LIGO measures a thousandth of a proton — then a thousand physicists stayed quiet for a month

Lex Fridman opens with the scale of the thing. LIGO, the Laser Interferometer Gravitational-Wave Observatory, is two detectors with four-kilometer vacuum chambers, sitting 3,000 kilometers apart and running in unison. Together they try to catch a motion ten thousand times smaller than the width of a proton — the smallest measurement science has ever attempted, he says — a ripple from the most violent events in the universe, tens of millions of light years away. Barry Barish, a Caltech physicist and a 2017 Nobel laureate for that work, is the guest who has to make the number feel real.

The prize is for contributions to the detector and to the first observation of gravitational waves. The rest of the conversation will spend two hours getting from an eight-year-old’s question about ice to the insides of black holes. The instrument is the through-line: a machine that listens to spacetime itself, built by more than a thousand people, aimed at catastrophes that happened when life on Earth was still learning to be multicellular.

Barish likes the ice story because it is ordinary. At about eight he asked his father why ice, which is made of water, floats on water. His father usually had answers. This time he did not. It may not have been the first unanswerable question, Barish says, but it is the first he remembers, and it was the moment he learned that curiosity has a method: the library, people who know more, and eventually what scientists call research.

Kids that age ask so many questions that adults treat them as pests and tell them to stop. School then trains students to study and finish assignments rather than pester anyone with questions. Barish thinks most working scientists are people who escaped that process — still children, or never quite beaten into shape. He teaches undergraduates and calls the lack of curiosity one of the biggest deficits in the room, because curiosity is an innate human quality that parents and schools keep trying to kill. Even the proverb is against it: curiosity killed the cat.

He stayed with water. Rain wets a pavement; later the pavement is dry. Where did the water go? Chemistry later told him water is hydrogen and oxygen, both gases — so how do they make a liquid? He was not especially scientific until college, but he did look up at the stars like everyone else. The more physics has learned, the more it has learned it does not know: most of the universe is dark matter and dark energy, questions that were not even on the table when he was a student. Dark energy, he says, is a complete puzzle that sits uncomfortably with Einstein’s cosmological constant. Dark matter is likely some exotic particle; supersymmetry was the most logical candidate and does not seem to work. He still hopes answers that look complicated are simple once you understand them.

Barish went to high school in Chicago and took classes at Fermilab, watching the buffalo on the grounds. Enrico Fermi was the influence that stuck. Fermi, he says, was the last physicist of note who was both an experimentalist and a theorist at the same time, and in a few months around 1933 he did two astonishing things. The neutron had only been discovered in 1932, so physicists had just learned that the nucleus is neutrons and protons with electrons outside. Fermi then wrote down a theory of beta decay — electrons coming out of a nucleus — that lasted for decades and has only been improved on. He sent it to *Nature*. It was rejected as too speculative. He went back to his desk in Rome, lengthened the paper, and published it in the local Italian physics journal and the German one.

The second act was experiment. In early 1934, Frédéric Joliot and Irène Curie produced artificial radioactivity by bombarding aluminum with alpha particles, exciting something that decayed with a half-life. Natural radioactive materials were scarce; if you could make them, medicine and industry could use them. Fermi’s insight, which Barish still cannot quite see the origin of, was that the right follow-up was not more charged particles but the newly discovered neutron. Neutrons had barely been seen and were hard to get in number, but they have no charge, so they go straight into the nucleus. That experimental program won Fermi the Nobel Prize and was the first step toward fission. He also learned, in the lab, that slow neutrons work better than fast ones, and he learned how to make them slow.

Was bombarding with neutrons an experimental hunch or an Einstein-style idea? Both, Barish says. It took a recognition of how a neutron interacts compared with an alpha particle, then a way to produce enough of them, and a team of associates who pulled it off quickly. Theory and engineering fed each other. No one has done both at that level since. Barish’s own youthful dream was to be like Fermi; he never liked experiments that ignored theory, or theory far from experiment, so he spent a career doing experimental work tightly tied to theoretical ideas — “half of half” of Fermi, as the Chinese outline puts it, and close enough to the spirit.

