When I was growing up, I watched a lot of sci-fi movies about aliens that come to Earth. The extraterrestrials in popular culture, however, always looked so familiar that I found them far-fetched. What are the chances that E.T., the Predator, or ALF would develop arms and legs, a humanlike face, and opposable thumbs? Perhaps as a result, I associated alien life more with fantasy than with science, and I never gave much thought to what a visit would really look like. But my attitude started to change in 2020, when I read Liu Cixin’s “The Three-Body Problem” and its two sequels. In Liu’s books, creatures called Trisolarans send a scouting mission of supercomputers to spy on and subtly disrupt human affairs. Although Trisolarans could do seemingly impossible things, such as program protons, Liu’s rigor got me thinking about aliens from a scientific perspective. Suddenly, I could imagine a sophisticated civilization coming into contact with humanity, perhaps in ways that we don’t immediately understand.

Then, in 2021, the Office of the Director of National Intelligence released a report on unidentified anomalous phenomena (U.A.P.)—essentially a rebranding of U.F.O.s. Several Navy videos had been made public a few years prior. In the so-called GOFAST video, recorded off the coast of Florida in 2015, a Navy pilot with an infrared camera follows an object zooming above the water and asks, over the radio, “What the fuck is that?” Another clip, deemed GIMBAL—“Look at that thing, dude”—showed a similar shape above some clouds. A third video, known as FLIR, was taken in 2004 from an aircraft in California. Navy pilots in two planes saw what looked like a large Tic Tac hovering over the water; it seemed to zip away at thousands of miles per hour. Military whistle-blowers subsequently claimed that the government knew more than it was admitting, leading to congressional hearings in 2022, 2023, and 2024. Last month, House Representative Eric Burlison, a Republican, introduced the U.A.P. Disclosure Act of 2025, aimed at preserving and eventually releasing U.F.O. reports.

None of the government’s disclosures demonstrated that Earth had welcomed interstellar house guests. And yet, after the releases and hearings, it seemed more acceptable to explore the possibility. In 2022, roughly fifteen hundred university faculty members replied to a survey about U.F.O.s; a majority said that recent governmental and journalistic reports had increased the topic’s credibility, and three-quarters said that it was of average importance, very important, or essential for academics to conduct more research about it. Tyler Cowen wrote about U.F.O.s for Bloomberg and Ross Douthat wrote about them for the New York Times; they compared notes on Cowen’s podcast. On the prediction platform Polymarket, the odds that the U.S. will “confirm that aliens exist in 2025” have ranged between four and fourteen per cent. (The detection of aliens on a faraway planet would count.)

I started to ask myself, How likely is it that we’ve ever been visited by aliens or their technology? It seemed improbable yet plausible. I wondered how scientists, engineers, and other thinkers would approach the question. What would count as evidence, and what kinds of educated guesses could we make? I decided to call Adam Frank, an astrophysicist at the University of Rochester and the author of “The Little Book of Aliens,” which looks to differentiate between science and fiction.

Frank doesn’t put much stock in U.F.O. videos, and he told a story to explain why. In February, 2023, photographs of a Chinese spy balloon over Billings, Montana, prompted speculation about aliens. The Air Force eventually shot it down, but first the pilot of an American U-2 spy plane flew past and took a selfie that showed the balloon out the window. “You can see it in exquisite detail,” Frank told me. “Where are all those pictures? Every U.F.O. picture is a fuzzy blob. Everybody carries a high-resolution camera in their pocket now, and it’s always fuzzy blobs.”

There are potential answers to Frank’s question, but most of them raise questions of their own. Maybe there’s a coverup. But, if so, wouldn’t whistle-blowers have turned up something by now? “Color me extremely skeptical that any government could keep a secret like this effectively for a week, let alone decades,” Austin Carson, a political scientist at the University of Chicago who has written extensively about government secrecy, told me. Steven Aftergood, who directed the Project on Government Secrecy at the Federation of American Scientists, pointed me to a 1970 report produced by a government task force on secrecy: “It is unlikely that classified information will remain secure for periods as long as five years, and it is more reasonable to assume that it will become known to others in periods as short as one year.”

