Welcome back to our ongoing series, a Brief-ish History of SETI. In our previous installments, we looked at the philosophical underpinning of SETI and the earliest experiments. We also examined the first modern SETI project and its lasting legacy, and the big ideas that remain integral to the discipline. Then, we looked at the first attempt at Messaging Extraterrestrial Intelligence (METI) and what is considered the best candidate for a signal detection. This was followed by a retrospective of the first physical messages humanity has sent to space.

Today, we will delve into one of the most daunting questions that continues to haunt SETI researchers. As Fermi famously said, “Where is Everybody?” Answering that question requires that we face some uncomfortable possibilities and address how little we know about life in our Universe. To recap on the “Lunchtime Conversation” we explored in Part I, Fermi’s question was motivated by some salient facts:

1. The Universe is Old: The Universe began roughly 13.8 billion years ago with the Big Bang and has been expanding ever since.
2. The Universe is Huge: The “Observable Universe” measures an estimated 96 billion light-years in diameter, and may be infinite.
3. The Universe is Packed: The most recent estimates indicate that there are over 2 trillion galaxies in the known Universe. The population of each ranges from thousands of stars (in the smallest dwarf galaxies) to over a trillion in larger galaxies.
4. The Universe is Abundant: The basic ingredients for life as we know it – carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur (CHNOPS), and water – are everywhere in abundance.
5. Our Solar System is Young: The Solar System formed ca. 4.6 billion years ago, and humanity has existed for only 200,000 years, making us a late addition to the party.

So… given the amount of time life has had to emerge, the sheer number of stars and planets, and the fact that the ingredients for life are so common, it’s a foregone conclusion that life is quite common too. By extension, it stands to reason that intelligent life would also have had enough time to emerge many times over and to explore the Milky Way galaxy. So why hasn’t humanity seen or heard from any advanced life forms yet?

When Fermi and his colleagues did the math on this question, they found that Earth should have been visited several times already. And yet, there is no definitive evidence of extraterrestrial visitors to Earth, and when our instruments are pointed toward the heavens, we encounter what scientists call the “Great Silence.” This is the essence of what came to be known as “Fermi’s Paradox,” referring to the discrepancy between the assumed likelihood of life and the absence of evidence. This gap has led to multiple proposed resolutions.

They Don’t Exist

The first formal proposals, which also formalized the Fermi Paradox, were published in the 1970s and early 80s by two physicists: Michael Hart and Frank Tipler. In 1975, Hart published a paper titled “Explanation for the Absence of Extraterrestrials on Earth,” where he made a controversial claim. According to Hart, if advanced civilizations had emerged in our galaxy in the past, they would have surely developed the technology for interstellar space travel.

By his estimates, such a civilization would only need two million years to colonize the entire galaxy and would have been to Earth many times. Ergo, the absence of evidence for extraterrestrials on Earth (what he called “Fact A”) implied that intelligent life did not exist beyond Earth. This was followed in 1981 by Tipler’s paper, “Extraterrestrial Intelligent Beings Do Not Exist,” in which he made similar arguments. However, Tipler gave a more liberal estimate, claiming an advanced civilization could colonize the galaxy in 300 million years.

This came to be known as the Hart-Tipler Conjecture. While they make some fatalistic conclusions, these arguments are not without merit. Using humanity as an example, both authors reasoned that aliens would be subject to the same exponential rate of population growth and technological progress. As such, it would not take them very long to develop advanced communications, spacecraft, and self-replicating (Von Neumann) probes.

Also known as “Universal Constructors,” Hungarian-American John Von Neumann proposed this last concept in the 1940s based on his research into the self-replicating nature of DNA. As described in the 1966 book Theory of Self-Reproducing Automata, written by Neumann’s colleague Arthur W. Burks after his death, these machines would be capable of harvesting resources and building exact copies of themselves.

He further reasoned that probes equipped with this ability would be an ideal means for exploring space, as they could proliferate endlessly across star systems. The absence of such machines in our backyard, said Hart and Tipler, proved that there were no advanced civilizations out there.

