einstein, physicist David Bohm felt sure there was a perfectly reasonable explanation for entanglement. Perhaps we couldn't see it quite yet, but the explanation might not be so spooky after all. It could be attributed to hidden variables. Physics simply had more work to do to find them. In 1957 Bohm and his graduate student Yakir Aharonov wrote about how photon research could harness the famous EPR paradox to reveal these hidden variables. [T]here has been done an experiment which, as we shall see, tests essentially for this point, but in a more indirect way, Bohm wrote.

That experiment, says Indianara Silva, a professor of physical sciences and history at the State University of Feira de Santana in Brazil, was the 1949 Wu-Shaknov experiment.

Silva is a historian who is acutely attentive to the missing stories of women in science. When Wu and Shaknov made the first precise measurement of Wheeler's pair-theory in 1949, Silva says, they became the first to document entanglement between photons, inspiring decades of later research in quantum foundations. Silva has identified a string of publications by other physicists and historians who acknowledge Wu's 1949 observation of entangled photons. She begins with Bohm in 1957 and continues through Zeilinger, one of the 2022 Nobel laureates, who wrote in 1999 that an earlier experiment by Wu and Shaknov (1950) had demonstrated the existence of spatially separated entangled states.

Bohm had good reason to trust Wu's findings. He was a few years junior to Wu when they were graduate students at Berkeley. Both had studied under Oppenheimer, and both worked in E. O. Lawrence's prestigious radiation laboratory. Bohm had every reason to know of Wu's stellar reputation. He acknowledged Wu in a footnote in his 1957 article.

Silva traces how Wu's experimental workin 1949 and later in 1971prompted later entanglement experiments. Silva's findings were published in The Oxford Handbook of the History of Quantum Interpretation in 2022. She points out how Bohm's article about hidden variables inspired John Bell, who proposed that the number of quantum coincidences between particles could be predicted and counted. In 1964, in an obscure journal called Physics, Physique, Fizika, Bell discussed Bohm's 1957 paper (which referenced Wu's experiment) and launched his own new theory. A few years later at Columbia, a young Clauser found Bell's Theorem in the library. The theory inspired Clauser to design a new experiment, one he hoped could prove Bell right, showing hidden variables were real.

Interestingly, the Wu-Shaknov letter to Physical Review in 1950 talks about Wheeler's pair theory, but it is silent about entanglement. In 2012 physicist F. J. Duarte called Wheeler's pair theory the essence of entanglement. Other physicists, and historians like Silva, clearly spotted the connection, too. So why did Wu not mention quantum entanglement in her 1950 letter?

Wu might have been hesitant to discuss evidence of entanglement because throughout the 1950s and 1960s, such quantum-foundations work was stigmatized as junk science. Back then, explains David Kaiser, a professor of physics and history of science at the Massachusetts Institute of Technology, the idea of using an experiment to prove or disprove theories about quantum physics or to test for local hidden variables was not even an inkling for most physicists. Researchers who explored questions about entanglement often disguised their research because backlash could stymie a promising career. We're left to wonder whether Wu might have done so as well.

Silva points out that Wu came back to her 1949 experiment more than 20 years later to refine it further. By then, Wu was far more professionally secure, and she addressed questions about quantum mechanics directly. She favored traditional quantum-entanglement interpretations, not Bohm's theory. In 1971, when she designed a new version of the 1949 experiment, Wu wrote that it should certainly quiet those proponents of the hidden variables.

When Clauser published his proposed test of Bell's theorem in 1969, he took care to distinguish the Wu-Shaknov experiment from his own. Clauser had wanted to prove hidden variables were real; instead, in 1972, he disproved the existence of hidden variables and demonstrated entanglement with even greater certainty. He had counted coincidences, much as Bell suggested, but there were far more coincidences than hidden variables could explain. Clauser's work prompted Aspect and Zeilinger's later experiments, which closed lingering loopholes and supported entanglement further. Together those experiments led to their 2022 Nobel Prize.

By the time Bohm's paper on hidden variables emerged, much had changed in Wu's life. She had married and moved to the East Coast. She had broken a glass ceiling at Princeton, had a child and had become a U.S. citizen. She was on the faculty of Columbia University, though still not a full professor.

