Let’s say that you’ve devoted your entire adult life to developing a carbon-free way to power a household for a year on the fuel of a single glass of water, and that you’ve had moments, even years, when you were pretty sure you would succeed. Let’s say also that you’re not crazy. This is a reasonable description of many of the physicists working in the field of nuclear fusion. In order to reach this goal, they had to find a way to heat matter to temperatures hotter than the center of the sun, so hot that atoms essentially melt into a cloud of charged particles known as plasma; they did that. They had to conceive of and build containers that could hold those plasmas; they did that, too, by making “bottles” out of strong magnetic fields. When those magnetic bottles leaked—because, as one scientist explained, trying to contain plasma in a magnetic bottle is like trying to wrap a jelly in twine—they had to devise further ingenious solutions, and, again and again, they did. Over decades, in the pursuit of nuclear fusion, scientists and engineers built giant metal doughnuts and Gehryesque twisted coils, they “pinched” plasmas with lasers, and they constructed fusion devices in garages. For thirty-six years, they have been planning and building an experimental fusion device in Provence. And yet commercially viable nuclear-fusion energy has always remained just a bit farther on. As the White Queen, in “Through the Looking Glass,” said to Alice, it is never jam today, it is always jam tomorrow.

The accelerating climate crisis makes fusion’s elusiveness more than cutely maddening. Solar energy gets more efficient and affordable each year, but it’s not continuously available, and it still relies on gas power plants for distribution. The same is true for wind power. Conventional nuclear power has extremely well-known disadvantages. Carbon capture, which is like a toothbrush for the sky, is compelling, but after you capture a teraton or two of carbon there’s nowhere to put it. All these tools figure extensively in decarbonization plans laid out by groups like the Intergovernmental Panel on Climate Change, but, according to those plans, even when combined with one another the tools are insufficient. Fusion remains the great clean-energy dream—or, depending on whom you ask, pipe dream.

Fusion, theoretically, has no scarcity issues; our planet has enough of fusion’s primary fuels, heavy hydrogen and lithium, which are found in seawater, to last thirty million years. Fusion requires no major advances in batteries, it would be available on demand, it wouldn’t cause the next Fukushima, and it wouldn’t be too pricey—if only we could figure out all the “details.” (A joke I heard is that fusion operates according to the law of the “conservation of difficulty”: when one problem is solved, a new one of equal difficulty emerges to take its place.) The details are tremendously complex, and the people who work to figure them out have for years been dealing with their own scarcities—scarcities of funding and scarcities of faith. Fusion, as of now, has no place in the Green New Deal.

In 1976, the U.S. Energy Research and Development Administration published a study predicting how quickly nuclear fusion could become a reality, depending on how much money was invested in the field. For around nine billion a year in today’s dollars—described as the “Maximum Effective Effort”—it projected reaching fusion energy by 1990. The scale descended to about a billion dollars a year, which the study projected would lead to “Fusion Never.” “And that’s about what’s been spent,” the British physicist Steven Cowley told me. “Pretty close to the maximum amount you could spend in order to never get there.”

“To be honest, I was feeling pretty despondent,” Dennis Whyte, the fifty-seven-year-old director of the Plasma Science and Fusion Center, at M.I.T., said. “And I was seeing that despondency in the faces of my students, too.” It was 2013, and M.I.T.’s experimental fusion device had lost its Department of Energy funding, for no clearly stated reason. The field of nuclear fusion, as a whole, was still moving forward, but agonizingly slowly. iter, an enormous fusion device being built in southern France, in an international collaboration, was progressing—the schedule is for ITER to demonstrate net fusion energy in 2035, and the majority of plasma physicists have high confidence that it will work—but Whyte knew that it wasn’t going to deliver affordable energy to the public in his lifetime, and maybe not in his students’ lifetimes, either. “ITER is scientifically interesting. But it’s not economically interesting,” Whyte said. “I almost retired.”

