dijous, 23 de novembre de 2006

The New Search for E.T.

Night watch: The new telescope at Oak Ridge Observatory, in Harvard, Mass., waits patiently for nanosecond light pulses that a distant civilization might be sending our way.
It doesn’t look like much: just a clapboard shed in a clearing, surrounded by tall pines. No plaque or other marking announces what goes on here. Inside stands a 4-meter-tall black metal frame covered in Mylar, looking sort of like a giant’s box kite ­waiting for a stiff breeze. Beneath the shroud of plastic are a largish mirrored disk, a second smaller mirror, and some cables and electronics.
Suddenly, the roof begins to glide backward on steel tracks, revealing the night sky overhead. Even as the contraption tilts slowly into place, its exact angle controlled by a computer in the next room, the true purpose of this unassuming apparatus might be unclear to the casual observer. But it constitutes the most sophisticated implementation of a concept that a few technologists, including me, have been pushing for more than four decades—a telescope dedicated to answering an age-old question: Is anybody out there?
With the unveiling last April of this new facility, the search for extraterrestrial intelligence, or SETI, entered a new era. Most previous SETI attempts have listened for radio ­signals, but after more than 40 years, none has detected anything of significance. This telescope—designed by Harvard University physics professor Paul Horowitz and his colleagues and constructed at the Oak Ridge Observatory, in Harvard, Mass., about 50 kilometers northwest of Boston—takes a new tack [see photo, “Night Watch”].
Horowitz is hunting for the briefest pulses of visible light that a far-off civilization could be sending toward Earth. His is the first telescope to be specially designed and dedicated to this purpose, and, many experts now agree, it represents the right direction for SETI to take. If E.T. is trying to talk to us, he’s probably beaming light our way, not radio waves.
It’s been said that the most important event in human history will be when someone discovers that we earthlings are not alone in the universe, that there are other beings smart enough to let us know they exist. For most of our history, the technology to look for extraterrestrial life was beyond our means. But the relatively recent advent of large dish antennas and extremely sensitive receivers gave us the tools to listen for radio frequency signals.
Starting in 1960, radio astronomers have mounted dozens of SETI experiments, some lasting only a few weeks or months, others running for years. Most of these searches were targeted at nearby star systems, those thought most likely to harbor life, while others encompassed the entire sky.
The two longest-running SETI projects to date,
Phoenix and Serendip, both rely on the 305-meter-diameter Arecibo Radio Telescope, in Puerto Rico. Project Phoenix collects broadband RF signals from the antenna, which computers then digitize and split into narrow frequency channels, measuring the strength in each. Anomalous results—anything that rises above the noise—are compared with a comprehensive database of terrestrial radio sources. If the transmission can’t be identified, it is then checked against data from other radio telescopes to see whether it truly comes from the target star system.
Project Serendip, meanwhile, is tuned in only to 1420 megahertz, the frequency of the neutral hydrogen atom, which is the most abundant substance in the universe and can be readily detected, even by small telescopes.
But neither
Phoenix nor Serendip, nor any other search, has turned up signals of extraterrestrial origin.
One reason for that failure is the sheer complexity of the task. Our galaxy contains more than a hundred billion stars, spread across an expanse of almost 100 000 light-years. The recent discoveries of extrasolar planets—which number more than 200 at last count—boost hopes that there is intelligent life out there. And yet, as astrophysicist and SETI pioneer Frank Drake famously postulated in equation form, the likelihood that any one of those billions of star systems hosts not only a habitable planet but also one that has evolved beings who are both willing and able to communicate with us is quite low.
Low, but not zero. If you assume, for example, that one in a million stars has a planet bearing intelligent life, that means our galaxy is home to at least 100 000 advanced civilizations. Even if only one in a hundred million stars qualifies, that still leaves more than 1000 civilizations that could be trying to contact us.
To date, though, radio astronomers have heard nothing. It’s too soon to conclude that nobody’s out there: maybe SETI researchers are just looking in the wrong place or in the wrong way. I believe they’ve made the latter mistake. No intelligent society would attempt to communicate with us over hundreds of light-years using radio waves when physics suggests other wavelengths would be the more intelligent choice.
As far as we know, only two types of waves can travel through the vacuum of space: electromagnetic and gravitational. Gravitational waves would be exceedingly hard to generate or detect, so any signals headed our way will probably lie somewhere in the electromagnetic spectrum—from X-rays at one extreme to frequencies lower than an ordinary AM radio at the other end. For years, the SETI community rejected the idea of looking for anything other than RF signals, even though they represent just a tiny portion of the electromagnetic spectrum [see illustration, “Casting a Wider Net”]. Limiting SETI to just radio waves is like losing a diamond ring on a football field and only searching for it at the 1-yard line.