In 1956 Wu's Columbia colleague T. D. Lee approached her for advice about an odd question. He and his research partner, Chen Ning Yang, wondered if some of the tiniest particles in the universe might violate long-established expectations. In response, Wu pointed Lee to a body of research, and she described a handful of possible experiments to address the questions he posed.

Yang and Lee were far from the likeliest of candidates to act on Wu's suggestions. Both were theorists, not experimentalists like Wu. In an oral history with the Simons Foundation half a century later, Yang confessed that neither he nor Lee had any sincere belief in 1956 that their hypothesis would hold up. In fact, physicists had assumed for decades that the opposite would be true: that symmetry would be among the immutable, consistent patterns in many building blocks of our universe. Mathematical conservation laws said that if you ran the same sequence of events forward and backward in time, the events would remain symmetrical. Yang and Lee's hypothesis, though, suggested that the behavior of nuclear particles in beta decay might not look the same if you flipped the events in an imaginary mirror. The idea simply did not align with conventional scientific thought or with common sense.

Like her father, Wu was willing to question mainstream thinking. She suspected the issue was important, and she knew how to approach it. So she designed and led an experiment to address her colleagues' ideas. It meant canceling a trip to China that would have been her first visit home since 1936.

To run the experiment she had in mind, Wu needed to reduce the temperature of radioactive cobalt 60 nuclei until the particles almost stopped moving. She wanted to study whether the daughter particles of nuclear decay shot out in a symmetrical patternas all of mainstream physics believed they wouldor if the radioactive patterns showed a preference for right-handed or left-handed behavior. She enlisted cooperation from the National Bureau of Standards (NBS, now the NIST) in Washington, D.C., because, unlike many other labs, they had the technology and expertise to work at temperatures close to absolute zero. For months Wu commuted between New York City and Washington, overseeing graduate student work that supported the experiment.

By January 1957, in close consultation with Yang and Lee, Wu and her NBSpartners made an astonishing discovery. Beta-decay particles were slightly left-handed, not symmetrical as all of physics had assumed. As soon as it was announced, Yang, Lee and Wu, along with other experimentalists who followed Wu's work, found themselves on a national conference circuit, their names and images splashed across the popular press. When the American Physical Society met at the New York Hotel that year, they presented their findings in what the New Yorker called the largest hall ... occupied by so immense a crowd that some of its members did everything but hang from the chandeliers.

That October, Yang and Lee became the first two Chinese Americans in history to win the Nobel Prize. Although Nobel rules allowed up to three award recipients each year, Wu was not included. It could hardly be more apt that the law of physics that Wu toppled was called the principle of parity. Like a prism, the 1957 Nobel Prize separated out elements of identity like bands of light, rendering the impact of gender more visible. The following year Columbia finally promoted Wu to the rank of full professor.

In his Nobel lecture that December, Yang told the committee and guests how crucial Wu's experiment had been, making a bold statement that the results were due to Wu's team's courage and skill. Lee would later plead with the Nobel Committee to recognize Wu's work. Oppenheimer publicly stated that Wu should have shared in the 1957 prize. Segr called the overthrow of parity probably the major development of physics after the war.

Other scientists criticized Wu's exclusion from the highest recognition of scientific achievement, too. In 1991 Douglas Hofstadter, the author of Gdel, Escher, Bach, organized scientists to write letters to the Nobel Committee recommending Wu for the physics prize. And in 2018, 1,600 researchers invoked Wu's name in an open letter to CERN challenging current-day sexism in physics. [T]here are at least four women whose work is relevant for particle physics who are widely viewed as having deserved the Nobel prize but who did not receive it, in some cases even though their male colleagues did, the letter says. Wu's name appears at the top of that list.

After overthrowing parity, Wu became the first woman to receive the Comstock Prize from the National Academy of Sciences; the first female president of the American Physical Society; the first physicist to receive the Wolf Prize; and the first living physicist to have an asteroid named in her honor. Her work pushed open doors to university teaching in the West for women and scientists of color. In China, she is revered. In 2021 the U.S. Postal Service released a Forever stamp with Wu's portrait. Today Wu's parity experiment is understood as an early step on the path to what would become the Standard Model of particle physics, and it points toward possible answers about why matter exists in our universe at all.

