Only a small fraction of space has been searched for aliens

How little? A volume equivalent to a hot tub’s worth of the Earth’s oceans

The Arecibo radio telescope in Puerto Rico (here) has been used for decades in the search for extraterrestrial intelligence (SETI). But these efforts have searched only a very tiny portion of the skies, new calculations suggest.

Courtesy NAIC-Arecibo Observatory, an NSF facility

Scientists have been searching for signals of aliens for 60 years. So far, they’ve had no luck in finding any. So you’d be forgiven for thinking, “Where is everyone?”

In fact, the lack of success may just reflect not looking widely enough.

A new calculation shows that if space is an ocean, we’ve barely dipped in a toe. The volume of observable space combed so far for E.T. is comparable to searching a volume the size of a large hot tub for evidence of fish in Earth’s oceans. That’s according to Jason Wright and his colleagues. Wright is an astronomer at Pennsylvania State University in University Park. His team presented its calculations in a paper posted online September 19 at arXiv.org.

“If you looked at a random hot tub’s worth of water in the ocean, you wouldn’t always expect a fish,” Wright says.

Still, that’s far more space searched than had been calculated back in 2010. That estimate had been done for the 50th anniversary of the search for extraterrestrial intelligence, or SETI.

SETI programs have searched for messages from aliens in radio waves. This is the same range of frequencies over which music and news are sent to your car. But it’s also the range that scientists think is easiest to monitor for communication from across the universe.

In the 2010 work, SETI pioneer Jill Tarter and her colleagues imagined a “cosmic haystack.” It was made not of hay, but of naturally occurring radio waves. The researchers considered what it would be like to sift through that haystack for the proverbial needle. That needle would be an artificial, alien beacon. Her haystack went beyond physical space. It also included factors such as a possible signal’s duration, frequency, variations and strength. And it accounted for the sensitivity of radio telescopes on Earth that would attempt to detect a signal.

Tarter’s group concluded that searches had covered about a drinking glass’s worth of seawater. That would hardly be enough to conclude an ocean is fishless.

Wright worked with his colleagues Shubham Kanodia and Emily Lubar to update Tarter’s calculation. They devised a slightly different haystack. It included factors like the frequency and bandwidth aliens might broadcast in. It also included more recent SETI searches such as the Breakthrough Listen project. It’s the largest project yet to search for signs of alien life.

The researchers converted the volume of space to liters of ocean for the sake of analogy. And SETI has covered the equivalent of 7,700 liters out of 1.335 billion trillion liters of water in Earth’s oceans, they now conclude. That’s like taking a hot-tub’s worth of water out of the ocean. You might find a fish in there, but if you don’t, it would not be all that surprising.

“We’re finally getting to the point today … that we have a chance of finding something, depending on how much there is to find,” Wright says. So there’s no point in giving up the search yet.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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