“We must be careful not to believe things simply because we want them to be true,” physicist Richard Feynman once observed. “No one can fool you as easily as you can fool yourself.”
This is true in private life as it is in science. Consider two timely examples from the past few days.
First, mysterious drones have been spotted over New-Jersey. Many of them fly like human-made drones and others appear to be airplanes or helicopters. It is standard practice for the U.S. military to notify law enforcement authorities of any plans to fly drones over residential areas. Therefore, unidentified drones must have originated from civilians or an adversarial nation.
The latter possibility should come as no surprise, since a Chinese spy balloon was spotted between January 28 to February 4, 2023, to fly at a high altitude across the United States. Even though the balloon was 45 meters in diameter, it took a while for the U.S. Air Force to shoot it down on February 4, off the coast of South Carolina.
Over the past few days, I was asked in many interviews, text messages and emails whether the mysterious drones could be extraterrestrial in origin. In response, I explained that all available data is consistent with the flight characteristics of human-made flying objects.
The primary question that needs to be clarified is whether these flying objects are used for espionage or pose any other national security threat. Alluding to an extraterrestrial origin is an inappropriate smoke screen to hide the incompetence of the U.S. intelligence agencies.
We must keep in mind the duck test: “If it looks like a duck, swims like a duck, and quacks like a duck, then it probably isa duck.” Common sense suggests, based on the available information about the mysterious New-Jersey drones, that they are human-made flying objects.
Entirely independent from this story, a new paper led by the astronomer Darryl Seligman appeared a few days ago in Proceedings of the National Academy of Sciences. The paper reports about a population of dark comets with no detected cometary tail. The authors highlight the significance of the paper by stating that this population of dark comets shows “non-gravitational accelerations explainable by outgassing of volatiles, analogous to the first interstellar object 1I/’Oumuamua.” The implied interpretation is that `Oumuamua was simply a dark comet, namely a natural comet which was accelerated by invisible outgassing.
Over the past five years, Seligman led a series of papers attempting to explain `Oumuamua as a hydrogen iceberg. When he shared with me the first version of his hydrogen iceberg paper, I immediately noticed without following the details of his calculations that the quoted evaporation rate is severely underestimated. The shortcut for my reasoning was based on momentum conservation. The measured non-gravitational acceleration of `Oumuamua near Earth was equivalent to 0.1% of its gravitational acceleration. This corresponds to a net velocity boost of order 0.1% of the orbital velocity of the Earth around the Sun, or about 30 meters per second.
Given that the typical speed of evaporated cometary gases is an order of magnitude larger, `Oumuamua had to lose about 10% of its mass to show the observed acceleration. Seligman thanked me for spotting the error in the acknowledgements at the end of the paper. Subsequently, I wrote a paper with Thiem Hoang, showing that hydrogen icebergs would not survive the journey from their conjectured birth sites in giant molecular clouds. Indeed, there is no evidence that chunks of hydrogen ice exist in interstellar space. They are expected to quickly sublimate when heated by starlight.
The new Seligman-led paper about dark comets provides measured values of non-gravitational accelerations that are at most 2×10^ {-8} meters per second squared. For comparison, the measured non-gravitational acceleration of `Oumuamua near Earth was about 3×10^{-6} meters per second squared. This is 150 times larger than the most accelerated dark comet and half a million times larger than the least accelerated dark comet in the new paper.
The primary question that needs to be clarified is whether the coma of dark comets would remain invisible if their outgassing rate was boosted by several orders of magnitude?
Specifically, if the mass and corresponding brightness of the cometary tail increases by a factor of half a million for the least accelerated dark comet 2001 ME_1, will this comet remain dark? Probably not. Conversely, if a comet loses about 10% of its mass – as needed to explain the non-gravitational acceleration of `Oumuamua, would this comet be dark?
If `Oumuamua does not look like a comet and does not accelerate as little as a dark comet, then it is probably not a comet. This is what common sense suggests about `Oumuamua based on the available information.
Unfortunately, common sense is not always common.
What is the lesson learned from the mysterious New-Jersey drones or the new class of dark comets? That Richard Feynman was wise and that people often forget to take his advice to heart.
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.