Whyte is a gentle giant from Saskatchewan, Canada. “If you’ve ever been to the middle of nowhere, that’s where I grew up,” he told me. His family were farmers and electricians. By the time he was in the fifth grade, he knew he wanted to be a scientist, and in the eleventh grade he wrote a term paper on that wild idea which often appeared in science fiction—near-boundless energy generated by the fusing of two atoms, as happens in stars. “I remember getting that paper back, and my teacher saying, ‘Great job, but it’s too complicated.’ ” Whyte went on to major in engineering and physics at the University of Saskatchewan; for his Ph.D., he attended a new plasma-physics program at the University of Quebec, where he worked in a government-funded fusion lab. “I thought, Great: I’ll learn French and get to work on a tokamak,” he said, referring to the large doughnut-shaped machine whose design is commonly used for fusion devices. Later, Whyte took a job at a lab in San Diego. He intended to return home eventually, but in 1997 Canada cancelled its fusion program. “I was stranded in the U.S.,” he said.

At M.I.T., Whyte teaches an engineering-design class for graduate students which he organizes each year around a different practical problem in fusion. “I’ve always wanted to expose my students not only to the science questions but also to the technology questions,” he said. In 2008, he asked his students to design a device that would pump helium but not hydrogen—in most approaches to fusion, hydrogen is the fuel, and helium is, in effect, the ash. “Helium is one of the hardest things to pump in the periodic table, because it’s so inert,” Whyte said. The class came up with several very clever ideas. None of them was successful. “We’re still working on that one,” he said.

The next year, something happened that Whyte credits with restoring his interest in fusion. “I had passed my colleague Leslie in the hall, and he was holding a bundle of what looked like the spoolings of a cassette tape,” he said. It was a relatively new material: ribbons of high-temperature superconductor. Superconductors are materials that offer little to no resistance to the flow of electricity; for this reason, they make ideally efficient electromagnets, and magnets are the key component in tokamaks. A high-temperature superconductor—well, it opened up new possibilities, in the way that the vulcanization of rubber opened up possibilities in the mid-nineteenth century. The superconductor material that Whyte’s colleague was holding could in theory make a much more effective magnet than had ever existed, resulting in a significantly smaller and cheaper fusion device. “Every time you double a magnetic field, the volume of the plasma required to produce the same amount of power goes down by a factor of sixteen,” Whyte explained. Fusion happens when a contained plasma is heated to more than a hundred million degrees. Whyte asked his class to use this new material to design a compact fusion power plant of at least five hundred megawatts, enough to power a small city: “I was not sure what we would find with H.T.S., but I knew it would be innovative.”

The physicists Bob Mumgaard, Dan Brunner, and Zach Hartwig were in that class. The power plant that they came up with was in most respects familiar. At its center would be a doughnut-shaped tokamak, not unlike the type that Whyte had worked with as a graduate student. They named their design Vulcan. In the next iteration of the class, those ideas evolved into a design called ARC, for “affordable, robust, and compact.” (This also happens to be the name of the personal fusion device of the billionaire industrialist Tony Stark, in the “Iron Man” movies.) ARC would use an ordinary salt to translate its heat onto an electrical grid. It would be modular, for easy maintenance. It would not be able to recycle its own fuel. It was a “good enough” machine. But the use of H.T.S. magnets made it about the size of a conventional power plant—a tenth the size of ITER.

Physicists from both classes later formed a group that modified the arc design. The new model was two-thirds the size and intended to be ready as soon as possible—SPARC. SPARC would be the prototype that demonstrated the concept; ARC would be a long-lasting power plant capable of delivering affordable energy to the grid.

There were real reasons for skepticism. H.T.S. is fragile—it remained to be seen if it could even be made into a hardy magnet, and, if it could, how well that magnet would endure bombardment by charged particles. Plus, H.T.S. was not yet commercially available at sufficient scale and performance. “But those were engineering barriers, not scientific barriers,” Whyte said. “That class really changed my mind about where we were in fusion.”

Fusion scientists often speak of waiting for a “Kitty Hawk moment,” though they argue about what would constitute one. Only in retrospect do we view the Wright brothers’ Flyer as the essential breakthrough in manned flight. Hot-air balloons had already achieved flight, of a kind; gliders were around, too, though they couldn’t take off or land without a catapult or a leap. One of the Wright brothers’ first manned flights lasted less than a minute—was that flight? An A.P. reporter said, of that event, “Fifty-seven seconds, hey? If it had been fifty-seven minutes, then it might have been a news item.”

Our sun is a fusion engine. So are all the stars.

