Where Is Everybody in Our Universe?

Two physicists contemplate life on other planets in this excerpt from “Exoplanets”

NASA's NExSS collaboration includes those who study Earth as a life-bearing planet (lower right), those researching the diversity of solar system planets (left), and those on the new frontier, discovering worlds orbiting other stars in the galaxy (upper right) NASA Ames/W. Stenzel

The story is that it all started one day in 1950, when a group of prominent physicists— all veterans of the Manhattan Project—were walking to lunch at the Fuller Lodge in Los Alamos. They were discussing the spate of recent UFO sightings that had been claimed in the area, and the conversation turned to the topic of extraterrestrial civilizations. Out of the blue, Enrico Fermi (1901–54), a man well known for his ability to see to the heart of a problem, asked a simple question: Where is everybody? In the years since then, scientists have come to realize that Fermi’s offhand question is, in fact, the deepest question we can ask about life in our galaxy. The fact that there is no evidence for the existence of extraterrestrials in spite of the calculations suggesting that they should exist is known as the Fermi paradox.

So why has his offhand question played such an important role in the debate about extraterrestrials? To understand this, we can go back to our old device of compressing the lifetime of the universe into a single year. In this scheme, the Sun and our solar system formed in the late summer (Labor Day is a convenient approximation), modern humans showed up a few minutes before midnight on New Year’s Eve, and all of recorded history took place while the ball is descending in Times Square, with modern science appearing in the last second of that descent.

The point is this: if there really are other technological civilizations out there, it is extremely unlikely that they developed science after we did—after all, they had the whole year to discover the laws of nature. To understand what follows from this statement, let’s look at a possible future for the human race.

Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life beyond Our Solar System

In Exoplanets, astronomer Michael Summers and physicist James Trefil explore remarkable recent discoveries: planets revolving around pulsars, planets made of diamond, planets that are mostly water, and numerous rogue planets wandering through the emptiness of space.

We’ll start at Princeton University in the 1970s, where physicist Gerard O’Neill (1927–92) was teaching a seminar centered around an interesting question: is the surface of a planet really the best place for a technological civilization? The answer the class came up with was “no,” and from their deliberations came the design for a structure now called an O’Neill colony.

Imagine a hollow doughnut, a mile or more across, rotating slowly in space. In O’Neill’s vision, people live inside the doughnut, and the centrifugal force associated with its rotation substitutes for gravity. Using solar or nuclear power, possibly with ancillary doughnuts for raising crops, such a system could be self-sustaining, a true move of humanity away from our home planet. It is almost within our technological capabilities to build such a structure right now, if not within our budgets. In any case, we should expect that any extraterrestrial race that has come to our level of technical sophistication should also be able to build something like an O’Neill colony.

Let’s imagine how something like O’Neill colonies might play out in our future. Eventually, we can expect that people in colonies like this would leave the space around Earth and move to the truly prime real estate in the solar system, the asteroid belt, where ample material and solar power are available.

It’s the next step that has enormous implications for the Fermi paradox. After a few generations have spent their lives in something like an O’Neill colony, will it really matter to them if their colony is on the way to another star system rather than in the asteroid belt? As the best locations in our own system fill up, it is reasonable to suppose that future space colonists will follow the example of their forebears and “light out for the territories,” except that, in this case, that would mean moving to other solar systems. In essence, we suggest that they would turn their colonies into interstellar starships. How hard would that be?

Let’s make two extraordinarily conservative assumptions. Let’s assume that (1) there is no way to get around the speed-of-light barrier—no “warp drive”—and (2) no major technological advances will be made in the next couple of centuries. The immense distance between stars would require travel times of a century or more, which would mean that the starship would be multigenerational—you get on, your grandchildren get off. Several propulsion systems for such a trip have been proposed—for example, one in which the ship scoops up rarefied interstellar hydrogen to run its power and propulsion systems. The idea of such a multigenerational starship is already a staple of science fiction.

