Combing the Cosmos at High Speed: The Allen Telescope Array

October 12, 2006
by Seth Shostak, Senior Astronomer

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

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

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

Four decades ago we might have imagined that tripping across an extraterrestrial broadcast would be an easy matter, but all the searches since then tell us it's not. The sky isn't cluttered with honking signals that anyone with a backyard satellite dish, a crystal set, and abundant spare time can find. If we hope to discover ET in the near future, we're going to need highly sensitive antenna systems that can check out large expanses of cosmic real estate quickly. That's simply the consequence of doing a discovery experiment with a universe of possible search locales.

The need for speed is a major impetus for the Allen Telescope Array (ATA), a specialized radio telescope now under construction by the SETI Institute and the University of California Berkeley - and the first such instrument designed with SETI in mind. Sure, making a search with someone else's telescope spares you the bother of building one, and using a loaner instrument is a modus operandi that has given SETI scientists access to some of the largest antennas in the world. But frankly, it's mightily inefficient: comparable to doing medical research with borrowed microscopes. Usually only a few weeks per year of a big telescope's observing schedule will be devoted to SETI, and some of that precious time is inevitably lost in the ritual of repeatedly setting up and turning 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 per year than was available for the SETI Institute's Project Phoenix, which ran on telescopes in Australia, West Virginia, and Puerto Rico.

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

The Allen Telescope Array's MMIC (Monolithic Microwave Integrated Circuit) receiver simultaneously picks up all cosmic static between 0.5 and 11.2 gigahertz - a spectral range equivalent to two thousand TV channels, side-by-side on the dial. In its first incarnation, only four selected sections of that spectrum will be examined. Nonetheless, that's a several-fold improvement over past SETI experiments. In a decade or two, as digital electronics become cheaper and more powerful, 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 a dozen or so pixels in place of that one spot beam, but those dozen are tightly clustered and in a fixed pattern. But an array of small antennas, such as the ATA, is able to simultaneously create pixels at arbitrary positions over many square 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 for earlier SETI experiments, which had to look at star systems one at a time. But as processing power becomes cheaper, those three pixels will eventually blossom to 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 any previous radio search. And that's just its opening gambit, since its speed will only increase.

Of course, if there's nothing to be found - if nowhere in the Galaxy are other beings stabbing the darkness of space with their radio beacons - then the capabilities of our telescopes won't matter. But if - as many think reasonable and likely - intelligent life is a phenomenon that is something less than miraculous, then a discovery experiment will benefit from the increased speed of new instrumentation. SETI scientists are combing the cosmos for a signal, and the ATA will be the mother of all combs.