Wu's early entanglement work, however, remained in obscurity. Sometimes by examining one part of a system, we begin to perceive a related link, at a distance. The 2022 Nobel Prize celebrated a set of connected experiments that took place at great distance from one another. Even though Wu couldn't have been awarded the prize posthumously, her early research is finally coming to light as a crucial part of that entangled history, thanks in large part to historians such as Silva. Society may prefer a hero narrative or the myth of a lone genius, but a closer look reveals that extraordinary science, like entanglement itself, depends fundamentally on connection.

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The Little-Known Origin Story behind the 2022 Nobel Prize in Physics - Scientific American

March 17, 2023• Physics 16, 46

Researchers have demonstrated quantum gate operations in a system where voltage pulses cause neighboring electron spins to swap with one another.

Twenty years ago, theorists proposed an approach for protecting fragile spin-based qubits against the decoherence from noisy inputs. The idea was to encode information in the qubits by swapping the spin states of neighboring electrons. Unlike the usual method of flipping spins, this swapping process would add no energy to the system. Researchers at HRL Laboratories in California have now realized that design in an electrically controlled, silicon-based platform [1]. Their devicewhich was presented last week at the APS March Meetingdemonstrates a low-error logic gate that can be used to perform any kind of quantum computational algorithm.

In most spin-based qubit designs, a qubit is a single spin with two states0 or 1which have different energies corresponding to the spins alignment with respect to an applied magnetic field. The qubit can be controlled by adding or removing energy from the system. Thats typically accomplished by irradiating the qubit with microwave photons at a frequency corresponding to the qubits energy level splitting. The qubits spin responds by flipping directionlike an on-off switch. This method is well established, but it suffers from decoherencethe qubit tends to lose its quantum information as the result of small inhomogeneities (noise) in the microwave radiation or magnetic field.

In contrast, the teams approach creates a spin-based qubit whose 0 and 1 states have the same energy. Here the qubit states correspond to whether two electron spins in the qubit have antisymmetric (0) or symmetric (1) spin wave functions. Control over these states is offered by voltage pulses that swap the directions of neighboring spins without aligning them in a particular direction. These swaps, which are energy-conserving operations, change nothing when the two wave functions are symmetric, but they introduce a quantum phase of 1 when the wave functions are antisymmetric. Such swaps are actually partial swaps, meaning the voltage pulse is tuned so that the swapping can occur but theres a certain probability that it doesnt. A partial swap is a quantum operation that leaves us in a superposition of swapped and not swapped, explains HRL team member Thaddeus Ladd. He and his colleagues use a complex sequence of partial swaps to encode information in a set of electron spins.

For the experiment, the HRL team fabricated six silicon quantum dots, forming two distinct qubits. Each dot traps a single electron, whose spin interacts with neighboring spins through voltage pulses delivered to metal gates. The researchers demonstrated two quantum operationscalled CNOT and SWAPwith the two qubits. Doing so required complex sequences of partial swaps across the six spins, involving thousands of precisely calibrated voltage pulses that switch on and off a hundred million times per second. The measured errors in these operations were low, characterized by a fidelity of around 97%. With a healthy dose of mathematics, one may show that this [method] of partial swapping of spins is sufficient to perform any quantum operation on a desired, limited set of states of many spins, says Ladd.

This approach offers two key advantages compared to conventional single-spin qubits. First, it avoids the need for various hardware integration to control magnetic fields and mismatched phases. Second, it avoids the crosstalk generated by a microwave input. These advantages avoid microscopic sources of error and improve the fidelity of qubit control. The price is that each qubit needs three quantum dots to form a single qubit, and each basic operation consists of a long complex sequence of pulses. Ladd says getting the device to work was no easy feat of hardware fabrication and software development.

