Physicist Avi Loeb: UFOs over Ukraine are not as weird as they seem

Over the past two weeks I was bombarded by a dozen requests to read a new report by astronomers on Unidentified Aerial Phenomena (UAP) in Ukraine. My response to all of these messages was the same: “I am not sure what to make of the report. Ukraine is in a military conflict with a lot of human-made activity in the sky. This must introduce a lot of noise for any search for objects that are not human-made. In science we aim to maximize the signal-to-noise ratio, and so Ukraine would be the last place on Earth where I would initiate UAP studies.”

But last evening I received a special request from a high-level official in the US government to summarize my thoughts on observable signatures of UAP. And so, this morning I checked the UAP report from Ukraine and wrote a paper about it a few hours later.

The Ukranian paper reports about two types of objects: luminous and dark. The dark objects with no visible emission were labeled as “Phantoms.” They were characterized by a size of three to 12 meters and speeds up to 15 kilometers per second at a distance of up to 10 to 12 kilometers. If real, such objects exceed the capabilities of human-made aircrafts or rockets. I quickly realized that the distance of these dark objects must have been incorrectly overestimated by an order of magnitude, or else their bow shock in the Earth’s atmosphere would have generated a bright fireball with an easily detectable optical luminosity.

The interest in UAPs stems from their potential non-human origin. Extraterrestrial equipment could arrive in two forms: space trash, similar to the way our own interstellar probes (Voyager 1 & 2, Pioneer 10 & 11 and New Horizons) will appear in a billion years, or functional equipment, such as autonomous devices equipped with Artificial Intelligence (AI). The latter would be an ideal choice for crossing the tens of thousands of light years that span the scale of the Milky Way galaxy and could survive even if the senders are not able to communicate.

It is likely that any functional devices embedded in the Earth’s atmosphere are not carrying biological entities because these would not survive the long journey through interstellar space and its harsh conditions, including bombardment by energetic cosmic-rays, X-rays and gamma-rays. Interstellar gas and dust particles deposit a kinetic energy per unit mass that exceeds the output of chemical explosives at the speed of tens of kilometers per second characterizing rockets.  However, technological gadgets with AI can be shielded to withstand the hazards of space, repair themselves mechanically, or even reproduce given the resources of a habitable planet like Earth. With Machine Learning capabilities, they can adapt to new circumstances and pursue the goals of their senders without any need for external guidance.

As argued by John von Neumann in 1939, the number of such devices could increase exponentially with time if they self-replicate, a quality enabled by 3D printing and AI technologies. Physical artifacts might also carry messages, as envisioned by Ronald Bracewell in 1960.

In principle, the fastest gadgets could be launched by lightsails, pushed by powerful light beams up to the speed of light. Natural processes, such as stellar explosions or gravitational slingshot near black hole pairs, could launch objects to similar speeds. However, it would be difficult for relativistic payloads to slow down below the escape speed of Earth, smaller by 4.5 orders of magnitude than the speed of light, without having around the same facilities that generated their high initial speeds.

A better suited propulsion technique that was used in all space missions from Earth is chemical rockets. Since rockets carry their fuel, they can navigate to a desired planet and slow down near it.

The tyranny of the rocket equation, requiring that the fuel mass must increase exponentially with increasing terminal speed, explains why all human-made spacecraft reached a speed limit of tens of kilometers per second — four orders of magnitude below the speed of light. Interestingly, this speed is comparable to the escape speed from the Earth’s orbit around the Sun, 42 kilometers per second, making it possible for humanity to launch probes to interstellar space by taking advantage of the motion of the Earth around the Sun at 30 kilometers per second. Chemical propulsion may not be sufficient for probes to escape from the habitable zone around dwarf stars, like the nearest star, Proxima Centauri.

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In summary, chemical propulsion allows escape from the habitable zone of Sun-like stars and enables slowing down near a destination. The Ukranian report suggests objects with comparable speeds of up to 15 kilometers per second.

