The Milky Way by Tom Hall (CC2 license)
“Where is everybody?” Enrico Fermi, physicist and Nobel Laureate, asked that question at Los Alamos, New Mexico, in 1950. He was referring to extraterrestrial life. Why aren’t ETs visiting the Earth in spaceships? Fermi had done the math and concluded that there ought to be many intelligent species in our galaxy. Some would have technologies far beyond our own. They’d have mastered interstellar travel. So where were they? None of Fermi’s colleagues could say.
The question has resonated down the decades. It is the general opinion of biologists—and screenwriters—that ETs are out there. Since Fermi’s time astronomers have searched the skies for radio signals from ETs. In ever case they’ve come up empty.
One man may have the answer. He is J. Richard Gott III, a Princeton astrophysicist. In a 1993 paper in Nature, Gott invoked the Copernican principle, a foundation of astronomy. This says that our situation in the universe is unlikely to be special or privileged.
Gott applied this idea to Homo sapiens itself. Suppose that we are not so different from most ETs. Then a typical ET species is not visiting us in spaceships for the same reason we’re not visiting them: They/we don’t have any interstellar spaceships (yet?)
We have a limited Search for Extraterrestrial Intelligence (SETI) effort that listens for signals. But we don’t broadcast any signals at the high power suited to reaching across interstellar distances. Maybe most ETs are like us, listening but not sending (“lurking”).
Now course it’s true that intelligence might have evolved elsewhere billions of years before it did on Earth. ETs could be that much more “advanced.” But, says Gott, let’s not jump to conclusions about what that means. Those old civilizations may have advanced themselves all the way to extinction. We have not one iota of evidence that interstellar travel and multi-million-year civilizations are common things.
Gott proposes that there are ETs out there, but long-lived, technologically advanced species may be rarer than we think. Perhaps most ETs do not, after all, go on to explore many planets, achieve immense populations, and proclaim their existence to the whole universe. We’ve got a nice planet here. For most of the universe’s species, that may be all there is.
Yes, but what about the Drake equation? Devised by radio astronomer Frank Drake in 1960, it is the classic attempt to estimate the number of intelligent extraterrestrial species. Drake proposed that the number of ET species in our galaxy equals the product of seven unknowns:
(1) how many stars come into existence in our galaxy per year;
(2) how many of those stars have planets;
(3) how many planets exist, in a typical star system;
(4) how many such planets develop life;
(5) how many of those life-bearing planets evolve intelligent life;
(6) how many intelligent species broadcast radio signals [or otherwise reveal their existence];
(7) the average lifetime of communicating intelligent species.
There is uncertainty in all the factors but above all in (4) through (7), as these entail speculation about ET biology, history, and motivations. In 1960 Drake and colleagues estimated the number of ET civilization in the galaxy as between 1000 and 100 million.
Obviously the error bars were wide. Yet even the low-end estimate of a thousand ET species is, well, “a lot.” The notion that there are probably many, many ET species has been with us ever since.
In my book The Doomsday Calculation I talk about Gott’s answer to the Fermi question. I mention a provocative 2018 paper by Anders Sandberg, Eric Drexler, and Toby Ord, all of the Future of Humanity Institute, Oxford. The Oxford group notes that in the Drake equation we are not just multiplying seven unknowns; we are also multiplying the uncertainties in those unknowns. Any Drake estimate inherits all these uncertainties.
Sandberg and colleagues collected estimates of Drake equation factors that had appeared in the scientific literature. They then did a computer simulation in which the value for each factor was chosen by drawing randomly from among the published estimates for that factor. Seven chosen factors were multiplied together to get a virtual Drake estimate, and this process was repeated many times to reveal the range of variation. The Oxford group found that the resulting estimates varied by over 40 orders of magnitude. Much of this was due to factor (4), the probability of life originating. Drake and colleagues believed this was a near-certainty. But in the decades since, some scholars have argued that this confidence is unjustified. Until and unless we find life elsewhere, we can’t rule out the possibility that the origin of life requires a highly improbable molecular accident.
It’s hard to wrap your head around the level of uncertainty the the Oxford group is talking about. But let’s try. Suppose you are asked to guess the wealth of the next person you will pass in the street in Manhattan. Well, New York is a rich city—with a lot of income inequality. The mean wealth in Manhattan is much bigger than the median, for a handful of billionaires raises the average. The small billionaire population does not much affect the median, the value such that half of Manhattan’s residents have less wealth and half have more. And there’s a fairly large chance that the next person you pass in the street will be broke! There are a lot of poor people in Manhattan too.
Drake equation estimates involve a similarly skewed distribution, only it’s unimaginably greater. According to the Oxford simulation, the estimated mean number of ET civilizations in the galaxy was a generous 53 million. However, the median was only 100. And because of the spread-out distribution, the Oxford group found that 30 percent of virtual estimates had zero ETs in the galaxy. Indeed, about 10 percent of the estimates implied that ET civilizations are so rare that there are unlikely to be any in the observable universe.
Accept this, and we have no reason to be surprised at the absence of ETs. We may well be alone in the galaxy or even the universe, and it’s not out of line with what scholars currently believe. Where is everybody? The Oxford group’s answer is “probably extremely far away.”