Quintillions and Quintillions of Alien Probes - Or None at All?

The question of SETI and Extraterrestrial Intelligence has now been debated at length for more than fifty years. Are there other intelligent civilizations in our galaxy, and elsewhere? If so, is it possible to communicate with them? Can they possibly even send spacecraft here? The controversy rages on.

Jonathan H. Jiang

A team of scientists working under a NASA contract at Jet Propulsion Laboratory has written a paper that is very pessimistic for SETI. A group of five researchers led by Jonathan H. Jiang of NASA's JPL have written a paper (which is not yet peeer-reviewed), "Avoiding the “Great Filter”: Extraterrestrial Life and Humanity’s Future in the Universe." From the paper's abstract, 

Coupled with logical assumption and calculations such as those made by Dr. Frank Drake starting in the early 1960s, evidence of life should exist in abundance in our galaxy alone, and yet in practice we’ve produced no clear affirmation of anything beyond our own planet. So, where is everybody? The silence of the universe beyond Earth reveals a pattern of both human limitation and steadfast curiosity. Even as ambitious programs such as SETI aim to solve the technological challenges, the results have thus far turned up empty for any signs of life in the galaxy. We postulate that an existential disaster may lay in wait as our society advances exponentially towards space exploration, acting as the Great Filter: a phenomenon that wipes out civilizations before they can encounter each other, which may explain the cosmic silence. In this article, we propose several possible scenarios, including anthropogenic and natural hazards, both of which can be prevented with reforms in individual, institutional and intrinsic behaviors. We also take into account multiple calamity candidates: nuclear warfare, pathogens and pandemics, artificial intelligence, meteorite impacts, and climate change.

What is the "Great Filter"?  In brief, it is the supposed tendency of intelligent civilizations to die off, either because of their own actions, or unavoidable disasters like pandemics or asteroid impacts. They conclude that the best way to try to escape the Great Filter "begins with collaboration," whatever that actually means.

The first person to propose the Great Filter in the late 1990s was Robin Hanson, an economics professor at George Mason University. And he doesn't agree with the recommendations in this paper. 

The global cooperation Jiang and company advocated as the means of our survival could be the very thing that ends up destroying us, Hanson told The Daily Beast. “Clearly they recommend more centralized control and governance of our civilization,” Hanson said. “But I actually see excess governance centralization as the most likely contribution to our future Great Filter.”

In Hanson’s conception, the more we decentralize, the more likely some of us to survive and thrive.

And I tend to agree with that. The more power held by a centralized government, the more likely it is to march us over a cliff. Seth Shostak, the senior astronomer with the California-based SETI Institute, isn't buying it, either. He told The Daily Beast, “The Great Filter theory depends on the assumed observational result that nobody is out there. But that conclusion is far too premature. We’ve just begun to search.”

But if you thought those guys were too pessimistic about SETI, wait until you hear this next one!  Dr. Edwin L. Turner is an Emeritus Professor of Astrophysics at Princeton University. He recently spoke at a colloquium of the UCLA Division of Astronomy and Astrophysics. Here is the abstract:

From Turner's Princeton page

November 16, Ed Turner (Princeton University)

The Hubble Volume May Well Be Entirely Devoid of Extraterrestrial Life, Intelligence or Technological Civilizations 

Abstract: The two most common and apparently compelling arguments for the existence of extraterrestrial life, intelligence and technological civilizations are the (probable) extremely large number of exoplanetary environments similar to the Earth's and the application of the Copernican Principle to abiogenesis, evolution and sociology. On closer examination both of these lines of reasoning are shown to have fundamental flaws. Thus, it remains entirely plausible that the Earth is unique in the observable universe as a home to any or all of these three astrobiological phenomena. The discussion will also illuminate a major unresolved question in our understanding of nature which deserves serious attention independent of the specific topic considered in this presentation.

At first I thought that "the Hubble volume" meant the entire volume of space that is visible to the Hubble Space Telescope. But researcher Nablator points out that actually references the much larger region of the universe as determined what is essentially an event horizon, where the expansion of the universe (from our viewpoint) would reach c, the speed of light. It is an enormous expanse billions of light years across, and filled with gazillions of galaxies, each one containing billions of stars. "Nobody there, except us," he says. I was not able to listen to Dr. Turner's talk on Zoom, and the recording is not yet posted on-line. However, I understand that he placed very heavy emphasis on biophysics, explaining the near-impossible odds against amino acids and  other molecules forming in exactly the right way for complex life to evolve. 

A 2011 paper by Turner, co-authored with David S. Spiegel, is titled "Bayesian analysis of the astrobiological implications of life's early emergence on Earth." Its somewhat confusing abstract states,

Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young Earth-like conditions. We revisit this argument quantitatively in a bayesian statistical framework. By constructing a simple model of the probability of abiogenesis, we calculate a bayesian estimate of its posterior probability, given the data that life emerged fairly early in Earth's history and that, billions of years later, curious creatures noted this fact and considered its implications. We find that, given only this very limited empirical information, the choice of bayesian prior for the abiogenesis probability parameter has a dominant influence on the computed posterior probability. Although terrestrial life's early emergence provides evidence that life might be abundant in the universe if early-Earth-like conditions are common, the evidence is inconclusive and indeed is consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors. Finding a single case of life arising independently of our lineage (on Earth, elsewhere in the solar system, or on an extrasolar planet) would provide much stronger evidence that abiogenesis is not extremely rare in the universe.

