Taking a Fresh Look

More than 45 years have passed since radio astronomer Frank Drake made history by turning a 26-meter dish toward the nearby Sunlike stars Tau Ceti and Epsilon Eridani to scan for artificial signals. Although Drake failed to hear any cosmic company, the astronomical community most assuredly heard Drake. The idea of searching for radio signals from interstellar civilizations caught hold quickly and firmly. By the 1970s even NASA joined the hunt, primarily because of the initiative of a transplanted British physician, John Billingham. In a landmark study, Billingham teamed up with Bernard Oliver (director of research at the Hewlett-Packard Company) and a slew of academics to consider the best approach toward detecting sophisticated extraterrestrials. Their effort, known as Project Cyclops, proposed constructing an array of large dishes, a magnificent radio telescope that would inspect vast numbers of stars in a sensitive and systematic search for microwave signals.

The Cyclops array wasn't built; the price tag was as imposing as the instrument. But ideas spawned by Drake's efforts and the Cyclops study are part and parcel of today's SETI projects.

One of them is Project Phoenix, on which I serve as a staff astronomer [as of 2001]. It uses the world's largest radio telescope to examine nearby, mostly Sun-like stars for very narrowband microwave signals. The Phoenix search is supported by private donations to the SETI Institute of Mountain View, California. It is reckoned to be 100 trillion times more efficient than Drake's pioneering effort, due to its vastly improved sensitivity (the best in the world) and computer circuitry that can listen to many millions of radio channels at once. But the basic strategy is similar. Other SETI searches take a somewhat different strategy, sweeping wide areas of the sky rather than targeting individual stars — an approach that trades away high sensitivity in favor of scanning more cosmic real estate.

Time marches on. In 40 years we've witnessed a growing suspicion among biologists that simple forms of life are as common as carbuncles. We've found plenty of evidence that planets are abundant, though we still don't know the abundance of nice, warm, wet planets that stay clement for billions of years. Scientific debate rages about whether complex animal life and intelligence arise commonly or are rare flukes. Of greater practical import for SETI, these twoscore years have ushered in powerful lasers and blisteringly fast computers. SETI Institute scientists concluded that it was time to review the situation. John Billingham summed up the general attitude: "Cyclops had a very considerable impact on everyone connected with SETI, and it gave us some very good general principles for engineering design concepts. But 30 years later, we needed to revisit the entire approach."

The revisiting took place in a series of workshops from 1997 through 1999. The participants, collectively referred to as the SETI Science and Technology Working Group (STWG), comprised about 50 astronomers, engineers, and high-tech strategists from private industry. It was a cerebral crowd and a largely irreverent one, since most of the participants were SETI outsiders. (As an example of their willingness to consider fresh thinking, the group immediately took up the question of whether the Drake Equation, a staple of SETI science since 1961, might be an impediment rather than a useful tool!) The sessions were chaired by Ron Ekers, director of the Australia Telescope National Facility, and Kent Cullers, a physicist with the SETI Institute.

The STWG's brief was simple, if challenging: examine future opportunities for SETI and recommend specific approaches through the year 2020. Although commissioned by and for the SETI Institute, most participants anticipated that their conclusions would influence the whole field, a sort of Cyclops redux.

So what did the STWG say? The group first decided that most of SETI's basic tenets cut the mustard even today. For example, our best bet is still reckoned to be a search for electromagnetic radiation. In other words, photons. They are easy to produce, easy to detect, and travel as fast as possible.

Of course, there's the obvious problem that a broadcast from another planetary system will have to compete with the enormous photon flux from its host star. The Sun, for example, spews 4 x 10²⁶ watts of light and radio energy into space. Fortunately, this incandescent cacophony is spread over a very wide band, so even a low-power transmitter can outshine it if confined to a narrow enough channel of the spectrum. Indeed, a 1-kilowatt signal squeezed into a 1-hertz channel — a transmission that can be done by a ham radio enthusiast — could be heard from far away right through the Sun's noise.

Radio works. And 30 years ago, researchers were convinced it works best — better than light, for instance. The argument was twofold. Microwaves handily penetrate interstellar dust, whereas visible light is blocked. But a subtler point is that radio requires less energy per bit of information, which ought to make it the communication medium of choice for any alien engineers. In the radio regime, the minimum background noise you'll encounter is the faint, 2.7-degree Kelvin afterglow of the Big Bang. In the microwave part of the spectrum this means you typically need to receive just 50 photons per bit to stand out from the noise. No problem. But at higher, optical frequencies, a photon is more energetic and expensive. Even a single infrared photon packs roughly 5,000 times more punch than the group of 50 necessary to send one bit at microwave frequencies. So higher frequencies mean higher energy costs.

In the past, this argument convinced most SETI scientists that any alien society attempting to broadcast a "hailing" or "beacon" signal would use radio. But just a few decades of advances in our own engineering have altered this picture. A laser, if attached to a big optical telescope, can easily produce a beam that is exquisitely well focused. Either of the two 10-meter Keck Telescopes, for instance, if used as a transmitter, could concentrate laser light into a beam a billion times tighter than a 100-meter radio dish could do. So even though optical photons are energetically expensive, a lot fewer of them would be needed — if the aliens know where to aim.

Consequently, the STWG decided to consider new SETI experiments at both radio and optical wavelengths.

They quickly realized that technology and physics conspire to encourage opposite strategies in these two different regimes — based on how extraterrestrials could punch a signal through the cosmic noise and make it stand out as obviously artificial. They could squeeze it into a very narrow frequency, as mentioned already, or squeeze it into brief pulses very narrow in time. Radio works for the former; optical works for the latter.

At microwave radio frequencies, a narrowband continuous signal is easy to recognize and travels well through space. Short radio pulses, on the other hand, get lost — smeared out by electrons in the interstellar medium. At optical frequencies the situation is the opposite. Continuous narrowband (monochromatic) signals are hard to make very strong; but extremely short, extremely powerful pulses are easy and relatively invulnerable to broadening by interstellar gas. The group's conclusion was straightforward: look for continuous, narrowband signals in the radio, and very short pulsed signals at optical wavelengths.

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