for extraterrestrial intelligence are about
to expand into new realms, thanks to new advances
in technology — and new thinking.
Adapted from Sky & Telescope, April 2001
As far as we know, we're alone in the universe. Sure, it's conceivable that bacteria-like organisms live in wet, underground soils on Mars. Maybe alien sea creatures even swim the dark waters beneath Europa's icy crust. But nearby life, if it exists at all, is undoubtedly dumb. If we want to look for smarter extraterrestrial biology — the kind that could rival or perhaps far surpass us humans for reasoning, inventing, and building — we have to look much farther afield, among the stars. And we don't know where.
Planets inhabited by high technologies (if they exist at all) are surely much rarer than planets inhabited by bacteria. After all, microbes were Earth's most advanced life forms for nearly a million times longer than the span of written human history. But a tantalizing prospect fires the imagination: the rare technological worlds may actually be the easiest to find. Unlike bacteria, intelligent creatures could choose to make themselves known across vast interstellar distances. They could do it with radio transmitters or lasers not much bigger than our own. Maybe they are doing it right now.
Searches for extraterrestrial intelligence (abbreviated SETI) have no obvious hunting ground and no clear route to discovery. Instead, after more than 40 years researchers have amassed a mountain of speculation and only a small hill of experiment. If sentient beings exist among the stars, they have remained beyond the grasp of our instruments.
But perhaps not for long.
New ideas are stirring in the world of SETI. A small group of scientists and engineers have been busy for several years discussing how to expand our ability to detect artificial transmissions from deep space. Their task was to reason out what new instruments should be used and how best we might rake the skies for plausible alien signals.
The task is daunting. To begin with, the sky is huge — with 400 billion stars (and possibly as many planets) in our galaxy alone. The radio spectrum is also huge — with roughly 10 billion channels to scan per star, and that's just in the "microwave window" of frequencies that come in best through Earth's atmosphere. The distance any signal has to travel is astronomical — so it will be weakened by an astronomical amount squared. It has been said that SETI is like looking for a needle in a haystack. But if the instruments proposed by this group are built, SETI scientists will stop sifting the hay with spoons; they'll have a pitchfork.
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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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Light Up My World
What would it take to send and receive a simple light signal? In answering this question, the STWG drew inspiration from a high-powered laser at the Lawrence Livermore National Laboratory. This leviathan light source (intended for triggering nuclear fusion rather than communicating with aliens) produces pulses that are a trillionth of a second long, brief indeed. But during that short pulse, the laser puts out a million billion watts. Note that the electric bill for this formidable flasher needn't be large. When pulsing once per second, its average power output is a measly kilowatt.
We could see a pulsed laser signal this bright beamed by a large telescope a few dozen light-years away, and a bigger brother could be used for communicating across a sizable chunk of the galaxy. For example, imagine a device 1,000 times more powerful than the Livermore laser affixed to a 10-meter telescope. If it packed infrared pulses into a billionth of a second, it would flash 25 photons per pulse into another 10-meter telescope 10,000 light-years away. Such a heavy-duty communication device could reach billions of stars! Even this little handful of photons, bunched into a single nanosecond, would stand out as artificial were anyone looking for it. Nature just doesn't do things like that.
Of course, flashing a billion stars would be a major effort. But even a modest device could be useful for hailing to the stellar neighborhood. Imagine an automated beacon consisting of a laser ten or a hundred times heftier than the Livermore model feeding a computer-steered mirror system. This system might be tasked with sequentially "pinging" a thousand, or even a million, of the most promising stars. Assuming the system is supple enough to take aim in a tenth of a second, it could repeat its pings every two minutes if a thousand stars were being flashed, and once a day if the targets numbered a million.
For laser beacons, the aiming accuracy has to be very good. If we assume that a reasonable target size to include any inhabited planets has a diameter of 10 astronomical units (the size of Jupiter's orbit) centered on a star, the pointing accuracy needs to be 0.03 arcsecond at 1,000 light-years. So the alien broadcasters will have to loft their laser beacons into orbit, where the atmosphere of their planet won't mess up the beams. In addition, they'll need to know the precise distance and motion of each star they're trying to ping in order to "lead" their targets by the right amount. But such aiming and tracking requirements are neither difficult nor particularly onerous. The aliens could do it.