Fridman, who is Russian, asks whether it is tragic that Fermi is remembered as an architect of the nuclear age. Barish widens the frame. A few minutes earlier he had praised curiosity; now he mentions a book called *The Ratchet of Curiosity*. A ratchet turns only one way. Knowledge is like that: once you know something, you cannot unknow it, and you may not have the sophistication to keep it under control. Nuclear physics was the example. Fermi himself appears, in his writing and his wife’s, as an apolitical person who saw the world through a scientist’s lens. The bomb on Japan was tragic, and he had a role in it. Barish would not want that as a legacy.

Fridman maps the same ratchet onto artificial intelligence and bioengineering: you ship a system to see what happens, and most of the time it is not yet powerful enough to do large-scale harm. The same curiosity, run forward, might. Barish agrees it is the same idea in another field. Fermi died young, not long after the war, already a little cynical about Teller’s hydrogen bomb and the endless U.S.–Soviet tension. People who had spent years at Los Alamos, Barish notes, had to convince themselves that dropping the bombs would bring lasting peace. It did not.

Fermi’s name is also on the Fermi Paradox. At lunch with Edward Teller and Herb York he sat down and asked, in effect, “Where are they?” — given the number of galaxies, stars, and Earth-like planets, why no obvious visitors or loud signals? Frank Drake later made the numerology more careful; the conclusion is still that it would be a miracle if we were alone. Barish’s view is plain: life is likely somewhere, intelligent life probably too, but communication is harder than people think. Every channel we know travels at the speed of light, and nothing we know goes faster, which makes back-and-forth talk possible only at modest distances. A century ago we did not even understand that limit. Gravity as a channel? “Why not,” he says, except that gravitational effects are brutally hard to detect unless they come from something like a black hole. Some respected colleagues spend their lives looking for a fifth force, perhaps longer-range than gravity and not falling as one over r squared.

Newton gave a formula: force proportional to the product of two masses, inversely proportional to the square of the distance, with a constant G measured a century later. It was the most successful theory in physics for more than two hundred years, and nobody saw a violation. It also never explained *why* the Earth pulls the apple. The first small crack was Mercury’s orbit, slightly wrong in Newton’s calculation. Einstein spent ten years on the problem — after solving three or four of physics’ biggest questions in months in 1905 — and released general relativity in 1915, a new theory of gravity.

In 1916 he wrote a short paper with no fancy derivation. He noticed that if you write the general-relativity formulas a certain way, they look like the formulas of electricity and magnetism. Electromagnetism had been shown, only about twenty years earlier, to have waves: light, radio, everything else. So he took the leap that gravity probably has waves too. Barish calls it a heck of a leap. The paper was poorly written and had a serious mistake, a factor of two in the strength of gravity. Two years later Einstein wrote a second paper, fixed the factor without admitting the error, and explained how you make the waves: not a dipole, plus and minus charge oscillating, but a quadrupole moment, a three-dimensional oscillation. Again, insight rather than derivation. The same year, Karl Schwarzschild — a German theorist who would die in the First World War — proposed black holes. Einstein did not.

Quantum mechanics then ate physics, and Einstein left the problem until after he immigrated to the United States in 1932. Working with Nathan Rosen around 1935, he tried to derive gravitational waves from the theory and kept hitting infinities — coordinate singularities, the student trap of four-dimensional relativity if you set the coordinates wrong. In 1936 they submitted a paper to *Physical Review* titled “Do gravitational waves exist?” twenty years after he had proposed them. The young editor, John Tate, sent it for review to Howard P. Robertson, on sabbatical at Caltech, otherwise at Princeton with Einstein. Robertson saw the coordinates were the problem, showed that in cylindrical coordinates the infinities vanish, and Tate wrote a polite note asking what Einstein thought of the referee. Einstein’s documented reply: he had sent the paper to be published, not to be shown to so-called experts; he withdrew it, and never published in that journal again. He rewrote it, changed the title, and put it in the Franklin Institute’s journal in Philadelphia. It was the last time he wrote about gravitational waves.

