Extraterrestrial

by Avi Loeb

  This, too, was not an entirely original thought. The concept of a laser-propelled sail was invented the year I was born (1962) by the visionary Robert Forward and subsequently developed by other scientists, such as Phil Lubin, to include miniaturized electronics and modern optical designs. But never before had it been so close to becoming a reality.

  We calculated that a 100-gigawatt laser beam capable of targeting a sail roughly the size of a person for a few minutes would propel the sail, along with its attached camera and communication device, to one-fifth of the speed of light by the time the spacecraft was five times as far away as the moon. This would be the spacecraft’s open runway, so to speak. Across this distance, the laser beam could launch the spacecraft with enough speed to allow it to reach the nearest star within our lifetimes.

  Everything we proposed was within existing technological bounds. Difficult? Yes. Expensive? Somewhat; it was on the order of one of the biggest science projects, like the Large Hadron Collider at CERN or the James Webb Space Telescope, but cheaper than the Apollo moonshot. (Many people who heard about the Starshot Initiative confessed that they hadn’t been as excited about space exploration since the Apollo mission fifty years earlier.) But it would also be efficient. Once built, the launch system could be used to send lots of such craft—which are perhaps better thought of as probes, though we fell into the habit of referring to them as StarChips.

  Top: Artist’s impression of Starshot, a project to push a lightsail from Earth with a powerful laser beam. Middle: An example of the lightweight electronic device (StarChip) that could be attached, along with a camera, to the sail. Bottom: Photograph of the solar LightSail 2 deployed by the Planetary Society on July 23, 2019, with the Sun showing through the thirty-two-square-meter sail.

  Breakthrough Starshot/A. Loeb (top and middle images) and Planetary Society (bottom image)

  Just a few months into this endeavor, in August 2015, I coauthored a paper with my postdoc James Guillochon on lightsails. In it, we mused that since humans had been able to come up with this technology, other intelligent life might too. Building on that hypothesis, we argued that scientists should seek out the sort of microwave beams that extraterrestrial life might use to send such craft of their own among the stars.

  When our article was published in the Astrophysical Journal in October 2015, there had not yet been a formal announcement of the Starshot Initiative, and James and I had broached only one consequence of my team’s exploration of the possible solutions to the Starshot challenge. But it was rewarding to have a published article arise out of our earliest evaluations of Yuri’s proposal.

  The paper’s publication also had an unanticipated consequence: the media paid attention. The media’s interest hadn’t been one of the goals of my research, which followed the same principles that had guided my previous hypotheses on the nature of dark matter, the first stars, and black holes. In retrospect, I realized this unexpected media attention was a sign of things to come.

  …

  Back at Harvard, we continued to work, and six months after I met Yuri and Pete Worden, I received the expected phone call from Pete. He and Yuri wanted me to report my team’s findings. They wanted me to do this at Milner’s California home. And they wanted me to do it in two weeks.

  I had been ambitious when I asked for a mere six months to draw up a plausible plan for reaching the nearest star in around twenty years. Now I had to quickly sum it all up in a way that would convince a small panel to fund it. And the board would be rounded out by Stephen Hawking, by then the most renowned living theoretical astrophysicist. He was not the only scientific luminary who would be reviewing what I submitted. Freeman Dyson corresponded with me routinely about my research, and he had started showing interest in Starshot as well.

  When Pete reached me, I was on vacation and just on my way out the door of my hotel room to visit a peaceful and secluded goat farm in the Negev region of southern Israel where my wife wanted to spend the weekend. So the following morning I prepared a presentation while sitting outside with my back against the wall of the office of the goat farm—the only place with internet connectivity.

  For me, it was ideal. The weather was cool and dry. I looked out on the goats that had been born the previous day. It was all very familiar, reminding me of the farm I had grown up on with my two sisters, Reli and Shoshi. Among my tasks had been collecting eggs and helping to corral the farm’s just-born chicks when they escaped their cages. It was in this familiar setting that I typed out my plans for humanity’s first interstellar probe using lightsail technology.

