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You cant divide the indivisible, unless you use quantum mechanics. Physicists have now turned to quantum effects to split phonons, the smallest bits of sound, researchers report in the June 9 Science.

Its a breakthrough that mirrors the sort of quantum weirdness thats typically demonstrated with light or tiny particles like electrons and atoms (SN: 7/27/22). The achievement may one day lead to sound-based versions of quantum computers or extremely sensitive measuring devices. For now, it shows that mind-bending quantum weirdness applies to sound as well as it does to light.

There was no one that had really explored that, says engineering physicist Andrew Cleland of the University of Chicago. Doing so allows researchers to draw parallels between sound waves and light.

Phonons have much in common with photons, the tiniest chunks of light. Turning down the volume of a sound is the same as dialing back the number of phonons, much like dimming a light reduces the number of photons. The very quietest sounds of all consist of individual and indivisible phonons.

Unlike photons, which can travel through empty space, phonons need a medium such as air or water or in the case of the new study, the surface of an elastic material. Whats really kind of, in my mind, amazing about that is that these sound waves [carry] a very, very small amount of energy, because its a single quantum, Cleland says. But it involves the motion of a quadrillion atoms that are all working together to [transmit] this sound wave.

Phonons cant be permanently broken into smaller bits. But, as the new experiment showed, they can be temporarily divided into parts using quantum mechanics.

Cleland and his team managed the feat with an acoustic beam splitter, a device that allows about half of an impinging torrent of phonons to pass through while the rest get reflected back. But when just one phonon at a time meets the beam splitter, that phonon enters a special quantum state where it goes both ways at once. The simultaneously reflected and transmitted phonon interacts with itself, in a process known as interference, to change where it ultimately ends up.

The lab demonstration of the effect relied on sound millions of times higher in pitch than humans can hear, in a device cooled to temperatures very near absolute zero. Instead of speakers and microphones to create and hear the sound, the team used qubits, which store quantum bits of information (SN: 2/9/21). The researchers launched a phonon from one qubit toward another qubit. Along the way, the phonon encountered a beam splitter.

Adjusting the parameters of the setup modified the way that the reflected and transmitted portions of the phonon interacted with each other. That allowed the researchers to quantum mechanically change the odds of the whole phonon turning up back at the qubit that launched the phonon or at the qubit on the other side of the beam splitter.

A second experiment confirmed the quantum mechanical behavior of the phonons by sending phonons from two qubits to a beam splitter between them. On their own, each phonon could end up back at the qubit it came from or at the one on the opposite side of the beam splitter.

If the phonons were timed to arrive at the beam splitter at the exact same time, though, they travel together to their ultimate destination. That is, they still unpredictably go to one qubit or the other, but they always end up at the same qubit when the two phonons hit the beam splitter simultaneously.

If phonons followed the classical, nonquantum rules for sound, then there would be no correlation in where the two phonons go after hitting the beam splitter. The effect could serve as the basis for fundamental building blocks in quantum computers known as gates.

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The next logical step in this experiment is to demonstrate that we can do a quantum gate with phonons, Cleland says. That would be one gate in the assembly of gates that you need to do an actual computation.

Sound-based devices are not likely to outperform quantum computers that use photons (SN: 2/14/18). But phonons could lead to new quantum applications, says Andrew Armour, a physicist at the University of Nottingham in England who was not involved in the study.

Its probably not so clear what those [applications] are at the moment, Armour says. What youre doing is extending the [quantum] toolbox. People will build on it, and it will keep going, and theres no sign of it stopping any time soon.

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This Microsoft illustration depicts a future quantum supercomputer operating in an Azure data center. (Microsoft Image)

Microsoft says it has achieved an important physics breakthrough representing the first milestone in its long-term initiative to build a quantum supercomputer capable of solving some of the worlds most difficult problems.

A peer-reviewed paper in Physical Review B, a journal of the American Physical Society, confirmed that the companys approach can create and control Majorana, a type of particle considered key to the future creation of scalable and stable qubits, the fundamental units of quantum information.

Its akin to inventing steel, leading to the launch of the Industrial Revolution, said Krysta Svore, Microsofts vice president of advanced quantum development, in a video outlining the companys quantum supercomputer roadmap.

Quantum computing uses the principles of quantum physics to process information in ways that traditional computers cant, potentially solving complex problems much more quickly. Unlike classical bits that can be either 0 or 1, qubits can exist in multiple states at once, allowing quantum computers to perform many calculations simultaneously.