But we discovered that fusion powered the stars only about a hundred years ago, when the British physicist Arthur Eddington put together two pieces of knowledge into what was seen at the time as a wild surmise. The facts he combined were that the sun is made up mostly of hydrogen, with some helium, and that E=mc².

Eddington noticed that four hydrogen atoms weigh a tiny bit more than one helium atom. If four hydrogen nuclei somehow fuse together, in a series of steps, and form helium, then a little bit of mass must be “lost” in the process. And if one takes seriously that most famous of equations, then that little bit of mass becomes a lot of energy—as much energy as that amount of mass multiplied by the speed of light, squared. To give a sense of this ratio: If you converted a baseball into pure energy, you could power New York City for about two weeks. Maybe that process—hydrogen crashing into hydrogen and forming helium, giving off an extraordinary amount of energy in the process—was how the sun and all the stars burned so bright and so long. Eddington, in a paper laying out this theory, closed with an unusual take on the story of Daedalus and his son Icarus. Eddington argued in defense of Icarus, saying it was better to fly too high, and in doing so see where a scientific idea begins to fail, than it was to be cautious and not try to fly high at all.

When most people think of nuclear energy, they are thinking not of fusion but of fission. Fission is when an atom—most commonly uranium or plutonium—breaks in two. Fission generates waste that remains radioactive for tens of thousands of years; in contrast, the little bit of waste that fusion generates remains radioactive for only a few decades. Fission is pretty powerful, as evidenced by atomic bombs; fusion is much, much more powerful. (In 1952, a fusion bomb, known as the H-bomb, was tested, though it has never been used in warfare; it worked by using a fission bomb to set off a giant uncontrolled fusion reaction. One of the fathers of the H-bomb, Edward Teller, an aggrieved Shakespearean villain in most tellings, had other incautious ideas, such as using fusion bombs to dig canals or make diamonds.) The process of fusion sounds dangerous to a layperson—a sun in a magnetic bottle?—but it is easier to extinguish than a match.

The allure of fusion has attracted brilliant, imaginative minds; it has also attracted a crowd of shysters, cranks, and false messiahs. In 1951, Juan Perón, Argentina’s President, announced that the country had harnessed fusion energy. It would soon be available in litre and half-litre bottles, like milk. Perón had made the mistake of distrusting his own country’s scientific community, instead putting his faith in Ronald Richter, an Austrian immigrant whose apparatus, when inspected by scientists, didn’t even have a functioning Geiger counter, the device he was using to claim evidence of fusion radiation.

A few decades later, two respected chemists at the University of Utah, Stanley Pons and Martin Fleischmann, convinced the public that they had produced nuclear fusion at room temperature, in what looked like a jar with a little mixer stick in it. They announced their results in a press conference before they published their data or methods. Pons and Fleischmann were featured on the cover of Time. Meanwhile, the work of Steven Jones, a respected physicist at Brigham Young University, was also receiving press attention; he, too, was working on producing fusion at a low temperature, and, though he seemed to be on a promising path, he was ultimately unsuccessful. When Pons and Fleischmann finally published a paper, they were suspected of having fudged their data. No one was able to reliably reproduce their results. Jones later turned to proving that Jesus had visited Mesoamerica, and after that to explaining that the destruction of the World Trade Center was an inside job. Zach Hartwig, now a professor of nuclear science and engineering at M.I.T. and part of the ARC/SPARC team, has said, “The biggest problem in fusion is perception. It’s the perception that fusion is a joke.”

Estimates of the cost of the Manhattan Project, which produced atomic weapons in four years, vary, but it is commonly said that the scientists were given a “blank check.” This year, the U.S. government will spend some six hundred and seventy million dollars on nuclear fusion. That’s a lot of money, but six hundred and fifty billion—the amount the I.M.F. estimates that U.S. taxpayers spent on fossil-fuel subsidies last year—is quite a bit more.

During the oil crisis of the nineteen-seventies, fusion research briefly received the sort of funding that goes to national-defense projects. M.I.T.’s Plasma Fusion Center was established in 1976. The Joint European Torus, at the Culham Center for Fusion Energy, in the United Kingdom, which has heated hydrogen to temperatures hotter than the inside of the sun, began operating in 1983, and by 1997 had set important records, some still not surpassed. “It was such an exciting time,” Michael Mauel, a professor of applied physics at Columbia University, who did his undergraduate and graduate work in fusion at M.I.T., said. “And we were sure we were going to be the ones to solve it all.”