The argument lasted until a 1958 conference of general-relativity experts. A British theorist produced a clean derivation from the heart of the theory. Richard Feynman, in typical Feynman style, said that if the waves exist they have to *do* something — they have to carry energy. His thought experiment was a bar with rings on it: a wave distorts the bar, the rings rub, friction is heat, heat is energy. After that, theorists believed the waves should exist and should be detectable. The remaining problem was the size. You need a quadrupole: two objects orbiting, Earth around the Sun, or two black holes. You could spin a barbell in the lab the way Hertz made radio waves, but the effect would be hopelessly weak. Nature has to supply something stronger. LIGO targeted neutron stars and got lucky: black holes were a stronger source.

What does a gravitational wave feel like? Barish’s amusement-park picture: one funhouse mirror makes you short and fat, the next makes you tall and thin. Walk back and forth between them once a second and you have a one-hertz wave; sixty times a second is sixty hertz. That is all that happens — you get taller, shorter, fatter, thinner at the frequency of the wave. The amplitude is tiny. Space itself, he says, is like a very stiff table; luckily for ordinary life the distortion is negligible. It is still in your pocket. GPS satellites triangulate your position from light-travel times; skip the general-relativity correction and, on a ten-meter-wide road, you drive off it.

Early searches used Feynman’s bar: a huge aluminum cylinder with sensitive detectors around it, looking for extra strain. It was not a good method for a signal this weak. The right instrument is an interferometer. You split a laser down two perpendicular arms, bounce it off end mirrors, and invert one return so that equal arms cancel. If a wave stretches one arm and squashes the other, the light comes back out of time and the cancellation fails. The strain is one part in 10²¹. Make the arms four kilometers and you are measuring 10⁻¹⁸ meters. A proton is about 10⁻¹⁵ meters. The effect is a thousand times smaller than the thing we are made of. Einstein himself did not think it could be measured; he had not anticipated modern lasers.

Put that instrument on the ground and it fails, because the Earth is moving all the time. Isolation is an engineering problem, not a physics one. LIGO works in the audio band — tens of hertz to thousands — which is where the Earth is quietest, and where our ears evolved to work. The first layer of the solution is ordinary: shock absorbers, like a car, four layers of the right “squishiness,” looking like little springs, each stage eating what the last let through. Passive isolation was not enough. They added active cancellation, the Bose-headphone trick: sense the repeating ambient noise and push back. Inside the shock-absorber array they placed seismometers to measure residual motion *and its direction*, then small actuators to cancel it. What gets through is one part in 10¹² of the Earth’s shaking. Fridman calls it one of the most precise robots ever. Barish does not disagree. Nobody, he says, seems to challenge the claim that LIGO is the most precise instrument ever built.

The worry that kept MIT’s Rai Weiss up was vacuum. The whole interferometer sits in the world’s largest high-vacuum system, four-kilometer tubes rolled from stainless steel with a spiral weld, at around 10⁻⁹ torr. CERN’s accelerator is a bigger vacuum system, but not this quality of high vacuum at this size. In a lab you hunt leaks with helium. You cannot surround four kilometers of pipe with helium. You can tell the vacuum has gone bad; finding and fixing a crack is another matter. They built test chambers, including for the active isolation, and worked with a small company near MIT that hoped to commercialize stable tables for microelectronics. Astronomers, Barish recalls, often thought the money was being wasted. NSF was paying.

The collision they eventually heard happened 1.3 billion years ago, when Earth was converting from single-cell to multicellular life. The signal crawled across that whole history to meet a collaboration that started around a hundred people and grew past a thousand. Fridman asks what it takes to lead theorists, engineers, and a lot of egos. Barish’s goal is the cliché everyone recites: the sum should be more than the parts. Put people together carelessly and you usually get *less*, because they overlap. The structure has to require combined talents — things neither person could do alone — so the team feels it is doing more than anyone could do themselves.