Our goal is to compress the next 250 years of chemistry and material science progress into the next 25, Microsoft CEO Satya Nadella said in a video Wednesday introducing the companys quantum announcements.

Nadella as far back in 2017 was identifying quantum computing, virtual and augmented reality and artificial intelligence as the three technologies he believed at the time were most likely to shape the future.

Microsoft announced the milestone along with a new service called Azure Quantum Elements, which uses AI and high-performance computing to accelerate scientific research; and an AI-powered copilot for its Azure Quantum service, letting researchers use natural language for difficult chemistry and materials science problems.

The company is competing against several other major tech companies pursuing quantum breakthroughs, including IBM, Google, and Amazon, in addition to quantum companies and research institutions.

The announcements build on Microsofts existing momentum in quantum computing with commercial partners such as Johnson Matthey and government agencies including the Pentagons Defense Advanced Research Projects Agency.

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Eminent theoretical physicist and Dirac Medalist Roman Jackiw, MIT professor emeritus and holder of the Department of Physics Jerrold Zacharias chair, died June 14 at age 83. He was a member of the MIT physics community for 54 years.

A leader in the sophisticated use of quantum field theory to illuminate physical problems, his influential work on topology and anomalies in quantum field theory (QFT) underlies many aspects of theoretical physics today.

Iain Stewart, the MIT Center for Theoretical Physics (CTP) director and Otto (1939) and Jane Morningstar Professor of Science, says that Jackiw served as an inspiration for what one can achieve as a theoretical physicist. He made profound contributions to physical problems in a wide range of areas, including particle physics, condensed matter physics, and gravitational physics.

Professor Jackiw was a pioneer in the field of mathematical physics, says Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics and dean of the MIT School of Science. His imaginative use of quantum field theory shed light on physical problems, including his work on topological solitons, field theory at high temperatures, the existence of anomalies, and the role of these anomalies in particle physics."

Says Frank Wilczek, a CTP colleague who is the Herman Feshbach Professor of Physics and a 2004 Nobel Laureate, Roman Jackiw had an uncanny knack for identifying curiosities that have grown into fertile, vibrant areas of physics research. His seminal contributions to the theory of anomalies, the interplay of topology with quantum theory, and fractional quantum numbers are a rich legacy which has become central to both fundamental physics and modern quantum engineering.

He was a major, major figure in theoretical physics, Wilczek said to his audience at a conference he attended a day after Jackiws death. Roman was a pioneer in all these subjects, and advanced them greatly, before they became so popular.

He is renowned for his many fundamental contributions and discoveries in quantum and classical field theories. Among his major achievements is the establishment of the presence of the famous AdlerBellJackiw anomalies in quantum field theory, a discovery with far-reaching implications for the structure of the Standard Model of particle physics and all attempts to go beyond it.

Jackiw shared the Dirac Medal with Stephen Adler of Princeton University for their celebrated triangle anomaly, one of the most profound examples of the relevance of quantum field theory to the real world, says the citation from the International Centre for Theoretical Physics. Jackiw made a major contribution to field theories relevant to condensed matter physics in his discovery (with Boston Universitys Claudio Rebbi) of fractional charge and spin in these theories. They received the medal in 1998 from the International Center for Theoretical Physics in Italy.

Roman's style was rigorous and mathematically sophisticated, but not pedantic, says Robert L. Jaffe, the Otto (1939) and Jane Morningstar Professor of Science, Post-Tenure. After his early groundbreaking work on the triangle anomaly, Roman for many years focused on the application of topological methods in quantum field theory. Although Jackiw was not directly involved in the creation of the Standard Model, which revolutionized physics in the last third of the 20th century, the methods of analysis that Roman invented were often essential to its development.

Bolek Wyslouch, professor of physics and director of MITs Laboratory for Nuclear Science, calls Jackiw a towering figure in theoretical physics one of the leaders that made MIT and the Center for Theoretical Physics world's first His foundational work was instrumental in establishing the Standard Model of particle physics, one of the most successful theories in physics.

Ukrainian roots

Born Roman Volodymyr Yatskiv in Lubliniec, Poland, to a Ukrainian family in 1939, his name was Romanized to Jackiw.

