Seti Searches

SETI Searches Today

by Alan M. MacRobert

---

Painting by Lynette Cook

Is life common in the universe? Biologists today tend to think so.

Are intelligent, technological species of life — like us — common or rare? Long-lasting or short-lived? No one knows, and scientific opinions are sharply divided.

Are any such civilizations broadcasting their existence to the cosmos? There's only one way to find out, and that's to listen.

Several large searches for extraterrestrial intelligence (SETI) are currently scanning the stars, looking for both radio and laser transmissions from distant civilizations. Either type of signal could be sent across interstellar distances fairly economically, scientists are convinced.

Radio searches have been going on the longest, and the theory behind them is well established. All radio searches follow the same basic strategy: they hunt through the microwave part of the spectrum for any extremely narrowband (single-frequency) signal coming from outside the solar system. According to conventional wisdom, this is the kind of broadcast that has the best chance of being detected across interstellar distances.

Of the entire radio spectrum, the band of frequencies from about 0.5 to 60 gigahertz has the least natural background interference in space. Any alien radio astronomers should realize this too — and perhaps they would build interstellar transmitters accordingly. Our atmosphere generally limits us to frequencies below about 12 gigahertz, but maybe other civilizations would have reason to choose the low end of the frequency range too.

Where should we tune the radio dial to listen for ET? Cosmic background noise is quietest in the band from about 0.5 to 60 gigahertz. Our atmosphere interferes the least from about 1 to 12 gigahertz. The black area shows the sum of all natural interference. Some searches have concentrated on the 'water hole,' the band marked off by strong emissions of hydrogen (H) at 1.42 gigahertz and hydroxyl (OH) around 1.72. These emissions are common throughout the universe, so the band between them might be a plausible place for alien civilizations trying to attract attention.

Courtesy Sky & Telescope.

The only kind of transmission that we have much chance of detecting is a "beacon" — a very strong signal that aliens somewhere have deliberately designed to announce "Here we are!" as clearly and loudly as possible to any listeners in the cosmos, such as us. The searches now under way are much too weak to pick up any plausible radio chatter from another civilization's internal traffic — its own broadcasts and point-to-point communications — no matter how advanced the civilization may be. (Indeed, there's every reason to think that internal communications will become less recognizable from a distance as a civilization advances, judging from trends in our own radio technology.)

Considering the huge size of our galaxy, the immense distances between stars, and the immense width of the microwave radio spectrum, it's a daunting task even to search for powerful beacons that are designed to help us out! SETI projects have advanced far in recent years, but we are still looking for needles in very big haystacks that remain almost completely unexplored.

Here is a complete rundown of all the major SETI efforts, both radio and optical, currently (or recently) under way worldwide.

Continued; click "Next Page" below.

---

1 2 3 4 5 6 7 8 Next Page >