by Chris Impey
Tarter refuses to be discouraged by the Great Silence. Rather, she thinks the search is only just beginning to get interesting. Our most powerful “send” capability is the radar transmitter at the Arecibo 305-meter radio dish in central Puerto Rico.12 The Allen Array will be able to detect analogous technology beamed from a planet around any star out to a thousand light years. Optical SETI’s ability to detect pulsed lasers is also getting better. In twenty years, SETI could detect the equivalent of our most powerful radio and optical transmitters if they are beaming at us from planets around 100 million stars in the galaxy. At that point, surely, continuing Great Silence means we’re alone.
Or perhaps the search is ill conceived. We might be looking in the wrong part of the cosmic haystack. Denizens of other planets may be using forms of data modulation and compression that look to us like pure noise. Radio transmitters and laser might be fleeting technologies, so the window of time when they get used by a civilization is small. Seth Shostak, a senior astronomer at the SETI Institute, jokes about people who say he should wait until he has more powerful technology, noting that Queen Isabella of Spain didn’t tell Christopher Columbus to wait until he had a jumbo jet to discover America.
Another possibility is that all other life is already dead. When it comes to putative alien physiology and psychology, answers to the Fermi question proliferate like weeds. As we see ominously from our own history, galloping technology might render a civilization unstable. If the factor L in the Drake equation is on average no more than a few centuries or a millennium, SETI is likely to fail. It doesn’t matter whether the civilizations self-destruct or degrade to a preindustrial state—the effect on the search is the same. Silence.
It’s also possible that extraterrestrial life is unrecognizable to us. Aliens in films and on TV are amusing because they’re usually thinly veiled versions of us: bipedal vertebrates with extra appendages or bad skin. Occasionally, they’re amorphous and weird. But life elsewhere might be organized quite differently at the level of the organism. It might communicate at speeds that are too slow or too rapid for us to recognize. It might not be culturally inclined to communicate at all, or it might be disinterested in space travel even if it had the suitable technology. It might have passed from a biological to a post-biological or computational state. We can try to think outside the box, but anthropocentrism permeates all discussions of advanced life beyond Earth.
The Great Filter
Does the lack of evidence for extraterrestrial technology say anything about us and our future?
Yes, especially if spacefaring civilizations are very rare, as opposed to just hard to detect or recognize. In 1998, economics professor Robin Hanson presented an idea called “the Great Filter.” If any of the Drake equation factors is very low, it will act as a filter to choke off evolution toward life venturing beyond its planet of origin. A filter can lie behind us (in our past), or ahead of us (in our future).13 In the past, the filter could be the transition from single- to multicelled organisms, or the steps required to develop a brain, or the instability of a technological species. When humans harnessed the power of the atomic nucleus in the middle of the twentieth century, it was in the unstable and destructive form of a bomb. For a decade, we teetered on the edge of nuclear holocaust. Regarding the future, this argument leads to the counterintuitive and unnerving conclusion that the easier it is for life to get to our stage of development, the bleaker our future chances for survival probably are.
Assuming that a Great Filter winnowed down billions of germination sites for life to zero observable extraterrestrial civilizations, it’s crucial to consider where the filter lies. If the filter is in our past, it means there is an extremely unlikely step in the progression from Earth-like planet to one that hosts a civilization with our level of technology. That step might even be the formation of life from simple chemicals. Whatever the filter is, if it’s behind us, we can explain the lack of observable aliens. Technological civilizations are intrinsically rare, so searches for them will fail.
On the other hand, if the Great Filter is in our future, it’s very unlikely that a civilization at our stage of development progresses to the large-scale colonization of space. One plausible scenario is that technology is the culprit because it includes the capability for self-destruction. Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, has done scholarly work on catastrophes. His partial list of existential threats faced by humanity includes nuclear holocaust, genetically engineered superbugs, environmental disasters, asteroid impacts, terrorism, advanced and destructive artificial intelligence, uncontrollable nanotechnology, catastrophic high-energy physics experiments, and a totalitarian regime with advanced surveillance and mind-control technologies.
Regarding existential threats that might act as a filter in our future, Bostrom makes another point. The requirement is not that it has a significant probability of destroying humanity. Rather, it must be able to plausibly destroy any advanced civilization. Asteroid strikes and supervolcanoes don’t qualify because they’re random events that some civilizations will survive and others won’t experience because their planet and solar system are different from ours. The technological innovations that drive the argument and act more effectively as filters are those that almost all civilizations eventually discover, where their discovery almost universally leads to disaster (Figure 55).
Figure 55. In the short history of the “nuclear age,” we have come close to a holocaust several times. The Doomsday Clock tracks our proximity to Armageddon. Civilizations may become unstable and destroy themselves. This issue impacts the prospect of companionship and contemporaneous communication in space.
Bostrom has said: “I hope that our Mars probes will discover nothing. It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirits.”14 Why would he be so grumpy about one of our best pieces of technology?