We stayed in Poland until it became clear that the Russians and the Communists would be the dominant force there, and my father didn't want to live under those conditions, recalled Jackiw in an oral history published by the American Institute of Physics. They went to live near his fathers other children, in Austria, and eventually moved to Germany before settling in New York City when Jackiw was about 10.

I was heartbroken to be leaving (Germany), said Jackiw. Its a town called Dingolfing, probably known these days to car buffs because BMW started in Dingolfing, or had one of its original factories in Dingolfing.

In New York, he was educated by Xaverian monks in junior high, and Christian brothers in high school. I became convinced I wanted to be a physicist after reading [George] Gamows One Two Three Infinity, recalled Jackiw. He describes people doing things that sounded fascinating to me and I wanted to do them. It was actually an act of faith because I didn't get to do them until graduate school.

After graduating from Swarthmore College in 1961, where he majored in physics with minors in history of science and mathematics, he went to Cornell University, where he worked with professors Hans Bethe and Kenneth Wilson and received his PhD in 1966. Jackiw recalled working on a thesis that went against Wilsons advice.

He wanted me to use the renormalization group to find the high-energy behavior of form factors in electrodynamics. It turns out that the renormalization group doesn't control that, but other approximations can be used to solve that problem, and I did. My thesis was published and its still referred to.

He had wanted to work with Bethe, but Bethe was doing nuclear physics while Jackiw was more interested in particle physics. However, Bethe asked him to co-author a textbook on quantum mechanics: Intermediate Quantum Mechanics. The popular book, most recently revised in 2018, was for many years the basic introduction to the application of quantum mechanics to atomic physics.

From 1966 to 1969, he was a junior fellow at Harvard University. In his second year he went to CERN, working with John Bell. I discussed current algebra a lot with him, Jackiw recalled, and then we fell into the problem of the decay of the neutral pion into two photons, which was a puzzle at that time, and we studied the properties of the axial vector current and discovered the axial vector current anomaly, and wrote a paper, which is my most cited paper and also John Bells most-cited Particle Physics paper, in fact.

At the time, theory seemed to predict that the neutral pion could not decay into two photons, but the decay had been observed in experiments. With the BellJackiwAdler anomaly, clarified later by Stephen Adler, they were able to explain the observed decays theoretically by adding an anomalous term resulting from the divergences of quantum field theory, according to an article in Physics World.

In his final year at Harvard, Jackiw had been working with other theorists at MIT. Physics professors Steven Weinberg and Sergio Fubini, together with physics department head Victor Weisskopf, helped to initiate Jackiws long career as a professor at the Institute, which began in 1969. In his first years at MIT, Jackiw and David Gross showed that cancellation of gauge anomalies implied an interesting connection between fermions in the Standard Model in particular, that fermions in two classes, those which are strongly interacting and those which are not, have to appear the same number of times. Over the years this cancellation continued to suggest the existence of new fermions before they were observed.

Jackiw held visiting professorships at Rockefeller University in 1977-78, at the University of California Los Angeles and the University of California at Santa Barbara in 1980, and at Columbia University in 1989-90. He became an emeritus professor in 2013.

An unusual kind of greatness

Jackiw had said he had two bodies of work. The first were mathematical investigations which fit Diracs criterion of beauty and have physical application because they are beautiful, like fractional charge phenomenon that I mentioned earlier, and like the anomaly phenomenon, like the Chern-Simons terms which I introduced with the help of [Stanley] Deser and students and later explored with So-Young Pi. Pi, currently a Boston University physics professor emerita, is a distinguished physicist who was a co-author on many of Jackiws papers, and is Jackiws widow.

But on the other hand, Ive also done kind of methodological investigations, which werent necessarily original but applied existing schemes to new context. Like for example, figuring out how to do quantum field theory at finite temperature and relativistic quantum field theory at finite temperature, taking over what they do in condensed matter physics and non-relativistic quantum field theory approach to condensed matter physics at finite temperature.

Jackiw was known for working on mathematically intricate physics without an application in mind. What Ive always liked is to do work which seems obscure but interesting, and then decades later it catches on, he said.

Roman Jackiw was a giant of theoretical physics, but of a somewhat unusual kind, recalls Daniel Harlow, the Jerrold R. Zacharias Career Development Associate Professor of Physics at the Center for Theoretical Physics. He was rarely working on the same thing as others, and indeed if something he was doing started catching on then he would often turn to something else. And yet his ideas had a way of growing up: He would leave them lying around, and then a decade or two later everyone else would realize that he had really been on to something.

