The endeavor known as the search for extraterrestrial intelligence (SETI) has long relied on radio telescopes to listen for broadcasts from potential alien callers. Yet in an expansive galaxy such as ours, how can we ever be sure that we have tuned in to the right station?
A new model simulating contact across the Milky Way suggests—perhaps unsurprisingly—that unless our galaxy is dense with long-lived intelligent species, the odds of stumbling across a signal are low. Yet the findings, which were published in the International Journal of Astrobiology, also point out that the probability of interaction could be greatest at the moment when a novel communication technology first comes online.
Along with providing fodder for imaginative scenarios—we flip the switch on some new listening device and, voilà, receive a transmission from E.T.—the results might encourage would-be alien hunters to innovate. Research efforts dedicated to discovering and developing new methods to communicate across cosmic distances may ultimately offer greater chances of making contact than long programs using a single technology.
For Marcelo Lares, the research began with a challenge. An astronomer at the National University of Córdoba in Argentina, Lares ordinarily works on data-rich statistical analyses involving stellar populations, the large-scale structure of the universe and gravitational-wave events.
Thinking about aliens offers no such informational abundance. “We have just one observation, which is that the Earth is the only known planet with life,” Lares says.
Scientific speculations about otherworldly life, intelligence and technology often rely on the Drake equation. This mathematical framework was first written down by astronomer Frank Drake in 1961. It estimates the number of communicating species by looking at the fraction of stars in the galaxy with planets, the percentage of those planets that develop life and the odds that such living creatures will grow curious about, and capable of, making interstellar contact with other beings.
Lares and his collaborators wanted something simpler. Rather than hazard guesses about the unknowns involved in life’s genesis and the development of intelligence and technology, they created a model with essentially three parameters: the moment when communicating species “awaken” and begin broadcasting evidence of their presence, the reach of such signals and the lifetime of any given transmission.
The resulting arrangement places a bunch of nodes—or intelligent message creators—at random throughout the Milky Way, where they sometimes broadcast and sometimes do not. “It’s like a Christmas tree,” says astronomer José Funes of at the Catholic University of Córdoba, who was Lares’s co-author. “You have lights going on and off.”
The team ran more than 150,000 simulations, each time with a different set of assumptions about these basic parameters, to see which scenarios favored interstellar contact. A galaxy full of technological aliens announcing themselves produced far more interactions than one where species were separated by vast distances or great amounts of time.
Such conclusions might not necessarily be shocking. “It’s just a statistical way of saying, ‘If you want to increase your chances of contact, you need greater numbers [of communicators] or have them last a long time,’” says planetary scientist Ravi Kopparapu of NASA’s Goddard Spaceflight Center, who was not involved in the work.
But Lares counters that quantifying our intuitive conceptions with mathematical models can be valuable, if only to serve as a reality check on our basic understanding. The findings set a kind of upper limit on the probability of contact under different circumstances, he adds.
In each case, the simulations showed that the odds of interstellar interaction are by far the largest just at the moment when a species “awakens” and figures out the right way to communicate. That result is because other nodes will have already come online and presumably found one another, essentially creating a large branch of “lit up” Christmas tree lights and increasing the chances of stumbling across this broadcasting network. But if the lights are flashing out of sync with one another or at vastly different times—a situation analogous to using the wrong contact technology or being separated by large time spans—intelligent species might never find one another.
After SETI’s historically preferred contact technology, radio waves, became commonly available in the early part of the 20th century, some discoveries were even initially thought to be alien transmissions. And in the 1960s, British astronomers Jocelyn Bell Burnell and Antony Hewish originally calledthe first detection of a pulsar, a rapidly spinning stellar corpse, LGM-1 for “little green men,” because the source’s pulses seemed too regular to be natural. Yet humanity has slowly been sending out fewer radio emissions over the decades as we have upgraded our technology to wired and fiber optic cables, which has lessened the chances that aliens might stumble across our leaking transmissions.
The new study’s authors see their findings as one possible answer to the Fermi paradox, which asks why we have not found evidence of intelligent aliens, given that in the long history of our galaxy, some technological species could have arisen and sent dispatches of its existence across space by now. The work suggests this absence is not very meaningful—perhaps E.T. is too far away from us in space and time or is just using some calling card that is unknown to us.
At the heart of the research is also an attempt to step away from some of the human-centric biases that tend to plague speculations about alien others. “It’s very difficult to imagine extraterrestrial communication without our anthropomorphic way of thinking,” Funes says. “We need to make an effort to exit from ourselves.”
Kopparapu concurs with this assessment. “Unexpected discoveries come from unexpected sources,” he says. “In our common-knowledge thinking, we are in a box. It is hard for us to accept that there could be something else outside it.”
SETI’s focus on radio waves developed under particular circumstances during a small slice of human history. Though the undertaking has sometimes tried other means to discover intelligent aliens, such as looking for high-powered laser beams or evidence of massive star-encircling artificial structures called Dyson spheres, any search still seemingly remains just as limited by the human imagination as it is by fundamental physics.
Yet looking for something as potentially fantastical as another cosmic culture requires the convergence of many disciplines, including physics, biology and even philosophy, Lares says. The effort to consider more creative messages, such as ones made by neutrinos, gravitational waves or phenomena that science has yet to discover, can help break down our parochial conceptions and give us a fuller understanding of ourselves.
Despite the small odds of contact, Lares is hopeful that attacking the problem in many ways will one day pay off. “I think that a SETI search is a high-risk bet,” he says. “The probability of success is actually very low. But the prize is really very high.”