The first job is attracting the best people in the world to spend years of their lives on it. The only pitch that works is that the work is more interesting than what they could do on their own. Managing egos is an art, not a science. The romantic goal helped: something never done, scientifically important, a huge challenge. The lasers LIGO needed were a specialty of three institutions that were otherwise competing; Barish brought all three in to work on one object they would not otherwise have touched, as part of a larger team that might discover something even the laser engineers were not aiming at. Publishing is the small, telling ritual: more than a thousand people have to agree on the phrasing of a sentence and the title of the paper, so that everyone actually feels like a contributor.

Failure is built in, Barish says, the way it is in mechanical engineering when you are doing things no one has done. NSF funded construction in 1994. About five years later, in 1999, they had a basic instrument without active seismic isolation and without several later features. They stuck at first to technologies they had tested in their own labs. The machine worked, not very well, and saw no gravitational waves. Then came almost a decade of the same loop: run for months looking for a signal, learn what limited them, fix for months, run again. Eventually they knew they needed a big step, including the active isolation. NSF gave them another hundred million dollars. They rebuilt. In 2015 they turned it on and almost instantly saw the first collision of two black holes.

Some colleagues had a eureka moment on the spot. The figure in the paper is raw data next to a numerical solution of Einstein’s equations; they look the same, and two detectors halfway across the United States saw it. That is already persuasive. Scientists still should not fool themselves. The instrument was new. They had to test that the rebuild itself could not generate a signal of that shape. Then, a concern Fridman as a computer scientist appreciates: they had to convince themselves they had not been hacked. The detectors are not on the internet; physical access is still possible. Disgruntled former graduate students are a long shot when the waveform matches theory, but the team checked anyway.

Within a month they had convinced themselves, and they kept a thousand collaborators quiet the whole time. They spent another month understanding what they had seen so they could do the science themselves rather than dump a mystery on the world. Fridman takes the silence as proof of a close-knit team. Barish jokes: tight-knit, or a strong dictatorship — you can rule by fear or by love, a line that goes back to Machiavelli. It did not have to be a success story on this timescale. It could have taken a century.

By the time of the conversation the detectors were off for the pandemic. Before the shutdown they were seeing some kind of gravitational-wave event each week. With the upgrades then underway, Barish expected a detection every couple of days when they turned back on, and the events are not all the same. The lucky fact, he says, is that they are limited by technology, not by science. Contrast the Higgs boson, found at CERN’s huge accelerator — a great discovery Barish also worked on. Since then, progress has been slow because the Higgs sits in a haystack of other physics you cannot make go away. LIGO’s signal is weak, but the background is isolation from the Earth and laser power. Improve those, and the science opens. Each upgrade since 2015 has been turning a detection into a new astronomy.

He reaches for Galileo. Galileo did not invent the telescope; he pointed existing lenses at the sky, saw moons around Jupiter, and started the habit of understanding the universe with something other than the naked eye. Telescopes have been getting better for four centuries. Gravitational-wave astronomy is still crawling. Barish is optimistic about engineers: as long as physics has not drawn a hard line, they will eventually build a device accurate enough. The next big instrument is still being designed — Europe and the United States have different ideas — but the aim is at least ten times better. Ten times the sensitivity is ten times farther, which is a thousand times more volume, and farther is also earlier: cosmology instead of astrophysics, the evolution of the universe, not just the objects in it.

The Nobel Prize, he says, cannot be a goal; they give one a year, a couple of hundred physicists since 1900, and luck has to break your way. What it is good for is putting science on the front page for a few days, and, unexpectedly, getting lawyers and businesspeople in government to listen. After the prize, people pay attention when he talks about global warming. The unfairness of singling out a few names from a huge experimental team is the other side of the coin. On the science, electromagnetic telescopes see only about four percent of what is out there. He would be shocked if gravity did not reveal objects that never show up in ordinary telescopes. Light was blocked in the early universe until it became transparent, a few hundred thousand years in; gravitational waves are not obstructed that way. In principle they can study the Big Bang itself. We cannot do it yet. Optical astronomy took four hundred years to get this far.

