When he plugs modern data into the formula, the number he comes up with is 10,000. And that just includes the ones he thinks we can detect, once we have the right tools searching the right places. Researchers examining data from NASA’s Kepler space telescope (including Marcy) announced in 2013 that they believe the Milky Way may harbor billions of planets that bear liquid water — a substance integral to the emergence of living things.
But if the universe is so full of the ingredients for alien life, why haven’t we found any yet? Or, more pertinently, considering how young humans are (100,000 years) compared to the age of the universe (13.8 billion years), why haven’t any aliens found us?
This question is known as the Fermi paradox, named for Italian physicist Enrico Fermi. According to scientific lore, Fermi was sitting around chatting about extraterrestrial life with fellow researchers (you know, as one does) when he asked, “So? Where is everybody?”
In the words of a University of Oregon professor’s lecture on the subject, Fermi reasoned that any civilization with “a modest amount of rocket technology and an immodest amount of imperial incentive” could colonize the galaxy by building artificially intelligent robotic probes that would self-reproduce as they journeyed beyond their home planet. While the distances between habitable planets are much too vast to be traversed in a lifetime, the theoretical robots have had millions or even billions of years to make the journey.
So far, notwithstanding the arguments of Roswell conspiracy theorists, we’ve seen no evidence of those probes. SETI experiments searching for radio signals or other broadcasts from possible alien civilizations have turned up empty.
There are three broad categories of explanations for Fermi’s paradox, each of which contains several of what you might call a “sub” explanation. Those range from reasonable — intelligent life is sending out signals, we just don’t know how to listen — to seemingly absurd — Earth is just a “zoo” built by aliens to hold us for their entertainment. Some come from astronomers and biologists, other from philosophers and economists. Still others seem to have more in common with science fiction than science. All of them are entertaining to think about, at least in the abstract sense.
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1. We really are alone.
This could be true for any number of reasons.
Life, as scientists have learned from repeated attempts to do so in a lab, is difficult to start from scratch. It requires a spontaneous sequence of events that somehow animates simple, non-living organic compounds and organizes them into more complex molecules capable of self-replication. So far, that’s only been known to happen once — when life on Earth began.
It’s possible that habitable planets like Earth are rarer than we think. Even if there are billions in the “Goldilocks zone” situated at the appropriate distance from their suns, maybe gamma rays or asteroid bombardment or other dangers from space prevented life from developing.
In a 1998 essay, George Mason economics professor Robin Hanson proposed the idea of a “great filter” — something “along the path between simple dead stuff and explosive life” that is very difficult or even impossible to move beyond. If the “filter” is somewhere in the early days of life’s beginnings, that would explain why no other planet has proven capable of nurturing life.
The notion that we’re unique in the universe is what paleontologist Peter Ward and astronomer Donald Brownlee call the rare Earth hypothesis. They argue that the presence of complex life on this planet may be a once-in-a-Big-Bang occurrence.
Earthlings may have to resign themselves to the fact that we’re on our own.
2. Life is out there; we just haven’t heard from it.
That said, there’s good reason to believe that life on Earth is not unique.
First of all, we’re pretty sure that water — that essential indicator of conditions for life — is fairly ubiquitous. NASA believes that there are large quantities of water in the atmospheres of at least four other planets in our solar system and ice on countless other celestial bodies. And in April, researchers announced in the journal Nature that they’d found the first evidence of organic molecules in an infant star system.
But if we do have otherworldly company, researchers like Marcy (one of the brains tapped for Hawking’s $100 million search) want to know why we haven’t heard from them.
“The absence of strong radio beacons, television broadcasts, robotic spacecraft, obelisks on the moon — all of those absences add up to give us the suggestion that our galaxy is not teeming with technological life,” Marcy told The Washington Post in February.
Which brings us to sub-explanation number one: The other life that exists isn’t capable of reaching out. This is a popular one.
Earlier this year, NASA’s chief scientist Ellen Stofan boldly predicted that we would find indications of life beyond Earth in the next 10 to 20 years. But what we find won’t be “little green men,” she cautioned at a public panel in April.
“We are talking about little microbes,” she said, according to the Los Angeles Times.
Another sub-explanation is that they are communicating, but either their signals haven’t reached us or they have and we don’t know how to recognize them.
Maybe, as astronomer Carl Sagan once pointed out, human brains don’t work at the right speed to comprehend an alien message — perhaps they work at a much faster pace than us, and their signals are gone in a blip, or maybe they are much, much slower, and their messages arrive at too sluggish a pace to be perceived as anything other than white noise. Or, as theoretical physicist Michio Kaku has proposed, super-intelligent aliens are out there and our brains are just too primitive to perceive them as such.
A third is that previous intelligent civilizations came and went before humans even arrived on the scene. They may have been annihilated by overpopulation or nuclear warfare or dangerous experiments or deadly disease or any number of other horrifying catastrophes. Any of these would suggest that Hanson’s great filter isn’t somewhere in our past, but rather in our future — that most civilizations destroy themselves eventually and we’re inexorably heading toward our doom.
This, as Oxford University philosopher Nick Bostrom has pointed out, is a chilling scenario, and perhaps a reason to hope that Hawking’s search doesn’t turn up remnants of any ancient alien civilizations.
“The silence of the night sky is golden,” Bostrom wrote.
A fourth proposal is that intelligent life is out there; it’s just smart enough to stay silent.
Humans have been cavalierly sending signals into space since before we launched the Golden Record on board the Voyager spacecraft in 1977, but it’s possible that’s a dangerous game to play. Perhaps there are vicious predator civilizations out there eager to feed on naive, idiotic species like ourselves, or even one super-advanced civilization that exterminates all competitors once they reach a certain level of intelligence. This is why many scientists — including Sagan, who orchestrated the Golden Record project — have advised against “active SETI,” or sending messages out into the unknown.
“ETI’s [extraterrestrial intelligence] reaction to a message from Earth cannot presently be known,” reads a petition signed by 28 prominent scientists and thought leaders, including Marcy and SpaceX founder Elon Musk. “We know nothing of ETI’s intentions and capabilities, and it is impossible to predict whether ETI will be benign or hostile.”
“Intentionally signaling other civilizations in the Milky Way Galaxy raises concerns from all the people of Earth, about both the message and the consequences of contact,” the petition concludes. “A worldwide scientific, political and humanitarian discussion must occur before any message is sent.”
3. They came, they saw, they conquered (or, uninterested in us puny humans, left us to our own pathetic devices).
Super-intelligent life may have already visited, but before humans were able to document it. They may even have left signs of their presence (ahem, people who believe that aliens built the pyramids). But they weren’t interested in humans for whatever reason — we’re too dumb, too useless, our planet doesn’t have resources they need — so they left Earth behind.
Or, they are here, totally in control, and we just don’t know it. Maybe we are descendants of aliens — given centuries of scientific evidence otherwise, that’s highly unlikely, but hey, you never know. Or maybe the entire concept of physical colonization is hilariously backward to a more advanced civilization, and they’re getting what they want from Earth in other ways. This could also fit with Michio Kaku’s theory that extraterrestrial intelligence operates on a higher plane than our own, limited consciousness.
Then there’s the unsettling “Zoo hypothesis,” proposed by researcher John Ball.
“Extraterrestrial intelligent life may be almost ubiquitous,” reads the blunt abstract to Ball’s 1973 paper, published in the journal Icarus. “The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.”
A bizarre theory, yes, but it’s a big universe out there, and we’ve only just made it to the edges of our solar system. You never know what’s possible.