Steven Cowley, the former head of the U.K. Atomic Energy Authority, who now heads the Princeton Plasma Physics Laboratory, recalled his days as a graduate student at Princeton, in the nineteen-eighties. “Fusion was all we thought about, from the time we woke up in the morning to the last beer in the basement of the graduate college,” he said. “I remember when we got to ten million watts of fusion power on T.F.T.R.”—Princeton’s fusion device. “I still have a photo of that moment outside my office.” It was a tremendous milestone, but it also, basically, created enough energy to light up a single bulb for a day. More needed to be done.

But, by the nineties, oil was cheap again. Fusion research funding declined. “We had learned to extract oil and gas from all kinds of places,” Cowley said. “Now we have to learn how to leave it in the ground in order to survive, to save civilization. It’s that simple.”

Bob Mumgaard, a thirty-seven-year-old plasma physicist from Omaha, gets animated when talking about the laying of the transatlantic telegraph cable, in 1858, or the founding of Genentech, in 1976. He studied engineering at the University of Nebraska, though his first love was physics, a field he saw as compelling but impractical. “A lot of the engineers who came out of my school took jobs designing tractors,” he said. In 2008, Mumgaard was working in a lab studying computer hard drives when the MacBook Air came out, with its solid-state hard drive: “I said to myself, ‘O.K., normal hard drives are dead now. I need to go and do something else.’ ”

He applied to graduate programs in physics. He was accepted at Stanford, where he could investigate questions of cosmology and dark matter; he was also accepted to M.I.T.’s P.S.F.C., where he could work on nuclear fusion. The Midwestern pragmatist in him chose fusion over foundational questions about the universe, though he was not particularly motivated by the climate emergency. “Sometimes I think about the way we talked about climate back then, and I can’t believe we wasted so much time debating, like, whether or not Penn State had the best climate model,” he told me. By the time he was a student in Dennis Whyte’s design course, his perspective had changed—he saw fusion as something that needed to have happened yesterday.

He was also a student in a program with an iffy future. After M.I.T. was told that it would lose funding for its experimental fusion device, the P.S.F.C. negotiated an extension to 2016, but it was clear there would be no further reprieve. “We had this opportunity forced on us,” Mumgaard said. “We lost our funding just at the moment that we had this big shiny new lever, this new superconducting material that could move fusion forward.” By 2014, Mumgaard and his colleagues could write down their plans for ARC/SPARC in the form of a concrete risk retirement plan—a venture-capital term for tightly focussed research, with discrete benchmarks. “At M.I.T., venture capital is something you learn about at the university bar,” Mumgaard said. As they saw it, the biggest risk to retire would be making an H.T.S. magnet for SPARC.

In 2015, the Institute of Electrical and Electronics Engineers Symposium on Fusion Engineering was held in Austin, Texas. Many key members of the plasma-physics community were there, and there were two especially noteworthy talks. The first was by the Austrian physicist Guenter Janeschitz, who not only sounds but also looks like Arnold Schwarzenegger. He gave a presentation on DEMO, a proposed fusion device that would be almost twice the size of ITER and produce five gigawatts of power. Janeschitz envisions that, if funded, a prototype could be built in twenty years. Demo is widely seen to be a clear-eyed, workable plan, and a step on the path to bringing practical fusion energy to your great-grandchildren.

Dennis Whyte gave a presentation on ARC. He estimated that it could demonstrate net fusion energy in 2025 and bring fusion to the electric grid by 2030, with individual plants producing a gigawatt of power each—about what a conventional power plant provides today. DEMO would cost an initial thirty billion dollars; ARC would be a million-dollar machine. “It was very dramatic,” Mumgaard said. “The difference was so stark. The room was split.” Roughly speaking, the younger people were buzzing with hope; the older people had perhaps been hopeful one too many times.