The researchers built their new six-dot device using a technique that theyve been developing called SLEDGE (single-layer etch-defined gate electrode). This platform uses an electron beam to pattern dot-shaped gates onto a plane and subsequently interconnect the gates via metal leads. Andrea Morello, a quantum physicist at the University of New South Wales, Australia, is impressed with the labs new device. [HRLs] state-of-the-art device fabrication capabilities allowed the researchers to fabricate quantum dots with exquisite precision and reproducibility such that even a complex six-dot device exhibited reliable behavior, Morello says.

Ladd clarifies that the technology wont lead to practical quantum computing until millions of qubits can communicate with one another. Although HRLs proof of concept avoids many problems associated with microwave control, there are other challenges, such as keeping the system cold and ensuring uniformity in the etched quantum-dot patterns, which will become more difficult as more qubits are included. Im not claiming ours is the best or fastest or smartest qubit design. But I think its one of the most interesting, not least because it connects to the fundamental computing problem of whether you must input energy to perform a computation, says Ladd.

According to Morello, the swapping approach would require big changes to the way people normally operate qubits, but he thinks a compelling argument can be made that removing the need for microwave signals may simplify the job of qubit control. The future will tell whether this bold choice pays off when enlarging the quantum processor to ever more qubits, he says.

Rachel Berkowitz

Rachel Berkowitz is a Corresponding Editor forPhysics Magazine based in Vancouver, Canada.

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Physics - Universal Quantum Logic through Spin Swapping - Physics

Thomas Hertog (left) collaborated with Stephen Hawking for many years

Courtesy of Thomas Hertog

IT WAS common knowledge among students at the University of Cambridge that whoever obtained the best marks in the final part of the mathematical tripos exams would be summoned to see Stephen Hawking. I had just got my results and had come top. Sure enough, I was invited for a discussion with him.

I made my way to his office deep in the labyrinth of the department of applied mathematics and theoretical physics, which was housed in a creaking Victorian building on the banks of the river Cam. Stephens office was just off the main common room, and even though it was noisy there, he liked to keep his door ajar. I knocked, paused and slowly pushed it open.

I didnt quite know what to expect on the other side of that door. I knew, of course, that Stephen was famous for his work on black holes and that he had even got into trouble for some of his ideas about what happens when they explode. But it turned out that he was musing on a different question: why is the universe just right for life to arise?

Pondering this question would turn into a long quest for us both. For the next two decades, until his death, Stephen and I worked shoulder to shoulder on novel ideas that suggest a radically new understanding of why the universe is the way it is. In our conception, the laws of physics themselves have, in a sense, evolved to be the

Link:

Stephen Hawking's final theorem turns time and causality inside out - New Scientist

The patient was elderly and lived alone. She was showing signs of depression, but it was clear that something more was amiss. She insisted she was trapped in the wrong timeline.

The ward to which shed been committed was unstuck in time, she told her doctors. Outside, the future had already arrived, and it was not a good one. She described then that the world outside the ward had been destroyed, reported the doctors in Exeter, England, who wrote a report about the case in a 2019 issue of the journal Neurology and Neurosurgery.

The woman was diagnosed with a variation of Capgras syndrome. First defined a century ago, Capgras typically describes a persons belief that someone close to him or her a spouse or a child has been replaced with a duplicate impostor. But in this case, the patient believed that the whole world everything she could observe of it was a duplicate, a fake.

I know a little bit how that feels.

So do you, probably.

It seems many of us have come to feel there are multiple realities and were stuck in the wrong one. For some, this sensation was occasioned by the outcome of the 2016 presidential election as it was for Arthur Darvill, known for his role in the British television series Doctor Who, who tweeted on Nov. 9, 2016, I think we landed in the wrong timeline.

The Rev. Traci Blackmon, a Christian minister and civil rights activist who was active in protests at Ferguson, Mo., and counterprotests at Charlottesville, Va., tweeted about this feeling in 2018: I believe I am trapped in an alternative universe. At the time, I reached out to her to ask her about it. She explained that the racism shed witnessed felt like a detour from the way shed assumed history would unfold. The gains wed made in social equity and humanizing people I thought these gains would result in a different world, she said.

