Last year marked the sixtieth year of the iconic Drake Equation, developed by astronomer Frank Drake aimed at stimulating the public to think about the prerequisites for life on other planets. Seth Shostak at Search for Extraterrestrial Intelligence (SETI) says it is the most famous equation after e = mc² and offers a bit of its history:

The Drake Equation was cooked up by astronomer Frank Drake in 1961 to serve as the agenda for the first meeting on the topic of SETI. In 1960, Drake had conducted a pioneering search for extraterrestrial signals – a several-week long effort he named Project Ozma. Somewhat unexpectedly, this modest experiment attracted a great deal of attention, and Drake was encouraged by J.P.T. Pearman, a staff officer at the National Academy of Sciences, to organize an informal gathering of accomplished researchers and engineers to discuss the prospects for finding a signal. Was listening for radio signals a worthy endeavor or not?

Approximately a dozen people attended this informal meeting, and they all were eminent. Among them was biochemist Melvin Calvin (who received a call during the meeting notifying him that he had just won the Nobel Prize), biologist Joshua Lederberg, physicist Philip Morrison, and planetary astronomer Carl Sagan, as well as Peter Pearman and self-invited guest Barney Oliver, a highly accomplished radio engineer. The conference took place at the Green Bank Observatory – the site of Project Ozma – in November 1961.

Seth Shostak, “Understanding the Drake Equation” at SETI (Updated July 2021)

He notes that the equation comprises seven factors (which he explains) that, if multiplied, might yield the number of extraterrestrial civilizations that might be broadcasting signals we could pick up.

The Drake Equation has functioned mainly as a discussion anchor over the years, as well as a way of determining what searches might be useful.

Drake’s daughter Nadia offers some insights as to how her then 91-year-old father got interested in the idea:

Dad first started wondering whether humans are alone in the cosmos when he was growing up in Chicago in the 1930s and his dad mentioned one day that “there are other worlds out there.”

My grandfather was talking about the other planets in the solar system—at the time, they were the only planets humans knew of—but eight-year-old Frank didn’t know that. To him, “other worlds” meant “other worlds like Earth,” places populated by smart, exotic beings who might be broadcasting their presence to the stars. The idea made sense to Dad, and he began thinking about how to detect such worlds.

“To find the existence of intelligent creatures that are conscious—that would be very exciting,” Dad says now. “I wonder how widespread that situation is, in the universe.”

Nadia Drake, “Alien Hunters Have Spent 60 Years Finding New Solutions for the Drake Equation” at Evon Media (November 21, 2021)

Drake was pretty daring. When he first started working on the idea, it would be over three decades before exoplanets began to be spotted in the 1990s. Now they are routinely catalogued.

Another change over the years is that, whatever people may think of the chances of identifying extraterrestrial intelligences, the Pentagon’s recent UAP (UFO) report signals a significant attitude shift. The brass admitted that many reported aerial phenomena do not have a conventional explanation and pledged to follow the evidence instead of merely discrediting it.

Harvard astronomer Avi Loeb seems to be following in Drake’s footsteps with his efforts to develop detection systems for intelligent signals, but some aspects of his work are proving controversial:

In July, Loeb unveiled the Galileo Project, which he says was designed in the spirit of the revolutionary Italian astronomer Galileo Galilei. (The tagline is “Daring to look through new telescopes.”) The overarching goal of the $1.8 million project is to search for evidence of extraterrestrial technology, and one branch is traditional: analyzing possible interstellar objects spotted deep in space by mountaintop observatories. More controversial is the construction of a network of rooftop cameras designed to capture any UFOs prowling through Earth’s atmosphere. After enlisting more than three dozen astronomers and engineers in the project—as well as some notorious nonscientists—Loeb hopes to solve the enduring UFO mystery once and for all. “Scientists have to come to the rescue and clear up the fog,” Loeb says.

Some researchers applaud Loeb’s endeavor. “He has mounted a scientific attack on a problem that is frustratingly fuzzy,” says Gregory Laughlin, an astrophysicist at Yale University. “A project like this would have been unthinkable 10 years ago.” But others say Loeb is tarnishing astronomy and undermining the search for extraterrestrial intelligence (SETI) just as that effort has started to acquire a veneer of respectability. In particular, they are bothered by the outspoken UFO zealots with no science background whom Loeb has welcomed into the project. “He’s intermingled legitimate scientists with these fringe people,” says Caleb Scharf, an astrobiologist at Columbia University. “I think you lose far more by doing that.”

Keith Kloor, “Why is Harvard University astrophysicist Avi Loeb working with ardent UFO believers?” at Science (Science, 27 January 2022)

Loeb replies to his critics, “We will not entertain fringe ideas that are outside the boundaries of the standard model of physics.” Of course, the Standard Model of physics does not rule out extraterrestrial civilizations; all we can infer from it is that they would be bound by the same laws as we are.

