Harvard's Avi Loeb more sure than ever we were visited by alien spacecraft

The renowned scientist made waves claiming the interstellar object Oumuamua might be extraterrestrial. Now he's doubling down.

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Avi Loeb says Oumuamua may have actually been an alien light sail.

The Planetary Society

Professor Avi Loeb has had a pioneering career in astronomy, astrophysics and cosmology. He's authored hundreds of academic papers on topics from black holes to the early days of the universe, collaborated on projects with the legendary Stephen Hawking and helmed Harvard's astronomy department for almost a decade, longer than anyone in the department's history. 

But despite an impressive resume that runs deep inside some of the world's most revered institutions, Loeb has found himself at odds with the mainstream of science in recent years over his most controversial hypothesis. He's become increasingly convinced a space object many other astronomers assume is just a peculiar space rock is really a piece of alien technology sent in our direction by some sort of extraterrestrial civilization.

"I submit that the simplest explanation for these peculiarities is that the object was created by an intelligent civilization not of this Earth," Loeb writes in the introduction to his new book about Oumuamua, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth

Back in 2017, astronomers (Loeb was not among them) spotted a strange object flying away from Earth with an unusual shape, tumbling end over end and accelerating as it sped out of the solar system. Even more remarkable was that it appeared to originate from beyond our solar system and was just passing through -- the first object we'd ever detected from outside our corner of the cosmos.

This first-ever interstellar object was nicknamed Oumuamua, a Hawaiian word that roughly translates as "scout" and is pronounced "oh-MOO-ah-MOO-ah." Scientists around the world got to work analyzing the limited data on the odd object. Because Oumuamua was only discovered after it had already entered, passed by the sun and Earth and begun to exit our solar system, few telescopes were able to get anything approaching a good close-up image.

Avi Loeb
Enlarge Image
Avi Loeb

Avi Loeb, chair of Harvard's astronomy department, ignited an academic firestorm by suggesting that a mysterious object that hurtled close to Earth in 2017 was perhaps an artificial object sent from an extraterrestrial civilization.

Lane Turner/The Boston Globe via Getty Images

"It's like having a guest for dinner and realizing the guest is weird only when he exits the front door and goes into the dark street," Loeb told me over Zoom earlier this month. 

Loeb is, in many ways, the picture of what you might expect a Harvard astronomy professor to look like. He speaks to me from his personal study dressed in a sharp suit and glasses, but his ideas about Oumuamua are far less conservative than his wardrobe. 

A central feature of Oumuamua's weirdness is how it appeared to accelerate as it departed our cosmic neighborhood, just like an outgassing comet might. The problem is Oumuamua had no visible cometary tail. Basically, it looked like a very odd asteroid, but acted like a comet.

This was all fascinating and perplexing for scientists to study. But the sight really grabbed the attention of the larger public when Loeb and one of his graduate students dropped a paper in late 2018 suggesting Oumuamua could be a "light sail" (a type of spacecraft pushed by the momentum of light particles in space) built by a technologically advanced alien civilization.

Suddenly, Oumuamua, Loeb and his hypothesis went viral, bringing a wave of media attention and a backlash from many space scientists. Some saw Loeb as calling forth images of E.T. primarily for publicity, while others accused him of seeing light sails everywhere -- Loeb is on the Breakthrough Starshot team working to send a light sail to the nearest star beyond the sun, Proxima Centauri.

In 2019, a group of astronomers, including those who originally discovered Oumuamua, published a paper of their own dismissing all alien theories and declaring their relative certainty in Oumuamua's natural origins.

"Assertions that Oumuamua may be artificial are not justified," the paper concludes.

"Well, I'm not surprised by this," Loeb said when I asked him about that rebuttal. "If you show a cellphone to a caveman that looked at rocks all of his life, the caveman would conclude that the cellphone is just a well-polished rock. ... You need to be open-minded in order to find wonderful things." 


Loeb's book is out now, from Houghton Mifflin Harcourt.

Houghton Mifflin Harcourt

This is one of the central messages in his new book, which lays out the light sail hypothesis again in lay language, but spends almost as many pages responding to the backlash against it and what Loeb sees as a crisis in science. He believes cadres of scientists sometimes come together to establish authority and push back at more far-out notions like distant advanced civilizations, which have often been considered unworthy of serious scientific study over the decades.

"Some of the resistance to the search for extraterrestrial intelligence boils down to conservatism, which many scientists adopt in order to minimize the number of mistakes they make during their careers," Loeb writes in his book.