Now five years and one pandemic later, Everything Everywhere All at Once, a film about the idea that there are multiple universes, each containing a different version of you, swept the Oscars and struck a chord. Apparently many of us have this sense that, as Waymond Wang, played by Ke Huy Quan, says in the movie, something is off.

Its easy to see the appeal of the multiverse, even as metaphor: the notion that were surrounded by a multitude of parallel selves, one of which might be living in a better timeline than the one were stuck in. Its probably no coincidence that the idea has become so popular during an era of pandemic, climate change and political turmoil, when so many of us have felt helpless and trapped. Who doesnt want to imagine a different world?

But it can also be a dangerous way of imagining the cosmos. Like the Capgras patient, we risk becoming detached from the world we can see and touch. Regardless of whether we can prove that the multiverse exists, the idea of it can distract us from doing the work we need to do to make this world better. This timeline is the only one we have access to, and its got to be enough.

As a species, weve long been haunted by spirit realms and ghostly domains. Plato conceived of an intangible world of forms realer than anything we can touch. Plutarch reported that Alexander wept when he heard the possibility of an infinite number of worlds, having not conquered all of this one.

C.S. Lewis was an early multiverse explorer with his Narnia books, in which siblings grow to adulthood as kings and queens on the other side of their magical wardrobe in a world that exists parallel to our own. He was also paying quite a bit of attention at the time to a new branch of science known as quantum physics. In 1957, a year after Lewis published his last Narnia book, a Princeton doctoral student, Hugh Everett III, published a dissertation bringing the ancient idea of the simultaneous existence of several worlds into the realm of modern science.

Everett was trying to solve a seeming paradox in quantum theory: Certain elementary particles (say, a photon) seemed to exist mathematically in many places at once but could be detected at only one location at a time.

Perhaps, Everett suggested, the act of detecting the particle splinters reality; perhaps the observer, and indeed the universe, splits into different possible timelines, one for each possible location of the particle. This would become known as the many-worlds interpretation. Physicists recoiled at the idea at the time.

It took a while for Everetts idea to trickle into popular culture, but once it did, pulp fiction writers fell in love with it. In 1961, DC Comics published Flash of Two Worlds! in which the superfast hero vibrates his way into another universe to meet an alternate Flash. More recently, the multiverse has become the perfect framework for superhero-centric entertainment entities like Marvel Studios (and its licensees) to reiterate a seemingly endless series of franchise reboots. (A.O. Scott, writing in this newspaper, called this a conceit that promises ingenuity and narrative abundance but instead has delivered an infinite recombination of clich.) Thus, Spider-Man entered the Spider-Verse; Dr. Strange got lost in the Multiverse of Madness.

I first encountered the idea of a parallel world as a kid in the 1980s, watching the She-Ra cartoon movie. Princess Adora is separated at birth from her brother, Adam, and sent to grow up on the other side of a tridimensional portal.

I was an only child, and I was fascinated by the idea of some alternate plane where, like Prince Adam, I might discover a secret sibling, an end to loneliness.

When I was 12, my mother met a man, and suddenly the family Id imagined for myself became real. I had an older brother who loved puns and an older sister who wrote poems.

But when I was 19, my stepfather died of melanoma; within a few years of recriminations and disputes, our blended family unblended itself.

I entered adulthood bereft and wrong-footed. I felt a horrible sense of vertigo as I watched the life Id been expecting to live tilting away from me. In this new timeline, my stepsiblings were no longer my siblings; they would become, instead, just people I knew for a while in high school.

All this because a photon of sunlight had collided with a segment of my stepfathers DNA. A quantum event a whole universe of grief unfurled from a minuscule catastrophe that could just as easily not have happened.

For years, I couldnt stop thinking about other, better timelines where it didnt happen, where my stepfather was still alive and my family intact. It helped me understand what was missing, but it did not allow me to mourn what Id lost.

And thats the peril of the multiverse; I was becoming unreal to myself, nostalgic not for a time before the death happened but for a timeline in which it never happened at all. At the climax of the Narnia series, Lewis renounces his beloved fantasy land as a shadow of a copy of a newer, realer Narnia. The new one was a deeper country, he writes. Every rock and flower and blade of grass looked as if it meant more. A shadow of a copy thats how I felt.