Suppose they have. Suppose such pulsed laser beacons exist? Shouldn't we look for them?
Some people have. Stuart Kingsley in Columbus, Ohio, was busy pioneering optical searches for a decade, using a backyard 10-inch telescope and high-speed photomultipliers to look for pulses. A few years ago Paul Horowitz of Harvard and Dan Werthimer at Berkeley entered the optical SETI fray after participating in the STWG. They've geared up clever searches that "piggyback" on two medium-size telescopes that were already engaged in other stellar research. Starlight collected by the telescopes that would otherwise be wasted is sent through a beamsplitter and fed to a pair of photomultiplier tubes; a coincidence detector looks for very short flashes appearing simultaneously in each tube. (The use of two detectors is essential to reduce false alarms caused by cosmic rays, scintillation, and radioactive decays in the photomultipliers' glass itself.) Such detectors are simple and cheap. Amateurs can fit them to their scopes.
Optical SETI is gaining adherents. Part of the appeal is that the telescope doesn't have to make good images. Paul Horowitz is building a 72-inch (1.8-meter) optical SETI telescope around a cheap "light bucket" of a mirror. The light will go through a beamsplitter to two arrays of 1,024 pulse detectors, each with nanosecond speed, covering a 1.6-by-0.2-degree rectangle of sky. Only recently have such arrays become available. This instrument, says Horowitz, will be able to examine every point on more than half the celestial sphere (not just selected stars) for at least 48 seconds every 200 clear nights — roughly once a year, considering the weather. It will sweep the whole sky from declination +60 degrees to 20 degrees, a region that includes more than half the visible Milky Way. [It's up and running as of spring 2006; see our article.].
One followup to the STWG's work is the development of an improved detector for targeted optical SETI. Remington Stone of Lick Observatory has teamed up with Frank Drake (SETI Institute), Shelley Wright (University of California, Santa Cruz), Richard Treffers, and Dan Werthimer to build a device that should help keep observers' blood pressure low. It does so by reducing the number of false alarms. In a two-detector system, noise and electronic glitches still typically produce one false alarm per night, complicating the search. By using three photomultiplier tubes instead of two, the new experiment should only have about one false alarm per year.
The new instrument is already checking out stars using Lick Observatory's 1-meter Nickel Telescope. "It's great," says Drake. "No terrestrial interference, such as complicates radio SETI. The only drawback is that you do require that the extraterrestrials are deliberately targeting you with their lasers."
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Radical Radio Visions
Radio SETI may no longer be the only game in town, but it's still the game to which most researchers belly up. That's because the odds of a jackpot, though quite unknown, are unquestionably getting better all the time — because the instruments are growing more capable by leaps and bounds.
The ideal SETI radio telescope can only be imagined. It would monitor every point on the sky, in every radio channel from one end of the microwave window to the other (about 1,000 to 11,000 megahertz), all the time — a true Omnidirectional Search System, or OSS.
Unfortunately, this ideal is a very long way off. But it's no longer impossible to work toward. The STWG team considered what it would take to build a reasonable interim OSS. They were seduced by the thought of a telescope able to find powerful but intermittent signals, the kind that none of the current large SETI experiments has a hope of detecting.
A baby prototype OSS has already been developed by radio astronomer Robert Dixon at Ohio State University. Known as Argus (a reference to the hundred-eyed monster in Greek mythology), Dixon's prototype uses an array of small, fixed antennas to synthesize many beams, or pixels, on the sky. Synthesis is a well-established radio-astronomy technique. Its most famous embodiment is at the Very Large Array (VLA), where 27 dish antennas are splayed across the New Mexico desert. By meticulously combining data from each of the array elements, one can artificially construct, or synthesize, the beam of a very large antenna, though the computing demands are tremendous.
If even greater computer horsepower is on tap, many different beams can be constructed simultaneously. In this way a field of simple antennas can form a simultaneous radio image composed of hundreds of sky beams, or pixels. That's what Dixon's Argus prototype does.