Quelle: SCIENTIFIC AMERICAN
REANALYSIS OF BREAKTHROUGH LISTEN DATA TO INCLUDE OTHER STELLAR OBJECTS IN THE FIELD PRODUCES MOST COMPREHENSIVE SETI SEARCH TO DATE
Independent team combines existing radio telescope data with new catalogs to search over 200 times more stars than before.
Manchester, UK – September 2, 2020 – Breakthrough Listen (the initiative to find signs of intelligent life in the universe) and the University of Manchester announced today a reanalysis of existing data that represents a new milestone in the search for extraterrestrial intelligence (SETI). SETI scientists search for technosignatures (indicators of technology developed by extraterrestrial intelligence) using cutting-edge instruments at some of the world’s most powerful telescopes. No technosignatures have yet been detected, but as more and more comprehensive searches are carried out, astronomers can place tighter and tighter limits on how many stars in our neighborhood might be home to powerful radio transmitters (and the minds that constructed them).
Breakthrough Listen’s previous strongest limits came from two analyses published in 20171 and 20202 by the Listen science team. Based primarily at the University of California, Berkeley, SETI Research Center, the team is responsible for carrying out Listen’s primary program of observations on the Green Bank Telescope (GBT) in West Virginia and the CSIRO Parkes Radio Telescope in Australia, in addition to other facilities around the globe.
In their earlier papers, the Berkeley team looked for technosignatures in radio data gathered when GBT and Parkes were pointed in the directions of 1327 individual stars. Their search focused on relatively nearby stars (within about 160 light years from our Sun) because less powerful transmitters would become more easily detectable the closer they are to our telescopes. However, as anyone who has looked at images of deep space knows, even small regions of the sky are full of stars at a range of distances from Earth.
When Breakthrough Listen searches for technosignatures coming from a nearby star, it is also sensitive to more powerful potential technosignatures from other stars within the telescope’s beam. Masters student Bart Wlodarczyk-Sroka and his advisor Prof. Michael Garrett at the University of Manchester in the United Kingdom, collaborating with Berkeley SETI Director Dr. Andrew Siemion (who is also a Visiting Professor at Manchester) took advantage of this fact to determine new, more stringent limits on the prevalence of technosignatures, without the need to gather any new telescope data.
Combing through the catalog produced by the European Space Agency’s Gaia spacecraft, which measured the distances to over a billion stars, the researchers recalculated limits on the prevalence of transmitters around additional stars within the GBT and Parkes fields of view. By selecting stars out to much larger distances (up to about 33,000 light years) than the original sample of nearby stars, they were able to expand the number of stars studied from 1327 to 288,315. A small fraction of these additional stars (around 196 in total) were closer than the 160 light year range of the original sample (some of them in binary systems, and some unrelated to the original primary targets). Most, however, were more distant than 160 light years, meaning that only more powerful transmitters could be visible at these increased ranges.
However, the sheer number of stars studied enabled Wlodarczyk-Sroka to place some of the most stringent limits to date on the prevalence of powerful radio transmitters in this region of our Galaxy. In addition, for the first time, the team have been able to do this as a function of stellar type – the extended sample includes not only a wide range of main-sequence stars, but also numerous giant stars and white dwarfs.
Team leader Mike Garrett had always been troubled that SETI searches didn’t usually take into account the many other cosmic objects that fall within the range of sky a telescope is sensitive to, in addition to the main target. According to Garrett, Gaia has changed all that. “Knowing the locations and distances to these additional sources,” he says, “greatly improves our ability to constrain the prevalence of extraterrestrial intelligence in our own galaxy and beyond. We expect future SETI surveys to also make good use of this approach.”
“Our results help to put meaningful limits on the prevalence of transmitters comparable to what we ourselves can build using 21st century technology,” remarked Wlodarczyk-Sroka. “We now know that fewer than one in 1600 stars closer than about 330 light years host transmitters just a few times more powerful than the strongest radar we have here on Earth. Inhabited worlds with much more powerful transmitters than we can currently produce must be rarer still.”
“This work shows the value of combining data from different telescopes,” noted Siemion. “Expanding our observations to cover almost 220 times more stars would have required a significant investment of our telescope time, not to mention the computing resources to perform the analysis. By taking advantage of the fact that we already had radio scans of stars in the background of our primary targets, and by reading their positions and distances from the Gaia catalog, Bart’s analysis has extracted additional information from the existing dataset. Work like this gets us one step closer to the goal of knowing the answer to humanity’s most profound question: Are we alone?”
The paper, “Extending the Breakthrough Listen nearby star survey to other stellar objects in the field”, has been accepted for publication in Monthly Notices of the Royal Astronomical Society. A preprint is available at arxiv.org/pdf/2006.09756.pdf. Supplementary material, including the catalog of Gaia stars used in the analysis, along with an accompanying video and artwork, are available at seti.berkeley.edu/deeper.
- Jodrell Bank Center for Astrophysics: www.jodrellbank.manchester.ac.uk
- Berkeley SETI: seti.berkeley.edu
- Breakthrough Initiatives: breakthroughinitiatives.org
Breakthrough Listen is a scientific program in search for evidence of technological life in the Universe. It aims to survey one million nearby stars, the entire galactic plane and 100 nearby galaxies at a wide range of radio and optical bands.
The Breakthrough Initiatives are a suite of scientific and technological programs, founded by Yuri Milner, investigating life in the Universe. Along with Breakthrough Listen, they include Breakthrough Watch, an optical search for Earth-like planets in the habitable zones of nearby stars; and Breakthrough Starshot, the first significant attempt to design and develop a space probe capable of reaching another star.
Quelle: Breakthrough Initiatives