Fermi and the Hart-Tipler Conjecture

Matt Williams
6 min readJun 7, 2021

Welcome back to the ongoing Fermi Series! This marks the fifth installment in our look at Enrico Fermi and the famous question that launched close to 100 proposed resolutions. Previously, we looked at the nature of the Fermi Paradox, the famous Drake Equation, the Great Filter, and what researchers are looking for when they engage in the Search for Extraterrestrial Intelligence (SETI).

Today, we’ll take a look at a key aspect of the Fermi Paradox, a proposed resolution known as the Hart-Tipler Conjecture. The theory is named for Michael Hart and Frank Tipler, who authored and refined the argument between 1975 and 1980. In response to Fermi’s question about why humanity has not heard from another civilization (aka. “Where is everybody?”), Hart and Tipler claimed it was because “humanity is alone in the Universe.”

For this reason, the Hart-Tipler Conjecture is often the first stop on the “possible resolutions to the Fermi Paradox” tour. Not only is it the earliest academic paper written that addresses the Fermi Paradox. It is also the most basic and fatalistic. When asking “is anybody out there?” there are few answers simpler than a flat-out “no.”

But as I said, the story is more complicated than all that. In truth, Michael Hart was not only responsible for offering the first resolution to Fermi’s famous question, He’s also the man who crafted the Fermi Paradox as we know it—with all the assumptions and suppositions that it entails — and thus framed the debate that’s ensued.

Credit: Mark Molnar

“Fact A”

In 1975, Michael Hart (a white nationalist and separatist in addition to an astrophysicist) published a paper titled “An Explanation for the Absence of Extraterrestrials on Earth.” In this paper, Hart summarized the crux of his argument — what he referred to as “Fact A” — as follows:

“If… there were intelligent beings elsewhere in our Galaxy, then they would eventually have achieved space travel, and would have explored and colonized the Galaxy, as we have explored and colonized the Earth. However, (Fact A), they are not here; therefore they do not exist.”

Naturally, the argument contains several assumptions, many of which have been seized upon by other astrophysicists who’ve offered their own resolutions. For starters, Hart assumed that since the Milky Way has existed for over 13 billion years (and the Solar System for only the last 4.5 billion years of it), life must have emerged elsewhere in our galaxy already.

With even a modest head-start of a few eons, Hart argued, any intelligent species that emerged in the Milky Way would have had plenty of time to grow and develop advanced technology, eventually leading to interstellar travel. Moreover, they would have had plenty of time to travel to and colonize the nearest star systems to their own, becoming an interstellar civilization.

These colonies would then launch their own ships, which would eventually allow the species to expand its civilization over much of the galaxy. Overall, Hart calculating that with a relatively modest velocity of one-tenth the speed of light (about 107.9 million km/h; 670.46 million mph), it would take a single species between 650,000 to 2 million years to expand across the entire galaxy, a feat that humanity would surely notice.

Tipler’s Addendum

In 1980, physicist and cosmologist Frank Tipler expanded on this argument in a paper titled “Extraterrestrial Intelligent Beings Do Not Exist.” Here, Tipler applied various arguments used by SETI researchers, the foremost being that ETIs would develop similar technologies since the principles of physics are the same everywhere in the Universe, etc. As he stated:

“In addition to a rocket technology comparable to our own, it seems likely that a species engaging in interstellar communication would possess a fairly sophisticated computer technology… I shall therefore assume that such a species will eventually develop a self-replicating universal constructor with intelligence comparable to the human level… and such a machine combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years.”

Here too, the same assumptions are at work. If the Hart-Tipler Conjecture could be boiled down to a single sentence, it would be that “our galaxy should have been colonized by now, but it hasn’t. Ergo, we have to conclude we’re alone.” Such an argument invites falsification, and Hart and Tipler found themselves on the receiving end of a proverbial whooping shortly thereafter.

Sagan’s Response

In 1983, renowned astrophysicist and science communicator Carl Sagan teamed up with his colleague William I. Newman (a professor of physicists and astronomy from UCLA) to write a rebuttal essay to Hart and Tipler. Titled “The Solipsist Approach to Extraterrestrial Intelligence” (aka. “Sagan’s Response”), Sagan and Newman took apart the Hart-Tipler Conjecture.

In fact, it was here that Sagan issued one of the quotes he’s famous for (indicated in bold):

“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 addition to criticizing the inherent assumptions Hart and Tipler made, Sagan and Newman also took a scalpel to the math they employed. One of Tipler’s main assumptions was that an extraterrestrial intelligence (ETI) would use self-replicated robots (aka. von Neumann probes) to explore the Universe, a far more efficient method than physically traveling from star to star.

For instance, 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 summary, the Hart-Tipler conjecture is flawed on its face because it employs assumptions that are just a little too convenient. Whereas Hart assumed an ETI would simply spread from one star to the next at a constant rate of 10% c (without any serious pauses to settle new worlds before sending out more ships), Tipler employed assumptions and values that were ultimately self-defeating.

In 1981, before releasing their “response,” Carl Sagan and William I. Newman produced another paper relevant to the discussion, titled “Galactic civilizations: Population dynamics and interstellar diffusion.” Here, they argued that based on how much time and energy it takes to travel between stars, it was likely that alien signals and probes may simply not have reached Earth yet.

Since “Sagan’s response” was penned, many astrophysicists have produced their own challenges to the Hart-Tipler Conjecture. And yet, just about all of these proposed resolutions have made the same assumption as Hart did with his “Fact A.” Basically, it is still widely assumed that humanity should have encountered an ETI or found evidence of intelligent life by now.

Decades later, Hart’s “Fact A” continues to frustrate scientists who would rather speculate that humanity is not alone in the Universe. Given the sheer immensity of the Universe, the long march of time, and the abundance of all the elements we associate with life, it feels presumptuous (not to mention painfully anthropocentric) to assume that humanity is the only species in our galaxy capable of asking “Where is everybody?”

The likelihood that life is out there right now and has been for some time reasonable. But why would we assume that we must have found evidence of it by now? Why do we assume that an intelligent species would want to colonize the galaxy, for that matter? Why, for that matter, do we assume they would want to leave their home planet and expand outwards?

The answer is simple: because that’s what we would do! When it comes right down to it, our predictions and theories about what extraterrestrial life would look like are qualitatively no different than our predictions about how they would behave. It all comes back to what humans are and what humans do because that’s what we know. At the end of the day, all we can do is search for life “as we know it” (though that could be changing soon!)

In our next installment, I hope to address this and other philosophical underpinnings that inevitably come up in astrobiology and the Search for Extraterrestrial Intelligence. Stay tuned!

Matt Williams

Space/astronomy journalist for Universe Today, SF author, and all around family man!