The point of this exercise in futurology is that once a civilization reaches our level of sophistication, it is only a matter of a few centuries before it can start colonizing other star systems. If we can imagine ourselves doing it, then there’s no reason extraterrestrials couldn’t do it as well. The important point for our discussion is that we are talking about a time span of only a few hundred years. In terms of our galactic year analogy, this amounts to only one second. Basically, as soon as the ball touches down in Times Square, Earth could be the center of an expanding wave of human colonization. No one would even have time to say, “Happy New Year.”

How long would it take that wave to engulf the entire galaxy? Most calculations give times on the order of 30 million years or so. And while this is an extremely long time on a human scale, it is only one day in our galactic year. So if extraterrestrial civilizations have been popping up throughout the galactic year, and if at least some of those civilizations are as scientifically adept as we are, there should have been multiple waves of colonization sweeping over the solar system.

So . . . where is everybody?

That, in essence, is a modern look at the question Fermi asked over a half century ago, one we still haven’t been able to answer. His point can be stated this way: we shouldn’t be looking for extraterrestrials out there, as we do in SETI (the search for extraterrestrial intelligence)—we should be looking for them right here. And if we ignore the silliness of UFOs and ancient astronauts, we can say that there is no evidence whatsoever for extraterrestrials being here now or in the past.

Where is everybody? Why the Great Silence?

Artist's rendering of potentially habitable exoplanets. NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

William of Ockham was an English scholar who is famous for one throwaway line in an otherwise turgid theological treatise. Called Occam’s razor, it says, “Plurality must never be posited without necessity.” In essence, it tells us that when we have a question to answer, the simplest solution is the one we should choose. The concept shaves away complexity; hence the word razor.

There is no doubt that the simplest answer to the questions “Why the Great Silence? Why don’t we hear any SETI signals?” is that we don’t hear signals because no one is sending them. There are a number of other explanations that have been put forward, and we can look at them briefly before taking William of Ockham seriously. Basically, the explanations can be divided into three categories:

1. They really are out there, but they’re not interested in us.

2. They really are out there, but they’re protecting us.

3. They really are out there, and we’re going to get it unless we mend our ways.

An example of the first category would be a race of extraterrestrials living in a Dyson sphere, happy as clams with their star’s energy and supremely uninterested in anyone else. Another possibility would be extraterrestrials on a rogue planet who can’t imagine a planet near a star being inhabitable. An example of the second item in the list is seen in the Star Trek series, where spacefarers obey the Prime Directive, which forbids them from interfering with the development of other life forms. The last category is portrayed in the classic 1950s film The Day the Earth Stood Still, in which an extraterrestrial visitor warns that Earth will be destroyed unless we control our use of atomic weapons:

Klaatu barrada nikto!

All these schemes have two things in common. First, there is no evidence to support any of them, and, second, they are all somewhat improbable in a galaxy with thousands of different advanced civilizations. Some might indeed retreat to Dyson spheres or refuse to go near stars, but to suppose that all of them would is something of a stretch. Similar arguments can be made for the other explanations. 

One way to approach the question posed by the Great Silence is to think of each term in the Drake equation as a gateway or valve on the way to an advanced technological civilization. If even one of those terms has a numerical value much less than we have assumed, the effect would be to greatly reduce our estimate of the number of extraterrestrials out there. In essence, that term would act as a kind of filter, blocking the orderly progression implied in the equation. To use a term introduced by economist Robin Hanson, our colleague at George Mason, somewhere in the chain of events in the Drake equation there might be a “Great Filter” that effectively blocks the development of civilizations that might be trying to communicate with us.

Some scientists have argued that the existence of periodic ice ages played an important role in producing the kind of social interactions needed to take humans beyond the hunter-gatherer stage. In one scenario, for example, the need to protect the nutritionally rich shellfish beds along the African coast—a dependable source of food—during an ice age is what led to both the kind of cooperativeness and the kind of aggressiveness that have characterized our species ever since. Again, if you accept this sort of argument, you are saying that the Great Filter is located at the point where intelligence progresses into advanced society. If this is true, there will be lots of planets with the equivalent of dinosaurs out there, but none (or very few) with radio telescopes.