For example, Harlow once asked him why he had been studying gravity in two spacetime dimensions. His response: Well, everyone else was thinking about gravity in more than four dimensions, so I figured I'd see what happens in fewer than four."

His work on low-dimensional gravity from the 1980s has really taken off in the last five years, says Harlow. His influence will be felt both here at MIT and around the world for generations.

David Kaiser, a physics professor and the Germeshausen Professor of the History of Science, says that, while working with a CTP doctoral candidate, It seems like every other day we discover that Roman had first published on this-or-that piece of what we are trying to figure out, many years ago, in greater generality and with far more elegance than we had ever aspired to. He and his work remain a major inspiration for us.

Indeed, besides Jackiws celebrated work on anomalies, other important examples of his contributions include providing the first example of charge and spin fractionalization with solitons, elucidating the periodic vacuum structure of the non-abelian gauge theories that form the core of the Standard Model of particle physics, launching the use of quantum field theory for the rigorous study of systems at finite temperature, and determining the nature of Chern-Simons terms for both gauge and gravitational theories.

This broad range of research influenced countless others. To get an appreciation of Romans impact on theoretical and mathematical physics, one need only look at how often people refer to him by name in their papers, with examples including Adler-Bell-Jackiw anomalies, Jackiw-Teitelboim gravity, Fadeev-Jackiw quantization, the Jackiw-Nohl-Ressen ansatz, and the Jackiw-Rossi, Jackiw-Rebbi, and Jackiw-Pi models, says Stewart.

Roman had over 30 PhD students, including Estia Eichten (Cornell), Joseph Lykken (Fermilab), and Andrew Strominger (Harvard); he was a very successful mentor to generations of PhD students who formed a school of theoretical physics focused on the use of sophisticated mathematical methods to explore the physical content of quantum field theories, recalls Jaffe.

Other awards and honors

From 1969 to 1971, Jackiw was honored as an Alfred P. Sloan Research Fellow, and from 1977 to 1978 as a John Simon Guggenheim Memorial Fellow. In 1995 Jackiw received the Dannie Heineman Prize for Mathematical Physics from the American Physical Society for his imaginative use of quantum field theory to throw light on physical problems, including his work on topological solitons, field theory at high temperatures, the existence of anomalies, and the role of these anomalies in particle physics. In 2007 he received the Bonnor Essay Prize from Queen Mary University of London.

He was a member of the American Academy of Arts and Sciences, the American Physical Society, and the National Academy of Sciences, and a foreign member of the Ukrainian National Academy of Sciences. Honorary doctorates were also awarded by Turin University, Italy; Uppsala University, Sweden; the Kyiv Bogolyubov Institute, Ukraine; and Montral University, Canada.

Professor Jackiw wrote six other books: Lectures on Current Algebra and its Applications (with S. Treiman and D. Gross); Dynamical Gauge Symmetry Breaking (with E. Farhi) 1982; Shelter Island II (with N. Khuri, S. Weinberg and E. Witten) 1985; Current Algebra and Anomalies (With S. Treiman. B. Zumino and E. Witten) 1985; Diverse Topics in Theoretical and Mathematical Physics, 1995; and Lectures on Fluid Dynamics, 2002.

I have immense respect for his legacy and achievement, and greatly appreciate the doors he has opened for the rest of us, says Stewart.

He is survived by his wife, So-Young Pi, and three children: Stefan Jackiw, a violinist; Nicholas Jackiw, a software designer; and Simone Ahlborn, an educator at Moses Brown School in Providence, Rhode Island. Funeral services will be private.

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When I was in my mid-30s, I was faced with a difficult decision. It had repercussions for years, and at times the choice I made filled me with regret. I had two job offers. One was to work at a very large physics experiment on the West Coast of the United States called the National Ignition Facility (NIF). Last year, they achieved a nuclear fusion breakthrough. The other offer was to take a job at a university research institute. I agonised over the choice for weeks. There were pros and cons in both directions. I reached out to a mentor from graduate school, a physicist I respected, and asked him to help me choose. He told me to take the university job, and so I did.

In the years to come, whenever my work seemed dull and uninspiring, or the vagaries of funding forced me down an unwelcome path, or worse the NIF was in the news, my mind would turn back to that moment and ask: What if? Imagine if I were at that other job in that other state thousands of miles away. Imagine a different life that I would never live.