  Two weeks later, I visited Milner’s home in Palo Alto and announced that we had a plan that fulfilled the mandates he had set out. Within our lifetimes, it was technologically feasible to send a craft to Proxima Centauri.

  Yuri was pleased and excited, as was Pete. After several months of extensive discussions, they decided to publicly announce the Starshot Initiative at the observatory atop One World Trade Center in New York City on April 12, 2016. This was Yuri’s Night, a commemoration of the launch of the first human into space, Yuri Gagarin, on April 12, 1961. There, on the stage with Yuri Milner, I was joined by Freeman Dyson and Stephen Hawking. The historic vision presented by the panel was recorded by TV crews and broadcast all over the world. The following morning, my wife brought our car in for an oil change, and the mechanic, who was used to seeing me perform this chore, asked where I was. Ofrit mentioned that I could not do it because of the announcement, and he replied: “This project is amazing; I read all the news reports about it.” The vision of visiting another star within our lifetimes captivated the public’s imagination in a way reminiscent of the Apollo 11 moon landing.

  Just seventeen months after that presentation, ‘Oumuamua was discovered by the telescopes of Pan-STARRS.

  …

  Let us pause here briefly to recap the evidence that emerged about ‘Oumuamua in the weeks immediately following its discovery. It was a small, oddly shaped, shiny object that deviated from an orbit shaped by the Sun’s gravity without showing any discernible cometary tail (caused by the outgassing of a comet’s ice turning to steam by friction and the warmth of the Sun) despite a deep search for it with the Spitzer Space Telescope and other detectors.

  These are certainties, and they allow us to declare confidently that the first three of ‘Oumuamua’s identified anomalies—its unusual orbit without a tail, its extreme shape, and its luminosity—make it statistically different, by a large margin, from all other objects cataloged by humanity. To put this distinctiveness in statistical terms, we can conservatively state that based on its extra push and its lack of a cometary tail, ‘Oumuamua is a one-in-a-few-hundred object. Based on its shape, it is, also conservatively, a one-in-a-few-hundred object. And based on its reflectivity, it is (again, conservatively) at least a one-in-ten object. When we multiply those three anomalous qualities, we can appreciate how much of an outlier ‘Oumuamua is. It is now a one-in-a-million object.

  Those three traits—orbit, shape, and reflectivity—do not exhaust ‘Oumuamua’s weirdness, as we know. Alone, however, these three traits do clearly defy the understandable but naive expectation that our first interstellar visitor would resemble the rocky asteroids and icy comets that we know have passed through our solar system.

  Yet even as these anomalies mounted, most scientists clung to the most familiar explanation: that ‘Oumuamua had to be a naturally occurring object, an asteroid or comet. Most scientists—but not all. Even our community, you see, has its anomalies.

  With my work on the Starshot Initiative fresh in my mind, I, for one, was finding myself pulled toward a different hypothesis.

  5

  The Lightsail Hypothesis

  In early September 2018, just about a year after ‘Oumuamua passed overhead, I wrote an essay for Scientific American on what the search for relics of alien civilizations—specifically, dead civilizations—might entail. Based on Kepler satellite data, I argued, we knew that about a quarter
of all stars hosted habitable Earth-scale planets. Even if only a small fraction of all habitable Earths led to technological civilizations like our own during the lifetime of their stars, there might be plenty of relics out there in the Milky Way for us to explore.

  Some of these habitable worlds, I theorized, might have evidence of previous civilizations, anything from atmospheric or geologic traces to abandoned mega-structures. But even more intriguing was the possibility that we would find flying through our solar system technological relics with no detectable functionality—for example, pieces of equipment that had lost power over the millions of years of their travel and become space junk.

  I went on to note that it was entirely possible we had already found one such technological relic. Noting the discovery of ‘Oumuamua the previous fall, I summarized the anomalous evidence we had accumulated about it and then posed a rhetorical question: Given its deviation from its expected orbit and its other peculiarities, “might ‘Oumuamua have been an artificial engine?”