Maybe the aliens are coy and want to stay hidden. If that’s the case, though, why are we seeing them at all? Mark Rodeghier, the scientific director of the Center for U.F.O. Studies, which is headquartered in his basement, told me, “They’re actually trying to slowly acclimatize us to the idea that aliens, in fact, exist.” Robert Hampson, a neuroscientist at Wake Forest University who has written over a dozen science-fiction novels, speculated that, perhaps, “what we’re detecting are the alien equivalent of graduate students who have been given an assignment to go and watch the humans and report back.” Plenty of human graduate students have been tasked with researching other worlds—and they don’t always get the best equipment.

The elaborateness of these explanations, in Frank’s view, is a reason to be skeptical of them. “If you’ve got to go through all those contortions to make your story work, you’re not doing science anymore,” he said. He noted that, when humans send spacecraft into the solar system, they tend to land safely. Alien craft, in contrast, “would have managed to cross interstellar distances, and these things seem to crash every fifteen minutes. It’s like everybody’s sending us their 1987 Dodge Omnis.” The 2021 national-intelligence report on U.A.P.s said that it could not explain a hundred and forty-three U.F.O. reports, but that many were probably “airborne clutter,” weather, or terrestrial technology. “Sometimes they’re hard to explain because we just don’t have the data,” Frank said. In his opinion, high-quality data (high-resolution photography, for example) has never “showed us that anything required an extraterrestrial hypothesis.”

Our galaxy contains at least a hundred billion planets, and biology finds a way in extreme environments on Earth, so it’s not unreasonable to suppose that life thrives elsewhere in the Milky Way. But if aliens wanted to visit us they would have a long way to go. The closest star to the sun, Proxima Centauri, is roughly 4.2 light-years away. NASA’s Voyager 1 space probe would take more than seventy-five thousand years to get there, if it were headed in the right direction. The closest inhabited solar system could be much, much farther away. Could aliens, or at least their tech, survive that journey?

Avi Loeb, a Harvard astrophysicist, thinks they may have already. In 2014, military satellites observed a half-metre-long meteor in the sky near Papua New Guinea. Several years later, Loeb and a student, Amir Siraj, concluded that the meteor had been travelling too fast to be orbiting the sun, and that, therefore, it was interstellar. Loeb went on to argue, over the objections of many experts, that it may have been alien technology, and that debris he recovered on a Pacific expedition was from the meteor. “Think of it as Amazon delivery service, but from interstellar space,” he told me during the expedition, from a ship that was incidentally called the Silver Star. He and numerous collaborators wrote up their findings in Chemical Geology last year. (Cosmochemists questioned the claim that the debris was interstellar; one told Science, “I’m surprised anyone would take it seriously.”) Loeb currently leads the Galileo Project, which is erecting ground-based sensors to look for anomalous phenomena.

U.F.O.s could contain biological aliens or machines powered by artificial intelligence. Either way, they might travel much faster than Voyager 1—but that would require a lot of energy. Les Johnson, a retired chief technology officer at NASA who has written fiction and nonfiction books about interstellar travel, gave me an example. To accelerate a pineapple to just a tenth the speed of light, he said, would require the energy of seven Hiroshima explosions. You would need the same quantity of energy to decelerate at your destination. “Suddenly, you’ve got the energy of fourteen Hiroshima bombs on a pineapple,” he said. “So I look at that, and I think it just doesn’t pass the giggle test.” At that speed, a speck of space dust would also have the impact of T.N.T. “I’d love to think we’re in a ‘Star Trek’ universe,” Johnson said. “But I don’t know if we are.”

Logically speaking, the chance that aliens are here right now must be slimmer than the chance that they have been here at some point in the past. Garry Nolan, a biotech founder and an immunologist at Stanford, has a hunch that we’ve been visited by aliens—he had an eerie experience as a child—and speculates that they or their technology may have been on Earth for millions of years. “So the open question might be, Is this even our planet?” he asked me. As outlandish as his theory is, it’s tricky to firmly disprove. If they came long ago, and left or died, would we even know? In 2018, Frank and a co-author published a paper in the International Journal of Astrobiology titled “The Silurian Hypothesis.” (In “Doctor Who,” Silurians are advanced reptilian humanoids who predated us on Earth.)

