For 21 years, between 1999 and 2020, millions of people worldwide loaned UC Berkeley scientists their computers to search for signs of advanced civilizations in our galaxy.
The project — called SETI@home, after the Search for Extraterrestrial Intelligence (SETI) — generated a loyal following eager to participate in one of the most popular crowd-sourced projects in the early days of the internet. They downloaded the SETI@home software to their home computers and allowed it to analyze data recorded at the now-defunct Arecibo Observatory in Puerto Rico to find unusual radio signals from space. All told, these computations produced 12 billion detections — “momentary blips of energy at a particular frequency coming from a particular point in the sky,” according to computer scientist and project co-founder David Anderson.
After 10 years of work, the SETI@home team has now finished analyzing those detections, winnowing them down to about a million “candidate” signals and then to 100 that are worth a second look. They have been pointing China’s Five-hundred-meter Aperture Spherical Telescope, a radio telescope referred to as FAST, at these targets since July, hoping to see the signals again.
Though the FAST data are not yet analyzed, Anderson admits he doesn’t expect to find a signal from ET. But the results of the SETI@home project — presented in two papers published last year in The Astronomical Journal — provide lessons for future searches and point to potential flaws in ongoing searches.
“If we don’t find ET, what we can say is that we established a new sensitivity level. If there were a signal above a certain power, we would have found it,” he said. “Some of our conclusions are that the project didn’t completely work the way we thought it was going to. And we have a long list of things that we would have done differently and that future sky survey projects should do differently.”
According to astronomer and SETI@home project director Eric Korpela, searches like SETI@home will inevitably turn up billions of possible signals. The challenge for researchers is to develop algorithms to cull the spurious signals caused by noise or radio interference without eliminating actual beacons from a distant civilization. Radio frequency interference, or RFI, comes not only from satellites orbiting Earth and scattered throughout the solar system, but from radio and TV broadcasts and even microwave ovens.
“There’s no way that you can do a full investigation of every possible signal that you detect, because doing that still requires a person and eyeballs,” he said. “We have to do a better job of measuring what we’re excluding. Are we throwing out the baby with the bath water? I don’t think we know for most SETI searches, and that is really a lesson for SETI searches everywhere.”
An early photo of some of the SETI@home team, with David Anderson seated in the front. Standing, left to right, are Jeff Cobb, Matt Lebofsky, Eric Korpela and Dan Werthimer. SETI@home
Anderson and Korpela addressed that issue by inserting some 3,000 fake signals — called birdies — into their data pipeline before combing through it to eliminate the RFI and noise. They blinded themselves to the nature of these fake signals, and calculated their sensitivity based on the signal power of the birdies they were able to detect.
Korpela pointed out that nearly all searches today assume a civilization would put lots of power into a narrow frequency band to get the attention of other civilizations, then send information or data through an adjacent broadband frequency. To increase the chances of being detected, the beacon should be around a frequency at which astronomers would be observing the universe, Korpela said — most likely around the radio wavelength of 21 centimeters, which is used to map hydrogen gas in the galaxy.
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