Somewhat ironically, he sees astronomers coming up with their own far-out ideas to explain Oumuamua as a natural object, such as a cosmic "dust bunny" being pushed by light or an iceberg made almost solely of pure hydrogen, both phenomena that have never been directly observed before.

"So when people try to attend to the details [of the data on Oumuamua]," Loeb told me, "they want to explore things we have never seen before. And my point is, if we have to contemplate things we have never seen before and artificial origin is one of them, why not put it on the table?"

Extraordinary claims

Scientists like Natalie Starkey, who analyzes comets and asteroids, say an alien explanation is on the table, just at the very far end of it.

"What we have to do first is rule out all the more natural ideas about what this thing could be," Starkey, from the UK's Open University, said of Oumuamua on the Jan. 18 episode of StarTalk Radio with Neil deGrasse Tyson.

Starkey also cites the popular maxim "extraordinary claims require extraordinary evidence," a standard established by famed astronomer Carl Sagan that I've often looked to myself when vetting the steady flow of purported UFO and alien evidence that lands in my email.

But in his book, and in conversation with me, Loeb makes a point of dismissing this time-honored phrase.

"The word extraordinary really is quite subjective. ... I think science should be based on evidence, period. Of course, if you want an airtight argument, you want more evidence, but we should not dismiss options because the evidence is not tight enough."

He says the past few years and the response of other scientists have actually made him more confident in the chances the light sail hypothesis for Oumuamua could be correct. He believes experts in the field have failed to come up with natural scenarios that make more sense.

The range of reactions from scientists to Loeb's hypothesis is interesting. He was never mentioned by name in the aforementioned StarTalk episode that was all about Oumuamua, or in the 2019 academic paper rebutting his widely circulated hypothesis (except in the end notes). Other big-name scientists, like CalTech cosmologist Sean Carroll, have given Loeb and his hypothesis a platform. The Jan. 25 episode of Carroll's Mindscape podcast was dedicated to a wide-ranging conversation with Loeb.

Toward the end of our conversation, I share my own hypothesis with Loeb, not on the origins of Oumuamua, but on how he's come to be so at odds with others in his field on the topic. (You can watch the full conversation below.)

My theory is that Oumuamua, with its vague weirdness and our limited data on it, is a perfect blank canvas for our own projections, a Rorschach test. Some of us hope and dream and even desperately want to know humanity isn't alone in the universe. Many of these same people might believe it's our destiny to reach farther out into the universe, to travel to other planets and beyond. My extensive experience writing, thinking and talking a lot about space tells me this is a minority outlook.

I think (based solely on my very unscientific polling of friends and family over the years) people are more likely to take a pessimistic view of things like building a city on Mars that completely captivate people like Elon Musk and his fans. Most people think humans have such a hard time managing our myriad problems on Earth that it makes no real sense to move to another planet or that it probably wouldn't work out if we tried.

I told Loeb I think he falls into the former category of dreamers. In his book he even writes about his desire to see humans send probes equipped with samples of human DNA out into the cosmos as a sort of backup for our species. As such, I say, I think he projects this more optimistic vision of a universe filled with intelligent civilizations traversing the cosmos onto Oumuamua.

Perhaps, I suggest, other scientists don't share Loeb's optimism about the prospects for intelligent life in the universe. Or maybe they're just more interested in fitting Oumuamua into a preexisting understanding of what the universe is and how it works. The point is that I wonder if the real mystery of the origin of Oumuamua lies in the eye of whoever beholds it.

"I've never heard that before from anyone," Loeb tells me. "But I think you got it."

Waiting for the next scout

Loeb and his critics agree on one key thing: Aspects of Oumuamua's weirdness are difficult to explain without entertaining phenomena we haven't seen before -- be they aliens or pure hydrogen icebergs. And we're unlikely to ever prove which theory might be correct.

At least not in the specific case of Oumuamua. But Loeb is hopeful its pass by Earth wasn't a once-in-a-lifetime sort of visit.

He's optimistic that sensitive equipment, like the upcoming Vera C. Rubin Observatory in Chile and its very wide view of the sky, could be able to find an object like Oumumua every single month.

"And then if one of these objects approaches us, we could send a camera close to it, take a photograph and I would be the first to agree that if we see a rock, then it's natural. But if we see something unusual, we should check it."

In other words, we'll likely never see Oumuamua again or figure out exactly what it was, but it still could be a piece to a much larger puzzle that eventually helps us see the much bigger picture of the universe and our place in it.

Loeb's book ends on a line in the same spirit, which even his most ardent critics would surely agree with:

"The detective work, in short," he writes, "goes on."