By Jeff Greenwald
|Searching the cosmic haystack: The 20-acre reflecting dish of the Areciboradio telescope in Puerto Rico listens for intelligent signals from space.|
National Astronomy and Ionospheric Center
Astrophysicist Dan Werthimer is looking for aliens. Or, to be precise, he is listening for them via a modified screen saver for a personal computer. All day long in Werthimer's office at the Space Sciences Laboratory at the University of California at Berkeley, spikes of green, blue, pink, and red pulse across his computer monitor. Each spike represents an incoming radio signal. The higher the spike, the greater the signal's intensity and power. Any sustained peak could be a shout from across the universe. "Why look at goldfish or flying toasters on your screen," says Werthimer, "when you could be doing something that answers the ancient question, Are we alone?"
|Astrophysicist Dan Werthimer says a signal must last 12 seconds to be "Interesting".|
Listening for aliens used to be a passion limited to folks with the biggest ears--radio telescopes with giant dishes cupped toward the heavens. Indeed, for nearly 40 years researchers like Werthimer have been scanning the cosmos with such high-powered instruments in hopes of stumbling upon an alien broadcast--something akin to an extraterrestrial version of I Love Lucy. The search has been slow and laborious. But what really frustrates Werthimer is that he doesn't have enough supercomputers to sort through the mountains of radio-transmission data they are collecting. Aliens may already have tried to contact us--but their message could be languishing on a hard disk, waiting to be decoded.
Now, thanks to Werthimer, anyone with a personal computer can join the search for extraterrestrial intelligence. Werthimer's brainchild--developed with colleague David Anderson--debuts this month. It's called SETI@home: a data-sorting software program that masquerades as a screen saver. Late this month, the software downloaded off the Internet (http://setiathome.ssl.berkeley.edu) will allow amateur computer jockeys to receive and collate chunks of data from the world's largest radio telescope--the thousand-foot-diameter dish nestled in the hills south of Arecibo, Puerto Rico. SETI@home users don't need to know a thing about astronomy. Nor do they need to know how to look for signals. The program does all the work whenever the screen saver is on.
The SETI@home program--so streamlined, so sophisticated, so economical--is a far cry from the first schemes for communing with aliens. In 1820, German mathematician Carl Friedrich Gauss wanted to announce our presence to extraterrestrial passersby by clearing a huge right triangle in the Siberian forest. His plan was to plant wheat in the triangle, then border each side with a square filled with pine trees. Aliens cruising by would glimpse this sylvan representation of a2 + b2= c2 and know the planet's inhabitants had mastered the Pythagorean theorem. Other visionaries favored more flamboyant displays. In 1840 the Viennese astronomer Joseph von Littrow proposed digging a patchwork of enormous ditches in the Sahara--and setting them aflame with kerosene. Nearly 30 years later the French inventor Charles Cros unveiled a plan to reflect sunlight toward Mars using seven carefully placed mirrors. In theory, amazed Martians would behold the shape of the Big Dipper: a wink of intelligence from nearby Earth.
|Radio signals from a minuscule sliver of sky bounce up from Arecibo's maindish and are focused on antennas inside the suspended dome. The telescope"listens" to different slivers of sky by moving the dome--not the dish.|
National Astronomy and Ionospheric Center
During the twentieth century the search for aliens has taken on a somewhat more pragmatic approach. In 1959 physicists Philip Morrison and Giuseppe Cocconi suggested that aliens attempting to broadcast a signal would probably do it at a significant spot on the electromagnetic spectrum. One obvious "marker," they concluded, lies at 1.42 gigahertz. That is the point on the spectrum at which energy released from hydrogen, the universe's most abundant element, shows up. Morrison and Cocconi argued that there was a practical reason aliens might choose to broadcast signals on a "channel" close to such a universal marker. That region of the spectrum gets little interference from other kinds of natural electromagnetic radiation.