One physics puzzle he wants that early view to help with: we do not know why we are here, and he does not mean it philosophically. In the laboratory, colliding energy makes as much antimatter as matter; they annihilate back into energy. The early universe should have done the same and left nothing. Instead the universe we see is all matter, with antimatter only in labs. There must have been an asymmetry, a little more matter than antimatter, and present physics cannot explain it. Fridman asks whether antimatter has a gravitational field you could use. In principle, Barish says, anti-neutron stars would look different — but we have looked hard enough, and everywhere we look it is ordinary matter. A space mission, eLISA, would be complementary rather than a rival: three satellites hundreds of thousands of kilometers apart, lasers between them, watching a triangle change shape at lower frequencies, like adding infrared and microwave to Galileo’s visible light. The hard part is isolating free masses from the satellites. A test mission suggests the engineering is in hand, for the 2030s. “It’s just engineering,” Barish laughs again.

A black hole, physically, is a region of space where the curvature of spacetime is so strong that nothing gets out, including light. Light already bends in a gravitational field; warp spacetime enough and it stays in. That is the concept, not yet the origin. The default story is stellar death. A star like the Sun makes heat and light by fusion, burning hydrogen, then helium, then heavier elements, until iron, where fusion no longer pays. The star has always been a contest between outward pressure from burning and inward gravity. When the fuel is gone it collapses — a supernova — and if the parent was big enough, the leftover mass in a much smaller space makes a black hole. The Sun is too small. The usual threshold is at least three solar masses. Binary black holes can come from binary stars. That is the first explanation. It is not necessarily what LIGO is seeing.

The puzzle is mass. They have seen a system of 140 solar masses. Stars that large are thought to be unstable in the environments they live in, so they should not exist as parents. Maybe smaller holes eat one another and grow. Maybe some are primordial, made in the Big Bang, a different object with the same gravitational costume. If there were a lot of those, they could account for some or all of dark matter. Gravitational waves can start to tell the stories apart: distributions of mass, of spin, whether the system wobbles, how the population changes as you look further back in time. The supermassive hole at the center of our galaxy was probably made still another way, tied to how galaxies form. The word “black hole” hides more than one origin.

Fridman finds infinitely dense, light-eating objects nightmarish. Barish finds them an opportunity. Physics has an embarrassment: two wonderful theories. Quantum field theory at CERN explains almost anything you can smash together. General relativity is fantastic at high speeds and long distances. That is not how it is supposed to be; we wanted one theory of physics. People have tried to unify them for decades, mostly on paper. Barish’s belief is the usual one in physics: we need experimental clues. At CERN, gravity does not matter. In ordinary black-hole astronomy, particle physics does not matter. Inside a black hole, both have to be satisfied. He does not think we are close. String theory is the most popular candidate: beautiful mathematics, almost no predictive power, because it lives in eleven dimensions and the extra seven have to curl up on unobservably small scales, and the curling is not unique. If you cannot go uniquely from the theory to our world, you cannot predict. Dark energy, which nobody understands, might someday offer a clue. On the horizon he can see, he cannot envision how the two theories come together.

Will humans colonize the galaxy, or travel near or past light speed? Barish calls that very futuristic. A one-way trip to Mars is not that far, and the question is whether anyone will go. Explorers, he thinks, will. He does not envision civilizations colonizing so much as something like the South Pole: we have not colonized Antarctica, because it is ice and cold, but we keep a self-sustaining station there. He has been. As a member of the National Science Board he got a VIP flight if he was healthy enough: United States to Australia to Christchurch, then a seven- or eight-hour military flight to McMurdo Station on the coast, about a thousand people. They cannot predict the weather for landing. Halfway there the pilot announced a “boomerang flight” and they turned back, waited in Christchurch, and tried again. Then they flew to the pole.

