Astrobiology—the study of extraterrestrial life—has made great strides since its 1960s origins, when the evolutionary biologist George Gaylord Simpson derided it as “a science without a subject.” Today it is booming as never before, driven by perennially high public interest and steadily growing scientific respectability.
In a press conference last week two senior NASA officials—Ellen Stofan, the agency’s chief scientist, and John Grunsfeld, the former astronaut and associate administrator for NASA’s science programs—predicted that astrobiologists would at last find their elusive alien subjects within only a decade or two. Not long ago the prediction would have been bold but now it seems almost passé, as more evidence mounts that the warm, wet conditions for life as we know it prevail throughout the cosmos. Surely simple, single-celled life should be common out there, waiting to be found by a rover in subsurface brines on Mars or by a mission sent to probe the oceans of the icy moons Europa or even via space telescopes gazing at Earth-like planets orbiting faraway stars. NASA generously funds all these efforts.
The possible existence of intelligent aliens and extraterrestrial civilizations, on the other hand, remains much more controversial and is scarcely funded at all. Even so, for more than a half-century a small, scattered contingent of astronomers has gone against the grain, engaging in a search for extraterrestrial intelligence (SETI). SETI chiefly looks for chatty cosmic cultures that might be beaming messages around our region of the galaxy using radio waves or laser pulses. But its interstellar eavesdropping has yet to detect any signals that withstand close scrutiny. Even if brimming with life, to us, the galaxy seems to be a very quiet, rather lonely place.
Now, new results suggest this loneliness may extend out into the universe far beyond our galaxy or, instead, that some of our preconceptions about the behaviors of alien civilizations are deeply flawed. After examining some 100,000 nearby large galaxies a team of researchers lead by The Pennsylvania State University astronomer Jason Wright has concluded that none of them contain any obvious signs of highly advanced technological civilizations. Published in The Astrophysical Journal Supplement Series, it is by far the largest of study of its kind to date—earlier research had only cursorily investigated about a hundred galaxies.
More heat than light
Unlike traditional SETI surveys, Wright and his team did not seek messages from the stars. Instead, they looked for the thermodynamic consequences of galactic-scale colonization, based on an idea put forth in 1960 by the physicist Freeman Dyson. Dyson postulated that a growing technological culture would ultimately be limited by access to energy, and that advanced, energy-hungry civilizations would be driven to harvest all the available light from their stars. To do that, they might dismantle a planet or two as feedstock for building star-enveloping swarms of solar collectors. A star’s light would fade as it was encased in such a “Dyson sphere,” but Dyson noted the constructions could be detected by the mid-infrared glow of their radiated waste heat—essentially the same phenomenon that causes your computer to warm up when it’s running. In 1963 the Russian astronomer Nikolai Kardashev extended these ideas by developing a tripartite classification system for a civilization’s energy use. A “type 1” civilization would harness all the energy of its home planet whereas a type 2 uses all the energy of its star, perhaps by building a Dyson sphere around it. A type 3 civilization would be capable of using all the energy of its galaxy, presumably by encasing all its stars in Dyson spheres.
Unable to secure funding from standard sources such as NASA or the National Science Foundation, Wright’s group instead turned to the Templeton Foundation, a private organization with a history of supporting controversial and speculative research. With that funding the team searched for type 3 civilizations in an all-sky catalogue from NASA’s Wide-field Infrared Survey Explorer (WISE). They looked for objects that were optically dim but bright in the mid-infrared—the expected signature of a galaxy filled with starlight-absorbing, heat-emitting Dyson spheres. After using software to automatically sift through some 100 million objects in the WISE catalogue, Wright’s student Roger Griffith examined the remaining candidates by hand, culling those that weren’t galaxies or that were obvious instrumental artifacts.
The result was about 100,000 galaxies, with about 50 in particular that emitted much more heat than light. Jessica Maldonado, a student at California State Polytechnic University, Pomona, then scoured the astronomical literature to determine what was already known about those top candidates. Many of them were well studied, and can be explained as pairs of galaxies in the process of merging or as isolated “starburst” galaxies—two processes that can heat galactic quantities of light-blocking dust to generate powerful infrared glows. According to the researchers, an additional 90 galaxies with less extreme heat-to-light ratios warrant further study but, by and large, the results are null. “On Kardashev’s scale, a type 3 civilization uses energy equal to all the starlight produced by one galaxy,” Wright says. That would equate to an infrared-bright galaxy seemingly bereft of stars. “We looked at the nearest, largest 100,000 galaxies we could find in the WISE catalogue and we never saw that. One hundred thousand galaxies and not one had that signature. We didn’t find any type 3s in our sample because there aren’t any.”
Even if advanced civilizations do not build Dyson spheres, Wright’s null result also applies to any other energy-intensive “astroengineering” taking place at galactic scales. “Looking for the absence of light as well as the waste heat like Wright and his colleagues have done is really cool,” says James Annis, an astrophysicist at the Fermi National Accelerator Laboratory who in the late 1990s used different methods to survey more than a hundred nearby galaxies for type 3s. “In some sense it doesn’t matter how a galactic civilization gets or uses its power because the second law of thermodynamics makes energy use hard to hide. They could construct Dyson spheres, they could get power from rotating black holes, they could build giant computer networks in the cold outskirts of galaxies, and all of that would produce waste heat. Wright’s team went right to the peak of the curve for where you’d expect to see any sort of waste heat, and they’re just not seeing anything obvious.”