Astronomer Frank Drake subsequently launched the first modern search for alien radio signals. Using the 85-foot-wide telescope at Green Bank, West Virginia, Drake analyzed two stars--Tau Ceti and Epsilon Eridani--for beacons at 1.42 gigahertz. The effort, called Project Ozma, marked the official birth of a new scientific field: the search for extraterrestrial intelligence.
Werthimer is articulate and optimistic, with a winning smile and contagious enthusiasm--just the sort of human aliens should meet first. For the past 15 years, he has directed the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations--SERENDIP, for short.
"A radio telescope is a curved mirror," says Werthimer. "You put your detector at the focus." The incoming radio-wave signals bounce off the reflecting dish and are collected at the focus and translated into electrical impulses. In most radio telescopes the dish moves; that's how reception shifts from one part of the sky to another. But the stadium-size dish at Arecibo is built right into the earth, so it's the receivers that must move. "Suspended above the dish is a latticework of struts from which receivers descend. Other groups attach their experiments, and our SERENDIP receiver goes along for the ride. It's focused as well--but on a different part of the sky. We have no control over where we're going to point. But that works out okay--because we don't know where to look anyway."
With its powerful but narrow beam (often compared with the view through a cocktail straw), the Arecibo radio telescope views just a millionth of the sky at once. After six months, though, this beam manages to scan the entire sky in its field (about one-third of the total sky). So the piggyback strategy, says Werthimer with a grin, "is almost as good as having the telescope to yourself."
During the 20-year history of SERENDIP, the project has seen an exponential growth in signal-detection capabilities. The reason, Werthimer claims, is Moore's Law--Intel cofounder Gordon Moore's time-tested axiom that computing power doubles every 18 months. When the first SERENDIP search was launched in 1979, the program could scan only 100 frequencies, or channels. Today that number is 168 million channels.
The bottleneck in this giddy progress, paradoxically, is the processing power of the program's mainframes. They simply can't analyze the information as fast as it's pouring in. One answer would be to continually upgrade the computers--but that would be outrageously expensive. Fortunately, the SETI@home screen saver offers a more ingenious solution. Users can receive 256-kilobyte data chunks--about 100 seconds of viewing--fresh from the Arecibo Observatory. With casual use on a typical home computer, the screen saver will take a week or two to crunch the numbers. At that point, users are cued to send the finished chunk in and download another. "SETI@home is going to allow us to ride Moore's curve of growth," Werthimer declares. "And we won't have to pay for it. Participants are going to want to upgrade their own machines. So we'll be able to do more and more powerful searches--without having to buy more and more powerful equipment."
The version of the screen saver on Werthimer's monitor, with its rainbow of spikes, is bare bones. Future renditions, says Werthimer, will show users precisely where in the sky they're looking and how many stars they have targeted so far. Best of all, users can watch the analysis unfold in real time--and be the first ones on their block to detect signals from a distant world.
In practical terms, SETI@home will increase SERENDIP's sensitivity by a factor of ten. As it is, the project looks at a very narrow band of some 100 million frequencies: a 100-megahertz range, centered right at 1,420 (the hydrogen line). The added computing power won't look at a different part of the haystack, but it will allow a deeper, more thorough search for the needle.
"SETI@home could help us find more distant signals from weaker transmitters," says Werthimer. Sorting through evidence of faint transmissions is critical, he adds, because electromagnetic leakage--like the TV and high-band radio signals radiating from our own planet for decades--tends to be very weak. "It'll also help us detect pulsing signals, drifting signals, and signals that don't stay at a steady frequency--which is what we would expect from transmitters located on planets that spin."
A hundred users tested the program last November. In February some 210,000 SETI@home volunteers were waiting to download the software--already by far the world's largest computer task force.
Meanwhile, Werthimer and his colleagues are continuing to gather more and more raw data. SERENDIP's tack has been to search the sky in a broad sweep, taking in millions of stars. The problem is that the search has been restricted to the thin sliver of the radio-wave spectrum in which an alien beacon seems most likely to appear. Project Phoenix, the centerpiece of the SETI Institute in Mountain View, California, is performing a more focused search. Phoenix is observing only 1,000 sunlike stars, all within 200 light-years of Earth. This study collects signals over a vast range of radio frequencies (between 1.2 and 3 gigahertz) that might carry alien radio or television leakage, as well as a formal beacon. In layman's terms, SERENDIP could find the radio-wave equivalent of a powerful lighthouse beam anywhere along a foggy coast. Project Phoenix could detect the lighthouse keeper's cell phone--but only if its antenna was aimed directly at the lighthouse itself.