“Sagan’s Response”

These conclusions prompted the famed astronomer, planetary scientist, and science communicator, Carla Sagan, to draft a formal response. In a paper he co-authored with fellow astrophysicist William Newman in 1983, “The Solipsist Approach to Extraterrestrial Intelligence” (aka. “Sagan’s Response”), Sagan argued that there were countless reasons why humanity has not found evidence of ETCs yet. As they summarized:

Seeking, in effect, a universal principle to explain the apparent absence of extraterrestrial beings on Earth, [Tipler] contends that if extraterrestrial beings exist, their manifestations will be obvious; conversely, since there is no evidence of their presence, they do not exist. But absence of evidence is not evidence of absence.

In particular, Sagan and Newman challenged the inherent assumptions Hart and Tipler made and were critical of the values they employed. For instance, Hart assumed an ETI would spread from one star to the next at a constant rate of 10% the speed of light without any serious pauses to settle new worlds before sending out more ships. Meanwhile, Tipler’s estimate of 300 million years was based on a replication rate of 10,000 probes a year and a modest travel velocity of less than 1% the speed of light.

But as Sagan and Newman pointed out, even if such probes only produced a single copy of themselves every time they replicated themselves, “the entire mass of the Galaxy would be converted into von Neumann machines within a few million years of their invention.” In addition, Hart and Tipler’s arguments assumed that an advanced species would pursue a policy of unlimited expansion and that its colonies, once established, would last for millions or even billions of years. If even one of these assumptions is incorrect, the entire Conjecture falls apart.

This echoed statements made by Sagan and Newman in a 1981 paper titled “Galactic Civilizations: Population Dynamics and Interstellar Diffusion.” Based on how much time and energy it takes to travel between stars, they argued, it was likely that alien signals and probes may not have reached Earth yet.

This response was one of many challenges and counter-proposals to the Hart-Tipler Conjecture, all of which sought special explanations for why humanity has not yet made contact with extraterrestrials.

Where’s the Filter?

One such explanation, which summarized many schools of thought, was the “Great Filter,” proposed by Robin Hanson, an associate professor from George Mason University and a former research associate with Oxford University’s Future of Humanity Institute (FHI). In 1996, he published a paper titled “The Great Filter – Are We Almost Past It?” where he proposed that something may exist in the cosmos that prevents intelligent life from achieving a high level of development on the Kardashev Scale.

Hanson argued that the “Filter” must lie somewhere between the point at which life emerges on a planet (abiogenesis) and the point at which it becomes an interstellar civilization. Using life on Earth and the emergence of humanity as a template, Hanson outlined a nine-step process that life would need to follow to reach the point of becoming a space-faring civilization. These included:

• Habitable star system (organics and habitable planets)
• Reproductive molecules (e.g. RNA)
• *Prokaryotic single-cell life*
• *Eukaryotic single-cell life*
• *Sexual reproduction*
• *Multi-cell life*
• *Animals capable of using tools*
• *Industrial civilization*
• *Wide-scale colonization*

In accordance with Hanson’s hypothesis, at least one of these steps must be improbable, which would constitute the “Filter.” Either life has a difficult time emerging from inorganic materials early on, or the odds of catastrophic failure increase as species become more and more complex and advanced. Examples of the latter include asteroid impacts and other Extinction-Level Events (ELE), which are statistically more likely the longer a planet hosts life, nuclear annihilation, or environmental destruction.

Either of these possibilities, said Hanson, has significant consequences for humanity:

Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding, lasting life, and humanity faces the ominous question: how far along this filter are we?

Nick Bostrom, a philosopher who also hails from the FHI, provided an excellent description of this hypothesis, which he described in his 2008 essay, “Where Are They? Why I Hope the Search for Extraterrestrial Intelligence Finds Nothing.” As he wrote:

The Great Filter can be thought of as a probability barrier. It consists of [one or] more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth-like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology.

By the closing of the 20th century, the field of SETI faced an uncertain future. On the one hand, Congress chose to cancel NASA’s formal SETI program, the High Resolution Microwave Survey (HSMS), in 1993. The move was led by Nevada Senator Richard Bryan, who argued the program was a waste of money, citing the Hart-Tipler Conjecture as the reason. On the other hand, the field had matured thanks to the many insightful ideas and frameworks introduced over the previous decades.

Within a decade and a half, SETI efforts would be revitalized thanks to renewed interest and the growth of public-private partnerships. But before we get into that, there are a few more notable ideas to explore. Stay tuned for those in our next installment!