Oumuamua Wasn't Supposed to Exist. Avi Loeb Wants a Photograph Next Time.

Lex Fridman opens with the question Avi Loeb poses in *Extraterrestrial*: throughout the expanse of space, have other civilizations ever explored the stars and left evidence? Loeb's answer starts from modesty, not from a radio signal. His daughters, as infants, thought they were unique; they went outside, saw other children, and recalibrated. The only reason humans still feel special, he says, is that the instruments are not yet good enough to find anyone else — and perhaps anyone better.

Kepler data, he notes, show that about half of Sun-like stars have a planet roughly Earth's size at roughly Earth's distance, which means liquid water and familiar chemistry are on the table. Roll those dice billions of times in the Milky Way, then again across tens of billions of galaxies, and uniqueness becomes a strange bet. The Sun itself formed relatively late. Many Sun-like stars are already white dwarfs. A civilization that survived around a typical one could be a billion years old, and a billion years of technology would look to us like magic, "an approximation to God."

Fridman asks whether consciousness is the essential ingredient. Loeb is unsentimental. Humans have built tools that can also destroy the climate or start a nuclear war; creatures like us may be short-lived, while crocodiles on other worlds last for ages because they never invent a way to kill themselves. The most common inhabitants of the cosmos, on that view, would be durable, unthinking animals. Superintelligent machines, he adds, might explore space without anything humans would recognize as inner life. Consciousness, which we treat as precious, could even be a drag.

The practical corollary is archaeology, not a live phone call. Dead civilizations still leave trash, the way the Maya left ruins. Finding technology we have not dreamed of, Loeb says, would be a better route to a fortune than Silicon Valley or Wall Street — cheating on the exam by looking over the next student's shoulder. He wants humanity to listen more than it broadcasts. If we are the inferior party, speaking too loudly has a downside.

On October 19, 2017, Pan-STARRS in Hawaii spotted an object already receding from Earth: the first visitor recorded from outside the solar system. It was named Oumuamua, Hawaiian for a messenger from afar. Astronomers' first guess was a rock, an asteroid or comet that happened to come from another star. Loeb was surprised it was seen at all. A decade earlier he and Ed Turner had asked whether that same survey would catch interstellar debris, assuming other systems eject rocks at solar-system rates. The answer was no. The telescope was not sensitive enough.

A later interstellar object, Borisov, discovered by amateur astronomer Gennady Borisov, behaved like an ordinary comet, which colleagues took as proof that Oumuamua must be natural too. Loeb's reply is a first-date joke: meeting many ordinary people afterward did not make his wife less unusual. The object was roughly a hundred meters across, football-field scale, and too small to resolve from Earth. The only way to photograph it was to send a camera close. By discovery it was already leaving faster than any rocket humans could launch.

He likens it to a dinner guest who looks odd, then is out the front door and down a dark street before anyone can say why. In July 2017, when Oumuamua was still approaching, Loeb was on vacation with his family near the observatories on Maui. Nobody there knew how close the object was. His campaign since then has a single operational point: the next time something this strange appears, take the picture. Science, he says, is based on evidence, not on prejudice. The Vera Rubin Observatory's survey, a few years out at the time of the conversation, might find an Oumuamua-like object every month.

The first anomaly was in reflected sunlight. Oumuamua tumbled every eight hours, and the brightness from Earth changed by a factor of ten, meaning the sunlit area — or a mix of area and reflectivity — swung that far as it spun. Even a razor-thin sheet of paper is unlikely to deliver a factor of ten on the sky. The geometry is more extreme than any comet or asteroid in the record. Cartoons made it a cigar; the light curve preferred a flat pancake, at 90 percent confidence. That was strange, Loeb says, but not yet enough to call it artificial.

The second anomaly was the missing comet. Spitzer stared hard for carbon-bearing molecules and found nothing: no dust, no gas, no tail. A comet is an icy rock. Near the Sun the ice does not melt into liquid; in vacuum it sublimes straight to gas and drags dust with it, which is the tail you can see from a backyard. Liquid water needs an atmosphere's extra pressure, which is why Mars lost surface water when it lost its air. Oumuamua showed none of that chemistry. The community's shrug, as Loeb tells it, was: fine, then it is just a rock.

