Searching for signals from extraterrestrials can be a ticklish business. Astronomer John Learned thinks tickling certain stars in just the right way might be a good strategy for ET to phone Earth.
Those stars, known as Cepheid variables, brighten and dim on a regular schedule. In 1908, after analyzing stars on photographic plates at Harvard College Observatory, Henrietta Swan Leavitt reported that a Cepheid’s maximum brightness depends on the timing of its bright-dim cycle. The longer the period, the brighter the star. Other astronomers soon realized that they could use the period-brightness relationship to measure distances to remote galaxies.
A century later, Learned and colleagues are proposing a new use for Cepheids. In an article recently posted online (arxiv.org/abs/0809.0339), the researchers suggest that tinkering with the core of a Cepheid variable using a beam of neutrinos could be an effective way for advanced civilizations to communicate. This modulation, or “tickling,” would alter the phase at which the star brightens and slightly shorten the time it takes for the star to wax and wane, creating a new pattern that distant observers might detect.
Although most SETI (search for extraterrestrial intelligence) programs use radio telescopes to look for alien broadcasts, fiddling with Cepheids has advantages for both senders and receivers, Learned and his colleagues note. Not only can the stars be seen from afar, but “Cepheids are great because any emergent civilization will surely find them and monitor them for the very same reasons we do,” says Learned, of the University of Hawaii at Manoa. And astronomers have searchable records of Cepheids that go back well over a century.
Another advantage, says Learned, is that a Cepheid star — unlike a directed radio signal — would radiate in all directions, making it more likely that the radiation would be recorded on Earth.
“If it could work, then this is an answer to one way to build an omnidirectional beacon,” says Jill Tarter, director of the Center for SETI Research in Mountain View, Calif. “It would be an example of an ‘almost natural’ signal that would get captured in a survey of the universe by an emerging technology, that’s us, and finally recognized in a database by some curious grad student.”
Neutrinos seem ideal for tinkering with Cepheids because these subatomic particles travel rapidly and interact with matter so weakly that they could penetrate all the way to the star’s core. If delivered at just the right time during the Cepheid’s cycle, when it’s in its compact, dim phase, the energetic neutrinos “would change both the pulsation rate and the peak amplitude” of the star using a minimum of energy, Learned says. “We leave it as an engineering problem for the star-tickling civilization out there,” to determine the optimal energy, the researchers wrote.
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Detecting the pattern won’t be easy. “One glitch tells us nothing,” Learned says. “It will take discerning a pattern of glitches, and some regularity to find something interesting.”
Although star tickling is beyond current human ability, “it’s silly to try to guess whether it’s feasible for some unknown, incredibly advanced civilization to be able to do this,” comments Jeff Scargle of NASA’s Ames Research Center in Moffett Field, Calif. “It’s a really smart idea because … the star generates the energy; all you have to do is change it a little bit. It’s a nice way to piggyback on what nature has supplied.”
He points out that researchers could start looking for artificially modulated signals by combing through the huge database on Cepheids amassed over decades by amateur and professional astronomers.
Neutrinos play a more direct role in another new way to look for ET. In a paper posted online in March, Zurab Silagadze of the Budker Institute of Nuclear Physics in Novosibirsk, Russia, details how neutrinos created by alien civilizations might be detected on Earth (arxiv.org/abs/0803.0409). The article is a follow-up to an idea that researchers first speculated about nearly three decades ago, Silagadze says.
Particle accelerators that collide beams of muons — heavy cousins of the electron — would produce intense neutrino beams. (Such accelerators are possible with known technology.) An advanced civilization with such a collider could communicate with other societies via neutrinos. The simplest possibility, Silagadze says, is to “periodically switch on and off the neutrino source to organize short and long signals, a kind of Morse code.”
“Neutrino communication schemes are broad band, and you do not have to know the transmission frequency,” he adds, allowing signals to be sent to the maximum number of unknown civilizations.
Suppose, Silagadze says, that a civilization within 20 light-years of the solar system produces 100-trillion-electronvolt beams of neutrinos directed toward Earth. He calculates that a neutrino detector called IceCube, now under construction beneath ice at the South Pole, could detect seven to 10 muons per year generated by neutrino impacts in the ice.
IceCube researcher Buford Price of the University of California, Berkeley, says that those muons, unlike random muons from space, “would all point closely in the direction of the muon collider that belongs to the advanced civilization.” The signal would be easier to discern if the senders turned it on and off.
About half finished, IceCube is now taking data. Says Price: “We will let the world know if we see a beam correlated in both direction and time.”