What lies outside the confines of our pale blue dot? Is there anyone else out there in the vast expanse of the universe — mostly void, partially stars — wondering the same thing?

In 1950, physicist Enrico Fermi famously discussed this idea with some colleagues over lunch. They agreed that we likely have some cosmic company. But if this is true, Fermi wondered, where is everybody? 

This question became the seed of the so-called Fermi paradox — the seeming contradiction between the likelihood that other intelligent life exists outside Earth and the fact that we have no evidence of it. It’s been a pivotal consideration for researchers involved in SETI, or the Search for Extraterrestrial Intelligence.

To calculate the likelihood that extraterrestrial civilizations exist, scientists use the Drake equation. According to Fritz Benedict, a senior research scientist at the McDonald Observatory, the equation includes several factors that could determine the number of technological civilizations in the galaxy.

Some of these variables are well-known and agreed upon. Astronomers have estimated there are 20 billion sun-like stars within our galaxy. About one in five have Earth-sized planets within their habitable zones — the surrounding region of a star where water can remain in liquid state. 

But, according to McDonald Observatory research scientist Michael Endl, other variables — such as the likelihood that life can arise given adequate conditions, how frequently life evolves intelligence, and how often intelligent life develops communication technology — are much shakier.

“The astronomical part is pretty well-covered,” Endl said. “Now the biological part has to catch up.”

Estimates of the number of extraterrestrial civilizations that should exist in the Milky Way range from as few as 20 to as many as 50 million. 

The Great Filter hypothesis proposes that one of the steps in the equation is particularly challenging. That’s why researchers don’t find communicating civilizations in every inhabitable planet of every sun-like star. In that case, the value assigned to this variable in the Drake equation would be especially low. 

“If the galaxy isn’t filled with galactic empires of some sort or another, then it must be very difficult to go from star to star,” Benedict said. “So, where does the filter kick in?”

Such a filter could lie in humanity’s past — in the inception of life or intelligence — or in its future. 

“Maybe once a human being has enough energy to obliterate a city, it just happens,” Benedict said. “We have so many ways to not just shoot ourselves in the foot, but shoot ourselves in the head.”

The average lifetime of a civilization is an especially important variable.

“If [the average lifespan] is large, the Milky Way should be teeming with other civilizations,” Endl said. “If [it] is short, then it’s no surprise we haven’t seen anything.”

Anatomically modern humans, for example, have been around for a cosmically insignificant amount of time — about 200,000 years — and started intentionally transmitting radio signals only 130 years ago.

“If you were an alien that visited Earth in any random time, chances are four to one that you would’ve only found microbes,” Endl said.

Many other hypotheses have attempted to crack Fermi’s paradox. Perhaps there is evidence of extraterrestrial civilizations, but the government is hiding it, or maybe visiting aliens were horrendously appalled by the fact that humans are made of meat and bailed, as Terry Bisson’s short story suggests.

Maybe, as observations so far suggest, we are truly alone in the universe.

“Is the absence of evidence evidence of absence?” Endl said. “It’s a dangerous conclusion.”

Endl said someone once compared our search for extraterrestrial civilizations to looking for a pizza restaurant somewhere in the galaxy.

“We always look for the thing that we have at that moment,” Endl said. “In the ’60s, we had radio, and we looked for radio signals. Now we have mostly laser-pulse communication, and we look for laser pulses.”

Most discussions of SETI are heavily contingent on what humans would be doing if they were able to travel intergalactic distances. Some scientists and speculators imagine that this ability would inevitably lead to the colonization of the galaxy. 

“Our main problem is that we are way too human-centered in our approach to SETI,” Endl said. “What is the motivation of an alien civilization anyways?”