“Would it be possible to detect an industrial civilization in the geological record?” Frank and his colleagues ask. Organisms can turn into fossils that last tens of millions of years, but most organisms don’t become fossils; metals and plastics might not stick around at all. And the planet’s surface constantly erodes and churns. “Our claim was that, after about a couple million years, anything on the surface is gone,” Frank said. “Even if some aliens came and built a pretty intense civilization, you wouldn’t have any evidence of it.” The best evidence one might hope for, he argued, was indirect: for example, an unusual proportion of certain isotopes at particular depths of rock. (In a 2019 paper, a physicist suggested that we should look for old probes, or “Lurkers,” on space rocks near Earth.) So far, humans haven’t found anything like that.

Science fiction often explains interstellar travel by imagining some kind of warp drive. Einstein said that nothing can travel faster than the speed of light, but, in 1994, the Mexican physicist Miguel Alcubierre found an exception: Einstein’s theory of general relativity actually allows for a craft to outpace light by squeezing space-time at its bow and stretching it at the stern. Nolan, who is a fan of this idea, likened the Alcubierre drive to “creating your own sub-universe, a warp bubble around yourself.” Of course, it is not at all clear that such a thing could be constructed. Alcubierre invoked what physicists call negative energy; later theorists thought that lots and lots of regular energy could be used instead. Nolan speculated that, maybe, sufficient energy could be harvested from space-time itself, using the zero-point field and quantum tunnelling. But he quickly admitted that “those are just fancy words that we throw at the unknown.” (On the plus side, a warp drive might quote-unquote explain the Navy’s U.F.O. videos; one paper estimated that the objects in the videos appeared to be pulling up to five thousand Gs, using no visible propulsion and leaving no wake. Stunt pilots max out at around ten Gs.) Other U.F.O. enthusiasts argue that aliens could reside in dimensions beyond the four that we know and love. Or maybe they’re time travellers.

I ran these notions by Arlan Andrews, a retired mechanical engineer and author who is the founder and director of SIGMA, a think tank of sci-fi writers that advises the government and N.G.O.s. He responded by throwing up his hands in exasperation. “If you start to do the woo-woo stuff with interdimensional and time travel, I can’t say they’re wrong, but there’s no place to start,” he told me. “As an engineer, I like to have a starting place.”

A civilization that’s millions or billions of years ahead of us would probably know physics that we don’t yet comprehend. Its technology might seem like magic to us. In that case, practically anything is possible. A NASA report on U.F.O.s, released in 2023, acknowledges that “it is difficult to put physical constraints on them at present.” How would you turn something without constraints from woo-woo into science?

The Society for U.A.P. Studies (SUAPS) is a think tank attempting to define U.A.P. studies as a field. “Not only is there not a science but there’s no academic field,” Michael Cifone, a philosopher of science at St. John’s University and one of the group’s founders, said. “How do we study this phenomenon? Who’s involved? What are the methodological principles that should guide us?” He cares less about people’s out-there theories than about what steps should be taken to resolve U.F.O. cases. He wants to avoid “endless, unconstrained, undisciplined speculation.” In his view, we’re at “a crucial transition point between the older ‘ufology,’ ” akin to forensic investigation, and modern scientific methods.

The first step to making something a science usually involves data. NASA’s report focusses on the agency’s potential role in collecting data, and in using A.I. to analyze it. According to the report, the agency’s sensors could be supplemented with crowdsourced data from apps; one example is Enigma, which uses algorithms to rate the credibility of people’s sightings and triangulate objects using video recordings. UAPx, a Florida nonprofit, has developed a suite of sensors specifically for analyzing odd aerial phenomena. And then there’s the data contained in past reported sightings, which could be regularized and collated somehow. Ryan Graves, a former Navy pilot who testified about U.F.O.s in Congress, founded Americans for Safe Aerospace, which encourages pilots to report U.F.O.s. The group vets the cases and has brought them to Congress; the All-domain Anomaly Resolution Office (AARO), which is part of the Department of Defense; and, lately, to an F.B.I. working group. But, as some argue, the plural of anecdote is not data. “I don’t trust pilots’ sightings,” Matt Mountain, the president of the Association of Universities for Research in Astronomy, told me. “They’re on a mission, not doing scientific exploration.” We should keep in mind, Mountain told me, that most criminal convictions that are overturned by DNA evidence were based in part on eyewitness testimony.

