New books and articles on the “great silence”

Credit: NASA

The great silence

As we have explained in previous Math Scholar blogs (see, for example, MS1 and MS2), the perplexing question why the heavens are silent even though, from all evidence, the universe is teeming with potentially habitable exoplanets, continues to perplex and fascinate scientists. It is one of the most significant questions of modern science, with connections to mathematics, physics, astronomy, cosmology, biology and philosophy.

In spite of the glib dismissals that are often presented in public venues and (quite sadly) in writings by some professional scientists (see MS1 and MS2 for examples and rejoinders), there is no easy resolution to Fermi’s paradox, as it is known.

Sociological explanations, such as “they have lost interest in research and exploration,” or “they are under a galactic directive not to disturb Earth,” or “they have moved on to more advanced communication technologies,” all fall prey to a diversity argument: Any advanced society that arose from an evolutionary process (which is the only conceivable natural process to explain the origin of complex beings), surely must consist of millions if not billions or even trillions of diverse individuals. All it takes is for one individual in just one of these advanced civilizations to disagree, say with the galactic directive not to or send probes or communications to Earth in a form that earthlings or other newly technological societies can easily recognize, and this explanation fails. Note, for example, that any signal (microwave, light or gravitational wave), once it has been sent by extraterrestrials (ETs) on its way to Earth, cannot be censored or called back, according to known laws of physics.

Similarly, technological explanations (i.e., it is too difficult for them) were once considered persuasive, but now founder on the fact that even present-day human technology is sufficient to communicate with and (soon) to send probes to distant stars and planets (see, for example, technical reports TR1, TR2, TR3 and TR4). If we can do this now, or within the next 20-30 years, surely civilizations thousands or millions of years more advanced can do this much better, and much more cheaply too. This same space technology, by the way, defeats the “all advanced civilizations destroy themselves” explanation, since as soon as human civilization (for instance) establishes permanent outposts on the Moon, Mars and beyond, its continued survival will be largely impervious to any calamities that may befall the home planet. By the way, the self-destruct explanation also falls prey to a diversity argument — even if most civilizations destroy themselves, surely at least a handful have figured out how to survive and thrive over the long term?

So are we alone, at least in the Milky Way? Perhaps, although many (including the present author) find such a conclusion rather distasteful, to say the least, since it goes against the Copernican principle that has guided scientific research for many years. See previous Math Scholar blogs MS1 and MS2 for a more complete discussion of these issues.

Recently two books and a Scientific American article (which summarizes a third book) have appeared on this topic. Here is a brief synopsis of each.

Webb’s If the Universe is Teeming with Aliens, WHERE IS EVERYBODY?

In his book If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, British physicist Stephen Webb updates his 2002 edition with a thorough discussion of 50 earlier proposed solutions to Fermi’s paradox, updated to reflect the latest scientific findings, plus 25 new proposed solutions for this edition. The discussion is very readable and focused, yet does not omit technical detail where needed.

Here are just a few of the proposed solutions presented (often with devastating rejoinders) in Webb’s book. Item numbers and titles are from Webb’s book:

  1. The Zoo scenario: We are part of a preserve, being tended by an overarching society of ETs.
  2. The interdict scenario: There is a galaxy-wide prohibition on visits to or communication with Earth.
  3. They have not had time to reach us: ETs may exist, but they have not yet been able to reach us or communicate with us.
  4. Bracewell-von Neumann probes: ETs could have explored the galaxy by means of self-replicating probes that arrive at a star system, find a suitable asteroid or planet, construct additional copies of themselves, and then launch them to other star systems, with updated software beamed from the home planet. Such probes could have explored the entire Milky Way in a few million years, an eyeblink in cosmic time. So where are they?
  5. They are signaling, but we don’t know how to listen: We are not using the right technology to receive and/or decipher their signals.
  6. They have no desire to communicate: ETs have settled into the good life and no longer have any desire to explore the cosmos or learn about other newly technological societies like us.
  7. Intelligence isn’t permanent: Intelligence has arisen elsewhere, but then degraded because it was not needed so much anymore.
  8. We live in a post-biological universe: ETs have progressed from biological to machine-based existence and no longer have interest in us.
  9. Continuously habitable zones are narrow: The Earth is nearly unique in maintaining a habitable environment over billions of years.
  10. The galaxy is a dangerous place: Much if not most of the galaxy is uninhabitable due to a steady rain of supernova explosions, gamma ray bursts and other phenomena that regularly sterilize their surroundings.
  11. Life’s genesis is rare: Although progress has been made, the origin of the first reproducing biomolecules on Earth is still a mystery. Perhaps biogenesis was a freak of nature, much more singular than we suppose.
  12. Intelligence at the human level is rare: Even if life is common, perhaps human-level intelligence, with its associated technology, is extremely rare.
  13. Science is not inevitable: Perhaps even if intelligence emerges, the development of science and advanced technology is rare.

Webb discusses each of these proposed solutions in detail, with numerous references to technical literature and other analyses. He also frankly discusses the weaknesses of each of these, and, in many places, offers his own assessment. In the end, Webb acknowledges that while we still have much to learn, he personally thinks it is unlikely that there are any other human-like technological societies in the Milky Way.

Cirkovic’s The Great Silence: The Science and Philosophy of Fermi’s Paradox

In his new book The Great Silence: Science and Philosophy of Fermi’s Paradox, Milan Cirkovic analyzes Fermi’s paradox from a somewhat deeper philosophical and scientific basis. He argues that many of approaches to the great silence are riddled with logical errors and other weaknesses. He then attempts to carefully examine many of the proposed solutions and to identify what can be safely said.

