The year is 1950. Physicist Enrico Fermi is eating lunch with a few colleagues outside Los Alamos National Laboratory in New Mexico. His shirt ripples in a hot desert wind.
He looks up at the sky and reportedly says, “Where is everybody?”
He is talking about space aliens. Known as the Fermi paradox, the question still hasn’t been answered. Despite numerous anecdotal reports, there is no convincing evidence of alien life or technology within our solar system (or, for that matter, in the cosmos at large).
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
The absence of evidence for aliens could be because they don’t exist or because our sampling depth is inadequate to detect them—a bit like declaring the entire ocean free of fish when none appear in a scooped-up bucket of seawater. Sampling depth refers to how thoroughly and keenly we can conduct a search. Fermi’s question is valuable because it narrows the possibilities down to two: either aliens are not present near Earth, or our current search methods are insufficient.
This dichotomy highlights a common challenge in science: determining when our sampling depth is sufficient to detect an effect, especially when that effect is not fully understood. Throughout the 20th century, for instance, astronomers faced this challenge while looking for planets orbiting other stars. Thousands of these exoplanets have since been found, thanks to dedicated surveys using bigger, more sensitive telescopes, but there was once a time when such searches were met with strong skepticism. Optimists posited that the exoplanets were just out of observational reach; pessimists predicted that if they existed at all, exoplanets were well beyond the measure of any conceivable telescope. Similar speculations can be made about most any field of inquiry in which data are sparse—including the search for interstellar spacecraft.
For many of us, the idea of scouring nearby space for interstellar travellers seems unnecessary. The Voyager 2 spacecraft, launched in 1977, took 12 years to reach Neptune. If it had instead been pointed at the closest star, Proxima Centauri, it would have taken about 84,000 years to reach it. Proxima is about four light-years away; the Milky Way is about 100,000 light-years in diameter. In such a vast galaxy, how could we expect an alien starship to overlap with us in both space and time? Perhaps the Fermi paradox is not so paradoxical.
Then again, in terms of cosmic time, our galaxy doesn’t seem quite so vast after all. The Milky Way is around 13 billion years old, while the solar system is about 4.5 billion years old—which implies that many rocky planets in our galaxy are eons older than ours. While technological life is probably exceedingly rare compared with bacterial life, if it arose on even one planet billions of years ago, that early start would offer ample time to develop interstellar probes before humanity appeared. If these probes could self-replicate using local materials to create and launch more probes, they could spread exponentially. Recent simulations indicate that even if such probes were limited to Voyager speeds, they could saturate most of our galaxy within a fraction of its lifespan.
So where are they?
If interstellar expansion is plausible, we owe it to science to reconsider the dichotomy underpinning Fermi’s famous question. As strange as it sounds, we must revisit our sampling depth. What are the chances we could detect an interstellar spacecraft if it were present nearby? Have we overlooked anything?
It wouldn’t be the first time. History is littered with discoveries that were only made because someone reevaluated the boundaries of what was considered “detectable.” In 1546 an Italian doctor named Girolamo Fracastoro proposed the existence of infectious microbes, a century before microbiologist Antonie van Leeuwenhoek’s microscopes allowed for their direct observation. At the time, the prevailing miasma hypothesis posited that diseases were caused by “bad air” or noxious vapors emanating from putrefying flesh. So when Fracastoro presented his idea for small infectious “spores” to his contemporaries, they understandably responded with skepticism. Without the benefit of microscopes—or of germs large enough to be glimpsed with the naked eye—the sampling depth was too shallow, and Fracastoro’s prescient idea was doomed to dismissal.
Such unfortunate tales should give us pause. Have we been too hasty to dismiss the possibility of interstellar spacecraft nearby? Are there limits to our sampling depth that we are not fully aware of?
To help find out, in 2022 NASA commissioned an independent study to determine whether current satellites and surveillance systems have sufficient sampling depth to detect “unidentified anomalous phenomena,” or UAPs (government talk for what could be alien spaceships). The researchers’ conclusions:
NASA’s fleet of Earth-observing satellites collect the most data within the Earth system, yet they typically lack the spatial resolution to detect relatively small objects such as UAP....
Commercial satellite constellations provide imagery at sub- to several-meter spatial resolution, which is well-matched to the typical spatial scales of known UAP.... The limitation on this data is that at any given time most of the Earth’s surface is not covered by commercial satellites at high resolution—for a particular UAP event, we will need to be fortunate to obtain high-resolution observations from space.
It seems that Earth’s atmosphere is unintuitively large, just as microorganisms are unintuitively small. While the atmosphere is so transparent and so close, we do not have a complete grasp of everything inside it. Consider that the average depth of Earth’s oceans is 2.3 miles, while the atmosphere extends up to about 6,200 miles, where it gradually transitions into space.
If Earth’s atmosphere is truly a rock unturned, why aren’t more astrobiology-minded scientists scrambling to take a peek? (One notable exception is Harvard University astrophysicist Avi Loeb, co-founder of the Galileo Project, an effort to search for and study evidence of UAP activity on or near Earth.) Just as Fracastoro’s colleagues made a series of assumptions about the nature of disease, so has the scientific community made a series of assumptions about technological species. Foremost among these is the idea that alien spacecraft in the solar system or cosmic civilizations percolating through the Milky Way would emit unmistakably prominent signals. This notion encourages us to look for displays of cosmic technological might that could be considered absurdly wasteful and impractical. In turn, it discourages us from seeking out quieter, more subtle forms of alien technology, even though they may be more common.
From our privileged position in history, we know that advances in energy use often come with increases in efficiency, not simply increases in size or expansiveness. Think of the modern miniaturization of smartphones versus the mid-20th-century trend of computers that filled up whole rooms. Perhaps we should be looking for sophisticated and compact alien spacecraft, rather than motherships spewing misused energy.
With this in mind, we can imagine going back to 1950 and rephrasing Fermi’s famed lunchtime question.
His shirt ripples in a hot desert wind. He looks up at the sky.
“Where are all the loud, obvious indicators of aliens?” he asks.
When phrased like this, the simplest explanation stands out like a sore thumb. Perhaps aliens don’t leave loud, obvious indicators. Perhaps their vehicles are nearby, and perhaps no one has bothered to check properly—yet.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.