The doubters weren’t simply killjoys—they were imaginative thinkers who had devoted decades of their lives to fusion research. It wouldn’t be easy to make H.T.S. into a magnet of sufficient size. And the powerful magnetic field created by H.T.S. was sure to have consequences, which hadn’t been fully studied. There was every reason in the history of experimental science to expect surprises. And funding for fusion projects was already tight; another idea might draw money away from projects that many scientists considered more promising. It was entirely reasonable to ask whether the members of the M.I.T. team were the Wright brothers or Samuel Pierpont Langley—the head of the Smithsonian who in 1903 crashed his very expensive Aerodrome into the Potomac, and then a couple of years later did it again.

After Whyte’s keynote, the M.I.T. crowd went out for lunch at Stubb’s Bar-B-Q. “It’s the kind of place with red-checked tablecloths and food that comes with a lot of napkins,” Whyte said. Everyone around the table knew that the primary funding for their work would end within a year. As Mumgaard recalls, “Basically, we all had pink slips, and yet we were still there. And the question was, Why? We had to learn to listen to ourselves. Did we really believe the field was where we were saying we thought it was?” Was H.T.S. really the shiny new lever that would move fusion dramatically forward? Whyte and his colleagues started to write on a napkin details of how they could make SPARC and then ARC a reality. They wrote down estimates of how much money it would cost to develop it. “It was like this collective dawning, that this thing was really possible,” he told me. Over ribs, they decided that they would fund their work with lottery tickets or with venture capital or with philanthropy—one way or another, they would make their good-enough fusion power plant real.

On September 30, 2016, M.I.T.’s old experimental fusion device, which had been running for twenty-five years, was obliged to shut down by midnight. “This device graduated more than a hundred and fifty Ph.D.s,” Whyte said wistfully. “It set records, even though it’s a hundred times smaller than ITER.” Although M.I.T. was never told why the device was shut down—the Department of Energy continued to fund two other tokamak projects in the U.S.—there was speculation that the reason was that it was the smallest. “Which is ironic, because smaller is where we’re trying to go,” Whyte said. The researchers ran experiments on the machine until the last permitted minute. At 10:30 P.M., they set a world record for temperature and pressure. At midnight, they shared champagne.

“I went home a little after midnight, but I couldn’t sleep,” Whyte said. In his home office, with his wife’s paintings of trees and flowers on the wall, he started going over the data from the final experiments: “I was just sort of plugging in what our results would mean in a machine with a higher magnetic field,” as would be produced with H.T.S. magnets. “It meant spARC could provide a hundred million watts.” This was even more than the team had speculated in Austin. Whyte was seeing fusion’s holy grail.

The M.I.T. team continued to dedicate its time to ARC/SPARC, quilting together fellowships and grants. At one point, to make payroll, technicians went into the basement and loaded trucks with scrap copper to sell. SPARC Underground was set up—a group of interested scientists who met regularly, to discuss plans and work through difficulties. They needed to buy as much H.T.S. as they could, in order to learn more about the material’s characteristics—hammer it, heat it, freeze it, send current through it. “I remember so well the first shipment of H.T.S.,” Mumgaard said. “We waited for months to get this reel of material. It was only five hundred metres. Now, if we’re not talking ten kilometres, we’re not talking anything. These days, you can order this stuff on Alibaba.com. But then—it was such a moment.”

The team had to solve engineering problems—it also had to solve business problems, including convincing suppliers that there was a market for the material, so that more would be made. “We met with them and asked them if they had considered fusion as a market,” Mumgaard told me. “They were, like, ‘No way, that’s not a real thing.’ ” After two years of extensive lab work and dreamy conversations over five-dollar pitchers of Miller High Life at the Muddy Charles Pub, SPARC Underground became Commonwealth Fusion Systems, a seven-person private fusion-energy company with an ongoing relationship with M.I.T. (C.F.S. funds research at M.I.T., which shares its intellectual resources and some lab space with C.F.S.; patents are filed jointly.) Some of C.F.S.’s funders are European energy companies, and some are philanthropists. By 2021, the company employed more than a hundred and fifty people, many of them veterans of SpaceX and Tesla.

“Energy is a market,” Mumgaard said. “If you knew there was a ten-trillion-dollar market out there—that is a pull. You couldn’t even have said there was a market that big for computers, or for social media. But you can say that about energy.”