Still, the OSS wouldn't be just a souped-up VLA. Unlike a conventional radio telescope, a SETI instrument also needs to divide the radio spectrum into very many narrow channels, each of which has to be examined separately. This is because E.T. hasn't sent us a fax revealing which hailing frequency he's using. Unless we make an incredibly lucky guess, we have to check them all.
A bare starting point for an OSS, the working group decided, would be to have 1,000 simultaneous channels per beam. Making a receiver capable of that much spectral resolution is primarily an exercise in computation, and computers are rapidly getting cheaper. But alas, the bit-busting doesn't stop there. Synthesizing about 4,000 pixels to cover the sky (each one a big 2 degrees across, and each with those 1,000 channels) requires galloping gigaflops. The imposing total is 1.5 million giga-operations per second, which at today's prices means about $3 million worth of computer.
Multiply that by 1,000 if you want a million simultaneous channels. (Even today's SETI receivers resolve tens of millions of channels at once.) If you want billions of narrowband channels to cover the whole microwave window, you're budgeting the gross national product of the United States for your computers. And that's before the electric bills arrive.
It gets worse. Unlike the VLA, whose 25-meter antennas typically restrict its imaging field to a patch of sky roughly 1/2 degree wide, the OSS would require very small antennas (about 0.2 meter) if it wants to extend its peripheral vision to two-thirds of the sky. But such diminutive antenna elements impose a price: even with an array of 4,096 of them, the OSS would sport no more collecting area than a single dish 7 meters across. This paltry aperture means that only very strong signals will get our OSS's attention.
Despite its poor sensitivity and mammoth computing requirements, a preliminary OSS is still an attractive SETI option. Even a 1,000-channel unit could be used to march through a billion channels, step by step, over a period of years. If we make a brilliant guess about some preferred frequency, the OSS will open a new region of search space by looking everywhere for radio blasts that persist for only an hour, a day, or a week. As an example, it could detect comet- and asteroid-tracking radars used by a prudent alien society 1,000 light-years away to check its planetary system for dangerous rubble. That's a seductive prospect.
But what's the price tag? The bill for the OSS is entirely dominated by computer costs, and these continue to plummet. So the STWG recommended investing a few hundred thousand dollars a year in OSS research and development for now, and awaiting much cheaper computing.
As everyone in the computer world knows, "Moore's Law" suggests that the computing power you get per dollar doubles every 18 months. That means a thousandfold improvement in 15 years, at least for computation that can be spread among a lot of chips. The OSS planners are betting that Moore's Law will keep holding true; they hope to have an actual instrument on the air by 2015. If computing power continues advancing beyond that, "all sky, all frequencies, all the time" will no longer be a SETI pipe dream.
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Targeting the Aliens
Frank Drake's first experiment was a "targeted" search, though that term wasn't in the SETI lexicon in 1960 (nor, for that matter, was the acronym SETI). Drake didn't slew his telescope across large tracts of celestial real estate in hope of a serendipitous hit from one of the distant millions of stars. Rather, he faced his antenna toward two Sun-like stars very nearby.
The only major targeted search under way now [as of 2001] is our Project Phoenix, a systematic scrutiny of about 1,000 nearby stars that are mostly similar to the Sun. The principal antenna we use is the giant Arecibo dish in Puerto Rico, which stares at the sky with an 18-acre eye 305 meters in diameter. But even with this prodigious collecting area, Phoenix is not sensitive enough to pick up the kind of radio leakage that Earth spews into space even if Earth were at the distance of Alpha Centauri, the closest star to the Sun. ("Leakage" includes TV carrier waves and other continuous transmissions. It does not include Earth's largest military radars, which would be visible to Phoenix from dozens of light-years away if they happened to be aimed just right at the right time.) This is discouraging, and more so when you consider that Phoenix is the most sensitive SETI experiment ever.
Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions. Therefore, what we're really looking for from the aliens is a beacon signal — a big, loud, continuous shout deliberately designed to attract the attention of interstellar listeners.
When it comes to sheer sensitivity, SETI's dream instrument is the proposed Square Kilometer Array, or SKA. In the years ahead radio astronomers hope to build this mother of all radio telescopes — a monstrous device with a total collecting area of one square kilometer, 14 times that of Arecibo. Although intended for conventional radio astronomy, it could easily have 50 to 100 times the sensitivity of Project Phoenix. Perhaps 10 percent of its synthesized beams could be dedicated to SETI research.