Graves said that he was agnostic about aliens. “I’m not jumping to conclusions,” he told me. “But I want to figure it out, damn it.” Still, he seemed skeptical of expert analysis. In November, 2024, AARO announced that it considered the GOFAST video mystery resolved; the object’s altitude, it said, gave it an exaggerated appearance of speed. “Through a very careful geospatial intelligence analysis and using trigonometry, we assess with high confidence that the object is not actually close to the water, but is rather closer to 13,000 feet,” the agency’s director told CBS. Graves wasn’t satisfied with that explanation. “Some of these cases aren’t quote-unquote debunked or no longer of interest,” he has said.

When I spoke to people who supported an alien hypothesis, I was often struck that relatively patchy evidence—what Frank had described as fuzzy blobs—had inspired such in-depth theories. I’m as puzzled by the U.F.O. videos as the next guy, but, as far as I can tell, nothing in them requires us to accept the existence of warp drives or time travel. Until someone produces high-definition videos of flying saucers and little green men, the evidence might not be extraordinary enough to demand such extraordinary explanations. And to the extent that U.F.O.s do need explanations it’s worth asking whether aliens are the best one. People have seen weird stuff in the sky for thousands of years, and only in the last eighty have flying saucers been a popular interpretation. Before that, we tended to credit the supernatural.

After all my conversations, I thought the odds that aliens or their tech had visited Earth were probably south of five per cent. Most U.F.O.s. are likely balloons, airplanes, weather events, visual illusions, or technical glitches. Even so, there are enough unknowns, and unknown unknowns, that the margin of error seemed enormous. A person who thinks the odds are much higher, I’d argue, shouldn’t be met with ridicule.

One more question: Why would aliens even visit us? Coming up with an answer forces us to speculate about alien motives. “I’ve always said that aliens are going to alien,” Andrews, the sci-fi writer who founded SIGMA, told me. “We have no idea what an alien person or being or intelligence or machine would be motivated by. . . . We don’t know what motivates the Kremlin, for God’s sake.” Extraterrestrials, he suggested, could be as different from us as we are from centipedes. (That could be an understatement.) Robert Powell, a chemist who serves on the board of the Scientific Coalition for U.A.P. Studies, has argued that some U.F.O.s are intelligently controlled and do not come from Earth. But, in his view, we shouldn’t try to study extraterrestrials as if they’re lab rats. “We’re the rat, and we’re trying to figure out what the doctor is doing,” he said.

It’s reasonable to think that there would be commonalities among intelligent species. One is the drive for self-preservation, which motivates the Trisolarans in “The Three-Body Problem.” Hampson, the neuroscientist and sci-fi writer, didn’t think that was a very good reason to come to Earth. “If they’re after resources, there are easier ways to get them,” he said. “Why would you go to an already inhabited world?” Meanwhile, a warlike species would probably just wipe us out. “If the idea is conquest, then I think we would already know,” he said. But he could imagine another reason: curiosity.

Nick Pope, who ran the U.F.O. program at the U.K.’s Ministry of Defense from 1991 to 1994, told me that the ministry had considered three main motives for alien visits: military reconnaissance, scientific study, and tourism. It’s not a given that aliens would enjoy travel. Maybe we’ve never met them because they’re homebodies. Still, what kind of intelligent life form wouldn’t want to see a sunrise on another world? “This might be the only place in the universe to see an elephant,” Pope said. “How many planets have a Stonehenge or a Machu Picchu or a Great Pyramids?” Aliens might want to survey the galaxy for all life forms—including us. They might even be having the same debate we are. Are we alone in the universe? The truth—whatever it may be—is still out there. ♦