It’s called the Fermi paradox, after nuclear physicist Enrico Fermi: if the galaxy is host to billions of stars and potentially millions of planets, a huge number of them capable of generating intelligent life, then why don’t we see signs of that life? Why haven’t we been visited? Where are the signals from planets broadcasting their own versions of the evening news, I Love Lucy, and Xenobia’s Got Talent out into the universe? Where are the space aliens?
Personally, I think it’s a naïve question. Considering the time scales involved, and then the distances, I would be very surprised if we ever met intelligent life forms other than ourselves and possibly other species on this planet like whales, dolphins, elephants, and gorillas.
Our solar system—the Sun, the Earth, and other planets—formed about four billion years ago. In a universe with an imputed age1 of approximately thirteen billion years, that’s about a third of the lifetime of the existence of everything we have ever known or can know. Our Sun is then either a third- or fourth-generation star—which is to be expected, since all the metals in the dust cloud that formed our system had to be born in the fusion reactions of earlier stars (for elements lighter than iron) or in the compression forces of supernovae (for the heavier elements). Given the generous amounts of lead, gold, and uranium found in just the Earth’s crust, and the huge proportion of iron and nickel that makes up its core, fourth generation sounds about right.
Earlier generations of stars would be different from ours. The clouds of dust and gas from which they formed would have had a much lower content of metals and of the minerals we value, like the silicon that makes up most of our soils and rocks, and the carbon, calcium, phosphorus, and other light elements that make up most of our bodies. Earlier-generation stars, which comprise the first two-thirds of the time span of the universe, would not be likely to have any kind of life that we would recognize. Even if they had plentiful water on planets within their solar system’s habitable zone, where that water remained liquid most of the time, they would still lack an abundance of the complex chemicals that support our kind of life.2
So we are only likely to find creatures whom we would view as intelligent, organized in complex, multi-cellular forms, and capable of recognizable communication out among the planets in solar systems of our own generation.
Now consider that, while we have evidence of life on this planet going back some three and a half billion of those four billion of the Earth’s life-span, such life is mostly in the form of bacteria and blue-green algae. These microbial forms were necessary to create the oxygen-rich atmosphere and sustainable ocean waters that allowed higher forms of life to develop. And those higher forms took a long time to develop. The Earth did not see the birth of multi-celled creatures, with nucleated DNA and differentiated cells and organs, until just half a billion—five hundred million—years ago. That was an explosion of life, to be sure, but none of it was what we’d call familiar or friendly.
Our kind of life did not come out of the oceans onto dry land until about three hundred million years ago, give or take—at least for the animals. The plants had taken over the land sometime earlier. But no one looking for extraterrestrial life forms is expecting to find plants.
Life on the land flourished and developed to fill all the available environmental niches: plant eating, flesh eating, walking, burrowing, and flying. Almost all of it was reasonably intelligent, if you consider intelligence as a spectrum and not the special capacity of H. sapiens. Frogs, turtles, lizards, reptiles including the dinosaurs and birds, and the early mammals all had the cerebral capacity to find food for themselves, recognize and move away from danger and toward comfort and safety, and bear and occasionally nurture their young. Survivals of those creatures today—think of dolphins, elephants, dogs, and cats—are able to recognize and respond to human beings and even understand some of our verbal commands.
That does not mean that any of these life forms—going back through the history of our planet—was capable of building a radio or television station able to broadcast the evening news or I Love Lucy out to the stars. Even human beings, who have been around for at least the past hundred thousand years—a mere fraction of the domain of life on this planet—have only had that capacity for the last hundred years or so. And, for all our science and technology, we have only sent a handful of probes out beyond our solar system and only landed human representatives half a dozen times on our nearest celestial neighbor, our own Moon, at a distance of a mere 240,000 miles.
So if it took life on this fourth-generation star this long to get to the very edge of interstellar space, why would we expect so much more from the life on other, similar planets? For one thing, though, that other life might be a lot older. A few million years of difference in the time frame of stellar formation might put the putative human beings of some not-too-distant planet a million years ahead of us. Imagine what we could achieve in a million years beyond the Apollo Moon landings, the deciphering of the human genome, or the invention of radio astronomy. But then, we might also be a million years ahead of our nearest interstellar neighbors. They might still be in their equivalent of the early Pleistocene, with the closest thing to a human mind still roaming the savannah and hooting at their moon, like our hominid ancestors.