The Plasma Science and Fusion Center, at the northwest corner of the M.I.T. campus, is only a few minutes’ walk from the Cambridge campuses of Pfizer and Moderna. In March, Whyte and Mumgaard met me at the front steps. Mumgaard is now the C.E.O. of C.F.S.; Whyte, a co-founder, remains at M.I.T. They wore T-shirts and had pandemic-untrimmed wavy hair, giving them the look of ambitious surfers. I was there to meet them, but also to meet their magnet, which was still under construction. Maybe it would work, or maybe it would send the team back to the planning stages for years. It was a warm and sunny day. If Kool-Aid had been on offer, I would have drunk not one glass but two.

Aristotle described magnetism as the workings of the soul inside a stone. Magnets have been used to navigate ships, to levitate high-speed trains, to image the inside of a human body, and to move iron filings to make a silly beard on a plastic-bubble-encased drawing of a face. In 1951, the physicist Lyman Spitzer suggested that a magnetic field could serve as a bottle in which to contain a plasma that re-created the pressure and the temperature inside a star. Magnets have been a centerpiece of fusion research ever since.

Mumgaard and Whyte gave me a tour of their lab spaces. The first stop was at what looked like a lectern, in a cubicled room. The room’s distant wall was the control board for M.I.T.’s first experimental fusion device, from the nineteen-eighties. The lectern featured pictures of common plasmas: the sun, lightning, the northern lights, magnetic fusion, and a neon sign reading “OPEN.” Mounted on the lectern was a hollow glass tube with copper wire coiled around it in two places. The wire was set up so that a current could be run through it, and the glass tube was suspended over a metal plate. You may remember a demonstration, from your high-school science class, of an electric current being run through coiled wires, generating an electromagnetic field—this was basically a fancier version of that. “You can turn it on,” Mumgaard said.

I pushed a black button. A purring noise began. “That’s the sound of the vacuum draining the air from the glass tube,” Mumgaard said. He turned a valve, releasing a tiny bit of hydrogen gas into the tube. A hot-pink glowing light appeared, nested within the glass tube like a matryoshka doll. The magnetic field that contained the pink plasma was visible in the form of empty space between the glass and the glow. “That pink is the superheated plasma,” Mumgaard said. “It’s at least a thousand degrees. But touch the glass.” The glass was cool. “Now touch the copper wires.” They were warm, but not hot. The warmth of the copper wires was not on account of their proximity to the superheated plasma but, rather, because copper is not a perfect conductor; some of the energy running through it is lost in the form of heat. Superconductors lose almost no heat—which is energy.

It seemed impossible that the pink plasma inside the tube, which was as hot as lightning, wasn’t in some way dangerous. Couldn’t some of it leak out of the magnetic bottle, with catastrophic consequences? As an answer, Mumgaard twisted a valve to let a tiny bit of air into the glass tube; the plasma vanished. “People think of fusion like they think of fission, as this overwhelming reaction, but, really, it’s such a delicate process,” Whyte said. “It’s like a candle in the wind. Anything can blow it out. Even a single human breath.”

Much of what Mumgaard and Whyte showed me at P.S.F.C. was the standard part of fusion science. A magnetic bottle is an old idea, and plasma is the most common state of matter; it’s the state that 99.9 per cent of the universe is in. Scientists have been studying plasmas, and magnetic bottles, for decades. Much of what seems difficult about fusion to a plasma physicist—How will tritium be produced and recycled? How can edge-localized modes be anticipated and countered? Will quantum computing enable the study of electromagnetic waves in a plasma?—is so much Greek to a layperson. In contrast, much of what seems difficult about fusion to a layperson—super-hot plasmas, magnetic bottles, toroidal coils—is bread and butter for a fusion scientist.

“As energy, fusion is in some sense very prosaic,” Whyte said. “It’s an intense source of heat.”

“And we’ve been turning heat into electricity since James Watt,” Mumgaard added, referring to the eighteenth-century Englishman whose development of the steam engine enabled the Industrial Revolution. Mumgaard often stresses that C.F.S. is building a “standard, even boring” machine, using “boring, non-innovative” technology, “but for very non-boring reasons.”

The one exception is the H.T.S. magnet—the most exciting element of the research, and the one that raises the most doubt within the scientific community. “I just wonder about the material stresses of such a powerful magnetic field,” one scientist said to me. “H.T.S. magnets will definitely be used in future tokamaks, no doubt, but I suspect they’ll be used with a weaker magnetic field.”