Whatever comes of the current ET detection projects, a design filter of the sort proposed is more useful than squabbling about the probability of ET without collecting any data.

You may enjoy these accounts of why we do not see extraterrestrials:

1.What if extraterrestrials can’t afford to take chances with us?
That’s the Dark Forest Hypothesis, riffing off the title of one of famed Chinese sci-fi author Liu Cixin’s novels. The Dark Forest Hypothesis assumes that we can use sociology to figure out what extraterrestrial intelligences might be like or might want. But can we?

2.Are the Aliens We Never Find Obeying Star Trek’s Prime Directive? The Directive is, don’t interfere in the evolution of alien societies, even if you have good intentions. Hence the Zoo hypothesis. Assuming the aliens exist, perhaps it’s just as well, on the whole, if they do want to leave us alone. They could want to “fix” us instead…

3.How can we be sure we are not just an ET’s simulation? A number of books and films are based on the Planetarium hypothesis. Should we believe it? We make a faith-based decision that logic and evidence together are reasonable guides to what is true. Logical possibility alone does not make an idea true.

4.Did the smart machines destroy the aliens who invented them? That’s the Berserker hypothesis. A smart deadly weapon could well decide to do without its inventor and, lacking moral guidance, destroy everything in sight. Extinction of a highly advanced civilization by its own lethal technology may be more likely than extinction by natural disaster. They could control nature.

5.Researchers: The aliens exist but they are sleeping… And we wake them at our peril. The Aestivation hypothesis is that immensely powerful aliens are waiting in a digitized form for the universe to cool down from the heat their computers emit.

6.Maybe there are just very few aliens out there… The Rare Earth hypothesis offers science-based reasons that life in the universe is rare. Even if life is rare in the universe, Earth may be uniquely suited to space exploration, as the Privileged Planet hypothesis suggests.

7.Does science fiction hint that we are actually doomed? That’s the implication of an influential theory, the Great Filter hypothesis, as to why we never see extraterrestrials. Depending how we read the Kardashev scale, civilizations disappear somewhere between where we are now and the advanced state needed for intergalactic travel.

8.Space aliens could in fact be watching us. Using the methods we use to spot exoplanets. But if they are technologically advanced, wouldn’t they be here by now? The Hart-Tipler conjecture (they don’t exist) is, of course, very unpopular in sci-fi. But let’s confront it, if only to move on to more promising speculations.

9.Is the brief window for finding ET closing? According to some scenarios (the Brief Window hypothesis), we could be past our best-before date for contacting aliens. Of course, here we are assuming a law of nature as to how long civilizations last. Can someone state that law? How is it derived?

10.What if we don’t see aliens because they have not evolved yet? On this view, not only did we emerge during a favorable time in the universe’s history but we could end up suppressing them. The Firstborn hypothesis (we achieved intelligence before extraterrestrials) lines up with the view that humans are unique but sees that status as temporary.

11.The aliens exist—but evolved into virtual reality at a nanoscale. That’s the Transcension Hypothesis, the latest in our series on science fiction hypotheses as to why we don’t see extraterrestrials.
On this view, after a Singularity the ETs become virtual intelligences, exploring inner space at an undetectably small scale.

12.Is intelligent life in the universe living in interior oceans of planets and moons? The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there. If intelligent life forms are trapped in the interior oceans of rocky moons and planets, Earth is a special planet—much better suited to space exploration.

13.Is real-world space travel just too daunting for ET? That’s the Percolation Hypothesis as to why we don’t make contact with aliens. They can’t overcome the laws of physics, any more than we can. If there is a purpose behind the universe, maybe the aliens and we weren’t intended to meet. That’s worth considering, given the physics barriers.

14.The Aurora Hypothesis: ET could risk only rare contact with us. Given the difficulties and risks of space travel, extraterrestrials with advanced technology may have visited Earth only one in a million years, researchers say. After centuries of modern science, we are just now looking for fossil bacteria on Mars, not without risk. ET may be in the same position.

15.Data analyst offers 15 reasons extraterrestrials aren’t seen. He estimates that there should be 100,000 civilizations in our galaxy. Some of Yung Lin Ma’s suggested reasons are ones we had not considered before, including flow of time and communication differences.

16.What if ET has morphed into what we now call the laws of nature? Astrophysicist Caleb Scarf has asked us to consider a daring hypothesis for conundrums around dark matter and dark energy. Scharf’s hypothesis highlights the genuine difficulty of accounting for a universe that comes into existence without any underlying intelligence at all.