Then again, perhaps I had dodged a bullet, who knows?

Every life contains pain. Even the perfect life, the life where you have everything you want, hides its own unique struggles. Writing in The Genealogy of Morals (1887), Friedrich Nietzsche said: Man, the bravest animal and most prone to suffer, does not deny suffering as such: he wills it, he even seeks it out, provided he is shown a meaning for it, a purpose of suffering. A life apparently perfect but devoid of meaning, no matter how comfortable, is a kind of hell.

In our search for meaning, we fantasise about the roads not taken, and these alternative lives take on a reality of their own, and, perhaps, they are real. In his novel The Midnight Library (2020), Matt Haig explores this concept. In it, a woman named Nora Seed is given the chance to live the lives she would have lived had she made different choices. Each life is a book in an infinite library. Opening the book takes her to live in that other world for as long as she feels comfortable there. Each possible world becomes a reality.

For centuries, philosophers have dreamed of possible worlds. But only with the advent of quantum physics and the need to interpret its counterintuitive predictions did it appear that these possibilities might be real. Introduced in the 1950s by a graduate student, Hugh Everett, to little fanfare, and promoted in the 1970s by the physicist Bryce DeWitt, the many-worlds interpretation of physics has captured the public imagination and flowered a burst of art and culture. Born out of a need to interpret the behaviour of the smallest building blocks of our Universe, quantum physics has powered a cultural conversation from the depths of academic philosophy and science, to the pinnacle of Hollywoods elite.

The modern concept of possible worlds is attributed to the German polymath, co-inventor of calculus, and rival to Isaac Newton, Gottfried Wilhelm Leibniz, in his work Theodicy: Essays on the Goodness of God, the Freedom of Man, and the Origin of Evil (1710). The phrase best of all possible worlds comes from this work and refers to Leibnizs attempt to solve the problem of evil by proposing that ours is the best possible world. In other words, any other possible world would contain more evil.

Could Socrates have been an alligator? Yes. His being a human is not necessary but contingent

Leibniz drew on the work of the 16th-century Spanish Jesuit priest Luis de Molina, who posited that God contains middle knowledge, the knowledge of what a person would do if placed in a given situation. In any given possible world, a persons actions are fixed but, from one world to another, they may act differently because of changes in their life circumstances. Hence, God gives us a kind of free will, which is essential to holding us responsible for our actions but, by his middle knowledge, places us in the best possible world for the greatest number of people; in this world, our choices are predetermined. Molinas theology proposes that even God requires some people to damn themselves to save others.

The contemporary American analytic philosopher Alvin Plantinga drew on Leibnizs theological ideas to produce his seminal work on possible worlds, The Nature of Necessity (1974). As in Haigs novel, Plantinga conceives of a library of books, each corresponding to a possible world. There, he defines a book on a world as everything that is true, including everything necessary (meaning true across all worlds) and everything that is contingent (meaning true only in some worlds). Each world has one, and only one, book of true things.

Plantinga illustrates the difference between necessary and contingent truths in this way: Could Socrates have been an alligator? Yes. There may be a possible world where Socrates wakes up, as in Franz Kafkas novella The Metamorphosis (1915), to find his body to be that of an alligator. Thus, Socrates being a human being is not necessary but contingent. It is not true in every book in the library. On the other hand, mathematical implications like 1 + 1 = 2 and logical proofs are true in all worlds. They are necessary.

Despite considering many possible worlds, like Leibniz and Molina, Plantinga asserts that there is only one real world. For him, alternative worlds are useful for philosophers to think about but do not actually exist.

The many-worlds interpretation (MWI) of quantum physics, on the other hand, says that all possible worlds exist, and the one we live in is no different from any of the others. According to one form of this belief, somewhere out there is an exact duplicate of you, your house, your family, but one small detail is different, perhaps something as tiny as a stray photon that went left instead of right, or maybe something big like you have a different significant other. Maybe a stray cosmic ray hit your DNA before you were born, and you have red hair instead of brown, or you developed a serious birth defect. Maybe you dont exist at all.

To the layperson, the idea of all these worlds existing out there might seem disturbing because it takes away from our own personal uniqueness. To philosophers like Plantinga, it is disturbing because it takes away from the uniqueness of truth.