In Everything Everywhere, Joy, the character played by Stephanie Hsu, has become aware of every possible timeline. She succumbs to nihilistic despair. If everything is happening, then nothing can matter. Its hard not to recall the real-life fate of Mr. Everetts daughter, Elizabeth, who ended her life in 1996, saying in her farewell note that she hoped she would go on to a correct parallel universe where both she and her father were still alive.

I reached out recently to Ms. Blackmon, the activist who tweeted about an alternate universe, to see how shes feeling about the timeline in 2023. Because she is still trapped here, she has no updates, a representative wrote back.

But its telling that Ms. Blackmon never stopped fighting for what she believes in, striving to improve this world, serving the United Church of Christ and, most recently, leading a group of religious leaders in an effort to block Missouris abortion ban. We can joke or wonder whether were in the wrong timeline. But we cant lose sight of the fact that this timeline is the only one weve got.

In my 30s, I knew I had to save myself from the enticements of alternate realities. So I envisioned a new cosmology of time. Instead of a linear, branching timeline with multiple, parallel possibilities so much more vivid than my real life I tried to imagine time as a sphere always expanding away from me in every direction, like the light leaving a star.

In this model of time, instead of the past receding behind me, it expands outward to surround me, always there and always present. The future is at the very center of the sphere, curled up infinitely small inside of me, waiting to be realized. That way, I can believe that there is nothing to come that I do not already contain.

As a cosmology, its no more tangible than the multiverse. But if we have to believe in something invisible, let me believe in a version of the universe that keeps my focus where it belongs: on the things I can touch and change.

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Opinion | Forget the Multiverse. Embrace the Monoverse. - The New York Times

Do universes pop up as bubbles from a multiverse?

Its easy to envisage other universes, governed by slightly different laws of physics, in which no intelligent life, nor indeed any kind of organized complex systems, could arise. Should we, therefore, be surprised that a universe exists in which we were able to emerge?

Thats a question physicists including me have tried to answer for decades. But it is proving difficult. Although we can confidently trace cosmic history back to one second after the Big Bang, what happened before is harder to gauge. Our particle accelerators simply cant produce enough energy to replicate the extreme conditions that prevailed in the first nanosecond.

But we expect that its in that first tiny fraction of a second that the key features of our universe were imprinted.

The Big Bang theory is the most widely accepted scientific explanation for the origins of the universe. It proposes that the universe began as a singularity, an infinitely dense and hot point that expanded rapidly about 13.8 billion years ago, and has been cooling and expanding ever since.

The conditions of the universe can be described through its fundamental constants fixed quantities in nature, such as the gravitational constant (called G) or the speed of light (called C). There are about 30 of these representing the sizes and strengths of parameters such as particle masses, forces or the universes expansion. But our theories dont explain what values these constants should have. Instead, we have to measure them and plug their values into our equations to accurately describe nature.

The values of the constants are in the range that allows complex systems such as stars, planets, carbon, and ultimately humans to evolve. Physicists have discovered that if we tweaked some of these parameters by just a few percent, it would render our universe lifeless. The fact that life exists therefore takes some explaining.

Some argue it is just a lucky coincidence. An alternative explanation, however, is that we live in a multiverse, containing domains with different physical laws and values of fundamental constants. Most might be wholly unsuitable for life. But a few should, statistically speaking, be life-friendly.

What is the extent of physical reality? Were confident that its more extensive than the domain that astronomers can ever observe, even in principle. That domain is definitely finite. Thats essentially because, like on the ocean, theres a horizon that we cant see beyond. And just as we dont think the ocean stops just beyond our horizon, we expect galaxies beyond the limit of our observable universe. In our accelerating universe, our remote descendants will also never be able to observe them.

Most physicists would agree there are galaxies that we cant ever see, and that these outnumber the ones we can observe. If they stretched far enough, then everything we could ever imagine happening may be repeated over and over. Far beyond the horizon, we could all have avatars.

