When it comes to the search for extraterrestrial intelligence (SETI), the biggest question for astronomers is: Where is everybody?
This question is the shortest way of describing the Fermi paradox, which contrasts the likelihood of intelligent life arising in the universe with the fact that so far we haven’t observed any evidence of its existence.
A number of solutions to the Fermi paradox have been proposed over the past 50 years, including the possibility that we’re alone in the universe, and that aliens are talking on a different radio frequency or are using a different mode of communication altogether.
The hard part of the Fermi paradox is getting a grip on how much SETI has actually been completed. For instance, if astronomers listened for signals from every star in the galaxy and didn’t hear anything, it would be more reasonable to conclude that ETs didn’t exist than if only a handful of stars had been examined. Yet far more criteria than merely the number of stars searched factor into understanding how much SETI has been completed, which makes this estimation far more complicated.
In a new paper recently posted to arXiv, researchers from Pennsylvania State University created a tool that helps determine the effectiveness of the search as well as an approximate estimate for how close we are to completing SETI.
The conventional wisdom is that so far, SETI has only searched a infinitesimal number of the stars in the universe for signs of intelligent life. In other words, the reason we haven’t found any intelligent extraterrestrials yet is because we’ve hardly started to look. Still, many critics argue that SETI’s failure to find anything even remotely resembling an intelligent signal suggests ET’s don’t exist or if they do, they’re not talking.
As the Penn State researchers note, however, the ability to determine whether a hypothesized thing does or does not exist requires being able to define the search space where that thing might exist. For example, if I am looking for a needle in a haystack, I can only conclude that the needle doesn’t exist in the haystack after I’ve examined every piece of hay. If I only look at the bottom of the haystack or only examine half the hay, I can’t conclusively say that a needle doesn’t exist.
The same principle is at work with SETI, except instead of looking for needles astronomers are looking for extraterrestrial “techno signatures”—signs of intelligent alien life, which include purposeful radio or laser messages, interstellar spacecraft, changes in a planet’s atmosphere due to industrialization, or mega-construction projects such as Dyson spheres.
The main problem when it comes to SETI is defining parameters so that astronomers know when they’ve exhausted all possibilities for searching for alien life is difficult because many of these parameters are unknown.
First off, the size of the “haystack” is uncertain because the minimum amount of time needed for an intelligent species to evolve is unknown. For example, if intelligence takes at least a billion years to evolve from a primordial soup of inanimate matter, then astronomers could limit their searches to solar systems that are more than one billion years old. Even then, things are not so simple because an intelligent species could hypothetically travel to a younger star system and colonize it.
This brings up another unknown: alien technology. Historically, SETI has mostly limited itself to searching the radio spectrum for extraterrestrial signals and occasionally expanded the search into the optical spectrum to look for extraterrestrial laser pulses. But what if extraterrestrials are communicating with a technology as yet unknown to humans? Radio is only 100 years old and lasters only 50 years old. Could extraterrestrials be using something far more sophisticated to chat like, say, neutrino beams?
Parameters of the SETI haystack or “search space” cannot be known in advance. Each search undertaken by astronomers must choose a limited set of parameters, such as the frequencies scanned, which stars to look at, and when to look at them. In this way, the possible space for aliens to exist in is slightly narrowed with each search.
What is more difficult to determine is how much of the possible search space is covered during a given SETI project, which requires creating a multi-dimensional framework that defines the parameters of possible searches. The Penn State researchers created a dynamic framework, provided as a Python script, that allows researchers to modify the search space by adding or subtracting search dimensions based on the nature of the SETI project.
Jill Tarter, a radio astronomer with the SETI Institute, for instance, has proposed a “nine-dimensional haystack” in which SETI searches range across the three spatial dimensions, time, two polarization dimensions of the radio signal, the central frequency of the radio signal, the sensitivity of receivers, and the way information is encoded in the signal.
Using the nine-dimensional haystack, Tarter has likened all SETI searches to date to extracting a cup of water from all of Earth’s oceans and looking for evidence that fish exist only in that cup.When the Penn State researchers plugged eight parameters into their tool, they found that all the SETI searches to date was more like looking for life in a hot tub-sized sample of ocean water. Even though this is significantly larger than a glass of water, it’s still small in the grand scheme of things and the researchers note that these types of calculations can be useful to rebut the misconception that “SETI can be said to have ‘failed’ to find what it seeks.”
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“We should be careful, however, not to let this result swing the pendulum of public perceptions of SETI too far the other way by suggesting that the SETI haystack is so large that we can never hope to find a needle,” the researchers concluded.
“The whole haystack need only be searched if one needs to prove that there are zero needles—because technological life might spread throughout the galaxy, or because technological species might arise independently in many places, we might expect there to be a great number of needles to be found.”