A good example is Schrdingers cat. In this classic thought experiment, a cat is placed in a box and the lid closed. Say I also put in the box a semi-reflective mirror that has a 50 per cent chance of letting light through, and a 50 per cent chance of deflecting light. Behind the mirror is Detector D (for Death), which can detect even a single photon of light and, if it does, it sends a signal that opens the lid of a vial of poison, filling the box with poison gas and killing the cat. Next to the mirror is Detector L (for Life), not hooked up to any poison. An automatic emitter inside the box is programmed to fire a single photon at the mirror at a certain time. We dont know which detector it will hit because it is random. Once it does, we wait a minute to ensure that the poison has had its effect.

Both are still possible a single world containing two contradictory facts

If the box is completely sealed and impenetrable by anything external, we wont know what happened inside until we open it.

All this seems very ordinary until I take the quantum nature of light into account. A quantum particle, experimental science has shown, can be in two states at once until it is measured. Thus, when the photon is fired at the mirror, it does not go through or deflect. Rather, it enters a state where, having gone through and having been deflected are both still possible a single world containing two contradictory facts. These facts are, hypothetically, passed on to the cat, although nothing as large and complex as a warm-blooded animal could be put into such a state in practice.

We know this is true for particles because of what physicists call the double-slit experiment. In it, a single beam of light is sent through two slits in a barrier to a screen on the other side. Even though the light originates as a straight beam, after it passes through the two slits, it emerges as two interfering waves hitting the screen together. This looks like alternating bars of light and dark.

We want to know if light is made of particles or a continuous wave. To do so, we fire the smallest amount of light we can, which are little packets called photons, at the double slit. We hypothesise that if these appear at individual points, then photons are particles; but if they appear spread across the screen, then photons are waves. We begin the experiment and see immediately that the photons appear at individual points on the screen: score 1 for particle hypothesis. If we continue firing photons, however, we find that the dots appear in the same alternating light and dark bars as if the photons were interfering with each other. Score 1 for the wave hypothesis.

The reason this happens is because, when the photon goes through the barrier, it enters what physicists term a superposition where it has, in a sense, passed through both slits at the same time, like a wave, but arrived at one point on the panel, like a particle. This is called wave-particle duality.

In standard interpretations of quantum physics, we do not say that the photon has passed through both slits at the same time; rather, we say that its wavefunction a kind of probability field has passed through both slits at the same time. That wavefunction then collapses or vanishes, leaving the one photon on the panel. This resolves the contradiction neatly because we can assert that the photon entered the left slit and the photon entered the right slit are never simultaneously true. Rather, we say the wavefunction passed through the slits and collapsed into the photons position on the screen.

According to the MWI of quantum physics, however, the entire wavefunction is a spectrum of alternative realities coexisting. These worlds are all connected and the photons in them interact weakly before they are measured but the very act of measurement causes them to either split apart or appear to do so. When that split happens, copies of you and the rest of the Universe split apart as well.

The MWI is controversial and is itself subject to interpretation depending on whether you believe there is a quantum mechanism for world splitting, or if it is simply how human beings experience quantum phenomena.

Real or not, possible worlds explain strange quantum paradoxes. For example, in the double-slit experiment, if I place a detector in front of each slit, it will detect only a single photon going through one or the other. Never both. If I take the detectors away, I get the interference pattern as if the photon went through both slits. This creates a paradox. Why can it be one way when I measure, and another when I dont?

This doesnt happen in classical physics. If I shoot an arrow at a bullseye, I can be absolutely certain that the arrow will follow a single trajectory from my bow to the target, whether I watch it fly or not. If I dont watch it but imagine a world where I did, that is called a counterfactual world. In classical physics, counterfactual worlds and real worlds are always the same but in quantum mechanics they are not. The world is really different if I look at a particle flying through space versus if I do not.

Physicists knew this to be true in the 1920s, but it took more than 60 years before anyone proposed a way to split the difference between looking and not looking. In 1988, the physicists Yakir Aharonov, David Albert and Lev Vaidman introduced such a method, called weak measurements. These measurements collect some information about particles and, over the course of many, many measurements, can give us statistical information that helps us understand what is going on inside a quantum superposition.

We are more like two-dimensional beings in a 3D world, perceiving only our little slice

Weak measurements let us detect traces of particles even when they are not present. If there is a trace of a particle, that means it had some measurement effect but was not necessarily there in any real sense. This is what researchers see during the double-slit experiment. A particle has a trace from both slits because of the pattern on the screen but has no presence in either. If a particle is present, that would be ascertained through a strong measurement where it is localised, literally appearing on a detector screen.