This vast (and mainly unobservable) domain would be the aftermath of our Big Bang and would probably be governed by the same physical laws that prevail in the parts of the universe we can observe. But was our Big Bang the only one?

The theory of inflation, which suggests that the early universe underwent a period when it doubled in size every trillionth of a trillionth of a trillionth of a second has genuine observational support. It accounts for why the universe is so large and smooth, except for fluctuations and ripples that are the seeds for galaxy formation.

But physicists including Andrei Linde have shown that, under some specific but plausible assumptions about the uncertain physics at this ancient era, there would be an eternal production of Big Bangs each giving rise to a new universe.

String theory, which is an attempt to unify gravity with the laws of microphysics, conjectures everything in the universe is made up of tiny, vibrating strings. But it makes the assumption that there are more dimensions than the ones we experience. These extra dimensions, it suggests, are compacted so tightly together that we dont notice them all. And each type of compactification could create a universe with different microphysics so other Big Bangs, when they cool down, could be governed by different laws.

The laws of nature may therefore, in this still grander perspective, be local by-laws governing our own cosmic patch.

The NASA/ESA/CSA James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant Universe to date. Known as Webbs First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail. However, we can only see a fraction of the universe. Credit: NASA, ESA, CSA, and STScI

If physical reality is like this, then theres a real motivation to explore counterfactual universes places with different gravity, different physics, and so forth to explore what range or parameters would allow complexity to emerge, and which would lead to sterile or stillborn cosmos. Excitingly, this is ongoing, with recent reseach suggesting you could imagine universes that are even more friendly to life than our own. Most tweakings of the physical constants, however, would render a universe stillborn.

That said, some dont like the concept of the multiverse. They worry it would render the hope for a fundamental theory to explain the constants as vain as Keplers numerological quest to relate planetary orbits to nested platonic solids.

But our preferences are irrelevant to the way physical reality actually is so we should surely be open-minded to the possibility of an imminent grand cosmological revolution. First we had the Copernican realization that the Earth wasnt the center of the Solar System it revolves around the Sun. Then we realized that there are zillions of planetary systems in our galaxy, and that there are zillions of galaxies in our observable universe.

So could it be that our observable domain indeed our Big Bang is a tiny part of a far larger and possibly diverse ensemble?

How do we know just how atypical our universe is? To answer that we need to work out the probabilities of each combination of constants. And thats a can of worms that we cant yet open it will have to await huge theoretical advances.

We dont ultimately know if there are other Big Bangs. But theyre not just metaphysics. We might one day have reasons to believe that they exist.

Specifically, if we had a theory that described physics under the extreme conditions of the ultra-early Big Bang and if that theory had been corroborated in other ways, for instance by deriving some unexplained parameters in the standard model of particle physics then if it predicted multiple Big Bangs, we should take it seriously.

Critics sometimes argue that the multiverse is unscientific because we cant ever observe other universes. But I disagree. We cant observe the interior of black holes, but we believe what physicist Roger Penrose says about what happens there his theory has gained credibility by agreeing with many things we can observe.

About 15 years ago, I was on a panel at Stanford where we were asked how seriously we took the multiverse concept on the scale would you bet your goldfish, your dog, or your life on it. I said I was nearly at the dog level. Linde said hed almost bet his life. Later, on being told this, physicist Steven Weinberg said hed happily bet Martin Rees dog and Andrei Lindes life.

Sadly, I suspect Linde, my dog and I will all be dead before we have an answer.

Indeed, we cant even be sure wed understand the answer just as quantum theory is too difficult for monkeys. Its conceivable that machine intelligence could explore the geometrical intricacies of some string theories and spew out, for instance, some generic features of the standard model. Wed then have confidence in the theory and take its other predictions seriously.

But wed never have the aha insight moment thats the greatest satisfaction for a theorist. Physical reality at its deepest level could be so profound that its elucidation would have to await posthuman species depressing or exhilarating as that may be, according to taste. But its no reason to dismiss the multiverse as unscientific.

Written by Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of Cambridge.

This article was first published in The Conversation.

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The Mind-Bending Multiverse: Our Universe Is Suspiciously Unlikely ... - SciTechDaily