The third anomaly arrived in June 2018, from the orbit. Besides the Sun's gravity there was an extra push. Comets get a rocket effect: gas thrown sunward shoves the nucleus the other way, like a jet engine. For Oumuamua to get the observed acceleration that way, about a tenth of its mass would have had to boil off. A hundred-meter body losing 10 percent of itself would have been impossible to miss. There was no sign of it. "It's super weird," Loeb says, and that is the point at which he stopped treating business as usual as an explanation.

Loeb says he proceeded like Sherlock Holmes: rule out what you can, then take whatever is left seriously, however odd. The leftover was sunlight bouncing off the object and giving it a push, the way wind hits a sail. For that radiation pressure to match the orbit, the body has to be extremely thin — less than a millimeter. Nature, he argues, does not manufacture sheets like that. People do. Lightsail spacecraft are being designed so a probe need not carry fuel, only a sail pushed by sunlight or a laser.

Critics noted that Loeb already worked on lightsails, so of course he saw a sail. He does not deny it. Imagination is bounded by what you know. Then he offers a control case from September 2020: object 2020 SO, named by the Minor Planet Center in Cambridge. It followed an Earth-like but not identical orbit, bound to the Sun, and it too drifted from a purely gravitational path. Tracing the orbit backward, astronomers found it had intercepted Earth in 1966 — the year of the failed Surveyor 2 lunar lander, which left a rocket booster in space.

2020 SO is that booster, hollow and thin enough for sunlight to shove it. No comet tail, plus a deviation from a gravity-only orbit, was enough to call it artificial, and in that instance we know the manufacturer. Oumuamua cannot be ours: it came through for a few months, moved faster than anything we can launch, and arrived from interstellar space. Walking on a beach, Loeb picks up beautiful seashells and, every so often, a plastic bottle. Maybe Oumuamua, he says, was a message in a bottle.

There was another peculiarity besides the shine and the lack of detectable heat, which already hinted at something small and reflective. Oumuamua arrived from a special frame: the local standard of rest, obtained by averaging the random motions of stars near the Sun. The Sun itself moves relative to that frame. The object was almost at rest in it. Only about one in 500 stars is that still. Loeb's picture is a public parking lot with a car sitting quietly while you walk into it. The relative speed is almost entirely yours.

A rock flung from another planetary system should usually carry the motion of its host star. The loosest objects live on the outskirts, moving slowly relative to that star, and they keep that residual speed when they are stripped away. Rest in the local standard of rest is the wrong inheritance. Loeb sketches alternatives: a grid of navigational posts, like road markers evenly spread so travelers can find the local frame; or relay stations, because deep-space communication need not be one huge beacon — Earth's networks already hop from node to node. After the solar system kicked it, this one would look like a billiard ball; most of its siblings would never hit a star.

It might be a probe aimed at the Sun's habitable zone, though the trip in from the Oort Cloud takes tens of thousands of years, and Earth at launch would still have been grass, with no technology worth visiting. Or it might be space junk: an outer layer ripped off some instrument, the kind of debris that would be everywhere. That last idea suits Loeb's "cocoon" speculation. A civilization millions or billions of years ahead may have built a comfortable closed habitat, with no interest in talking. You still find them, he says, the way investigative journalists go through Hollywood trash cans. Thermodynamics guarantees waste.

Fridman asks whether, as a scientist, Loeb hopes the trash is a water bottle or a smartphone. Loeb wants something more sophisticated than a phone. Fridman wants the bottle: a caveman can learn from plastic; a smartphone is inscrutable, and the caveman will conclude it is a rock, "just like my fellow astronomers said." Loeb grants the analogy. If it is a phone we may not decode the message. He still wants to get close enough to see what it is made of. Tracing a source is almost hopeless anyway. Oort Clouds of neighboring stars overlap, packed like billiard balls halfway to the nearest sun, so a direction on the sky crosses too many systems to pick a home.

Loeb's warning is that consensus on Twitter does not move Oumuamua. Reality does not care whether you ignore it. A crowd can agree it was a rock, award itself honors, and still be looking at an artifact. Science, in his telling, is not an echo chamber of students repeating a professor's mantras for prizes. It is a dialogue with nature, and working at the frontier means being wrong. Einstein, late in life, argued that black holes do not exist, that gravitational waves do not exist, and that quantum mechanics has no spooky action at a distance. All three were wrong. Avoiding mistakes to protect an image, Loeb says, makes you a boring scientist who repeats what is already known.

