SCIENCE: Illuminating the search for extraterrestrial intelligence

Nanosecond lasers look like the most feasible way of spotting space alien transmissions

Searching for extraterrestrial intelligence isn’t as simple as scanning for the space alien equivalent of "I Love Lucy" reruns. We have to think about how hypothetical space aliens would try to communicate.

Ultrashort laser pulses have become a favored contender, according to veterans of what insiders call SETI, for "Search for Extraterrestrial Intelligence." A panel of these longtime searchers spoke at the recent annual meeting of the American Association for the Advancement of Science in San Diego.

These laser pulses, lasting just a billionth of a second, can outshine the sun, said panelist Paul Horowitz of Harvard University and Frank Drake of the SETI Institute, a private nonprofit based in Mountain View. Such signals from an alien planet should be detectable at vast distances.

Nanosecond lasers are cheap, a byproduct of research into nuclear fusion, Drake and Horowitz said. The assumptions follow naturally: Nuclear fusion as a power source would be obvious to any technologically advanced alien civilization. That means such aliens would have developed such lasers. If they are in a talkative mode, such aliens would consider the advantages of using these lasers.

Where to look?

To be sure, it’s always risky to assume that another person will think the same way you do, and that is especially true for any technologically advanced aliens, if they even exist.

But with just one example of intelligent life to study ---- us ---- the search for intelligent extraterrestrial life is mainly focused on finding aliens on Earth-like planets, Drake said.

Ground-based searches for electromagnetic radiation are limited by atmospheric absorption. There are two main "windows," one for visible light, and another in the microwave region. Until recently, alien hunters have given most of their attention to microwaves.

The microwave region encompasses radiation given off by hydrogen atoms and hydroxyl (OH) ions. Because of its quietness and the importance of hydrogen as the most abundant element in the universe, SETI has dubbed this region the "water hole," as an obvious spectral location for interstellar civilizations to send signals.

Despite the vast distances of interstellar space, detecting signals is quite feasible because not much power is needed, Horowitz said. A powerful signal capable of being generated by humans today, sent 1,000 light years, might equal one-trillionth the energy of a falling snowflake at the receiving end.

"But it does the job," Horowitz said, "because it has to compete only with whatever background (noise emissions) there are, which are even smaller than that amount. And in fact we do about 10,000 times better than noise. So interstellar telegrams are cheap, and cost something like half a cent per word."

Going quiet

That was the promising state of technology in the 1960s. However, Drake said the advance of technology since then has reduced hopes of detecting alien intelligence from radio signals, especially of the unwittingly transmitted "I Love Lucy" variety.

Drake said the shift from analog to digital communications technology has quieted radio transmissions, contrary to what one might expect in a communications-soaked society.

Traditional analog technology uses brute force in a narrow spectrum range to get its signal across. Digital technology is more efficient and carries more bandwidth than analog. And as a result, less power is used, diffused across more of the spectrum. So with less energy generated, less is left to leak into space.

"We use much less power with our modern digital TVs to get a good picture than we used to," Drake said. "What's happened here is the Earth is growing quiet ... If we are the model for the universe, that's bad news."

Searching for brief optical-frequency pulses such as from nanosecond lasers is much more promising, Drake said.

"We don’t need to know the right frequency," Drake said. "We can detect the flash, because it will be 1,000 times brighter than the star."