For example, Cirkovic argues that discussions of the Drake equation, which is used to estimate the number of space-communicating civilizations in the Milky Way, are typically fallacious. Drake’s equation, which was first proposed by Frank Drake in the 1960s, is:

N = R* fp ne fl fi fc L

where N is the number of predicted ET civilizations, R* is the star formation rate in the Milky Way, fp is the fraction of stars possessing planets, ne is the average number of habitable planets per planetary system, fl is the fraction of habitable planets possessing life, fi is the fraction of inhabited planets developing intelligent life, fc is the fraction of intelligent societies developing space communication technology, and L is the lifespan of civilizations that have developed technology.

Cirkovic argues that because of the huge uncertainties involved, each term should be given by a probability distribution, and the product integrated over the multidimensional parameter space. In any event, since virtually all of these terms are matters of active astrobiological research, writers who cite Drake’s equation without analyzing it more carefully are on shaky ground.

In the end, Cirkovic argues that in addition to identifying and correcting prejudices and fallacies in this arena, we need to realize that substantive progress awaits more real scientific data. But he insists that we must not retreat from Copernican principle, namely that ultimately our existence is not unique. He concludes on a positive note:

Even when [space exploration] is achieved, it will not be the beginning of the end — but it might be the end of the beginning of the greatest of all journeys, the wildest adventure of all adventures. … After all, this is what science has stood for in its most brilliant moments: courage, conviction, and the spirit of great adventure. These qualities might, in the final analysis, withstand the test of cosmic time, repulse the last challenge to Copernicanism, and ultimately break the Great Silence.

Gribbin’s Are Humans Alone in the Milky Way?

Astrophysicist and well-known science writer John Gribbin has weighed into the debate with an article in the August 2018 Scientific American entitled Are Humans Alone in the Milky Way? Why we are probably the only intelligent life in the galaxy. His article presents a brief summary of his 2011 book Alone in the Universe: Why Our Planet Is Unique, with a number of updates reflecting recent scientific discoveries and findings.

Gribbin argues that we are the product of a long series of remarkably fortuitous (and unlikely) events that might not have been repeated anywhere else, at least within the Milky Way. These include:

  • Special timing: Given that all elements heaver than hydrogen, helium and lithium were created in supernova explosions, this means that a metal-rich environment like our sun and solar system could not exist until several billion years after the big bang, yet not so long after that the sun begins to die.
  • Special location: While the Milky Way seems vast, regions much closer than our sun to the center of the galaxy are bathed in high-energy gamma particles and gamma ray bursts, which are lethal to any conceivable form of biology. Much further away from the center than our sun, metallicity and other conditions are not met. In other words, we reside in a Goldilocks zone in the Milky Way, estimated to span only about 7% of the galactic radius.
  • Special planet: In spite of all the talk about exoplanets in the Milky Way, only a few are rocky like Earth, fewer reside in the habitable zone, and even fewer still (or perhaps none at all) have these features plus a combination of a molten core generating a magnetic field together with plate tectonics to regulate climate. In our solar system, Mars is too far away, has no magnetic field and is too cold (although primitive organisms might exist underground). Venus lacks a magnetic field and plate tectonics, and, because of runaway greenhouse effect, is much too hot for any life. The collision with Earth by a Mars-size object in the early solar system, and the presence of a large moon, also appear to be key to Earth’s long-term, life-friendly environment.
  • Special life: Once Earth’s original molten state settled, life appeared with “indecent rapidity,” in Gribbin’s words. But then not much happened for the next three billion years. More complex cells, known as eukaryotes, resulted from the chance merger of two primordial organisms, bacteria and archaea, but even after this crucial milestone event, not much happened for another billion years, until the Cambrian explosion roughly 540 million years ago. So the rise of complex life forms, much less intelligent creatures such as Homo sapiens, was hardly inevitable.
  • Special species: Homo sapiens appeared about 300,000 years ago, but nearly went extinct 150,000 years ago, and again about 70,000 years ago. Thus our subsequent domination of the planet seems far from assured.

Gribbin concludes as follows:

As we put everything together, what can we say? Is life likely to exist elsewhere in the galaxy? Almost certainly yes, given the speed with which it appeared on Earth. Is another technological civilization likely to exist today? Almost certainly no, given the chain of circumstances that led to our existence. These considerations suggest we are unique not just on our planet but in the whole Milky Way. And if our planet is so special, it becomes all the more important to preserve this unique world for ourselves, our descendants and the many creatures that call Earth home.


As mentioned above, the question of extraterrestrial intelligent life, and why we have not yet found any, surely must rank at or near the top of the most significant scientific questions of all time. In his foreword to Webb’s book, noted British astronomer Martin Rees explains what is at stake:

Maybe we will one day find ET. On the other hand, [Webb’s] book offers 75 reasons why SETI searches may fail; Earth’s intricate biosphere may be unique. That would disappoint the searchers, but it would have an upside: it would entitle us humans to be less “cosmically modest.” Moreover, this outcome would not render life a cosmic sideshow. Evolution may still be nearer its beginning than its end. Our Solar System is barely middle aged and, if humans avoid self-destruction, the post-human era beckons. Life from Earth could spread through the Galaxy, evolving into a teeming complexity far beyond what we can even conceive. If so, our tiny planet — this pale blue dot floating in space — could be the most important place in the entire Galaxy, and the first interstellar voyagers from Earth would have a mission that would resonate through the entire Galaxy and perhaps beyond.

[This appeared on the Math Scholar blog.]

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