Tenure, he told the *Harvard Gazette*, should be the chance to behave like a child. Something happens to people when they get it: ego starts to outrank curiosity. Meanwhile a *Nature* paper declared Oumuamua unusual but natural, period. Other mainstream attempts filled the gaps with things never seen: a dust bunny pushed by sunlight, or a hydrogen iceberg, transparent as it evaporates, which would hide a tail. Loeb's group later argued that neither object would survive the interstellar trip. His complaint is procedural. If you are willing to discuss unseen natural mechanisms, why is an artificial origin out of bounds?

Dark matter is the comparison he keeps using. Most of the universe's matter is a placeholder. Weakly interacting massive particles, axions, and other candidates have been proposed, and experiments costing hundreds of millions of dollars have ruled many out. Nobody called that speculative. Extraterrestrial technology, by contrast, is known to exist on Earth, and Earth-like conditions are reproduced in billions of systems in the Milky Way. "What's more conservative," he asks, than expecting a similar outcome from similar conditions? Extra dimensions, supersymmetry, string theory, the multiverse, and living in a simulation sit comfortably in the mainstream with, in his view, no experimental grounding.

Without an experimental ATM, Loeb says, a theory is as useful as imagining you are richer than Elon Musk until you check the balance. He then does something unexpected: he gives theology three tests. His book had inspired a Harvard English Ph.D. student; at her exam a professor asked why Giordano Bruno was burned. Loeb guessed Bruno was merely obnoxious. The professor said no: Bruno claimed other stars are suns that could host Earth-like planets and life. If that life sinned, Christ would have to save it, which implied multiple Christs, which the Church could not accept.

Loeb told the professor that this was, at last, an experimental setup. We now know other stars have Earth-like planets. Suppose we find life there, establish that they sinned, and ask whether they witnessed Christ. A no would rule that theology out. A second test is Abraham in the Hebrew Bible, commanded to sacrifice his son. Had Abraham opened a voice memo, Loeb says, the recording would have been evidence that God exists. He did not. A third, a story Elie Wiesel attributed to Martin Buber: Christians say the Messiah came and will return; Jews say he never came and will arrive. Both sides agree on a future visit. When he comes, ask whether he came before.

The sting is aimed at physics. If theology can put skin in the game, why cannot string theory? Alan Guth of MIT, one of the inventors of cosmic inflation, has argued the idea is not falsifiable. A theory that cannot be falsified, Loeb says, is not helpful, because you cannot improve your picture of nature. Knowledge has to be a dialogue, not a monologue in extra dimensions that we do not inhabit. Fridman calls the pairing of open-mindedness and rigor the point of the whole conversation.

The Drake equation, Loeb says, mostly quantifies ignorance. Star-formation rates and the frequency of Earth-like planets are now measured; the rest is assumption, and the equation only covers civilizations transmitting while you happen to listen. Artifacts and industrial pollution outlive the broadcasters. He is not speaking as an outsider. He chaired Harvard astronomy for nine years, the longest tenure in the department, and chaired the National Academies' Board on Physics and Astronomy. The disappointment is with the conservatism of a community he belongs to.

Astronomy was then writing its next-decade wish list, including telescopes costing billions of dollars whose selling point is oxygen in exoplanet atmospheres as a sign of life. Earth had life for roughly its first two billion years with almost no oxygen, and oxygen can also come from splitting water without biology. Even a detection would never be conclusive. Five years earlier Loeb had suggested using the same instruments to look for CFCs from refrigeration and industry — a much cleaner technological signature. Nobody will put that in the official goals. It sounds peripheral. It is a taboo.

Young people, he says, stay out because of bullying on social media. If you step on the grass it never grows; then the mainstream notes that nothing was found. Gravitational-wave astrophysics, pioneered by Rainer Weiss at MIT, was ridiculed until National Science Foundation administrators funded it over resistance. Now it is a Nobel-winning frontier, and the postdocs who dismissed it claim they were always supportive. Exoplanets have a similar scar. In 1952 Otto Struve proposed hunting hot Jupiters close to their stars, which would tug the star detectably. Time-allocation committees said no for forty years because theory forbade Jupiters that close. The theory was wrong. The first detection came four decades later; a Nobel followed. The baby was born, Loeb says, but late, and he wonders how many equally good ideas never were.