“Most of the criticism we hear is not about the science but about the timeline,” Mumgaard said. The magnets inside ITER took thirty years to develop. “It took us three years.” He could barely repress a grin; it was the one moment of boyish bullishness and ego that I saw in him.

SPARC will have eighteen H.T.S. magnets; each will be composed of sixteen “pancakes”—eight-foot-tall stackable D-shaped slices. I met a pancake in the West Cell, an enormous open laboratory space at M.I.T. which resembles an airplane hangar. What with all the pancakes and doughnuts being tested there, the West Cell has come to be called the West Cell Diner. The pancakes were given names in alphabetical order. The first production pancake was named Egg. When I was there, I saw Strawberry. “We originally planned to have a pancake breakfast for the team when we finished,” Whyte said. “COVID is making that look less likely.”

Strawberry was, incidentally, beautiful. It comprised coils of steel, copper, H.T.S., and helium coolant, because even a high-temperature superconductor has to be kept very cold. (In its internal structure, the magnet was more croissant than pancake.) “I remember when the first pancake was done, and we were moving it so delicately,” Whyte said. “Our hearts were in our mouths—it was, like, Holy cow. Then, the other week, it was the fifteenth pancake. We rolled it over, connected it, like we’d done it a thousand times.”

C.F.S. is not the only enterprise trying to be the Wright brothers. In 2001, Michel Laberge left his job as a physicist and engineer at a printing company and began work on a fusion project that evolved into General Fusion, a Canada-based company developing a technology called magnetized target fusion. General Fusion has the backing of Jeff Bezos, though some plasma physicists note that they haven’t seen enough published work to know how the fusion device is progressing. The U.K. Energy Agency has commissioned General Fusion to build a demonstration plant in Culham, Oxfordshire, where major fusion records were set in the nineteen-nineties. General Fusion has announced its intention to open the plant in 2025, the year that C.F.S. plans to turn on its switch at a SPARC demonstration plant being built in Devens, Massachusetts. There are at least twenty fusion startups now, all benefitting from technological advances in 3-D printing and artificial intelligence. The companies have different risks. TAE, in Orange County, California, uses a fuel, boron, that requires higher temperatures but generates no radioactive by-products. Physicists describe boron fusion as “elegant” and even “perfect,” if also, in certain ways, more difficult. Michl Binderbauer, the head of TAE, told me, “I don’t call these other companies my competitors, I call them my compatriots. We have the same goals, and it will be wonderful for any of us to get there.”

C.F.S.’s seventh hire was Joy Dunn, an aerospace engineer recruited from SpaceX and made head of manufacturing. Dunn, who is thirty-five, has a youthful face and short, rockabilly hair; she loves scuba diving, which made leaving California difficult. She had attended M.I.T. as an undergraduate, and at one of the early C.F.S. meetings she found herself seated next to her fluid-dynamics professor. “I was thinking, I hope he doesn’t remember what grade I got in his class,” she said.

One of Dunn’s main tasks has been producing the magnets, including the pancakes I saw in the West Cell Diner. When I met her, a test of the magnets was imminent, but Dunn told me that she wasn’t really worried about failure. “When they were hiring me, they stressed that it wasn’t a physics problem but an engineering problem,” she said. “That appealed to me. You can’t change the laws of physics, but an engineering problem—that can be solved.”

Dunn showed me around the C.F.S. headquarters, a modest one-story building a fifteen-minute walk from the M.I.T. campus. There were wooden presses and lazy Susans and people spooling H.T.S. wire onto metal plates in what I can only describe as an artisanal atmosphere. There was no hum of machinery. The pancakes that were being tested in the West Cell Diner had evolved from being hand-fabricated here to being made by repeatable mechanized processes.

Dunn said that her time at SpaceX had accustomed her to productive failure. “We’d all watch the early rocket-landing attempts,” she said. “One would miss the boat entirely. The next one would land on the boat, but then slide off into the water. Another would land, then tip over.” She went on, “But I remember having a good feeling before the first time we landed successfully. I made sure to go to the front row for the viewing.” The spirit in the crowd that day was something that still motivates her. Dunn sees her work at SpaceX as not very different from her work at C.F.S.: “It’s large metallic structures under stress.”