Against the empire
The idea that there may be galactic empires out there to find at all comes from seemingly reasonable extrapolations of our own situation here on Earth.
We know that on at least one planet microbial life emerged, and that life then ascended the evolutionary ladder to build large bodies, brains, societies and eventually technologies that could take it to other planets—maybe even other stars. If it happened here, why not on any of the billions of other habitable planets astronomers now estimate fill each galaxy? “Life, once it becomes spacefaring, looks like it could cross a galaxy in as little as 50 million years,” Annis says. “And 50 million years is a very short time compared to the billion-year timescales of planets and galaxies. You would expect life to crisscross a galaxy many times in the nearly 14 billion years the universe has been around. Maybe spacefaring civilizations are rare and isolated, but it only takes just one to want and be able to modify its galaxy for you to be able to see it. If you look at enough galaxies, you should eventually see something obviously artificial. That’s why it’s so uncomfortable that the more we look, the more natural everything appears.”
Over the years, researchers have created a vast assemblage of possible explanations for SETI’s failure to find any aliens. Perhaps we are alone or some restriction imposed by astrophysics and biology makes intelligent life vanishingly rare or technological civilizations always self-destruct or interstellar travel is simply too hard, too slow or too boring. Annis suspects galaxy-sterilizing astrophysical explosions called gamma-ray bursts, which were more frequent in the cosmic past, until recently suppressed the rise of advanced civilizations and that we inhabit “the beginning of history.”
But as rich as the scientific literature is with ideas, some of the most fascinating ones come instead from science fiction. Drawing from Arthur C. Clarke’s famous quip that “Any sufficiently advanced technology is indistinguishable from magic,” in 2011 the science fiction author Karl Schroeder coined an all-too-plausible reason for the apparent absence of aliens: “Any sufficiently advanced technology is indistinguishable from nature.” In this view the future of technology would not consist of star-hopping civilizations spreading like wildfire through galaxies, disassembling planets and smothering suns, but rather of slow-growing cultures becoming more and more integrated with their natural environments, striving for ever-greater efficiencies and coming ever-closer to thermodynamic equilibrium. Simply put, profligate galaxy-spanning empires are unsustainable and therefore we do not see them. “SETI is essentially a search for technological waste products,” Schroeder has written. “Waste heat, waste light, waste electromagnetic signals—we merely have to posit that successful civilizations don’t produce such waste, and the failure of SETI is explained.”
According to David Brin, an astrophysicist who also authors best-selling works of science fiction contemplating the “Great Silence” of the universe, Wright’s expansive study leaves the possibility of Dyson spheres open but may be the nail in the coffin for antiquated notions of all-encompassing empires that tap entire galaxies for energy. “Why would advanced beings need or want that?” Brin asks. “Only a stunningly vast project would justify such greed…. There would be no more gentle, welcoming systems like ours; all would be converted to industrial use. It’s the trait of rapacious users who either have a big reason or a big insatiability.”
Things to come
In 1973 Carl Sagan devised a more empirical formula for Kardashev’s scale that allowed for finer gradations. Our current planetbound civilization, he calculated, rated as a type 0.7 and would reach type 1, given a few more centuries of sustained growth in energy use, which for the past few centuries has increased by about 3 percent per year. To sustain that trend, humanity would need to build a Dyson sphere of our own within about a millennium, becoming a type 2, and would need to encase most of the Milky Way’s stars in Dyson spheres a millennium after that, becoming a type 3.
The implications are clear: Within the span of relatively few generations—a brief moment in comparison with all of human history—either we must rework the solar system and then a great deal of the galaxy itself or our civilization must shift to a radically different, less energy-intensive trend of growth. In the 1960s, during the dizzying early days of the space age, it was easier to believe the former was more likely; today, calamities both ecological and economical make the latter seem more certain. This numerical prophecy of quieter, slower growth, even stasis, is consistent with the null results from traditional SETI searches as well as the rare, unorthodox Dysonian searches of Wright, Annis and a few others.
Provided he can get more funding, Wright intends to perform follow-up work investigating some of his survey’s strangest galaxies, looking for civilizations further down the Kardashev scale. He also hopes to examine a curious cluster of optically dark point sources just outside the Milky Way’s galactic plane that his team discovered by their infrared glows in the WISE data. The cluster is probably a previously unknown giant molecular cloud, an unmapped stellar nursery filled with protostars, Wright says. But “it’s also almost exactly what you’d expect a cluster of Dyson Spheres to look like.” It seems Dyson’s dream is still alive.
Dyson, now 91 but still eager to talk about SETI from his office at the Institute for Advanced Study in Princeton, N.J., says the null results aren’t surprising but shouldn’t be discouraging. “Our imaginings about the ways that aliens might make themselves detectable are always like stories of black cats in a dark room,” Dyson says. “If there are any real aliens, they are likely to behave in ways that we never imagined. The WISE result shows that the aliens did not follow one particular path. That is good to know. But it still leaves a huge variety of other paths open. The failure of one guess does not mean that we should stop looking for aliens.”