Voicing a very different opinion on the matter is Harvard's Dr. Avi Loeb, head of the Galileo project, which plans to search for evidence of alien space probes in our own solar system. Dr. Loeb seems to think that the strange object called Oumuamua, which drifted into our solar system five years ago before drifting out again, was likely an alien probe from an interstellar civilization. He plans to set up cameras to detect such objects, as if nobody had ever before set up cameras to survey objects in the sky.

In a recent paper that has not yet been peer-reviewed, Dr. Loeb and his Harvard colleague Carson Ezell calculate that, based on our ability to detect such objects,

our estimate for the total quantity of ‘Oumuamua-like objects bound by the thin disk if they are not targeted is 1.91 × 10^^26 objects, which aligns with previous estimates for the abundance of similar objects. This estimate applies both in the case of ‘Oumuamua being of natural origin, and ‘Oumuamua being artificial space debris that is not targeted towards a particular location in space.

However, the inferred abundance of probes is distinctly different in case of ‘Oumuamua-like objects being targeted towards particular regions of the galaxy, specifically habitable zones containing planets. ‘Oumuamua was detected at a distance of 0.2 AU from Earth, and it passed through the habitable zone of our solar system. The estimated total number of ‘Oumuamua-like objects would then fall to 1.91 × 10^^10.

By "the thin disk," they mean the relatively thin disk of our Milky Way galaxy, i.e. stars near the galactic plane. What this says is: If  ‘Oumuamua-like objects are wandering randomly in the thin disk of our Milky Way, then we calculate that there are 191,000,000,000,000,000,000,000,000 of them; but if they are targeted toward habitable solar systems such as our own, then that number falls to just 19,100,000,000. Doing the same calculations based on "Interstellar Meteor 1," a much smaller object, 

We then estimate 7.59 × 10^^34 IM1-like objects bound by the thin disk of the Milky Way. However, if objects with the properties of IM1 were targeted towards habitable zones containing planets, we estimate 7.59 × 10^^18 such objects.

The Daily Beast misreported the conclusions of this paper, saying that Ezell and Loeb calculated that "in and around the solar system" there could be "as many as 4,000,000,000,000,000,000 (or 4 quintillion) of them." Actually, that was the calculated number of interstellar meteors (or spacecraft!) of one meter size or larger in our Milky Way galaxy, not just our solar system.

Given the aforementioned properties of chemically-propelled rockets and a hypothetical detection rate of  c = 0.1 yr-1 for interstellar meteors of meter size that collide with Earth, equation (21) estimates a total of 3.65 × 10^^34 such objects bound by the Milky-Way thin disk if they are not targeted, or 3.65×10^^18 objects if they are targeted.

But an article just published by Sky and Telescope pours cold water on the idea that ‘Oumuamua is artificial. Recent analysis of light curves of the object suggest that its shape is more like that of a pancake, rather than a cigar. The best available evidence suggests that ‘Oumuamua is a fragment of a Pluto-like object, composed primarily of nitrogen ice:

 if `Oumuamua were made of nitrogen ice, it would have the right albedo and the right mass to produce the exact amount of non-gravitational acceleration observed by astronomers as it retreated from the Sun. And if it were nearly pure nitrogen ice, it would exhibit this cometary behavior without any of the hallmarks of comets, neither reflecting sunlight from dust nor lighting up with emission from water or other gases.

Hypothesizing pure nitrogen ice for Oumuamua’s composition solves some other puzzles, too. The body passed within 0.2 astronomical units (a.u.) of the Sun (20% of the distance from the Sun to Earth), and yet it survived to exit the solar system. But only barely, according to Desch and Jackson’s model. A nitrogen-ice `Oumuamua would have lost 95% of its mass by the time it exited the inner solar system; evaporative cooling would have insulated the remaining morsel through the harrowing passage.

That much mass loss also explains the extreme shape. If you add 20 times the present mass in concentric layers around the present pancake, reversing its evaporation by the Sun, the original body would have had a much more normal 2:1 aspect ratio.

The pancake-like ‘Oumuamua, as depicted by William K. Hartmann

So, what is the number of space probes wandering the disk of our galaxy (not counting our own probes)? Is it in the quadrillions in each galaxy? Or is it zero in the entire Hubble Volume? Or likely something in-between?

A trivia question for UFO buffs: Who is William K. Hartmann, what is he known for in UFOlogy? No fair to look him up. Who can provide the first correct answer in the comments, from their own memory? 😏

And here is something that amazed me when I saw it: Edwin Turner is a member of the Galileo Project's "research team" (along with Jacques Vallee, Garry Nolan, and many, many others). I can just imagine what the Zoom call of the Galileo Project's researchers and consultants must be like:

Avi Loeb: We need to set up cameras, lots and lots of cameras, to capture the quadrillions of alien probes floating around.

Edwin Turner: No, you're wasting your time. There are no aliens out there, not in the entire Hubble volume.

Jacques Vallee: I heard that an alien craft crashed at Trinity in 1945. We should try to find the wreckage of that one!

Luis Elizondo: I can't confirm this officially, but I remote viewed an alien craft at an undisclosed location, some inspecific time ago.

Seth Shostak:You guys are all jumping to conclusions!

Happy Thanksgiving, everybody!