  Like my idea of eavesdropping on alien civilizations, it was just a passing thought. And I might have been content to let it stay that way if I could have gotten the StarChips out of my head.

  …

  About that time, a new postdoctoral fellow, Shmuel Bialy, arrived at Harvard’s Institute for Theory and Computation, where I serve as director. I proposed to him that we collaborate on a paper explaining ‘Oumuamua’s excess acceleration through radiation from sunlight. Because of my previous work on lightsails in conceptualizing the Starshot Initiative, I was familiar with the scientific constraints and possibilities that interstellar travel by lightsail technology presented. The relevant formulas were all fresh in my mind and ready to be applied to possibly explain the peculiar force exerted by sunlight on ‘Oumuamua. To be clear, my attitude at the time was simply That might work. The astronomical world had been presented with an exciting discovery, an interstellar object, about which we had collected a trove of confounding data. We confronted facts that were hard to match to a hypothesis that accounted for all of them. When I proposed that Bialy and I explain ‘Oumuamua’s deviation by way of sunlight, I was following the same scientific tenet I had always followed—a hypothesis that satisfied all the data ought to be considered.

  Bialy checked the numbers and his excitement grew; the idea I had proposed looked like a viable possibility. This led to a new question: What would we need to assume about ‘Oumuamua’s size and composition to explain its deviation? The key question was how thin ‘Oumuamua had to be in order to possess the extreme area-to-volume ratio that accounted for its excess acceleration. We determined that ‘Oumuamua needed to be less than a millimeter thick for the force of sunlight to be effective.

  Artist’s impression of ‘Oumuamua as a lightsail (left) alongside a conventional rendering of the object as an oblong, cigar-shaped rock (right).

  Mark Garlick for Tähdet ja avaruus/Science Photo Library

  The implication of this was obvious: Nature had shown no ability to produce anything like the size and composition of what our assumptions suggested, so something or someone must have built such a lightsail. ‘Oumuamua must have been designed, built, and launched by an extraterrestrial intelligence.

  It is an exotic hypothesis, without question. But it is no less exotic than other hypotheses that have been proposed to explain the outlier characteristics of ‘Oumuamua. Nature has shown no propensity to produce pure-hydrogen comets or fluffy clouds of materials that are both more rarefied than air and structurally cohesive. The extraordinary nature of our conclusion rested almost entirely on the presumption that it wasn’t a naturally occurring object.

  The lightsail inference may seem outlandish, but getting to it did not require any wild leaps. Shmuel and I went down a logical path. We followed the evidence, and, in the grand tradition of the detective work of science, we hewed closely to a maxim of Sherlock Holmes: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” Hence our hypothesis: ‘Oumuamua was artificial.

  We laid out these ideas in a paper titled “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” In it, we confronted a range of other questions about ‘Oumuamua. We described the likely damage it would sustain as it soared across the universe, for instance, from colliding with space dust or from the continuous strain of centrifugal force caused by its rotation. We discussed the impact that such damage might have on the object’s mass and speed and found it to be minimal. Laying out equation after equation, we drew conclusions from the available data about the object’s thickness and mass, which dictated its surface-to-volume ratio. And then, at the end of the paper, we put forth our hypothesis: “If radiation pressure is the accelerating force,” we wrote, “then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally . . . or is of an artificial origin.

  “Considering an artificial origin,” we continued, “one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as debris from advanced technological equipment.”

  We submitted the paper to the prestigious scientific journal the Astrophysical Journal Letters, which specializes in timely and high-impact papers, in late October 2018. Our intention was to engage the attention of our fellow scientists who we knew were weighing hypotheses against the evidence. This, too, should be considered. In that spirit, we also posted the manuscript before it had been peer-reviewed on the online preprint site arXiv.org. Science journalists regularly scour arXiv for stories, and in short order two of them found our study and raced to report our hypothesis. Their pieces went viral, and by November 6, 2018, roughly a year after ‘Oumuamua was discovered, everything blew up.