The MWI interprets trace and presence in a unique way. A trace is when particles in different worlds have not been measured strongly enough to stop interacting, so the worlds are not split. When the worlds cease interacting (split), then trace becomes presence.

Real-world studies of weak measurements have been designed with atoms, photons and other elements of the quantum world. For example, a lens can deflect photons in a laser slightly and cause them to interfere differently with another beam of photons than if the lens is not present. You can imagine, therefore, if you were to put lenses in front of the slits, they would have a measurable effect but, if the deflection is very slight, it would not be enough to collapse the wavefunction or split the worlds. Using that fact, you can construct experiments that allow you to see traces without presence.

Real experiments measure bizarre effects inside superpositions. For example, experiments with both photons and atoms have been done that show that sometimes a particle duplicates so that it can be in two places at once but each with 100 per cent probability, not the 50 per cent probability of the double slit. The particle will compensate by spawning a negative copy of itself, also with 100 per cent probability, somewhere else, so that the total still adds up to one.

These results are counterintuitive unless you believe the wavefunction is a real thing, in which case the particle is a wavefunction that has 100 per cent probability peaks in two spots and a (-100 per cent) trough in another.

For this reason, some flavours of the MWI, such as Vaidmans, maintain the primacy of the wavefunction over the concept of having multiple copies of the world that split. In other words, the multiverse isnt many worlds but one world, and we are more like two-dimensional beings in a 3D world, perceiving only our little slice. Worlds are like pieces in a jigsaw puzzle, fitting together in a commonsense way when together, but defying intuition when left apart.

This suggests that our lives too might be a jigsaw puzzle. Perhaps they make sense only when we look at them across a multiverse of possible lives and, if we could only talk to those other copies of ourselves, we could understand our experiences. Consider that, when we imagine ourselves in other possible worlds, we dont just want to know how our alternative selves are getting along. We want to know what they would think of us, what it would be like to speak to them, and we want to know what it might be like to live in those other worlds that those other selves inhabit. More than that, we want to resolve the uncertainty we have in our own past decisions by asking them: How did it work out? The only way to do that is to uncover the looking glass and glance through.

One means of connecting with our alternative selves is through literature, film and the arts. The MWI first appeared in Michael Moorcocks novella The Sundered Worlds (1962), a space opera that ranges across a vast multiverse. In this Star Wars-like action novel, the hero Renark von Bek undertakes to save the multiverse from Armageddon. This novel also hosted some of the earliest uses of virtual reality, computer tablets, digital displays and, of course, quantum physics, and it also launched Moorcocks long career.

Since then, numerous novels, movies and TV shows have made use of the concept, including childrens fiction. The first book about a parallel universe that I recall reading was the childrens book The Double Disappearance of Walter Fozbek (1980) by Steve Senn, about a boy who somehow swaps places with his dinosaur counterpart in a world where people are all dinosaurs. As a child, I was blown away by this idea of parallel worlds, and that remained my favourite book for many years.

A rupture opens a doorway, a necessary trope for reaching our parallel selves

The idea has captured the movies, too. Among the many multiverse films are those in the Back to the Future trilogy (1985-90), about what happens when we go back in time, change the past, and find the future is another world entirely. Theres also Spider-Man: Into the Spider-Verse (2018), a computer-animated smash hit about a high-school student, Miles Morales, who becomes a Black Spider-Man in his own universe and teams up with Spider-people (men, women, and even Spider-Ham, a pig) from other universes to defeat his nemesis Kingpin. Also, Doctor Strange in the Multiverse of Madness (2022), a Marvel Universe battle between good and evil in parallel worlds; and the Academy Awards Best Picture winner, Everything Everywhere All at Once (2022), about a heroine who learns that she can draw skills and powers from her alternative selves to battle villains who threaten the world.

In each work, a rupture opens a doorway, a necessary trope for reaching our parallel selves. Yet the MWI actually tells us that worlds are generally unreachable. The work on weak measurements means that worlds can diverge without completely disconnecting. A better device might be a hidden passage that already exists, more like the wardrobe portal in C S Lewiss Chronicles of Narnia series (1950-56) than a dangerous rip in space and time. I have yet to read a story where the plot revolved around keeping worlds from separating rather than worlds accidentally and catastrophically merging, but that might be more realistic.