A good argument can be made that the optical spectrum is a more likely place to find alien signals than are either RF or microwave frequencies. For one thing, it’s much easier to deal with noise at optical wavelengths. People who measure radio waves have to contend with interference from radar antennas, radio stations, and other terrestrial sources. The receiver itself also adds noise, which is why the detectors attached to advanced telescopes are typically cooled to near absolute zero. Yet there is always some residual thermal noise to contend with, and for certain wavelengths the cosmic microwave background—a ­vestige of the big bang—makes SETI searches difficult.
For optical observations, the only significant terrestrial source of interference is lightning, which is at worst a sporadic problem. In the early days, many investigators discounted optical SETI because they imagined that the sender’s star would be an overwhelming noise source. But they didn’t appreciate that it is actually quite easy to arrange a transmission that outshines whatever sun you’re circling: just use a pulsed laser rather than the continuous-wave type.
The development of laser communications systems for military satellites, submarines, and aircraft has proved that short bursts of light are far more efficient than continuous waves at carrying information over great distances. Each pulse has a high peak power, but most of the time the laser isn’t active, so the overall power consumption is low.
Presumably a distant intelligent civilization would have figured this out as well. With transmissions in brief bursts, each pulse could easily be 1000 times as bright as any nearby star in the receiving telescope’s field of view. The shorter the pulse, the less background light there is per pulse to compete with the signal. Reducing the pulse to nanosecond intervals makes the signal even more distinct, because there’s no source in nature that generates flashes that short.
The sender could vary the interval between pulses to convey information. Think of each interval as a roulette wheel. Each slot in the wheel represents a number: if the pulse fell in slot 36, it would be conveying the number 36; if it fell in slot 1, it would mean the number 1. The roulette wheel could have more or less than 36 slots, of course, and as the number of slots changes, so does the amount of information you can send per pulse. With just two slots, you could send just one bit per pulse; with 256 slots, you could send 8 bits; and with 1024 slots, you could send 10 bits. This technique, called pulse-position modulation, was used for many years in both radio and optical communications. So, if you were to detect a pulsed signal coming from space, the next step would be to analyze it for any repeating sequences.
Another reason to prefer optical methods over radio SETI is that it’s much easier to form a narrow beam of light. Remember, any message will have to travel many trillions of miles through space to reach us. If the sender were to broadcast a signal in all directions at once, the power needed would be prohibitively high, regardless of what wavelength was used.
George W. Swenson Jr., professor emeritus of astronomy and of electrical and computer engineering at the University of Illinois at Urbana-Champaign, has calculated that if a radio transmitter were 100 light-years away and projecting its energy omnidirectionally, it would require 5800 trillion watts to provide a detectable signal—an amount, Swenson points out, that is “more than 7000 times the total electricity-generating capacity of the U.S.”
SETI researchers therefore generally assume that the transmitter will point toward specific star systems and that the beam will be as tight as possible. The ratio of the wavelength being transmitted to the diameter of the antenna used is roughly proportional to the width of the beam. And the wavelength of visible light is six orders of magnitude smaller than that of microwaves, allowing the beam to be considerably narrower. So the physics of optics over RF wins out again.
Charles Townes—the coinventor of the laser—and Robert Schwartz first suggested the idea of searching for optical signals from extraterrestrials in 1961. Their paper, published in Nature, theorized that beings in a nearby star system, “some few or tens of light-years away,” could use laser (or maser) beams to communicate with us earthlings.
It took several more decades for optical SETI to catch on, in large part because laser technology wasn’t nearly as mature as radio technology. That’s no longer the case. Photodetectors today have quantum efficiencies of 40 percent or more; that is, for every 100 incoming photons, 40 are actually counted. The detectors are also much faster now, and picking up nanosecond pulses is no problem.
The new Harvard telescope takes advantage of these and other technological advances. Unlike a regular imaging telescope, Horowitz’s brainchild has no lens, nor is it designed to pluck pristine images of celestial bodies orbiting overhead. Its mission is a bit cruder. It uses a 1.8-meter-diameter mirror and a 0.9-meter secondary mirror to scoop up raw photons from the sky—much as a wooden barrel collects rainwater. For that reason, the telescope is more properly, though less glamorously, known as a photon bucket [see photos, “A Closer Look”].
Photon buckets are less expensive to build and operate than imaging telescopes, because all you’re really worried about is that the photons arrive at the detector. If a given photon ends up traveling a few extra millimeters because it hit an air pocket in the atmosphere, it doesn’t really matter. An advanced imaging telescope, by contrast, employs all kinds of sophisticated mechanisms to compensate for distortion in the incoming signal, so that it can accurately reproduce what it sees.
A key feature of the Harvard telescope is its use of ­multipixel photomultiplier tubes. Photomultipliers work by converting incoming photons into electrons, which then get amplified until the electrical signal can be distinguished from the noise. A multi­pixel photomultiplier divides the collection area up into tiny squares—64 per tube in this case—each acting like a separate detector. This setup lets you look at more star systems at a time.