The pole itself, in summer, was minus 51 degrees Celsius, zero humidity, about 11,000 feet of altitude because it is never warm enough to melt; snow packs up even though it rarely snows. You land high, cold, and dry, and your body has to adjust. Fewer than a hundred people; fifty or sixty when he was there, half of that in winter. On the coast, McMurdo has greenhouses and can feed itself. Inland, settlement has not gone much further. Fridman offers the computing analogy of an off-site backup: Mars, unlike Antarctica, is a copy of the species if Earth burns. Barish does not argue. He just cannot yet see people living there the way they live in cities, as opposed to a station. That returns him to Drake’s missing factor: how long civilizations last. We have barely learned to communicate with electricity and magnetism. He is hopeful in the near term and unsure about a couple of thousand years — a virus we create, the equivalent of nuclear war, something we cannot control.

Barish wanted to be a writer before he wanted to be a physicist. Nobody in the family had gone to college; he had no mentors, but he always carried a paperback. His mother read mysteries; he got bored and, around twelve or thirteen, found what he calls real literature. There is nothing better than Russian literature, of course. Dostoevsky mattered twice over: he was the first serious author to use multiple narrators, polyphony, and he began existential literature. In pandemic isolation Barish reread Camus’s *The Plague* and found a book about love for humanity, the right book for a plague year. Unlike most existentialists, Dostoevsky was at least in part religious. Barish’s favorite is *The Idiot*. Prince Myshkin is a Christ-like bumbler, not quite of this world. Barish often calls himself an idiot because he feels naïve about the world, maybe genetically; he has a spiritual connection to that character.

Solzhenitsyn gives him something closer to Viktor Frankl: the cruelty and indifference of human nature, and the ability to find happiness in very small joys. The description of getting through a day in a prison camp and actually enjoying it is, he says, pretty amazing. The political lesson is sharper. Stalin likely believed he was doing good for the world. The best way to do evil is to believe you are doing good; otherwise evil is too hard. The Gulag is the clearest picture of that, and Solzhenitsyn is one of the best people to have revealed it. The most recent book Barish mentions is nonfiction: a woman who won the Nobel Prize in Literature within the previous five years, interviewing people of every age and occupation about the Soviet system they had lived under. What struck him is how many look back and feel it was better before. In America, he says, we think we know the right answer for building a better world. He is not so sure.

Advice for the young is not deep, he admits: have dreams and follow them to the extent you can, and have fun at the work that takes most of your time. He did not follow the first dream. He took the most serious literature course in high school, at about fifteen, and *Moby-Dick* cured him of being a novelist. He remembers a chapter that felt a hundred pages long describing the white whale’s fins in crushing detail. If that was great literature, he thought, screw it. Rereading after thirty or forty, he diagnosed the problem: unlike the realistic Russian novels, *Moby-Dick* is one huge metaphor, and at fifteen he probably did not know the word, let alone why he should care about a fish. The teacher may have given him the wrong book, or the right one, because he went into physics.

The philosophical why — why we are here, beyond matter and antimatter — he is “probably not really good at.” The individual version, put simplistically, is whether you have made a positive difference for anything besides yourself: other people, or a discovery like gravitational waves that matters to others, something beyond existing as an individual. He is not exactly afraid of dying, but saddened by it. He is old enough to know he has lived most of his life; he enjoys being alive; he can imagine being sick and not wanting to continue, but he is not. He would like the good ride prolonged. He does not fear dying itself. What he finds hard to deal with is the mystery of consciousness, the fact that we can think. Asked what he hopes an encyclopedia would say, he wants to have helped move science forward and to have been a decent role model for how to live. Fridman adds the instrument: not a human creation but a collection of humans, humanity at its best. If we do not last, at least we built that. Fridman closes with Heisenberg: the universe is not only stranger than we think, it is stranger than we can think.