The day of the crucial magnet demonstration came about six months after I met Dunn. At around 5:30 a.m. on September 5th, Dunn gathered with much of her team at an outdoor tent—on account of COVID—near the magnet she and her team had worked for three years to develop. The magnet had spent the past week being cooled down to twenty degrees Kelvin; the air inside it had been pumped down to a vacuum. The plan was to run a current through it, resulting in a magnetic field of twenty tesla. (A kitchen magnet is about 0.001 tesla; an M.R.I. machine operates at about 1.5 tesla; the magnets that levitate high-speed trains are about five tesla.) Under the tent, a screen displayed a reading of the amps into the magnet, and of the magnetic field out.

As both the current and the magnetic-field numbers rose, Dunn said, “Our anxieties were about the pumps, the valves, the vacuum system, all that—but really it was about the unknown unknowns.” The magnetic field reached twenty tesla. There were hugs, cheers, high fives, and a crowd of very happy people. Whyte made remarks, as did Mumgaard. Dunn and her colleague Brandon Sorbom hosted “The Joy and Brandon Show,” in which they interviewed members of the team about their contributions. “I think for me, personally, a lot of the nervous excitement—it was existential,” Dunn said. “I feel we proved the science. I feel we can make a difference. When people ask me, ‘Why fusion? Why not other renewables?,’ my thinking is: This is a solution at the scale of the problem.”

Soon after the demonstration, Paul Dabbar, the former Under-Secretary for Science and a visiting fellow at Columbia University’s Center on Global Energy policy, declared in an op-ed for The Hill that “the fusion age is upon us.” He urged more government support for the field. Dabbar, like many fusion scientists, takes seriously C.F.S.’s claims that by 2025 it will be demonstrating a fusion device that gives out considerably more energy than it takes to run.

But many, many technological challenges remain before fusion will turn on the lights in your kitchen. Will these fusion devices sustain plasmas for sufficient periods of time? Will they solve their daunting fuel-cycle issues, and manage their exhaust, and will the stresses of the extreme conditions destroy the devices themselves? Will there come a time when there is jam today, and the day after, and the day after that?

“This is difficult to judge,” Cowley, of Princeton, told me. “What C.F.S. has done—it’s a big contribution, absolutely.” He went on, “I’m always cautious. That’s my personality. I do worry that this is fitting luxury seats into a hot-air balloon—and that won’t take you across the Atlantic. I do worry that if this doesn’t work, after all this attention, then the whole field will have a pall over it again for a long time.”

Cowley wavered between seeing his perspective as sober and seeing it as too cautious. He was the one who drew my attention to the argument, in Eddington’s fusion paper, that there is something to be said for Icarus. “My feeling is that there’s still an idea that we haven’t had yet, and that once we have it we’ll think what fools we were not to have had it earlier,” Cowley said. “But the Wright brothers weren’t like me. They weren’t scientists in a lab—they were mechanically minded people who had some new ideas but also who had some luck on their side in terms of other technologies that came of age at the right time. C.F.S. has that youthful spirit. C.F.S. thinks, We know more than we think we know.” The realm of science and invention is not free from psychology. Cowley circled back over his doubts, then suddenly said, “I can’t believe there aren’t a series of steps that will get us there. I can’t believe that we won’t be able to do it eventually.”

In 1901, the chief engineer of the United States Navy wrote, of heavier-than-air flight, “A calm survey of natural phenomenon leads the engineer to pronounce all confident prophecies for future success as wholly unwarranted, if not absurd.” At the time, the Wright brothers were studying aerodynamics in a makeshift wind tunnel; after one particularly disheartening summer at Kitty Hawk, Wilbur confided to Orville his feeling that “not in a thousand years will man fly.” Two years later, they flew their plane for twelve seconds; not too many years after that, they were flying for hours, performing figure eights for large crowds. In response to a report that President Theodore Roosevelt intended to fly with Orville soon, Orville said that, though he wouldn’t turn down a request from the President, he did not think it wise for the President to take such chances. ♦

An earlier version of this article misstated the number of people employed by Commonwealth Fusion Systems, and the period in which an earlier M.I.T. experimental fusion device was invented.

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