Earth can be a really loud place if you’re searching for an extraterrestrial signal. Television broadcasts, radio transmissions, cell phones - we’re beaming noisy signatures of our technology into space all the time. But we’re not the only noise maker in the universe. Black holes crashing into each other, fast radio bursts, and other celestial phenomenon create lots of astronomical background noise. So if there are extraterrestrials out there, how do we find the signal through all that noise?
"The question of whether or not we're alone in the universe is a question that's been with humanity since you know we stared up at the sky and wondered what was out there. But it's only been in the last about 50 or 60 years that we've had the technology and the understanding of astrophysics in order to really search for these signals."
Andrew Siemion is the director of UC Berkeley SETI Research Center.
"SETI is an acronym, it stands for the Search for Extraterrestrial Intelligence."
To find a needle in our galactic haystack, SETI uses a network of radio telescopes to search for alien technosignatures. Which would look like a specific, purposeful signal.
"Technology exhibits a very high degree of what we call coherence which means that ultimately electrons usually move in a very regular way through some piece of technology. That means a signal from an extraterrestrial would be consistent and distinct. That property of coherence is fundamentally the way in which we tell the difference between a natural astrophysical object and an example of technology. When SETI first started we could only search maybe 100 radio channels, maybe 200 radio channels and we had to tune by hand. Now today 60 years on we can search 10 billion radio channels and we can search them instantaneously."
With recent advances in machine learning & artificial intelligence, SETI experiments can look for a wider class of signals, and helping sift the data. This is one of the biggest questions in SETI, what part of the electromagnetic spectrum should we search.
"We have radio light followed by infrared light, optical light, ultraviolet light, x-rays and gamma rays. The truth is that we don't know what part is the best, so we operate experiments in the radio and the infrared and the optical."
While we haven’t found a clear sign of extraterrestrial intelligence yet, there was one notable close call that some consider first contact — it’s called the Wow signal.
"The day after an observing run one time, one of the scientists came in and looked at the output from this dot matrix printer and saw some sort of evidence of a signal on there and draw a circle around it and wrote Wow. Today if we saw that signal we would have so much more data on the signal that we could very rapidly identify it as interference."
"By far the biggest challenge in radio SETI is what we call radio frequency interference. Because we use our own technology as an example of what we should be looking for we in fact find many many examples of our own technology and those examples actually pollute the signal that we see especially with radio telescopes. Satellites that are whirling overhead — people with cell phones, microwave ovens, and Wi-Fi routers. One of the most famous examples of radio frequency interference in radio astronomy in the last decade or so actually was ultimately attributed to a microwave oven in the visitor center of the observatory that didn't have the proper shielding around it."
"It's telling the difference between the very very distant probably very very weak signal from an extraterrestrial intelligence and the incredibly numerous nearby examples of technology that are close to the radio telescope that get into the data. So it remains a big challenge, but if we see something, there are kind of very specific tests that we can do and all the signals that we found so far have been ruled out by those tests."
To date, SETI has only a few thousand star systems, out of over 100 billion stars out there. So the failure to find a signal through the noise isn't evidence enough that there isn't alien intelligence out there. At least, not yet.
"Despite 60 years of SETI, 30 or 40 years of astrobiology, 20 years of exoplanet science, sending landers to other bodies in our own solar system — nothing. But, as long as we're able to continue the search, it's only a matter of time."