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Avi Loeb is no stranger to controversy. The prolific Harvard University astrophysicist has produced pioneering and provocative research on black holes, gamma-ray bursts, the early universe and other standard topics of his field. But for more than a decade he has also courted a more contentious subject—namely, space aliens, including how to find them. Until relatively recently, Loeb’s most high-profile work in that regard was his involvement with Breakthrough Starshot, a project funded by Silicon Valley billionaire Yuri Milner to send laser-boosted, gossamer-thin mirrorlike spacecraft called “light sails” on high-speed voyages to nearby stars. All that began to change in late 2017, however, when astronomers around the world scrambled to study an enigmatic interstellar visitor—the first ever seen—that briefly came within range of their telescopes.

The object’s discoverers dubbed it ‘Oumuamua—a Hawaiian term that roughly translates to “scout.” The unavoidably cursory examinations of this celestial passerby showed it had several properties that defied easy natural explanation. ‘Oumuamua’s apparent shape—which was like a 100-meter-long cigar or pancake—did not closely resemble any known asteroid or comet. Neither did its brightness, which revealed ‘Oumuamua was at least 10 times more reflective than one of our solar system’s typical space rocks—shiny enough to suggest the gleam of burnished metal. Most strangely, as it zoomed off after swooping by the sun, the object sped up faster than could be explained by our star’s waning gravitational grip alone. Run-of-the-mill comets can exhibit similar accelerations because of the rocketlike effect of evaporating gases jetting from their sunlight-warmed icy surfaces. But no signs of such jets were seen around ‘Oumuamua.

To Loeb, the most plausible explanation was as obvious as it was sensational: taken together with its possibly pancakelike shape and high reflectivity, ‘Oumuamua’s anomalous acceleration made perfect sense if the object was in fact a light sail—perhaps a derelict from some long-expired galactic culture. Primed by years spent pondering how we might someday find evidence of cosmic civilizations in the sky’s depths, he became increasingly convinced that, with ‘Oumuamua, the evidence had instead found us. In late 2018 Loeb and his co-author Shmuel Bialy, a Harvard postdoctoral fellow, published a paper in the Astrophysical Journal Letters arguing that ‘Oumuamua had been nothing less than humanity’s first contact with an artifact of extraterrestrial intelligence.

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