In August 1977, a radio telescope operated by Ohio State University detected a 72-second signal that appeared to originate from the direction of Sagittarius. Astronomer Jerry Ehman discovered the signal when reviewing the data a few days later, famously writing “Wow!” in red ink in the margin of a computer printout.
The Wow! signal, as it’s now known, was never seen again and though it remains a candidate for an intercepted alien radio transmission, it could have been caused by radio interference of some sort on or just above the Earth. No one knows.
But researchers have now come up with a technique that can tell the difference in future observations by analysing subtle changes in a radio beam caused by passage through the interstellar medium. The changes are too small to assess for targets closer than about 10,000 light years, but given the vastness of the Milky Way, it still represents a major step forward.
“I think it’s one of the biggest advances in radio SETI in a long time,” said Andrew Siemion, principal investigator for Breakthrough Listen and director of the Berkeley SETI Research Center (BSRC), which operates the world’s longest-running SETI program.
“It’s the first time where we have a technique that, if we just have one signal, potentially could allow us to intrinsically differentiate it from radio frequency interference. That’s pretty amazing, because if you consider something like the Wow! signal, these are often a one-off.”
If a signal does not repeat, “there’s not a lot we can say about that,” Siemion said. But the new technique, described in a paper published by The Astrophysical Journal, shows the method, with the right instrumentation “capable of recording data at sufficient fidelity such that you could see the effects of the interstellar medium, is incredibly powerful.”
Graduate student and co-author Bryan Brzycki developed a computer algorithm that looks for signal scintillation, that is, slight changes in amplitude over periods of less than a minute, that are caused by passage through the cold plasma of the interstellar medium.
“This implies that we could use a suitably tuned pipeline to unambiguously identify artificial emission from distant sources vis-a-vis terrestrial interference,” said Imke de Pater, UC Berkeley professor emeritus of astronomy and Brzycki’s thesis adviser.
“Further, even if we didn’t use this technique to find a signal, this technique could, in certain cases, confirm a signal originating from a distant source, rather than locally. This work represents the first new method of signal confirmation beyond the spatial re-observation filter in the history of radio SETI.”