A team of astronomers is now looking for Dyson Spheres, massive star-scale solar power plants that extraterrestrial hunters hope alien civilizations employ.
In 1960, mathematician, physicist, and all-around genius Freeman Dyson predicted that every civilization in the Universe eventually runs out of energy on its home planet, provided it survives long enough to do so. Dyson argued that this event constitutes a major hurdle in a civilization's evolution, and that all those who leap over it do so in precisely the same way: they build a massive collector of starlight, a shell of solar panels to surround their home star. Astronomers have taken to calling these theoretical megastructures Dyson Spheres. Dyson's insight may seem like nothing more than a thought experiment, but if his hypothesis is sound, it has a striking implication: if you want to find advanced alien civilizations, you should look for signs of Dyson Spheres.
Last month a trio of astronomers led by Penn State's Jason Wright began a two-year search for Dyson Spheres, a search that will span the Milky Way, along with millions of other galaxies. Their project was just awarded a sizable grant from the Templeton Foundation, a philanthropic organization that funds research on the "big questions" that face humanity, questions relating to "human purpose and ultimate reality."
So how do Wright and his team aim to find a Dyson Sphere? Though the word "sphere" summons to mind a solid structure, Wright says his team won't be looking for solid shells. "Even though there is enough mass in our solar system to construct a solid sphere, such a structure would not be mechanically feasible," Wright told me. "It would probably have to be more like a swarm of collectors."
This wild speculation about futuristic alien tech probably seems unscientific, but the search for extraterrestrial civilizations has always depended upon such speculation. Think of all the predictions that are baked into SETI, the Search for Extraterrestrial Intelligence, which uses telescope arrays to scan the heavens for alien radio communications. At present, humans have nowhere near the excess energy you'd need to send the kind of radio signal that SETI is looking for. Earlier this year, astronomer Robert Gray told me that "to operate a radio beacon that is on all the time, broadcasting in all directions, strong enough to be picked up from many light years away, you need an enormous amount of energy -- something in the range of thousands and thousands of big power plants." SETI is betting that advanced civilizations will value communicating with other civilizations a lot, or at least enough to justify huge energy expenditures. It's also betting that such civilizations will communicate via radio waves, and that they will transmit their signals on one of the frequencies that we monitor. For us to find intelligent extraterrestrials, it's not enough that they exist; they have to develop and use technology in predictable ways.
Compared with SETI, a search for Dyson Spheres assumes a lot less about the goals of futuristic alien civilizations. In fact, most of its assumptions proceed directly from simple biology. As Wright, the project leader, explained to me, "life, by definition, uses energy, which it must reradiate as waste heat." The larger the civilization, the more energy it uses and the more heat it reradiates. Life also (by definition) reproduces, which introduces the possibility of exponentially increasing energy demands. If left unchecked, those increases will eventually outstrip the available energy on a planet. That would leave a growing civilization no choice but to mine energy from other planets and, eventually, their stars.
Let's use the Earth as a test case. As Oliver Morton has pointed out with a lovely metaphor, the sun beams a total of 120,000 terawatts per day onto our planet. That's 10,000 times the amount that flows through our industrial civilization. That's a lot of energy, but remember that our industrial civilization is young, and growing fast. In just the past 30 years, we've doubled our global energy supply. At that doubling rate, in 400 years we will be collecting or generating enough energy to match the total sunlight that comes to our planet. At that point, it may be time to draw up plans for a Dyson Sphere.
It's conceivable that an advanced alien civilization could be exponentially more energy-intensive than ours, especially when you consider that its industrial revolutions and energy doublings may have begun billions of years ago. Dyson Spheres could be an ancient and prolific phenomenon in our universe.
Dyson Spheres also fit squarely within with another theoretical model of civilizational advancement: the Kardashev Scale. In 1964, Soviet astronomer Nikolai Kardashev tried to plot out a theory of technological advancement based on a civilization's mastery of larger and larger energy sources over time. Under the Kardashev Scale, a Type I civilization uses all of the energy available on its home planet, a Type II civilization uses all of the energy from its local star, and a Type III civilization makes use of all the energy in its galaxy. The search for Dyson Spheres is, in essence, a search for Type II civilizations. And because it is premised on a civilization's energy usage, it has another advantage over efforts like SETI's: It allows us to find aliens that aren't necessarily interested in talking to us.
That's because if Dyson Spheres exist, they promise to give off a very particular kind of heat signature, a signature that we should be able to see through our infrared telescopes. The solar energy collected by a Dyson Sphere would heat it, the same way that your computer heats up when it uses electricity. That heat would radiate off the sphere as infrared light rather than visible light. "A Dyson Sphere would appear very bright in the mid-infrared," Wright explained to me. "Just like your body, which is invisible in the dark, but shines brightly in mid-infrared goggles."
A civilization that built a Dyson Sphere would have to go to great lengths to avoid detection, either by getting rid of its waste heat in some novel way, or by building massive radiators that give off heat so cool that it would be undetectable against the cosmic microwave background, the faint afterglow of the Big Bang. Wright told me that the latter solution would involve building a sphere that was a hundred times larger than necessary. "If a civilization wants to hide, it's certainly possible to hide," he said, "but it requires massive amounts of deliberate engineering across an entire civilization."
Wright's project won't be the first search for Dyson Spheres. In the 1980's, researchers at Fermilab looked for Dyson Sphere signatures in the data generated by IRAS, the first ever space-based infrared survey of the sky. They found several candidate sources, but on closer inspection they turned out to be giant stars, or else dusty objects that absorb starlight and then reradiate it.
Wright's group will have access to data that Fermilab's researchers could only dream of. They'll be scanning three different infrared sky surveys, including NASA's Wide-field Infrared Survey Explorer (WISE) (pictured up top) which is hundreds of times more sensitive than IRAS. They'll be looking for Dyson Spheres in our galaxy, but also for whole galaxies with excess waste heat -- galaxies that may contain a large number of stars enshrouded in technological megastructures.
If Wright and his team find something outside the range of expected astronomical phenomena, a lengthy confirmation process will begin, a process that will likely involve astronomers and telescopes across the world. Wright was careful to note that no matter what the initial data indicates, he won't be jumping to any conclusions. "More than once some inexplicable object has been discovered that looked like aliens, and then slowly it became clear that it was a very interesting, but totally natural, phenomenon," he said. Indeed, Nikolai Kardashev once thought he'd identified several good candidates for Type III civilizations, which operate on a galactic scale. But in the end, they turned out to be quasars.
Near the end of our conversation, I asked Wright if Dyson Spheres and the Kardashev scale had any competitors, if there were other theoretical models that described what extraterrestrial civilizations might look like. "I'm not aware of any other scales in the refereed scientific literature," he said, "but there probably are some." In astrobiology, the line between science and science fiction is blurry. "Often the best discussions of these issues are in paperback novels," Wright noted. "I can tell you, it's strange to write a serious research proposal and have half of your bibliography be science fiction."