If life is discovered on Mars or any other place in the Solar System, it suggests that the emergence of life is not an improbable event (and it doesn’t matter whether the life found is ancient or current). If biology emerged twice independently in our backyard, then surely there are many biological experiments in the galaxy. The same logic will apply if we one day find that a significant number of habitable exoplanets have had their atmospheres altered by microbial life. Either discovery would imply that the Great Filter is less likely to occur in the early history of planets and is more likely ahead of us. In other words, dead rocks and lifeless planets will be good news since they’d tell us we’d survived the tough part of our evolution.
This framing of the argument is a simplification. There may be more than one Great Filter. We might have cleared one, only to be faced in the future with another. Also, we should be wary of positing that life elsewhere has to follow the path traveled by life on Earth or that other civilizations progress with the single-minded purposefulness of humans. Let’s allow Nick Bostrom to have the last word. For someone who dwells on apocalypse and hopes that the search for life in the universe fails, he’s strangely optimistic:
If the Great Filter is in our past . . . we may have a significant chance . . . of one day growing into something almost unimaginably greater than we are today. In this scenario, the history of humankind to date is a mere instant compared to the eons of history that still lie before us. All the triumphs and tribulations of the millions of people who have walked the Earth since the ancient civilization of Mesopotamia would be like mere birth pangs in the delivery process of a kind of life that hasn’t really yet begun.
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A Universe Made for Us
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Our Far Future
If we make it through our troubled adolescence as a species, what lies in store? We’re curious and creative but also prone to tribalism and needless competition. I’ve sketched a scenario for the next century, when we establish homesteads on the Moon and Mars, project our tourism and c
ommerce off-Earth, and get used to traveling throughout the Solar System.
By overcoming our self-destructive tendencies, we might achieve the normal evolutionary lifespan of a mammal species, a million years or more. To see how hard it is to project ourselves forward that far, let’s play at “futurology.” Compressing time in orders of magnitude, we first look backward. Roughly ten years ago, there was no Internet. Roughly a hundred years ago, there was no mass transit and most people lived and died close to where they were born. Going back a thousand years, there was no medicine and life was short and brutal. Ten thousand years ago, agriculture would soon be invented but most humans were nomadic hunters and gatherers. Approximately a hundred thousand years ago, we hadn’t learned how to use tools or harness fire. A million years ago marks our emergence as a species. Leaping back in factors of ten quickly finds us in an unfamiliar and primitive state (Figure 56).
Now play the game forward. It’s fairly safe to predict that in ten years we’ll have sophisticated genetic engineering and a growing commercial space industry. In a century, we should have routine travel within the Solar System, robots doing our bidding, and artificial intelligence that rivals human capabilities. It’s very difficult to predict a thousand years hence, but I’m going to go out on a limb and assume that rapid technological progress will continue such that some of us will be heading for nearby stars. Ten thousand years from now, as far ahead of us as early civilizations lie behind us, the crystal ball is cloudy. A hundred thousand years and onward, it’s anyone’s guess. To venture further seems impossible. In the anthology Year Million, economists, science fiction writers, computer scientists, and physicists speculate freely, in tones that range from giddy to glum.1 The exercise is made more difficult by our position on the cusp of exponential technological change.2 The far future, with all its potential for our greatness and failure, and the possibility that we might not exist at all, is as haunting as deep space.
Figure 56. The human past becomes primitive viewing history in orders of magnitude of time. Landmarks in the past are labeled, along with speculations about the next thousand years. If humans persist for millions of years, our capabilities could be profoundly more advanced than they are now, and very difficult to predict.
When we finally leave the Solar System, those voyagers will represent a slender green shoot from the sturdy human tree. There’s no need to break laws of physics, or even to travel at a substantial fraction of the speed of light. They will never rejoin the tribe. Once they leave home, there’s no returning. The first European settlers to America knew that they would never go home; the commitment of the first star travelers will be just as absolute. They will, however, need to be kept alive for the duration of the trip.
Exploration of space beyond the Solar System is only possible if we persist as a species. It will also require innovations to extend the lives of the travelers.
The pig is named 78-6. She’s ruddy pink and weighs 120 pounds, and her beating heart is exposed in the operating theater. The surgeon cuts the aorta, watches the EKG flatline, and connects external tubes to replace the pig’s blood with a chilled saline cocktail. With her vital organs preserved, 78-6 isn’t quite dead. She’s in a state of cryogenic suspension or suspended animation at Massachusetts General Hospital in Boston. This surgeon has suspended 200 pigs for one or two hours each, and they all survived as long as they were given optimal treatment. A few hours later, 78-6 will wake up in a recovery room with classical music on the radio and a healthy pig in an adjacent stall for company. Postmortem exams on other pigs showed no cognitive damage from the procedure.3
Research in suspended animation is in its early stages. Pigs are critical test subjects because their physiology is close to that of humans. Also at Mass General, mice have had their metabolisms slowed by factors of ten or twenty. In other research labs, dogs have been revived after hours of being clinically dead. In a few cases, humans have survived being subjected to extreme hypothermia and near death for several weeks.4
But we’re taking the long view now, so let’s assume that we eventually master the art of suspended animation. This finesses the obstacle of long travel times—these starship Rip Van Winkles would wake up and continue with their new lives, oblivious to the centuries it took to get to their destination. Suspended animation would create an irrevocable rift between the voyagers and the Earth. Everyone they knew and loved, and their descendants, would have lived and died while they silently sailed through the void.