  …

  Within hours after the first media reports appeared, I was surrounded by cameras. While most of America was filing off to vote in the hotly contested midterm elections, four television crews crowded into my office on Garden Street in Cambridge, Massachusetts. I tried to field their questions while simultaneously responding to a steady stream of phone calls and e-mails from newspaper reporters.

  I had some experience with the popular press because of earlier papers I had written on an array of subjects, but this level of attention was new to me, and a bit overwhelming. It didn’t help that I was mentally preparing to depart for Berlin to give a public lecture, long in the works, at the Falling Walls Conference—an appropriately named gathering dedicated to celebrating breakthroughs that broaden society’s interest in the latest advances in science and technology.

  I raced home and grabbed my suitcase but didn’t make it back out to the car before another film crew arrived, having tracked down my home address. As I stood at my front door, the reporter asked, “Do you believe there are alien civilizations out there?”

  “A quarter of all stars host a planet the size and surface temperature of the Earth,” I said into the camera. “It would be arrogant to think we are alone.”

  By the time I deplaned in Berlin, members of the international media were responding much as the American media had. All of this before our paper was even published.

  Given the media’s attention and the need for us to present more of the factual basis for the hypothesis, the Astrophysical Journal Letters published our paper on November 12. They accepted it for publication just three days after I submitted it, the fastest turnaround time in my entire scientific career.

  I was grateful for the paper’s publication; it meant that an ever-widening circle of scientists was considering our hypothesis to explain the evidence left by ‘Oumuamua. But I was under no illusion that any appreciable part of the scholarly field would approach the theory that ‘Oumuamua had originated in an extraterrestrial civilization as just one exotic hypothesis among many. I presumed a majority would be reluctant to consider the idea and that some scientists would even be hostile. I was well aware of the prevailing suspicion regarding any argument that kept company with SETI scientists’ thinking.

>   The outpouring of popular interest—which only grew with the release of our paper—seemed ironic, too, when I considered the relative tameness of the hypothesis. Just a year earlier, following reports of an anomaly having to do with hydrogen atoms (which had been found to be colder than expected in the early universe), I published a paper with another Harvard postdoc, Julian Munoz, showing that if the dark matter was made of particles that possessed a tiny electric charge, they would cool the cosmic hydrogen and account for the reported anomaly. Even though this hypothesis was published in Nature and even though it was far more speculative than my and Bialy’s hypothesis about ‘Oumuamua being alien technology, it garnered much less attention.

  To be clear, although I made myself as available as my commitments allowed, I neither sought the limelight nor particularly enjoyed it. In the past when I had set out to draw attention to my work, as with the Starshot Initiative, I was grateful when even a few members of the press responded. And while I had undergone extensive professional training in various fields throughout my life, no one, especially me, had thought to include media training. In hindsight, maybe someone should have. Astronomy and astrophysics are fields that frequently require substantial commitments of time and money, and harnessing the public’s awareness of what is possible and what might be necessary cannot be an afterthought.

  …

  To say that my raising the possibility that ‘Oumuamua was artificial technology met with disapproval is putting the matter mildly. To be sure, the popular media was delighted, and the broader public was fascinated. But my fellow scientists were, shall we say, more circumspect.

  In July 2019 the ‘Oumuamua Team of the International Space Science Institute (ISSI) published their unambiguous conclusion in Nature Astronomy: “we find no compelling evidence to favor an alien explanation for ‘Oumuamua.” The immediately preceding paragraphs declared that the extraterrestrial-technology theory that Bialy and I had put forward was provocative but baseless. Yet the article also ends with a list of unanswered ‘Oumuamua anomalies, what the authors termed “open questions.” They also acknowledged that only after the advanced telescope at the Vera C. Rubin Observatory in Chile was fully operational might we have sufficient data to determine “how common—or rare—the properties of ‘Oumuamua are.”