In some cases, the literary purpose of the multiverse is not so much to connect parallel worlds as to tell different stories with the same characters. Star Trek, for example, depended on the multiverse for its James T Kirk reboot movies (2009-16), allowing the director J J Abrams to skirt around canon and change details to reimagine the young Kirk and his adventures on the USS Enterprise.

Using the multiverse to reboot Spider-Man in the movie Spider-Man: No Way Home (2021), MWI explains how the different actors Tobey Maguire, Andrew Garfield and Tom Holland who have played Spider-Man over the years might all exist simultaneously in different universes, and how they might meet up to fight as a team. The multiverse is not only a fun way to have all three actors appear in the movie but also a means of exploring how their characters differ and what they thought of the choices they made and the challenges they each faced, both similar and unique.

The multiverse has also opened up new ways of looking at the human condition. One of the most fascinating areas where culture, philosophy and possible worlds collide is in the work of Robert Lanza on biocentrism, which is a philosophical approach to physics through the lens of living beings. Lanza, a professional biologist, proposes that the Universe arises directly from an individuals conscious observation of it. He hypothesises that, for this reason, a conscious being cannot cease to be conscious. This leads to the potential fact that it is impossible to be dead. Instead, ones consciousness simply splits off, by quantum processes, into worlds where that consciousness can continue to exist. Every wavefunction collapse or world splitting leaves us in a world where we remain alive.

Another novel, The Doors of Eden (2020) by Adrian Tchaikovsky, explores parallel worlds through the phenomenon of branching evolution. For each parallel Earth in the story, a different species dominates, having continued on, rather than suffering extinction. For instance, the author imagines what a society of trilobites might look like. As in many multiverse stories, reality collapses and the different worlds bleed into one another. The book contains many detailed and imaginative scenarios about speculative evolution, and, from an MWI perspective, it is perfectly reasonable to imagine many different potential evolutionary outcomes, since evolution is highly dependent on randomness, including quantum variations in cosmic rays striking DNA.

Even the art world has taken notice of the multiverse. In response to the COVID-19 pandemic, the Burning Man in the Multiverse experience in 2020 showcased the multiverse with immersive visual styles in a virtual event. In this project, eight teams developed different virtual universes, with a unique Burning Man in each. You could traverse the Burning Man Playa the dry lake bed where it normally takes place at Black Rock City in virtual reality as an avatar, explore art and sculpture created within a virtual world, and imagine the parallel realities of the annual festival itself.

What greater despair than to believe you are living the wrong life?

The most powerful reason why the multiverse has infiltrated culture is because people are storytellers. Research shows that this tendency is universal and appears in early childhood. It is written in our DNA. Implicit in storytelling is the modification of details such that one possible world becomes another. Such narratives are essential to how our species has understood the world for millennia. Meta-stories containing conflicting possible worlds simultaneously become not only plausible but essential to how we interpret our perceptions: personal, nonlinear and qualitative, rather than objective, linear and quantitative.

The human mind even creates its own multiverses through dreams, where alternative realities appear. Who hasnt dreamed of a loved one acting in ways they never would, or living in a house that theyve never seen before? Fundamentally, the human mind has evolved to imagine multiple possible futures branching out from the present. Whether this is actually the case is an open question that physics still must resolve, if it ever can.

While the many-worlds interpretation has at times been overused, the pervasiveness of the multiverse in culture is a shift with benefits. There is more than one way to see the world, and every conscious mind may create its own version of reality. In a world awash with data, hard facts have become difficult to come by, and everyone needs to have their minds open to the possibilities that what they believe or have been told is only one of many possible worlds.

On the other hand, when we start longing to live in one of those alternative realities, it can make us desperately unhappy. This is the curse of imagining all these branching pathways in our lives. As the American novelist James Branch Cabell wrote in The Silver Stallion (1926): The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true. What greater despair than to believe you are living the wrong life? Yet, how can we claim a life is wrong? A life full of suffering is not a meaningless one as Nietzsche points out.

As Nora understands at the end of Haigs The Midnight Library:

This Essay was made possible through the support of a grant to Aeon+Psyche from the John Templeton Foundation. The opinions expressed in this publication are those of the author and do not necessarily reflect the views of the Foundation. Funders to Aeon+Psyche are not involved in editorial decision-making.

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Multiple worlds has been given artistic impetus by physics - Aeon

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