The search for extraterrestrial intelligence, SETI, at its core, is after one question: Are we alone in the universe?
The query is simple, but the answer requires a monumental effort. SETI often focuses on radio signals because radio waves travel relatively unimpeded by gas and dust. SETI researchers typically look for narrowband emissions — that is, ones involving just a few radio frequencies — as natural sources of radio waves in space are typically wideband.
"It is a needle-in-a-haystack problem, where our powerful telescopes can detect literally millions of radio transmitters on the Earth, even though they're pointed into space, and we need to try to pick out a perhaps very faint signal among them from space," Jason Wright, an astrophysicist and director of the Penn State Extraterrestrial Intelligence Center, who did not take part in the new study, tells Inverse.
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Distinguishing potential alien radio signals from ones generated by the countless antennas on Earth is also a huge challenge. On top of this, an ideal SETI emission is one that multiple telescopes detect coming from the same location in the sky. However, like artificial emissions on Earth, alien signals may come in pulses. This means one telescope could miss one of the signal's pulses, so automated filters would likely rule out the entire signal. As such, astronomers could miss important one-off signals that appear once and are never seen again, such as the legendary 72-second-long "Wow!" signal of 1977, study lead author Bryan Brzycki, an astrophysicist at the University of California at Berkeley, tells Inverse.
This led Brzycki and his colleagues to investigate whether there was a way to detect whether a narrowband signal did come from outside Earth without needing to have multiple telescopes pointing at the same location in the sky to confirm that fact. They focused on whether anything might happen in the depths of interstellar space to make distant radio signals look significantly different from radio emissions on Earth.
They zeroed in on scintillation, or twinkling. On Earth, stars at night twinkle or vary in brightness due to how the atmosphere interferes with their light. Regular fluctuations in atmospheric density can lead the air to scatter visible light to greater and lesser degrees.
Interstellar space is largely empty but does possess ionized plasma that could interfere with radio waves. This led the scientists to explore whether interstellar scintillation could leave enough of an imprint on extraterrestrial narrowband signals to distinguish them from terrestrial radio emissions.
"If we were able to observe this fluctuation during a single observation, that would imply that interstellar material is causing those fluctuations, in turn implying that the signal is extrasolar," Brzycki says.
The researchers found turbulent ionized plasma could lead to scintillations in extraterrestrial radio signals. "We now have a way of looking at a signal's brightness over time to assess the likelihood that it originated outside of our solar system," Brzycki says. "We now have an additional filter for compelling signals."
The new study has shown "that for a sweet spot of radio frequencies—not far from the optimum frequencies favored by the pioneers of radio SETI—this scintillation will be obvious in the kinds of data taken by the Breakthrough Listen search for radio technosignatures," Wright says — that is, radio evidence of alien technologies. "This provides a new, independent way to confirm that a signal is from space and not from Earth, meaning we now have another filter to use when sorting through all the hay of radio signals, looking for an alien needle."
Previously, radio SETI methods did not take a detailed look at the fluctuations in brightness associated with scintillation because they are random in nature. "It's certainly more difficult to identify the presence of a specific kind of randomness with a limited number of samples," Brzycki explains.
However, with the help of a $100 million, 10-year Breakthrough Listen initiative that began in 2016, the researchers possess state-of-the-art hardware capable of this analysis. "A lot of the initial work for Breakthrough Listen was setting up all this hardware, and that eventually enabled new techniques like this," Brzycki says.
One limitation of this approach is that it depends on twinkling due to interstellar material. As such, "the radio signals would have to pass through enough material in the galaxy with the right turbulent conditions in order to exhibit the target effect," Brzycki says.
The best direction to detect this scintillation is primarily the galactic center along the galactic plane. The core of the Milky Way "is already a very interesting target for SETI," Brzycki says. For example, it possesses a very high density of stars, "and therefore more possibilities for extraterrestrial intelligence," he says.
The scientists are now looking for scintillated potential alien signals. "We're well underway in taking radio telescope observations with the Green Bank Telescope towards the galactic plane and galactic center," Brzycki says. In addition, "I've been working on analyzing data as they come in and developing software that would allow other SETI researchers to quickly analyze their data for scintillated signals."
The scientists detailed their findings in July in the Astrophysical Journal.