And then there is the question of distance. If the speed of light is an immutable boundary, and the currently available forms of energy are all that humankind will ever invent or discover, then crossing the gulf to our nearest stellar neighbor, the Alpha Centauri group, is going to take a long time. Chemical rockets shooting out reaction mass just can’t achieve the speeds needed to cover such a distance in less than many human generations. Travel by interdimensional means, such as artificial wormholes and warp drives,3 are still figments from the minds of science fiction authors. There may still be some magical—in the sense of Arthur C. Clarke’s “Any sufficiently advanced technology is indistinguishable from magic”—means of energy release, like Star Trek’s controllable matter-antimatter reactions, and some fantastic reactionless drive, like the gravity polarizer from Larry Niven’s Known Space universe. Maybe one day, in a lot less than a million years, we will discover such wonders. After all, it’s only recently that we’ve stumbled upon materials that work as high-temperature superconductors.
But until we make some significant scientific breakthroughs, the distance between the stars is still a limiting factor. We aren’t getting there—and by “there,” I mean someplace more friendly and habitable than Alpha Centauri—anytime soon. And we can’t expect intelligent aliens at our level of technology or even a bit more advanced to come visiting here.
But what about those radio and television signals? Shouldn’t we have heard the alien versions of the evening news and I Love Lucy by now? After all, we’ve been listening with radio telescopes to every likely star for more than sixty years, and analyzing signals with the SETI (Search for Extraterrestrial Intelligence) program for almost as long. And yet not a peep.
Well, there’s that distance thing again. Our random programs—and presumably theirs—are all by broadcast: the radio or television station sends out a signal in all directions. Any such signal is going to be subject to the inverse-square law, which says that signal strength diminishes with the square of the distance. By the time one of our programs—or one from the alien broadcasting network—reached past Alpha Centauri, its signal would be barely a mouse squeak, drowned out by the clanging together of two hydrogen atoms in space. And our signals—and presumably theirs—are not that strong to begin with, usually about fifty thousand watts on Earth, because we limit the power of broadcast stations to prevent them overpowering other users in the same band of the electromagnetic spectrum. And, finally, many of our broadcasts—certainly in the UHF and VHF television bands—are now converting to cable, or to signals beamed by geosynchronous satellites down toward the Earth. So they would become invisible to interstellar aliens.4
To have any chance of communicating with a civilization around another star, you would need to know where they are and then beam your signal—direct it with a parabolic antenna or similar device—to that location. Given the vastness of this galaxy, let alone the whole universe of galaxies, and the distribution of four-generation stars likely to have advanced civilizations, the chances of our planet crossing a coherent beam and interpreting it, are diminishingly small.
And finally, there are accidents. Many science fiction authors and others responding to the Fermi paradox believe that advanced civilizations necessarily destroy themselves. These are minds bruised by catastrophic thinking: first the vision of a Cold War nuclear holocaust—something I and most of my generation grew up with—and then the succeeding visions of totalitarian takeover, population explosion, and climate collapse, and finally culminating in the presumed death of computerized commerce in the Y2K fright. The popular notion is that organized civilization is a force for stupidity and always poisons and destroys itself. But somehow we humans have managed to survive.
Still, there have been accidents beyond human control. The Earth has supposedly undergone five mass extinctions, whether from asteroid strikes like the Chicxulub meteor that took out the dinosaurs and most of the other animals on the planet, or from natural variations like Earth’s orbital perturbations. In the lifetime of the human species, we have experienced and survived the last of the great glaciations, which covered parts of the planet in ice a mile deep. Humanity is still in no position to ward off a large incoming asteroid, and our civilization might not survive another glaciation, although our species just might.
It doesn’t take the human stupidity of a nuclear war or the mismanagement of a population explosion to set back a developing planet. It happened five times around here. And who knows, without the Chicxulub event, the dinosaurs might eventually have grown smart and started building radios and rockets a couple of million years ago.
But given all these factors, I’m not surprised that we haven’t seen any aliens yet. We’ve only been thinking about them and listening for their signals in the last sixty years or so. And maybe we’re just not that interesting a species, or not yet, to make visiting us worthwhile.
1. Working backward through the expansion rate following the Big Bang—if, in the first place, you actually believe in a “Big Bang” and its consequences.
2. This would discount, of course, energy beings, ghosts from other and older dimensions, and similar denizens of the science fiction canon.
3. See Warp Drive from March 17, 2019.
4. Of course, some of our Earth stations communicating with satellites in geosynchronous orbit are beaming up, away from the planet’s surface and toward those satellites. Those transmissions might be strong enough and focused enough to travel through interstellar space and still be recognizable to alien intelligences—although they are not aimed at any particular star.