California astronomers are broadening the search for extraterrestrial intelligence (SETI) by looking for powerful light pulses coming from other star systems. Scientists from the University of California's Lick Observatory, the SETI Institute, UC Santa Cruz, and UC Berkeley are coupling the Lick Observatory's 40-inch Nickel Telescope with a new pulse-detection system capable of finding laser beacons sent by alien civilizations.
|Frank Drake and his team search for radio frequencies, using large telescopes such as the Arecibo Observatory, but now utilize much smaller telescopes for the optical search.
Credit: David Parker / Science Photo Library
"We are looking for very brief but powerful pulses of laser light from other planetary systems, rather than the steady whine of a radio transmitter," says Frank Drake, Chairman of the Board of the SETI Institute and a co-investigator on the new experiment.
SETI is usually associated with large radio antennas connected to specialized, multi-million channel receivers. But while radio SETI costs millions of dollars each year to operate, optical SETI (OSETI) will only have a total cost of about $30,000.
Lasers are a more efficient type of beacon than radio signals, says Monte Ross, former director of the McDonnell Douglas Laser, Space Communications, and Radar Observatory. Ross also has been involved with OSETI efforts for many years.
"Lasers are more efficient because of their narrow beams," says Ross. "The laser beam would be one microradian or less compared to radio frequency beams of one milliradian or more." [A milliradian beam spreads one foot for every thousand feet of travel, while a microradian beam spreads only one foot for every million feet of travel] "In effect, all the transmitted energy goes into a much smaller area than the radio frequency signal."
This narrower beam, however, creates a limitation in transmission. Unlike its radio counterpart, which can be broadcast broadly, laser beacons need to be sent in the direction of our solar system for us to be able to detect them. Presumably, advanced alien civilizations sending out such beacons would know exactly what sort of star systems to target.
"They would understand enough from bioastronomy to know which star systems have a chance for life," says Ross. "It is possible that weak laser signals are reaching us on a regular basis."
|The optical SETI detector is a triple photometer located at the Cassegrain focus on the Nickel one-meter reflecting telescope.
Credit: Lick Observatory
This new OSETI experiment will look for beacons coming from systems with Sun-like stars, as well as other star systems that are thought to have habitable planetary zones. OSETI will also target stars that are already known to have orbiting planets. The detectors will be able to look for beacons coming from star systems as far as 1,000 light years away.
This is not the first OSETI search -- scientists have been conducting OSETI experiments since the 1980s -- but we have only recently developed the technology to look for extremely quick flashes of light. The new light detectors (photomultipliers) are designed to look for short laser pulses lasting for less than a billionth of a second. By looking for nanosecond pulses of light, we will be able to distinguish alien beacons from other, naturally occurring astrophysical sources.
Such astrophysical phenomena plagued earlier OSETI searches with almost daily false alarms. Starlight, cosmic rays, muon showers, and radioactive decays in the glass of photomultiplier tubes can all create confusing signals.
The new experiment uses three photomultipliers to search for quick, bright pulses, whereas previous OSETI experiments had only used one or two. Light from the observed star will still trigger the detectors, but seldom will all three photomultipliers be hit by photons within a billionth of a second time frame. The expected number of false alarms for the stars being observed is about one per year.
According to Ross, not only could alien civilizations notify us about their existence with the light pulses, they could also send us information.
"Information can be sent by varying the interval between pulses," says Ross. "With nanosecond-type pulses, there are one thousand million such intervals in each second. There is no reason that the time between pulses should be greater than tens of seconds and probably will be more like a second or less."
|Shelley Wright, a physics student at the University of California Santa Cruz, lines up the Lick Observatory's Nickel 40 inch reflector to search for light pulses from extraterrestrial civilizations.
Credit: SETI Institute
Dan Werthimer and Richard Treffers of UC Berkeley designed the hardware and software for the new, three-tube photomultiplier system. It was built by Shelley Wright, an undergraduate physics student at UC Santa Cruz, under the direction of principal investigator Remington Stone, a research astronomer at Lick Observatory. The astronomers expect that the new approach will produce an experiment that can be run automatically, and with results that will be far less ambiguous than previous OSETI searches.
Says Ross about this new OSETI effort, "I have been promoting the concept of short laser pulses since the early 1960's. It is gratifying to see real action starting to take place."
Astronomers at the Lick Observatory have examined about 300 individual star systems thus far, as well as a few star clusters. The intention is to continue the search at least on a weekly basis for the coming year. Each star system is observed for only 10 minutes, but that is all the time needed to detect the light beacons of an alien civilization. The astronomers have singled out over 5,000 stars to target for observation.
Rather than observe each star only once, Ross believes astronomers should repeatedly check on the candidate stars for laser pulses. The current search methods, he says, assume an alien laser is directed at our solar system and is continuously sending the brief pulses.
"What is more likely is that a laser system on the source planet is aiming at one star system, and then is rapidly moving to point at another star system for a limited amount of time," says Ross. "This enables one powerful laser to cover many hundreds of stars -- rather than one laser per star -- which objectively we must consider unlikely."
According to Ross, an alien source would assume that an intelligent society would design an intelligent receiver. Such a receiver would look at all the most likely candidate star systems over the course of a couple nights, and then continually check up on these candidates, either over a few days, months, or years. Eventually, we would observe a star system at the same time that an alien laser pulsed light in our direction.
"One has to assume any laser pointing towards us will only be pointing for a short time before moving on," says Ross. "Therefore, receivers need to look at all the few hundred most likely candidates. Then we need to keep doing this so that when a beam is pointed at Earth, it will be detected."