In Israeli military service there was a saying that someone sometimes has to lie on the barbed wire so others can cross. He is willing to take that pain so younger researchers can talk about technological signatures in the sky. The public funds science and cares whether we are alone. Refusing to look through a telescope you already have, he says, is like refusing Galileo's. Science fiction's physics violations are irrelevant if you have a method. Treating the question as unsophisticated while you do gymnastics in extra dimensions is, in his word, a crime.

Fridman asks about the hundreds of thousands of UFO reports and the childlike excitement around them. Loeb starts from the same modesty: we are probably not interesting enough to spy on. Fuzzy images from fifty years ago should, with modern cameras, have become crisp; they have not, which makes instrument artifacts or misunderstood natural phenomena the default. Military and pilot reports still deserve examination as possible national-security threats from other countries. Pinning them on aliens, without multiple detectors and a scientific process, is a leap he will not take. He is in favor of funding better instruments and going to places with frequent unusual events. Until the evidence is in, he remains doubtful.

A more concrete path is interstellar meteors. Small ones burn up; tens of meters and larger can deliver a core you can hold. With a student, Loeb searched the historical record and thought they had one candidate from interstellar space. A more imaginative archive is the Moon. It has no atmosphere, so incoming rocks do not burn, and no plate tectonics or weathering to erase them. Earth remixes its surface every hundred million years. A civilization with electronics a hundred million years ago would be archaeologically gone, buried in the crust. Impacts on the Moon punch about ten meters down, kick up dust, and then the museum keeps the rest.

On how long technological civilizations last — the noisiest term in Drake's equation — Loeb again uses Copernicus: we are not at a special time. Humans have been transmitting radio for about a hundred years, so a few centuries is the unsentimental forecast unless we get smarter than the daily newspaper. That short window is also why he doubts a reported radio signal from Proxima Centauri. The Parkes telescope in Australia once picked up a lunchtime "signal" from a microwave oven opened early. A narrow-band transmission from the surface of Proxima b should also show a larger Doppler drift than was observed, unless the senders knew our exact line of sight. The Copernican odds of overlapping radio windows, he says, are about ten to the minus seven.

In May 2015, Silicon Valley investor Yuri Milner walked into Loeb's Harvard office and asked if he would lead a project to reach the nearest stars in a human lifetime. Chemical rockets would take tens of thousands of years; Voyager 1 and 2 would have needed to launch when the first humans left Africa in order to arrive at Proxima now. Loeb took six months with students and postdocs and concluded that only a lightsail could do it. The sketch: a 100-gigawatt laser array on Earth, focused on a few-meter, few-gram sail in space. Shine for minutes and the sail is five times as far as the Moon, at a fifth of the speed of light.

Milner later called and asked for a briefing at his home in two weeks. Loeb was already walking out of a hotel in Israel, headed with his wife to a goat farm in the Negev that she had chosen for being far from civilization. The farm had internet only in the office. The next morning at six he sat with his back to that office, newborn goats in front of him, typing a PowerPoint about visiting the nearest star. The pairing of primitive origins and stellar ambition, he says, felt surreal.

He dislikes the phrase space race. In a debate on whether a U.S.–China contest in space is good for humanity, the other speakers talked military threat. Loeb could not parse it. Threats come from hovering over Earth's two-dimensional surface. Mars and other stars are not a battlefield in that sense; going there is how a species notices it is one civilization. Treaties, he notes, did not prevent world wars, and commercial actors such as Jeff Bezos and Elon Musk will fly anyway. The underplayed engineering problem on Mars is cosmic rays. Earth's magnetic field blocks them; Mars has none, and a significant fraction of brain cells could be damaged within a year. Tunnels or shielding can be engineered. It is not a showstopper, he says, and it is not as rosy as Musk makes it sound.

A few months after Einstein's November 1915 paper on general relativity, Karl Schwarzschild — director of the Potsdam Observatory, fighting on the German front — mailed a postcard: he had found a solution, the black-hole solution. He died shortly afterward. Einstein the pacifist lived. Loeb's lesson is grimly practical: to work out a theory's consequences, you have to survive. In 1939 Einstein still wrote in the *Annals of Mathematics* that the solution would not be realized in nature, because stellar spin would stop a collapse to a point. Loeb defines a black hole as the ultimate prison: you can check in, even light cannot leave, and past the Schwarzschild radius you will never tweet what you saw.

