Benjamin Zuckerman, a researcher at the University of California, has proposed a new approach to the search for extraterrestrial civilizations. He believes that aliens will not transmit signals in all directions, but rather in a narrow, directed beam.
A new approach to the search for extraterrestrial civilizations
When searching for signs of extraterrestrial intelligence, it’s helpful to know exactly what you’re looking for and to do so in the most effective way possible. However, as Benjamin Zuckerman, an astrophysicist and professor emeritus in the Department of Physics and Astronomy at the University of California, Los Angeles (UCLA), writes, this has generally not been done until now.
Zuckerman suggests examining an approach to the search using the electromagnetic spectrum, drawing on astronomical surveys conducted independently of the search for extraterrestrial intelligence (SETI), and proposes an improved search methodology developed based on his research.
Since, by all accounts, there are no alien probes in our Solar System, his methodology leads to the conclusion that “no alien civilization has passed within 100 light-years of Earth over the past few billion years.” His work was published in The Astrophysical Journal.
Limited capabilities for detecting isotropic signals
Most published works (at least those written by humans) have been based on the assumption that extraterrestrial intelligence (ET) would have limited energy resources—the electricity available to extraterrestrials would become increasingly difficult to generate, and its supply would ultimately be limited. It was believed that, to optimize the signal-to-noise ratio on the receiving end, ET would use very narrow bandwidths for both transmission and reception.
That is why people searching for radio signals use algorithms that analyze very narrow frequency bands, spanning just a few hertz. “Even with the use of modern equipment and software, the total bandwidth that has been achieved today amounts to only ‘a tiny fraction of the entire radio and microwave frequency spectrum,’” writes Zuckerman.
In 1964, Soviet astrophysicist Nikolai Kardashev, known for his Kardashev scale—which classifies civilizations based on their energy consumption—proposed the hypothesis that any signal transmitted by extraterrestrials would be isotropic, meaning it would have the same intensity in all directions rather than being directional. In any given direction, an isotropic signal would be weak, which limits its power, and this view has since influenced scientific research within the SETI program.
Why directed signals change everything
Zuckerman puts forward the exact opposite hypothesis—that any extraterrestrial civilizations would choose to send highly directional signals, so limited power would not be a serious problem. In this case, he writes, the most uncertain factor for us will be the signal’s wavelength (or, alternatively, its frequency), so we should investigate a much broader range of the electromagnetic spectrum—not only radio waves, but at least infrared and visible light as well. But this is “very difficult” to do using existing SETI radio search programs.
As Zuckerman writes, for neighboring technological civilizations that engage in targeted communication and make “every technological effort to establish contact,” power limitations—and consequently, filtering through very narrow bandwidths on Earth—are “irrelevant.”
This scenario allows civilizations conducting searches and receiving signals to “accidentally discover an alien transmitter during electromagnetic surveys of the sky conducted for reasons unrelated to SETI.” Any such signals would be calling out to us—if we were looking in the right direction—simply by conducting routine astronomical observations with modest telescopes.
Narrowing the search to stars similar to the Sun
Zuckerman took into account five relevant observational parameters: distance, direction, sensitivity (power received per square meter per hertz), wavelength (or frequency), and time.
He suggested that any extraterrestrial life discovered would, like ours, be based on water; therefore, their planet must be located in the habitable zone around its star, where liquid water exists on at least part of its surface. The planet’s star must be similar to the Sun; only stars with a mass less than 1.25 times that of the Sun live long enough (4.5 billion years) for such a technological civilization to evolve on the planet.
According to Zuckerman’s estimates, a targeted SETI search program would likely need to observe 300,000 stars within a radius of 200 parsecs (650 light-years) to cover all the old stars—those that have existed in the Earth’s atmosphere over the past two billion years, during which oxygen has existed in Earth’s atmosphere as a result of the Great Oxidation Event—about two million such stars have passed within 100 light-years of our solar system.
Based on many such astronomical, astrophysical, and SETI considerations, and given that many other all-sky surveys of stars in the radio and optical spectrum could detect continuous, strong signals deliberately transmitted (via highly directional transmissions) with the aim of attracting attention, Zuckerman arrives at the aforementioned limit: “No alien civilization has passed within 100 light-years of Earth over the past few billion years.”
Need for further research
Although some parts of the radio and infrared bands have not yet been explored, according to Zuckerman, a broad-band study of old, Sun-like stars—from radio frequencies to the visible spectrum—is needed. This will allow us to expand the scope of the search and, for the first time, obtain a numerical estimate of the number of extraterrestrial civilizations capable of communication. 100,000, or perhaps even 10,000.
Zuckerman said he had spoken with some of SETI’s leading astronomers, and none of them had criticized him. The same applies to other astronomers who are not SETI experts but are sufficiently qualified to evaluate its work and claims. But, he adds, “although the jury has not yet reached a decision, various pieces of evidence suggest that the verdict will be this: we are alone in our little corner of the Milky Way galaxy.”
According to phys.org
