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.