One problem hindering SETI is a lack of resources. "At present we're very inefficient because we observe only as guests on big antennas," says Frank Drake, president of the SETI Institute. His famous equation, designed to calculate the number of "observable civilizations" in the Milky Way, underlies all the institute's research projects (see "The Drake Equation," on page 66). "Our efficiency would go up by many factors if we had our own telescope."
To this end, the SETI Institute, along with U.C. Berkeley, is working on a revolutionary project called 1hT--a one-hectare telescope to be built by 2005. The 1hT will take advantage of breakthrough technology to link hundreds of small, consumer-size dishes. And the project will be surprisingly affordable. Drake estimates the price tag will be about $20 million. That's just about a fifth the cost of a single-dish antenna of the same size.
Many things will make the 1hT remarkable. A radio telescope's beam depends on the diameter of its dish; the bigger the dish, the narrower the beam. In other words, a big dish takes a very close look at a tiny sliver of sky. A little dish offers a larger field of view but little sensitivity. The only way to get sensitivity with little dishes is to have a lot of them, all looking at the same large field of view.
Unlike standard antennas, the 1hT can form multiple beams easily and survey hundreds of stars at once. But there's one drawback: the 1hT will be only one-tenth as sensitive as the 1,000-foot dish in Arecibo. If the project succeeds, however, it will pave the way for SKA--a one-square-kilometer radio telescope. Such an array, with the power of about ten Arecibos, would be capable of detecting an alien television program on a planet orbiting Arcturus.
The plans are ambitious, but well within the realm of current technology. A more far-fetched plan on SETI drawing boards is to build a radio telescope on the farside of the moon. That would help researchers overcome a major problem: the best frequency range for terrestrial radio telescopes--1 to 10 gigahertz--is becoming polluted by interference from satellite, defense, and cell phone transmissions. The plan would involve suspending a powerful antenna, similar to Arecibo, in a huge spherical crater. The favored location--Saha Crater--is near the moon's equator and only 3 degrees onto the lunar farside. Cables could link the antenna to a transmitter on the near side, which would relay the data back to Earth.
Despite new technology coming onstream, Werthimer and his colleagues are hoping the chances of finding extraterrestrial intelligence will be enhanced by the increasingly democratic nature of the quest. For example, H. Paul Shuch, director of the SETI League's grassroots Project Argus, has already been helping amateur astronomers set up small radio telescopes--and networking their efforts into a global search for extraterrestrial beacons. The project has more than 900 members in 51 countries, with 67 Argus observing stations online--just a small fraction of Shuch's ambitious goal of 5,000.
The opportunities for the amateur SETI researcher have never been better. Now, with the advent of SETI@home, it's even possible that the first proof of extraterrestrial intelligence may begin as a spike on someone's screen saver. When we do find that signal (SETI researchers dislike the word if), there's likely to be planet-wide jubilation. But will the long-awaited ETs be equally delighted to hear from us?
"The first civilizations we find are likely to be far more advanced than ours," notes Werthimer. "So it really becomes a question of motivation. Do they want to get in touch with us? They're probably in direct contact with millions of civilizations already. Whether we'll be welcome on that galactic Internet, I don't know."
The Drake Equation
In 1961 astronomer Frank Drake, then 30, came up with an equation for estimating the number (N) of observable civilizations in the Milky Way:
N = R*
= the number of stars like Earth's sun born in the Milky Way each year
= the fraction of those stars with planets
= the number of planets in each planetary system that are Earthlike
= the fraction of Earthlike planets that actually develop life
= the fraction of planets with life that develop intelligence
= the fraction of planets with intelligent life that develop technology
L = the lifetime, or number of years, an observable civilization exists
The astronomer's best estimate these days is that 10,000 civilizations exist. The recent discoveries of planets in other solar systems have not affected his estimate. Finding these planets only reinforced his earlier estimates of fp, the fraction of stars with planets. And the number of Earthlike planets, ne
, also remains unchanged, he adds, because current techniques can detect only Jupiterlike planets.