Arguments that say, in effect, that there is something special about Earth that is unlikely to be duplicated elsewhere in the galaxy—go under the name of the Rare Earth Hypothesis. They are put forward most completely in a book titled Rare Earth, by geologist Peter Ward and astronomer Donald Brownlee. Ward and Brownlee’s central thrust is that we have been blindly accepting the Copernican principle—the idea that Earth is not special—and ignoring the fact that there are many unusual things about our home planet. In essence, they look at all the things that are unique about Earth and argue that if they are all necessary for an advanced civilization to develop, then we could well be the only such civilization in the galaxy. For example, if, besides an Earth-sized planet in the CHZ of its star, you need a star located a certain distance from the galactic center, a Jupiter farther out, plate tectonics, the right planetary tilt to produce ice ages, and a large moon to stabilize the planet’s axis of rotation and produce tidal pools (Darwin’s warm little pond), Earth might well be the only planet like that in the galaxy. The Rare Earth answer to the Fermi paradox is thus quite simple: there’s nobody here because there’s nobody there. We are indeed alone.

Those who don’t accept the Rare Earth Hypothesis assert that any specific event you want to talk about is extremely unlikely, and that simply reciting that fact proves nothing. Think, for example, of the chain of unlikely events that led to your reading these words. Your parents had to meet, you had to attend a certain school, learn to read, acquire an interest in science, and so on. There’s no point in harping on this improbability, though, because if you weren’t reading this book, you’d be doing something else equally improbable. In the same way, other types of improbable intelligences could have developed in the galaxy following their own improbable chain of events, and there could be an infinite number of those improbable paths. For these critics, all the Rare Earth Hypothesis proves is that there is at least one improbable path to an advanced civilization (our own); it says absolutely nothing about the possible existence of other paths.

The scenarios we have considered all have one thing in common: they all assume that the Great Filter is behind us, that by some combination of luck or providence, Homo sapiens has made it through all the filters and bottlenecks that stood in our way. But there is another, much more frightening possibility. What if none of these events in our past constitutes the Great Filter? What if the Great Filter is still in front of us?

To understand the importance of this question, let’s think for a moment about the nature of the evolutionary process. Natural selection is driven by one criterion and one criterion only: the need to get an organism’s genes into the next generation. Winners in the evolutionary game, in other words, are not determined by moral or ethical considerations. Consider the history of our own species as an example of this statement. The appearance of Homo sapiens in any region once we left Africa was accompanied by the disappearance of competing hominids (think Neanderthals and Denisovans) and just about every large animal (think woolly mammoths and giant tree sloths). We became the dominant life form on the planet by wiping out our competitors, either directly or indirectly. Given this history, we think it’s fair to say that Homo sapiens is not the sort of species you’d want to meet in a dark alley, and the same will be true of any other winner of the evolutionary game who became the dominant species on their planet.

The “Great Filter is in front of us” argument goes like this: despite the Rare Earth Hypothesis, there really doesn’t seem to be anything all that special about the way that life developed on Earth, and given the abundance of planets out there, there is no reason that complex life shouldn’t be quite common. On the other hand, from what we know about the process of evolution, we can expect the winners of the evolutionary game on other planets to be no more benevolent than Homo sapiens. In this case, the coming Great Filter is easy to see. Once an aggressive, warlike species discovers science, they are likely to turn their discoveries against one another and, in essence, wipe themselves out.

The picture of galactic history that comes from this argument is a disturbing one. From the very beginning, intelligent, technologically advanced societies have appeared only to disappear in a short time as they succumb to their own dark inner nature—a nature produced by the laws of natural selection. No one is out there, in other words, because they’ve all wiped themselves out long ago, before we started listening.

Exoplanets is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt condensed for print from Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life beyond Our Solar System © 2017 by Michaels Summers and James Trefil