But the SKA is at least a decade or two away and could cost $500 million or more. So the STWG advocated building a more modest instrument that could be probing the universe within a few years: the One Hectare Telescope, now renamed the Allen Telescope Array (for its major benefactor, Microsoft cofounder Paul Allen). This instrument will be both a prototype for the SKA and a dedicated SETI machine — one that, unlike Arecibo, could be devoted to looking for alien signals 24 hours a day, 7 days a week.
The ATA will have as much collecting area as a single antenna 100 meters square (one hectare). But it will be made of 350 relatively small, cheap dishes. This way a large collecting area can be put together for a fraction of the usual cost of a big radio telescope. In addition, compact low-noise amplifiers are commercially available for these dishes at prices so low that they cause radio astronomers to salivate.
Of course off-the-shelf TV dishes must be modified to slew and track astronomical objects, and they will need to be tightly integrated by fiber optics to create a functioning aperture-synthesis array. Once again, the real cost is the huge computational load of synthesizing beams and splitting the radio spectrum into many channels. But the telescope can start small and grow as technology and money allow.
Even in its first incarnation, the ATA will have impressive specs. Its frequency coverage will stretch from 500 to 11,200 MHz, spanning the whole microwave window, with at least 100 MHz of this range being examined at any given moment. At least three synthesized beams would be computed to begin with, allowing for triple targets on the sky. The number of beams can be increased as computing costs ease, so the ATA might someday examine 100 spots simultaneously in a 3-degree patch of sky. Indeed, one of the principal attractions of an array is the ability to examine multiple targets.
The bottom line gets the heart pounding: while Project Phoenix will examine 1,000 nearby stars, the full ATA could check out 100,000 in the same time and with comparable sensitivity. Ultimately, it might survey as many as a million targets. The ATA is such an appealing idea that implementation was under way even before the STWG banged the gavel at its last meeting.
How will the antennas be arranged? There are tradeoffs. A compact pattern — cramming all the dishes into a small circle or oval — would be best for SETI, since this arrangement would create wide beams encompassing many stars at once. But conventional radio astronomers prefer the opposite: that at least some of the dishes be scattered far across the landscape, allowing the synthesized beams to be small and sharp for high resolution. The final design was a compromise between both.
One long-standing issue facing the SETI community is the relative merits of star-by-star targeting, as done by Phoenix, and wide-sky surveys, which cover a great deal more celestial real estate but with much poorer sensitivity. However, to some extent this issue is destined to go away — because there comes a point when any targeted search will grow into a wide-sky survey.
The synthesized beams of the ATA telescope will surely be no bigger than 10 arcminutes across. That's pretty tiny. It would take 2 million such beams to cover the entire celestial sphere. But if the eventual observing program is stretched to include the nearest 2 million good stars, there's not much difference between selecting targets and scanning the whole sky.
The reason for this is that each time a target star is examined, our instrument is also listening to the rest of the little 10-arcminute patch of sky around it. A patch this size will include an average of 100,000 distant background stars — and that's just in our own Milky Way. Ironically, our best targeted searches are destined to become sky surveys within a decade or so. [For the other side of this debate, see Smarter SETI Strategy].
There's no denying that SETI is an uncertain enterprise. No one can tell when or if success will ever come. But the next generation of instruments proposed by the STWG can assay our galactic environs star by star out to 1,000 light-years or more, sifting for signals from a million potential solar systems. That's an imposing number, and one that stirs the imagination of all who dare to look for company among the stars.
(The STWG's final report has been published as a 551-page book, SETI 2020, available from the SETI Institute.)
Seth Shostak is a radio astronomer at the SETI Institute in Mountain View, California. His current occupation is a consequence of a sudden realization (while still a graduate student) that the hardware he was using to study galaxies could also be used to prove the existence of cosmic company. His guess is that new instruments for SETI "will reveal extraterrestrial intelligence before two decades have passed." He is author of Sharing the Universe: Perspectives on Extraterrestrial Life.