For decades, researchers in the Search for Extraterrestrial Intelligence (SETI) have strived to capture narrow-band radio signals. A new study suggests the failure to detect them may be due not to the absence of alien civilizations, but to signal distortion by the stellar environment. Provided by Getty Images Bank
Scientists have long predicted that as long as there are countless stars like the Sun in the universe, intelligent life should exist on other planets, just as it does on Earth. However, for decades, no radio signals from extraterrestrial intelligence have been detected by scientists. A new study suggests this may not be because the signals don’t exist, but rather that the environment around stars, such as stellar winds, could be distorting the signals, preventing existing detection equipment from capturing them.
A research team led by Vishal Gajjar, a researcher at the SETI Institute in the U.S., has established a theoretical framework showing that the interstellar medium surrounding exoplanetary systems significantly distorts narrow-band radio signals—the primary target in the search for alien civilizations. The team announced the results of a simulation of one million nearby stars on the 5th. The study was published in the international academic journal ‘The Astrophysical Journal’.
Signals from alien civilizations are primarily sought through radio telescopes, which look for narrow-band radio signals below 1 hertz (Hz). Narrow-band signals are considered suitable ‘technosignatures’—technical traces of an extraterrestrial civilization—because they are energy-efficient and distinguishable from natural phenomena.
Existing detection algorithms were designed on the premise that the signal maintains a very narrow spectral width. While the effects of stellar activity, such as stellar winds or coronal mass ejections (CMEs), on signal width have been studied within our solar system, this is the first systematic analysis extending to exoplanetary systems.
The research team established a theoretical model to calculate the degree of spectral broadening caused by the interstellar medium, using variables such as stellar wind speed, turbulence intensity, observation frequency, and the geometry of the signal’s path. They validated the model using actual measurement data from carrier signals of solar system probes and applied measured values for Sun-like stars and scaled values for M-dwarf stars. They then conducted a simulated search by randomly varying orbital conditions, stellar types, and interstellar medium environments for one million nearby stars.
The simulation results showed that at a 1 gigahertz (GHz) observation frequency, the signal width exceeded 1 Hz in over 70% of the target star systems and surpassed 10 Hz in more than 30%. In low-frequency observations at 100 megahertz (MHz), over 60% of the star systems showed broadening of more than 100 Hz. M-dwarfs, which account for about 75% of all stars and are particularly active, were especially affected by this broadening.
The probability of a signal intersecting with a CME was low, at less than 3% per hour of observation, but when it did occur, it almost invariably caused additional broadening of over 1000 Hz. This is far beyond the roughly 1 Hz range primarily searched by existing detection equipment, meaning that signals originating from most star systems could be altered beyond detectable limits before reaching Earth.
The research team believes these results partially explain the ‘Great Silence’ phenomenon—the fact that no definitive signal has been detected despite over 60 years of searching for extraterrestrial radio signals.
When a narrow-band radio signal sent by an alien civilization passes through the interstellar medium, its energy is dispersed widely into a bell shape, significantly reducing the signal’s peak intensity. If a 1 Hz signal is broadened to 10 Hz, its peak intensity drops to about 6% of the original. This means that existing detection programs are not designed to filter such distorted signals, so a detection failure does not necessarily mean an alien civilization does not exist.
“The starting point is to acknowledge that the shape of the signal we are looking for may be different from the shape of the signal that actually reaches Earth,” Gajjar said.
doi.org/10.3847/1538-4357/ae3d33
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