Images of the new interstellar object 3I/ATLAS (posted in papers here, here, here and here) show a slight elongation along its direction of motion. However, the elongation beyond the angular width of background stars (the so-called `point spread function’) is exactly at the level expected from multiplying the object’s speed of 60 kilometers per second by the exposure time of the telescope of about a hundred seconds. This elongation is not mitigated by freezing the object’s image and letting the background stars move relative to it. It results from the fact that a single snapshot of the image takes a hundred seconds. The product of this exposure time and the speed of 3I/ATLAS yields a scale of order 6,000 kilometers (comparable to Earth’s radius), extending over an angle of ~2 arcseconds in the sky given the object’s distance of 4.5 times the Earth-Sun separation. While 3I/ATLAS may well be a comet, this elongation should not be taken as evidence for its cometary tail. So far, spectroscopic data on 3I/ATLAS (published here, here and here) do not reveal the spectral features of cometary gas but only show reddening of reflected sunlight, consistent with a compact dust cloud or the surface of a solid object.
The brightness of 3I/ATLAS implies a diameter of 20 kilometers for an asteroid with a typical reflectance (albedo) of 5%. As I showed in a published paper shortly after 3I/ATLAS was discovered, the detection of this object over 5 years of the ATLAS telescope’s survey of the sky, requires an untenable mass supply of rocky material from the Milky-Way galaxy. If 3I/ATLAS is 20 kilometers in diameter, it might have targeted the inner Solar System as expected from alien technology. This possibility was discussed in a follow-up paper that I wrote with Adam Hibberd and Adam Crowl, where we highlighted the anomalous properties of the trajectory taken by 3I/ATLAS.
Indeed, the simplest interpretation that I provided in my paper is that 3I/ATLAS is an extended dust cloud with a cometary nucleus that is smaller than a kilometer in size. But it is worth contemplating alternatives in case future data will indicate a much larger solid object. Resistance to multiple interpretations and bullying those who suggest them is anti-scientific.
There is another lesson to be learned from the elongation in the images of 3I/ATLAS. Imagine a spacecraft moving a hundred times faster than 3I/ATLAS, at 6,000 kilometers per second. This corresponds to 2% of the speed of light, an order of magnitude lower than the goal of the Breakthrough Starshot Initiative, which I led over the past decade. If humans dream of launching a spacecraft at 20% of the speed of light only a century after the discovery of quantum mechanics and general relativity, why would aliens — who might have enjoyed the benefits of many millennia of science, not aim to launch spacecraft at 2% of the speed of light?
Unfortunately, our existing telescopes would not alert us to a spacecraft moving that fast because the spacecraft image would be smeared into a faint line that would be missed by observers, given the background of scattered light from the Sun and the Milky Way. If 3I/ATLAS was moving at 2% of the speed of light, then over a 100 seconds exposure, its image would have been smeared to a faint line, spreading its light over a length of a few arcminutes and making its surface brightness too small to be detectable even with our largest telescopes.
In other words, we are blind to spacecraft moving ten times slower than our own most ambitious technological goal in the context of Breakthrough Starshot.
In case relativistic spacecraft are flying through the Solar System, the answer to Enrico Fermi’s question: “where is everybody?” is “perhaps right here, but we are blind to them.” This blindness has nothing to do with stealth technologies or new physics that the aliens employ. The images of near-Earth relativistic spacecraft would be smeared to undetectable brightness levels even if they were to reflect sunlight like asteroids that are 20 kilometers in diameter, twice as large as the Chicxulub impactor that killed the non-avian dinosaurs on Earth 66 million years ago.
With better technologies, we can overcome this blindness. For example, the above limitations hold for observatories sensitive to light. In addition to those, the National Science Foundation also funded the Laser Interferometer Gravitational Wave Observatory (LIGO) which detects gravitational signals at a frequency of order 100 Hertz. In a paper published last year, I showed that LIGO is sensitive to the tidal gravitational signal from relativistic interstellar objects on the scale of tens of kilometers as long as they pass within a distance comparable to Earth’s radius. Since no unusual signal with the time profile that I calculated was reported by LIGO so far, we can conclude that no massive relativistic object passed near Earth over the past decade. Unfortunately, we cannot say much about larger distances, because the gravitational tidal signal would have been weaker than LIGO’s sensitivity.
It comes as no surprise that what we perceive as the observable Universe is limited by the sensitivity of our instruments. It is indeed the case that 95% of the cosmic mass budget is unknown. Nobel prizes were awarded to observers who exposed our ignorance about the unknown cosmic constituents, even though it would have been far more satisfying to know what dark matter and dark energy are. Given this backdrop, mainstream astronomers and SETI advocates must pause before claiming that the existence of aliens is an extraordinary claim that requires extraordinary evidence. It may well be an ordinary claim that requires ordinary evidence, but we fail to find this evidence because of our limited detector sensitivity.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.