Quasars, found in the mid-1960s, were the first strong hint they are real: point-like sources a hundred times brighter than their host galaxies, sitting at galactic centers. Ed Salpeter in the West and Yakov Zeldovich in the East argued that infalling gas swirls, rubs, and heats until the darkest objects in the universe make the brightest lights. Those quasars are visible to the edge of the observable cosmos. When the universe was about 7 percent of its present age, billion-solar-mass black holes already existed — giant babies in a nursery. In 2015, a century after Einstein's theory, LIGO heard gravitational waves from two black holes colliding. The messenger was new; the message matched Schwarzschild spacetime. Nobel Prizes in 2017 and 2020 followed, the latter split between Roger Penrose's 1960s proof that collapse makes black holes — Stephen Hawking contributed but had died — and Andrea Ghez and Reinhard Genzel's movie of stars whipping around a four-million-solar-mass object at the Milky Way's center at thousands of kilometers per second.

With undergraduate Amir Siraj, Loeb asked where the nearest black hole is, and whether Planet Nine — a hypothesized five-Earth-mass perturber in the outer solar system — might be one, perhaps a leftover of early-universe dark matter. Icy Oort Cloud rocks that graze it would flare; Vera Rubin could see those flares or set a limit if they never appear. A basement flood at his house, clogged by tree roots in the sewer, became an essay on the singularity: water goes to a reservoir, but where does matter go inside a horizon? A Planck-density object at the center, or a tunnel somewhere else? As founding director of Harvard's Black Hole Initiative, he once closed a conference by wishing for a field trip and recommending that string theorists go in to test their theory. A theorist in the audience accused him of an ulterior motive. He did not deny it.

Unifying quantum mechanics and gravity, he says, will fall into our lap once there is evidence from regimes where both matter — black-hole interiors, or the beginning of the universe — not from sitting in an office the way Einstein's luck with relativity suggested. Hawking radiation turns the information paradox into a live puzzle: throw the *Encyclopaedia Britannica* into a hole that, classically, has only mass, charge, and spin, then watch it evaporate into radiation that seems to carry no information, against quantum rules. String theory, in Loeb's view, is a multitude of unconstrained stories that still cannot predict those regimes. Until the data arrive, banging on a theory of everything is the wrong wall. Dark matter is the same assignment: look for anomalies, or a laboratory detection, rather than betting the Large Hadron Collider would reveal supersymmetry. It did not.

Loeb enjoys working with young people more than with senior colleagues, because they are not yet loaded with prejudice or self-importance. The lesson that took him too long, and cost him ideas, was believing experts. More than once he took an original suggestion to someone established in the field; the expert dismissed it because it was new, not because it failed on the merits. Someone else overheard, developed it, and it became the hottest topic in the area. After enough of those, he decided he was done: experts repeat what they already know and feel threatened by invention. He does not care about likes or about what they say. He will use his own judgment, and if he is wrong, that is part of the work.

His mother, born in Bulgaria, immigrated to Israel after the war, left university for farm work, and finished her degree only after the children were grown. She planted the valuation of knowledge; his taste for philosophy came from sitting in on her university classes as a teenager. Physics was circumstance: at eighteen, military service, the Talpiot program, a Ph.D. at twenty-four, then a five-year Institute for Advanced Study fellowship that required switching into astrophysics — an arranged marriage that became, he says, the love of his life. His father, a man of few words on the farm, taught that doing things matters more than talking about them. Among philosophers he names Sartre, Camus, and Nietzsche. What he takes from Nietzsche is the line that the best way to corrupt the young is to tell them they should agree with the common view.

On death he quotes Epicurus: while he is here, death is not; when death is here, he is not; they never meet. Finitude still has a job. It should not depress you into losing vitality. It should produce two things: appreciation of being alive, and modesty, because an arrogant person who remembers he is short-lived is a contradiction. Roman generals, he notes, kept someone at their shoulder to whisper that they were mortal. Asked for the meaning of it all, he would like to put the question to an alien, and he expects silence. Things exist; civilizations perish; a slogan will not be around in a billion years. What remains is to enjoy the process, the way you enjoy a good meal. Science is opening the watch. It does not replace the meaning you assign. It lets you appreciate the merchandise.