Devices which need to refuel would favor a habitable planet where liquid water or combustible organic fuel are available. Planets can be identified from a distance as they transit their star or through direct imaging. Once an Earth-like planet is targeted, an interstellar device can plunge into its atmosphere. In principle, a multitude of tiny devices can be released from a mothership that passes near Earth.

At a final speed of 30 kilometers per second, a probe would cross twice the distance of the Sun from the Milky-Way center within a time of half a billion years. The fraction of all Sun-like stars that host Earth-like planets in their habitable zone is in the range of three to 100%.  This implies that self-replicating probes could reach ten billion habitable planets around Sun-like stars in less than a billion years.

Since most stars formed more than a billion years before the Sun, it is possible that other technological civilizations predated ours by the amount of time needed for their devices to reach Earth. My paper points out that any supersonic motion of such devices through the Earth’s atmosphere would inevitably be accompanied by optical emission.

I showed that an object with a frontal cross-sectional area of 10 square meters, moving at a supersonic speed of 10 kilometers per second must create a bow shock in the Earth’s atmosphere and dissipate a mechanical power of 1.5 terrawatts at an elevation of 10 kilometers. Data on meteors implies that about a tenth of the kinetic power which is radiated away in the optical band implying that the reported properties of the phantom objects above Ukraine would result in fireball of visible luminosity above 150 gigawatts. For a path length 10 kilometers, the emission would last at least a second and cannot be missed.

I conclude that the reported speeds and sizes of the “phantom” objects would have generated fireballs of detectable optical luminosity at their suggested distances, and so these objects could not have appeared dark (as they did). However, if the phantom objects are ten times closer than suggested, then their angular motion on the sky corresponds to a physical velocity that is ten times smaller, 1.5 kilometers per second and their inferred transverse size would be 0.3-1.2 meters, both characteristic of artillery shells.

The inferred fireball luminosity scales with distance to the 5th power, and is reduced to a modest level of a few megawatts if the distance is shorter by a factor of ten than suggested by the Ukranian astronomers. If the artillery shells have a frontal diameter of only 10 cm, then the inferred fireball luminosity is merely 10 kilowatts, which at a kilometer distance would appear extremely faint — like a 100 watt light bulb at a distance of 100 meters.

In other words: After correcting the factor of ten overestimated in distance, everything falls into place with the parameters of artillery shells.

As the Nobel Laureate physicist Richard Feynman noted in the title of his book: there is a great pleasure in finding things out. There is no way out of the above-mentioned arguments because the Ukranian astronomers saw the objects as dark, meaning that they block the background light from the sky. The required electromagnetic cross-section for interaction with light implies that the phantom objects must also interact with air molecules.

The Ukrainian astronomers also identified a luminous and variable object at an altitude of 1,170 kilometers, which was detected through two-site observations above Ukraine. This object is likely a satellite.

In other words, “down to Earth” explanations can account for the reported unidentified objects above Ukraine. But in salute to my colleagues in Ukraine, let me conclude with a quote from Oscar Wilde: “We are all in the gutter, but some of us are looking at the stars.”

* * *

If the government finds evidence for an extraterrestrial technological origin of Unidentified Aerial Phenomena (UAP), the President will be the first to know about it. But such an event will be no different from the President being the first to know that the most abundant element in the Universe is hydrogen. It makes little sense for scientific knowledge of reality to adhere to national borders. Science should be done in an open and a transparent way, so that all of humanity will benefit from it. To make a contemporary analogy: In the case of COVID-19, many lives would have been saved if the detailed scientific information about the outbreak in Wuhan, China, would have been immediately shared throughout the world.

My paper provides a quantitative scientific calculation, implying that the dark objects identified as “phantoms” by a team of Ukrainian astronomers led by Boris Zhilayev, are likely artillery shells. The objects were characterized by the astronomers as having sizes of three to twelve meters and speeds of up to 15 kilometers per second at a distance of up to ten to twelve kilometers. I showed that these characteristics would result in huge fireballs around the objects as a result of their unavoidable friction with air. The power of the fireball scales as the inferred distance to the fifth power. If the distances are overestimated by a factor of ten, the size and speed of the dark objects would match those of artillery shells.