The Harvard telescope breaks up the sky into 1.6- by 0.2
degree patches, observing each patch for about 48 seconds before moving on to the next patch. A mere 48 seconds doesn’t sound like a long time, but remember that the associated electronics are sampling the data in nanosecond intervals. At that rate, the instrument should be able to cover the entire sky above the northern hemisphere in 150 nights of observation.
All those signals from all those pixels are then fed into 32 microprocessors, which were custom-designed for the project by Horowitz’s graduate student Andrew Howard. These PulseNet chips crank through the data—3.5 trillion bits per second—searching for a large spike in the photon count, which may indicate a possible light pulse from afar.
The telescope’s photodetectors are divided into two arrays, so that if one of them receives an interesting signal, you can check it against the second array. Ideally, you’d like to have an entirely separate telescope. In a previous search, Horowitz collaborated with David Wilkinson’s group at
Princeton University, using a 1.5-meter telescope at Oak Ridge and a 0.9meter telescope at Princeton’s FitzRandolph Observatory. The paired instruments conducted a targeted—rather than an all-sky—optical search, examining more than 6000 stars over a six-year run. In its first three nights of observation, the all-sky search took in 200 times as many stars as the targeted search did during the entire experiment.
The Harvard photon bucket isn’t looking at any particular wavelength, but SETI investigators have long speculated that intelligent beings would choose to send their signals at a special frequency, one that is somehow fundamental to the universe. If we knew what that frequency was, it would narrow our search dramatically, because we’d have to observe just a tiny fraction of the spectrum. That’s why Project Serendip and a number of other radio SETI efforts focused on the hydrogen line.
In the optical regime, there are equally interesting frequencies that an intelligent civilization might choose. Called Fraunhofer lines, these are naturally occurring gaps or holes within the spectrum of visible light given off by stars. At these frequencies, the stars’ background energy drops considerably—in some cases, to only one-tenth its normal value. So if an alien sent a signal at the wavelength of a Fraunhofer line, the transmitter wouldn’t have to be nearly as strong, and at the receiving end, the search for the correct wavelength would be greatly simplified.
The operators of the Harvard telescope assume that the transmitter is pointed our way continuously. But what if the source is transmitting only sporadically—pointing toward our solar system for a few nights, say, and then moving on to another star system? Only a lucky coincidence would have everything lined up at exactly the right time. Yet, the only way to cover the entire sky at once would be to construct many thousands of receivers, each looking in a different direction.
And what if the alien’s signal requires a much larger receiver than what we currently have? There are several options here. You can simply build a bigger photon bucket, which would increase the collection area and thus boost the number of incoming photons. Or you could network together lots of smaller telescopes. Stuart Kingsley, an optics engineer and SETI enthusiast, and I proposed such a scheme several years ago.
We were inspired by the enormous popularity of the SETI@home program, which is harnessing the power of 5 million personal computers to crunch through the data collected by SETI radio telescopes.
Our proposal involved using hundreds or even thousands of amateur telescopes, equipping each of them with a low-cost photodetector and then aiming all of them at a given star system at the same time. Of course, each telescope would be a slightly different distance from the star, but you could compensate for those differences by using a GPS receiver to pinpoint each telescope’s position to within a few centimeters. You would also need an Internet connection to some central control center, which could collect all the data and coordinate when the telescopes were observing and where they were pointed.
For the best observations, though, you would need to place your telescope out in space. Earth places some limits on how sensitive a detector you can build. Manufacturing and maintaining a telescope’s large mirrors is costly, and wind vibrations and the pull of gravity also constrain the size of the apparatus. What’s more, the atmosphere filters out photons of certain frequencies, and you can’t observe at all in cloudy weather.
A photon bucket out in space or on the moon, by contrast, would experience low gravity and no wind (although the solar wind is still a consideration). You could therefore construct it from lightweight materials, so it could be much larger than any terrestrial receiver. And, of course, there would always be clear weather.
Such endeavors, though, will have to wait for a generous benefactor to come along. Nearly all of the SETI research today is funded by individual donors or by nonprofit groups, such as the Planetary Society, in
Pasadena, Calif., and the Bosack/Kruger Charitable Foundation, in Natick, Mass., both of which helped finance the new Harvard facility. The U.S. government officially ended its support of SETI in 1993. Unless we get very lucky, we will need to conduct larger-scale, dedicated searches, not just in the RF and visible light spectrum but at infrared and ultraviolet wavelengths, too. We are, in other words, still a long way from being able to declare SETI a success or a failure. But broadening the scope of the search beyond radio waves is a significant step forward.

Font: Monte Ross, IEEE Spectrum (Novembre 2006).