Such a machine sounds absurd, and physicists I talked to disagreed over whether we have the technology to build something sturdy and energy-efficient enough to withstand such a deep expanse of time. I was at a dinner with a couple of plasma physicists a few weeks ago who thought we could create one right now if the funding came through. Others think we don't yet have the necessary know-how—though most concede we could figure it out down the line—but say that such a device wouldn't violate any steadfast physical law. In any case, a group of serious scientists has already started to attack the problems associated with building an ultra-long-term beacon.
The most fruitful work takes place on the blog Centauri Dreams, which is affiliated with a foundation that promotes interstellar travel. The charge is led by a pair of twin brothers, physicists James and Gregory Benford (the latter is also a science fiction author), who approach the field of alien-directed beacon-design first from an economic approach. In a paper in the journal Astrobiology, they lay out how one might build such a device without spending the whole GDP. No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs, they argue. But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon's message arrived. This more sensible approach—a sort of Energy Star specification for SETI—would save enough power to keep the beacon running for millions of years.
The question of how much power you can feed a beacon, however, depends a lot on where you put it. When I spoke with Gregory Benford, he proposed sticking the “funeral pyre” in a broad orbit around the sun, at a radius that's roughly half the distance to the Earth. That would provide plenty of energy for the beacon's solar cells but not so much heat that the equipment would be in jeopardy. This degree of solar energy could probably sustain the sort of Energy Star beacon Benford imagines, but there’s also the possibility that you could augment the cells with the sort of generator used on the Voyager 2 spacecraft, which generates heat from radioactive decay. This would function as a backup system, said Benford. “You never build in a single point of failure.”
Even if the beacon had enough juice to blast out its signals until the year 100 million, it would face a constant and recurring threat from micrometeorites and man-made space debris. Such hazards could be dealt with, said Benford, by installing some advanced robotics—which don’t necessarily exist yet—that would repair the regular damage to the solar cells and other wear-and-tear of any mechanical device. Of course, there's robotics we can imagine that would protect a beacon from a major collision, but this is a risk with anything you put in space, and the odds are in the favor of small objects. If you figure that the planet Earth has sustained only a few disastrous impacts over its 4.5 billion-year lifetime (albeit with an atmosphere to fry many otherwise dangerous asteroids), then a tiny beacon stands a decent chance of staying out of trouble.*
The technology we'd need to build a Benfordian beacon would seem to be mere decades in the future, not centuries, said senior SETI astronomer Seth Shostak. “There’s an engineering challenge here, no doubt. But we could build such a thing.”
Still, given the present state of geopolitics, the prospects of our ever coming around to such an idea seem dismal. But the concept of erecting monuments to outlive our civilization is not new. From the pyramids to the Easter Island statues, one might call this the Ozymandias syndrome, in honor of the poem by Percy Bysshe Shelley. There is, of course, the danger with all such projects of falling victim to the hubris of Shelley’s pharaoh, that all we’d leave behind would be some crumbled statue to our glory, “two vast and trunkless legs of stone.” That’s why I’ve left aside the question of what message to send to the aliens. The content of any beacon signal would have to be a matter of intense international deliberations. It wouldn’t have to be anything simple, necessarily—when we’re dealing with thousands and millions of years, you can take your time and say your piece. But frankly, I don’t really care. It’s the thought of total silence that frightens me. To break that silence, all we’d really need to say is “Earth was here.”
This is the second half of a two-part article on Earth's legacy and the search for extraterrestrial life. (Read Part 1.)
Correction, Nov. 29, 2011: This article originally referred to Earth's age as 4.5 million years. (Return to the corrected sentence.)