The reporter Matthew Gault from VICE sent me the response from the aforementioned Ukrainian astronomer Boris Zhilayev:

“Avi Loeb is a theorist. We are experimenters. We observe, process, and determine the characteristics of objects. Our publication contains just such data. We are not in the business of interpretation. Avi Loeb is trying to interpret our data. The work contains a discovery. Bright and dark objects. Our work can be repeated and verified. Although this is a challenging experiment. Our characteristics of the objects are very similar to those of US military pilots and Canadian civilian pilots.”

I wrote back to Matthew:

“Being an experimentalist or a theorist is not relevant. All scientists, whether they are experimentalists or theorists, must use logic. Here is how my argument cannot be refuted by anyone who uses logic. The Ukrainian astronomers saw the phantom objects as dark. This means that the objects blocked background light from the sky. The required electromagnetic interaction with light implies that the phantom objects must also interact with air molecules. There is no logical way for the phantom objects to block light but not air molecules, because the cross-section for electromagnetic interaction of air molecules with matter is larger than for light with matter. If we accept this premise, then the parameters inferred by the experimentalists would create fireballs of several terawatt brightness that illuminate the sky. This is comparable to the entire electric power consumption on Earth coming from one of these objects. But the experimentalists claim the object are darker than the sky. This violates logic and means that the distances of the phantom objects were overestimated by a factor of ten, as I show in my paper.”

Immediately afterwards, I received an email from the so-called “UAP debunker,” Mick West. He argued that the dark objects are most likely insects, because they change their speed in the sky unlike artillery shells. Consider, for example, Figure 13 from the Zhilayev et al. paper. It shows snapshots of a dark object, at three different moments, separated by a constant interval of 0.02 seconds. West argued that the separation between the top and middle positions on the sky is larger than between the middle and bottom positions — hence, the object must change its speed very rapidly, unlike artillery shells.

I explained to Mick that this data is fully consistent with an object moving at a constant speed. Imagine filming an artillery shell that is approaching or receding from us at a nearly constant speed. The angle on the sky that is traversed by the object per unit time will be inversely proportional to distance. At a larger distance the object will traverse a small angle per time period and at a closer distance it will traverse a large angle for the same period.

We see this phenomenon routinely when a train approaches us from a distance and moves much more rapidly across our field of view when it passes by. The object should also get bigger when approaching us, but the observed angular size of the dark object in the images may be blurred by resolution, atmospheric turbulence or its motion.

It is often said that when caught in the crossfire between two sides, one should simply duck down and let the bullets cross and reach both sides. This is a wise strategy unless science provides a bulletproof shield. 

On October 6, 2022, Rakéta, a Hungarian magazine, wrote this on the situation:

The relevant Kyiv observatory (Main Astronomical Observatory of the National Academy of Sciences of Ukraine/MAO NASU) issued a statement that confirms Loeb’s conclusion. Accordingly, the sightings were made in the observatory’s test mode during a planned meteor observation, and the results were not discussed or reviewed with the observatory. Due to the media uproar, the observatory convened a seminar on the case on September 15, at which the following important conclusions were reached:

“The observations of Zhilyaev and his colleagues are original, but the processing and interpretation of the results was done at an inadequate scientific level and with significant errors in determining the distance of the observed objects. Also, the dates of the sightings are missing from the article; the authors do not indicate which events were observed from two locations simultaneously; the authors do not provide arguments that the observed UAPs may include natural phenomena or artificial objects of terrestrial origin (meteors; objects carried by the wind over long distances; space debris, etc.). Instead of a critical analysis of the observations (possible errors, the adequacy of the models, the accuracy of the post-processing), the authors postulate unjustified conclusions about the characteristics of the observed objects as UAPs. The MAO Academic Council of NASU believes that the above-mentioned B.E. Zhilyaev’s conclusion was hasty and did not meet the professional requirements for publishing the results of scientific research.”

A previous version of this story originally appeared on Avi Loeb’s personal blog.