The seeming infinity of stars we see in deep exposures of the Milky Way belies the fact that our galaxy has been dead silent when it comes to detecting a radio or optical signal saying “hello” from any neighboring extraterrestrial civilization.
Maybe extraterrestrials are out there but they might not have gone to the effort and expense of building a powerful radio beam and aiming it at us. This may not be in their annual science budget. Or they simply may not want to make their presence known to the universe, as astrophysicist Stephen Hawking recently warned us not to do.
Therefore, the idea of eavesdropping on the normal telecommunications of a civilization has always been compelling because it doesn’t require that aliens do anything special for our sake.
Any civilization within a few dozen light-years of Earth — having comparable technological capabilities — could be detecting our artificially produced electromagnetic radiation right now. The major component would be from the radar of our defense systems, which pours out 2 billion watts in all directions.
Nearly 40 years ago SETI astronomers were thinking big, really big, about how to do this kind of search. A design study at Stanford University in the summer of 1971 looked at the feasibility of deploying a vast array of steerable radio dishes in an attempt to listen in on the radio transmissions leaking off into space from the day-to-day telecommunications of extraterrestrial civilizations.
The scientists and engineers envisioned a vast “orchard” of one thousand 100-meter antennas covering a total area some 10 kilometers across (pictured top). They gave it a cinematic name: "Project Cyclops." But the "big cinema" $10 billion price tag scared away NASA. More modest SETI eavesdropping experiments took place in the subsequent decades.
Fast-forward to the near future. The planned monstrous $1 billion Square Kilometer Array of 1,000 radio antennas would be powerful enough to pick up on telecommunications signals leaking off a civilization’s home planet 300 light-years from us.
But the dilemma is that our world is already growing fainter and harder to detect by extraterrestrials. We are rapidly moving into digital communications, cable and fiber optic transmission. Within 100 years Earth will be "radio quiet," save for defense radars.
Any nearby extraterrestrials would have to get extremely lucky to be able to find us in such a comparatively short window of time.
This dilemma has been revisited in a recent paper by Duncan Forgan of the University of Edinburgh and Robert Nichol of the Institute of Cosmology and Gravitation, Portsmouth, U.K. They conclude the chances of a SETI eavesdropping strategy being successful are one in ten million! You’d have better luck winning a fortune with a lottery ticket.
Because the number of variables and uncertainties in estimating the abundance of extraterrestrial civilizations is so huge, the researchers ran a string of Monte Carlo simulations. These simulations generated a catalog of hypothetical planets that were home to civilizations that developed at various times and locations in the galaxy.
In one set of simulations, the team imagined a civilization that puts out detectable radio signals for the duration of its existence. This is as unlikely as us still using horse-drawn trolleys for mass transit today. The simulation results, not surprisingly, were optimistic.
The second, and far more realistic, set of simulations assumed that a civilization leaks radiation into space for only about 100 years. The chances for success plummeted. Even if a dedicated Cyclops-array ran for 10 years nonstop, odds were reduced to one in ten thousand.
The big conclusion here is that for SETI success, someone has to be targeting us with radio or optical transmissions. Or, an even more exotic medium we have not even realized: neutrino particles?
We now have the privately funded Allen Telescope Array (ATA) that is operating and expanding to a total size of 350 commercial TV-satellite sized antennas. SETI astronomers are using the Allen Array to look for targeted signals from 250,000 stars (including stars with known exoplanets).
The dilemma is that the aliens first have to suspect intelligent life is here on Earth. This would most likely be attempted with astronomical observations that would lead an extraterrestrial civilization to conclude that Earth is habitable and old enough for advanced forms of life to evolve.
Those civilizations lying along the ecliptic plane would see Earth transiting the sun and be able to do spectroscopic studies of our atmosphere. We are planning to attempt just such observations with NASA’s upcoming Webb Space Telescope and much larger envisioned optical space observatories.
Alien civilizations located at stars along the ecliptic plane might be aiming SETI beam at us right now, as veteran SETI astronomer Seth Shostak and I proposed in a paper in 2002. We’ve teamed up with other astronomers — who independently came to the same conclusion — to do an ecliptic search with the ATA.
It seems that the only indirect way to remotely observe evidence of intelligent life would be to see some sort of immense astroengineeering project, such as a Dyson sphere, around a star. And, the odds of coming across such a super-civilization within just a few hundred light-years are probably astronomically unlikely.
So today we remain in cosmic isolation, like a prisoner in a cell waiting to hear a tap on the wall.
Image credits: NASA, SETI Institute