Inauguration Day for Alien Signal-Hunting Telescope

October 11, 2007
By Seth Shostak

Today, in the remote northeast corner of California, technology innovator and Microsoft co-founder Paul Allen will hit the big red button.

No, he won't be throwing heavy-duty machinery into an emergency shutdown, nor will he be sending ICBMs screaming from their silos (traditional functions for ruddy buttons). Instead, he'll be christening a new telescope that, in its significance, could eventually outpace the Nina, Pinta, and Santa Maria.

The famous technologist will be inaugurating the initial 42 antennas of his namesake, the Allen Telescope Array (ATA) – the first major radio telescope designed from the pedestal up to efficiently (which is to say, rapidly) chew its way through long lists of stars in a search for alien signals. Within two decades, it will increase the number of stellar systems examined for artificial emissions by a thousand-fold. The ATA will shift SETI into third gear.

This telescope is truly a geek's barn-burner. In the last two decades, high-performance radio amplifiers have gotten smaller and, more importantly, much cheaper. This has changed the recipe for building radio telescopes, and the ATA is taking advantage of the new formula.

Consider: the single most consequential characteristic of a radio telescope (at least, for SETI) is its collecting area: the number of square meters boasted by its "mirror." There are two ways to increase this area: either build a bigger antenna, or build lots of smaller ones and hook them together. As an example of the former strategy, imagine doubling the diameter of the antenna's "dish", thereby increasing the collecting area by a factor of four. A good thing, surely. But since an antenna is a three-dimensional device, the amount of aluminum and steel necessary for the larger antenna has gone up by a factor of eight. Expensive. It's cheaper by half to build four of the original-size antennas.

This is a simple scaling argument, but it boils down to this: it's always more economical to assemble a large collecting area by constructing small antennas, rather than large ones.

In past practice, this elementary fact of antenna life was routinely diluted by the high cost of the receiver equipment. Check out the cryo-cooled, quiet-as-death receivers at the focus of any other radio telescope, and you're looking at a million dollars' worth of electronics. That's why the Very Large Array – the iconic radio telescope in New Mexico that you've seen in a raft of sci-fi films – has only 27 antennas. That number was a compromise between structural and electronic costs.

Today, you can festoon the focus of your antenna with high-grade receivers for about one percent of the old price. So the paradigm has changed, and today it's better to build a large number of small antennas, rather than a small number of large antennas.

The individual dishes of the ATA are 6 m in diameter, small enough that you can't see them from California state route 89, even though they're barely a mile beyond its eastern berm. Like slow-growing lotus blossoms, these antennas have methodically erupted on a lava-littered heath 300 miles northeast of San Francisco during the last four years. Eventually, 350 dishes will grace the Hat Creek Observatory site. But the 42 now up and running are equivalent in collecting area to a 40 m single-dish antenna – and that's large enough to start doing some serious science.

What it can do

A lot of that science will be vanguard radio astronomy. Thanks to the ATA's small individual antennas, the instrument has a wide field of view. That is to say, like wide-angle sports binoculars, it sees a big chunk of sky all at once. (In contrast, most radio telescopes look at the heavens with a field of view comparable to what you'd see through the tiniest of soda straws.) The University of California radio astronomers, who – together with the SETI Institute – are building the ATA, will fill that large field of view with pixels to produce high-resolution radio photos of large tracts of cosmic real estate.

The combination of wide-angle view and high resolution allows rapid surveys of our local chunk of the cosmos. And it's a hallowed axiom of astronomy that surveys nearly always pay off with unexpected, major discoveries.

In addition, by looking at lots of the sky, and looking at it often, radio astronomers can find transient phenomena: things that go burp in the night, and that otherwise would never be seen.

The new possibilities might best be understood by analogy. Consider making a time exposure photo of Manhattan from the Empire State Building. That's comparable to what astronomers do now – collecting data with their radio telescopes for hours, while staring at one patch of space. A time exposure reveals lots of subtle detail – cars parked on the streets, the filigreed facades of the skyscrapers, and so forth. But anything that changes – the taxis, the pedestrians, or even the stoplights – gets blurred or lost by the long exposure. Well, with the ATA's snappy radio picture mode, things in the universe that change will finally be seen. Prepare to be surprised.

For SETI, the ATA will be as revolutionary as a Parisian mob. Most folks, indoctrinated by Hollywood's sleek, blue-lit view of science – with its immaculate laboratories and aimlessly wandering engineers – imagine that SETI researchers spend their days with earphones on their heads, straining to pick out ET's transmissions from the fuzzy din of cosmic static. It isn't that way, and if it were, SETI scientists would have long ago checked into the funny farm.

The reality is more complex. Even the ATA-42, the first incarnation of this new instrument, will be able to simultaneously observe several star systems at once, while monitoring at least 40 million radio channels. You can't analyze all that data with earphones, and so a sophisticated, custom software system carefully screens all incoming static for the tell-tale whistle of an extraterrestrial transmitter.

For its first foray into SETI, the ATA-42 will be used to scan 20 square degrees of sky in the direction of the center of our Galaxy. It will spend several months looking for signals coming from the direction of the Milky Way's star-clotted, inner realms. Eventually, the ATA will start a massive campaign to examine approximately a million nearby star systems. That's a thousand times more than all those carefully scrutinized in the past.

Today, when Paul Allen hits the big button, 42 radio ears will pivot toward the sky like synchronized swimmers, and the first official observations with the ATA will begin. There'll be applause and smiles all around.

But the true excitement is yet to come. It's hard to imagine that such a modest collection of small metal contrivances, pinned to the earth, and each no bigger than a delivery truck, could somehow reveal the activities of unknown, unseen beings on a planet a thousand trillion miles away. But a simple calculation on a small sheet of paper shows this to be true. And perhaps someday soon, that discovery will be made.