Drake admits that no one knows the values of any factor in the equation for sure. But the first values on the right side of the question--like the rate of star birth--are more certain than the rest. "The Drake equation doesn't give us a very accurate numerical answer, but it does give us guidance," he says. "That tells us it's not easy. We have to search many, many stars before we succeed. No matter what numbers you put in, you come up with that result."
One of the wildest of the new SETI (Search for Extraterrestrial Intelligence) strategies involves using our sun as a "gravitational lens" capable of focusing alien transmissions with astounding power.
All stars bend space around them. (This is a basic principle of general relativity.) As a result, rays from distant star systems graze the edge of our sun and come to a focus. This image is very bright, and the magnification is on the order of a million. If you put a ten-meter-wide dish at that point--at the gravitational focus of the sun--the effective collecting area would be 10,000 times that of the most powerful radio telescope. "Want a telescope with the power of a square-kilometer radio telescope?" asks SETI Institute president Frank Drake. "You can have it--with a receiver the size of a card table.
"My thought," he adds, "is that this is the way all space-faring civilizations search for extraterrestrial intelligence."
Why aren't we doing it? Well, there's a hitch. The nearest focused images, it turns out, lie at a distance of 550 astronomical units (AU) from our sun. Pluto is at 40 AU--less than a tenth that distance. Nonetheless, the European Space Agency is seriously weighing a mission to carry a receiver to that remote acre of space. Using novel techniques like electric ion propulsion and solar sails to power the spacecraft, the journey would take an estimated 40 years.
|Physicist Kent Cullers thinks aliens are more advanced.|
Like many other SETI strategists, Kent Cullers is a physicist. But he brings a unique perspective to his work: he is blind. He was born prematurely, and the pure oxygen in his incubator destroyed his vision. So there's a grain of irony in Cullers's attraction to the newest tactic in the search for alien intelligence: optical SETI.
The best guess of how aliens might communicate, says Cullers, depends on our own level of technical advancement. When the first SETI radio-telescope search was launched in 1960, lasers had just arrived on the scene. It hadn't occurred to physicists that an otherworldly civilization might use high-end laser technology to send a beacon toward the stars.
"In radio SETI," says Cullers, "we're interested in finding pure tones." He whistles, low and long. "That is a frequency. The longer a tone exists, the better you can measure its frequency. But if I clap my hands sharply, you can't assign a frequency. It has many frequencies, spread over the spectrum. It turns out this is a fundamental property of waves. If waves exist for a very short time, their frequency is undefined."
While traditional radio astronomy detects signals at well-defined frequencies, optical SETI looks for signals emitted in short bursts or pulses at well-defined times. A powerful laser, tightly focused at a specific wavelength, can outshine its sun in a given direction. If that bright laser emits a very brief burst of light--say, a billionth of a second long--the pulse will span a huge range of frequencies.
All that SETI researchers have to do, says Cullers, is build a receiver that detects photons--and divide each second of observation into billions of tiny time gates. "In one particular interval, you might get 100 photons. Now, the chances that background starlight will give you more than one or two photons in this time are vanishingly small," he says. "So your detector would get excited about the sudden burst of photons."
An optical SETI could search for signals at many frequencies--from infrared to visible light--and it could search them all at once.
"But the great novelty of this sort of a survey," says Cullers, "is that it assumes the guys on the other end are doing something different from us--and that they're a little more advanced than we are."
--Jeff GreenwaldStay up-to-date with the latest science discoveries -- Click here to subscribe to Discover!SETI@homeThe SETI Institute
SERENDIP ProjectThe Planetary SocietyColumbus Optical SETI ObservatoryAreciboObservatory
from the NAIC