How to Signal Aliens

The most practical takeaway for contacting hypothetical aliens is that “signaling” doesn’t have to mean blasting space with radio or lasers for decades. A third approach—using artificial, geometric signatures that would be visible to astronomers already hunting for exoplanets—could act like a long-lived billboard with far less ongoing energy cost.

Radio leakage from Earth since around World War II is widely considered too weak and too hard to decipher for nearby civilizations to notice, even if they had space-based receivers with large effective collecting areas pointed at Earth continuously. To make a detectable radio beacon, one analysis suggests an enormous phased array of many radio telescopes transmitting narrow, high-powered microwave pulses (10 gigahertz or higher) while sweeping the sky about once a year. Detection from a few thousand light years might be possible if power reaches gigawatt levels, but the price tag is staggering: roughly $100 billion for an omnidirectional transmitter, or about $10 billion if aimed along the galactic plane. Ongoing operating costs—personnel, power, and maintenance—would add billions more, making “going loud” a sustained, civilization-scale investment.

Lasers offer a different tradeoff. Modern lasers can generate ultra-short, high-brightness pulses—brief enough to look clearly artificial because natural sources don’t produce stellar-brightness nanosecond pulses. They’re also detectable with instruments like photomultiplier tubes, though repeated flashes are needed to rule out false positives such as cosmic rays. The downside is range: interstellar dust attenuates optical signals, limiting practical reach to roughly 1,000 light years. Still, the beam spreads slowly enough that a laser could be targeted to individual star systems, and multiple systems could be flashed per second by re-aiming a sufficiently large laser array. Like radio, however, the need to keep transmitting continuously drives high long-term energy costs.

The “billboard transit” idea sidesteps continuous transmission. Exoplanet hunters using the transit method—such as NASA’s Kepler satellite—monitor vast numbers of stars for tiny dips in brightness when an object passes in front of its host star. That same technique could reveal artificial structures if the transiting object has a distinctive shape. Work by French astronomer Luke Arnold showed that transits of non-round objects could be distinguished from those of ordinary planets, with patterns from louvered slits especially hard to mistake for natural worlds. The concept imagines an ultra-thin, lightweight, opaque material launched into solar orbit and unrolled like a sail, then periodically adjusted for solar wind and radiation pressure. If feasible, it would function as a long-lasting geometric signal visible within a few light years (or perhaps a couple thousand with very sensitive instruments).

For farther targets near the galactic center, the discussion returns to high-powered microwaves, since optical/nearby methods won’t reach. But for nearby stars, the billboard approach stands out as a relatively cheap, ongoing signal that aliens might notice simply by doing exoplanet astronomy—even if they aren’t specifically searching for alien artifacts. The segment ends with a challenge to SETI: beyond radio and laser pulses, should searches also look for geometric transit signatures consistent with orbiting alien “billboards,” using public Kepler data?