Combing the Cosmos at High Speed: The Allen Telescope Array

Rememberstudying the (heavy chords) Scientific Method in middle school? According to your dour-faced science teacher, this was the secret formula bywhich legions of clipboard-carrying, lab coat-attired researchers pushed backthe frontiers of knowledge. The scheme was simple: Scientists sat arounddreaming up hypotheses--possible new truths--which they torture-tested in the labor in the field. Experiment would arbitrate, either by validating the truth ofa hypothesis, or by sending the scientist back to the blackboard to thinkagain.

Indeed,some research is done like that; investigations that proceed by testing afalsifiable premise. But there's another way to learn about the world whichyou might call "discovery" science. Consider X-rays or penicillin. Theyweren't first hypothesized by tweedy academics; they were simply found, andtheir nature and significance worked out after the fact. The same is true forquasars, pulsars, dark matter, dark energy, and nearly every object you'll finddescribed in an astronomy textbook.

SETI isakin to discovery science, despite its obvious presumption that theextraterrestrials exist, because that hypothesis is not falsifiable. A failureto receive a radio beacon from space doesn't say a whole lot about whetheraliens do, or do not inhabit the 'hood. But while we can't prove that aliensare not there, we can prove they are. We just have to find asignal.

Fourdecades ago we might have imagined that tripping across an extraterrestrialbroadcast would be an easy matter, but all the searches since then tell us it'snot. The sky isn't cluttered with honking signals that anyone with a backyardsatellite dish, a crystal set, and abundant spare time can find. If we hope todiscover ET in the near future, we're going to need highly sensitive antennasystems that can check out large expanses of cosmic real estate quickly. That'ssimply the consequence of doing a discovery experiment with a universe ofpossible search locales.

The needfor speed is a major impetus for the Allen Telescope Array (ATA), a specializedradio telescope now under construction by the SETI Institute and the University of California Berkeley--and the first such instrument designed with SETIin mind. Sure, making a search with someone else's telescope spares you thebother of building one, and using a loaner instrument is a modus operandi thathas given SETI scientists access to some of the largest antennas in the world. But frankly, it's mightily inefficient: comparable to doing medical researchwith borrowed microscopes. Usually only a few weeks per year of a bigtelescope's observing schedule will be devoted to SETI, and some of thatprecious time is inevitably lost in the ritual of repeatedly setting up andturning on specialized hardware that has been dormant for months.

The ATA,however, will be available 24/7. That's a factor of ten more search time peryear than was available for the SETI Institute's Project Phoenix, which ran ontelescopes in Australia, West Virginia, and Puerto Rico.

Inaddition, the ATA benefits from startling new developments in receiver design.Most receivers used for radio telescopes can tune a band that's a few hundredmegahertz wide. That beats the heck out of your AM radio, but it's still apretty small chunk of the radio spectrum--which means that if you want to searchfor ET's transmission, but don't know where on the dial to listen, then youhave to keep changing out receivers to cover different frequency ranges.

The AllenTelescope Array's MMIC (Monolithic Microwave Integrated Circuit) receiversimultaneously picks up all cosmic static between 0.5 and 11.2 gigahertz--aspectral range equivalent to two thousand TV channels, side-by-side on thedial. In its first incarnation, only four selected sections of that spectrumwill be examined. Nonetheless, that's a several-fold improvement over past SETIexperiments. In a decade or two, as digital electronics become cheaper and morepowerful, even these modest bandwidth limitations will seem quaint.

Finally,and undoubtedly most importantly, the ATA is an imaging telescope. In contrast,a single-dish instrument such as the Arecibo antenna in Puerto Rico is basically a one-pixel radio camera, its metal eye stares at a single spot on the heavens.Yes, you can bolt multibeam receivers to the focus of such a scope and get adozen or so pixels in place of that one spot beam, but those dozen are tightlyclustered and in a fixed pattern. But an array of small antennas, such as theATA, is able to simultaneously create pixels at arbitrary positions over manysquare degrees of sky. Again, the initial configuration of the ATA is modest:three pixels will be its limit. Still, that's three times better than forearlier SETI experiments, which had to look at star systems one at a time. Butas processing power becomes cheaper, those three pixels will eventually blossomto ten, a hundred, a thousand, or more. The speed of the search will increase accordingly.

In sum,when the ATA is completed, it will be about a hundred times faster than anyprevious radio search. And that's just its opening gambit, since its speedwill only increase.

Of course,if there's nothing to be found--if nowhere in the Galaxy are other beingsstabbing the darkness of space with their radio beacons--then the capabilitiesof our telescopes won't matter. But if - as many think reasonable and likely--intelligentlife is a phenomenon that is something less than miraculous, then a discoveryexperiment will benefit from the increased speed of new instrumentation. SETIscientists are combing the cosmos for a signal, and the ATA will be the motherof all combs.

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."