For thousands of years, we’ve cast our gaze toward the stars, opining about what awaits us out there. Our telescopes have scanned the skies in search of life, and, in some instances, we thought we’d discovered it.

In 1877, astronomer Giovanni Schiaparelli discovered a series of deep trenches carved into the Martian surface. He called them “canali.” His discovery was mistranslated, leading people to believe that aliens had constructed a series of canals on the Red Planet.

⚠️ Spoiler: there are no canal-carving aliens on Mars.

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There are a lot of galaxies out there in the universe. We know life exists in ours. In this sprawling universe, the chance that some form of life exists out there on some exoplanet circling a distant star is pretty high. So why haven’t we found them? Or, conversely, why haven’t they found us? Enter the Fermi paradox.

Child-prodigy-turned-physicist Enrico Fermi invented the atomic reactor, sparked the first controlled nuclear chain reaction, and won the Nobel Prize in Physics in 1938. Just 12 years later, he posed the famous question while lunching with colleagues at the Los Alamos National Laboratory: “Where is everybody?”

Our galaxy consists of 400,000,000,000 stars. The Fermi paradox works under the assumption that the number of possible locations where extraterrestrial civilizations might exist—a seemingly infinite number of cosmic hotspots—is high enough that we should have detected at least one of them.

Do You Read Me?

china starts assembling the world's largest telescope
China’s 984-foot-long single-aperture spherical telescope (“FAST”) began scanning the skies in 2020. For decades, scientists have used radio astronomy to search for signs of faraway civilizations.
Barcroft Media//Getty Images

The SETI Institute in Mountain View, California, for example, has set out to explore this very question. The nonprofit has probed the universe for the call signs of intelligent life since it was founded in 1984 by Jill Tarter and Thomas Pierson. (SETI stands for the Search for Extraterrestrial Intelligence).

Researchers like those at the SETI Institute have used radio telescopes like the one at Arecibo Observatory in Puerto Rico that collapsed in 2020, or those at the National Radio Astronomy Observatory in Green Bank, West Virginia, to track down signals. And the hunt is on.

In 2020, China’s newly-built Five-hundred-meter Aperture Spherical radio Telescope (FAST) began poking around the cosmos for signs of extraterrestrial life. NASA also recently funded research on an especially bold plan to construct a radio telescope dish in a crater on the far side of the moon. This giant E.T.-hunting soup bowl, proponents say, could probe farther out into the universe than those on Earth.

But it’s not just radio astronomy that’s doing the heavy lifting. There have also been efforts to physically get the message out there. Both of the Voyager spacecraft have left the solar system and are carrying a shiny gold record etched with snapshots of humanity. And the Breakthrough Initiatives have funded Breakthrough Message, a competition to develop ways to broadcast our existence out into the universe.

The Nitty-Gritty on SETI

this illustration shows hd 189733b, a huge gas giant that orbits very close to its host star hd 189733 the planet's atmosphere is scorching with a temperature of over 1000 degrees celsius, and it rains glass, sideways, in howling 7000 kilometre per hour winds at a distance of 63 light years from us, this turbulent alien world is one of the nearest exoplanets to earth that can be seen crossing the face of its star by observing this planet before, during, and after it disappeared behind its host star during orbit, astronomers were able to deduce that hd 189733b is a deep, azure blue — reminiscent of earth's colour as seen from space
NASA, ESA, M. Kornmesser

The radio astronomer Frank Drake’s ground-breaking SETI experiment, Project Ozma, ran from AprilJuly 1960 and marked the very first attempt to detect radio transmissions from extraterrestrial life. In 1961, he developed the Drake Equation (an alphabet soup of the ages) to explain the probability that life might exist elsewhere in our galaxy.

N = RfpneflfifcL

In the above equation, R is the rate of star formation per year, fp is the fraction of stars with planets, ne is the number of habitable planets per planetary system, flis the fraction of these planets with life, fi is the fraction of life that develops intelligence, fc is the fraction of intelligent civilizations that we can see or contact, and L is the average longevity of detectable civilizations in years.

It’s a lot to digest. We know there are over 400 billion stars in our galaxy alone, 20 billion of which are sun-like. A fifth of those sun-like stars, according to recent estimates, might have planets lying within the habitable zone, where conditions that enable life exist.

But these variables are constantly shifting. Research published in 2020 in the journal Nature Astronomy found, for example, that certain types of bacteria here on Earth could actually survive on a planet with a hydrogen-dominated atmosphere, thus expanding our definition of “habitable.” Still, if just 0.1 percent of the habitable planets in our galaxy harbored alien life, we’d still have one million planets teeming with life.

Now, let’s say that life is more than just a couple of microbes swimming around in a toxic soup. What might a complex civilization look like? According to the Kardashev Scale, developed in 1964 by the Soviet astronomer Nikolai Kardashev, there are three different types of potential civilizations we might come into contact with.

Type I civilizations have mastered all of the energy on their planet. Here on Earth, we’re pretty close to achieving Type I status. Type II civilizations have harnessed the energy of their home star. (Dyson sphere, anyone?) And Type III civilizations control all of the energy within their galaxy.

Given what we know about the Milky Way so far, there might be anywhere between 1,000 and 100,000,000 civilizations lurking in the distant corners of our galaxy who have the ability to “phone home.”

Okay ... But Where Are They?

the bubble nebula, snapped by hubble space telescope
NASA, ESA, and the Hubble Heritage Team (STScI/AURA), F. Summers, G. Bacon, Z. Levay, and L. Frattare (Viz 3D Team, STScI)

Since Fermi proposed the idea in 1950, researchers have spent decades proposing solutions to the Fermi paradox, hoping they can find the best answer as to why aliens haven’t reached out to us yet.

Scientists have put forth a number of bizarre hypotheses about our status in the universe. Some—like the astronomer Frank Tipler, or astrophysicist Michael Hart—have suggested that the solution to the Fermi paradox might be that complex civilizations elsewhere in the universe simply don’t exist.

Others believe that complex extraterrestrial civilizations exist, but blame logistics for their lack of communication. Maybe they’re just too far away or, like us, don’t have the technology yet to travel or communicate across great distances. The astronomers Carl Sagan and William Newman suggested just this in a landmark 1981 paper. Could it be too costly for other worlds to travel to greet us? Or perhaps their methods of communication are too complex for our puny brains to understand.

🛸 Alien Central

One hypothesis, the zoo hypothesis, suggests that our cosmic companions are observing us from a distance like caged animals. Another hypotheses suggests that maybe we’re trapped in some sort of simulation created by these ultra-powerful beings.

A quirky 2018 hypothesis posited by Alexander Berezin and dubbed the First In, Last Out solution suggested that perhaps “the first life that reaches interstellar travel capability necessarily eradicates all competition to fuel its own expansion.” Berezin argues in his paper, which was posted to the preprint site (meaning that it has not been peer-reviewed), that these beings likely wouldn’t notice the consequences of their actions in “the same way a construction crew demolishes an anthill to build real estate because they lack incentive to protect it.”

Others have suggested something even more disconcerting: that maybe they’ve been here the entire time and we just haven’t noticed. Call it the “Men in Black” hypothesis. Perhaps they’ve mastered a way to tap into our consciousness, as stated by the transcension hypothesis.

And then there’s the Great Filter Theory, which suggests that life has had to slip past a series of “filters” in order to evolve. Perhaps we’ve gotten lucky and surpassed the majority of these great filters while other civilizations have not? Or maybe the final filter is just ahead of us.

How’s that for bleak?