Let’s also assume that human cloning will one day be perfected. Since the pioneering experiment that gave birth to Dolly the sheep in 1996, cloning has been performed uneventfully on rabbits, goats, cows, cats, and fifteen other species.5 Primate reproductive biology appears to be more complex, but it’s only a matter of a few years before humans are cloned. Cloning is ethically fraught, but it would provide a way for us to propagate in the vastness of space. Instead of one set of colonists, chosen to be a minimum viable population with a good genetic mix, there would be a suite of colonies, each composed of the same set of cloned individuals. Each cloned colony would disperse to a different destination. Each would grapple with a different environment. Despite identical DNA, the evolutionary paths of the colonies would diverge. Taken together, they would play out natural selection on a new cosmic stage.
How then will we head for the stars?
Conceptually, there are four approaches: The travelers live and die on the spaceship, they travel in suspended animation, they’re carried as embryos or single cells, or they’re transported digitally at the speed of light. These four scenarios are ordered in increasing level of technical sophistication but decreasing order of resources required for the trip.
We’ve seen that vast Gerard O’Neill pinwheels loaded with thousands of passengers are ruinously expensive, and teleportation is far beyond current and projected technology. Suspended animation is promising, and it need not be for the whole trip. Subsets of the crew could be revived periodically to monitor life-support systems and carry out routine maintenance. Embryo transport may also be possible one day.6
Echoing Arthur C. Clarke, we’ve imagined the emotional impact of the cry of the first off-Earth infant. But what if that baby is animated after a journey of millennia and tens of light years? It would have traveled as a frozen zygote, the earliest developmental stage of an embryo. It would then have been brought to term in an artificial womb, and reared to self-sufficiency by robotic nannies, all to be part of a new human colony. The starship would also carry frozen cells of useful livestock and crops, serving as a miniaturized Noah’s ark.
Living in the Multiverse
We leave a tiny footprint in the universe. The sum of all our industry and striving is a spherical ripple moving out into the void. We’ve had powerful radio and TV transmitters operating for fifty years, and in principle that expanding sphere of radiation has swept over thousands of habitable worlds. In practice, all the pop-culture messages carried by these waves are diluted to a level below the hiss of the cosmic background radiation before they exit the Solar System. The Pioneer and Voyager spacecraft carry information about our civilization, and they’re our first artifacts to reach interstellar space, but it will be hundreds of thousands of years before they reach another star.
Other creatures to have left their planet are likely to be considerably more advanced than us. What would that imply?
Since it’s impossible to anticipate the function and form of an alien species, the simplest way to categorize hypothetical civilizations is by their energy use. This was first done by the Russian Nikolai Kardashev. Kardashev studied astronomy while both of his parents were in Stalin’s slave-labor camps in the 1950s. He heard about Frank Drake’s Project Ozma, which inspired him to write his influential paper “Transmission of Information by Extraterrestrial Civilizations.”7 In it, he defined three levels based on the amount of power available to a civilization. Type I civilizations utilize all the solar energy arriving at their planet’s surface, abo
ut 1017 watts for a planet like the Earth and a star like the Sun. The next tier is 10 billion times higher—a Type II civilization harnesses all the energy from its star, about 1027 watts. A Type III civilization is 10 billion times hungrier, consuming energy at the phenomenal rate of 1037 watts, the luminosity of a galaxy like the Milky Way. Beyond the original Kardashev scale is Type IV: masters of the universe (Figure 57).
Kardashev created his scale to categorize technologically mature civilizations. We’re so feeble that we don’t even make it onto the scale. Stuck getting energy from dead plants, our vaunted civilization runs on a measly 0.001 percent of the energy that arrives gratis from the Sun. Theoretical physicist Michio Kaku has noted that with our energy consumption growing at 3 percent per year, we’ll rise to Type I status in a few centuries, Type II status in a few millennia, and, if we make it that long, Type III status in a million years.8
Type I civilizations will elude detection, giving off extra waste heat but not enough to be detected from many light years away. Civilizations that harness most of their star’s energy might be detectable because they would have to build something like a Dyson sphere.9 Freeman Dyson published this thought experiment in 1960, based on a 1937 Olaf Stapledon science fiction novel. The idealized concept of a hollow sphere around a star is physically unstable (in Larry Niven’s Ringworld series of science fiction novels, this instability causes a collapse of the civilization), but a civilization might build a swarm of orbiting satellites to envelop the star and capture most of its energy. The visible light is captured and reradiated as infrared emission, so Dyson spheres are detectable as excess infrared emission from an otherwise normal star. Several SETI projects look for anomalous infrared radiation. Researchers at Fermilab, near Chicago, sifted 250,000 stars down to seventeen candidates, of which four were declared “amusing but still questionable.”10