The physicist's simple question is enshrined now as Fermi's paradox — one of the two coolest paradoxes of all time, along with the crocodile paradox — and it continues to puzzle scientists to this day. Indeed, the mystery has deepened considerably over the years. For one thing, we're not just talking about the lack of visitation anymore. In 1960, 6 years after Fermi's death, astronomer Frank Drake pointed a radio telescope at West Virginia's Green Bank Observatory at the nearby sun-like stars Tau Ceti and Epsilon Eridani, kicking off the search for extraterrestrial intelligence (SETI).Nearly 60 years later, SETI scientists are still hunting for the first confirmed peep from ET. 

Then there's the exoplanet revolution. Alien worlds were purely hypothetical objects in Fermi's day and for decades afterward; scientists didn't announce the first confirmed detection of a planet beyond the solar system until 1992. But in the last decade or so, NASA's Kepler space telescope and other instruments have revealed that the cosmos is teeming with possibly life-supporting worlds. Kepler's discoveries suggest that about 20 percent of the Milky Way's sun-like stars host an Earth-sized world in the "habitable zone" — that just-right range of orbital distances that would allow you to walk around in flip-flops pretty much year-round. The proportion appears to be similar for red dwarfs, the small, dim stars that dominate our galaxy. (About 75 percent of Milky Way stars are red dwarfs, whereas just 10 percent or so are similar to our sun.) 

"There's a lot of real estate out there, and we now know that," said radio astronomer Jill Tarter, who co-founded the SETI Institute in Mountain View, California, and served as the inspiration for Ellie Arroway, the lead character in Carl Sagan's novel Contact and the movie based on it. 

Not all of this real estate is way out in the boonies, either. The sun's nearest neighbor, the red dwarf Proxima Centauri, hosts an Earth-size planet in the habitable zone. Seven rocky planets circle the dwarf star TRAPPIST-1, which isn't much farther away from us in the cosmic scheme of things — and three of those worlds may be able to support life as we know it. (Proxima Centauri and TRAPPIST-1 lie about 4.2 light-years and 39 light-years from Earth, respectively. The entire Milky Way is about 100,000 light-years wide.) [The 7 Earth-Sized Planets of TRAPPIST-1 in Pictures]

So, again: where is everybody? Nobody knows. The Fermi paradox is tougher than a Brazil nut, and scientists haven't cracked it yet. But it's not for lack of trying. They've advanced hundreds of hypotheses to explain it. As varied as these ideas are, they all encompass just a few basic possibilities, as physicist Stephen Webb noted in his book If the Universe Is Teeming with Aliens, Where Is Everybody? Let's take a look at each of these three explanation families. 

Possibility 1: What paradox? Intelligent aliens have already messed with us 

You may already have wandered off, irritated or incensed that I put the Fermi paradox on equal footing with the beloved crocodile paradox. Perhaps you're now thumbing through a dog-eared copy of Chariots of the Gods? or watching YouTube clips of that "alien autopsy" TV special that Fox aired in the 1990s. 

Indeed, one possible resolution of the Fermi paradox is that it's no paradox at all, because ET has already journeyed to Earth. Adherents of this explanation often point to UFO sightings and alien abduction stories, topics that you can read about in chapter 10. For our purposes here, suffice it to say that scientists generally don't regard any of these reports as convincing evidence of alien life. (If they did, you definitely would have heard about it.) 

There are more subtle possibilities in play as well. For example, what if ET came to our planet long ago, before people were around to be probed? Unless the voyaging aliens were particularly interested in us, this is much more likely than a documented visit, given that our species has existed for just the last 200,000 years of Earth's 4.5-billion-year history and has been capable of capturing encounters on blurry, low-light video for only a few decades. 

Let's indulge in some wild speculation, because it's fun! Say Earth has been colonized many times over the eons by greedy, grabby alien civilizations, each of which ground the planet's native species into the dust in the process. (Don't get too high and mighty: pioneering humans have tended to wreak ecological havoc as we've explored the globe.) As astrophysicist and sci-fi author David Brin has pointed out, a history of such oppression could explain why it took intelligent life so long to arise on our planet as well as the radio silence in our galactic neighborhood. Maybe Earth is the only planet for light-years around to have recovered from the ravages of invasion. 

If you squint a little, this scenario lines up with the five mass extinctions that scientists have identified in the fossil record. These great purges occurred about 450 million years ago, 375 million years ago, 251 million years ago, 200 million years ago, and, most famously, 66 million years ago, when an asteroid strike wiped out three-quarters of all Earth's species, including the dinosaurs. "It may not be preposterous," Brin wrote in a seminal 1983 paper, to compare the intervals between these extinction events and the time it might take for different waves of invasion to wash over Earth. The dino-killing asteroid could even have been a weapon of war, slung by a space-dwelling alien faction with a beef against their brethren on Earth. 

Brin didn't mean to suggest that any of this actually happened, and neither do I. There's no evidence that it did — no spacecraft entombed in ancient amber, no ruins of a 200-million-year-old city — and I certainly wouldn't put any money on it. But it's possible. [5 Bold Claims of Alien Life]

Possibility 2: They're out there, but we haven't found them yet 

As scientists and other logically minded people often point out, absence of evidence isn't evidence of absence. It's entirely possible that intelligent aliens are (or were) out there, and we just haven't spotted any signs of them yet. 

For example, maybe ET hasn't visited Earth because getting here is just too hard. The distances involved in any interstellar trek are mind-boggling. Proxima Centauri is "just" 4.2 light-years from the sun. But that's almost 25 trillion miles — equivalent to circling Earth 1 billion times, going to Pluto and back 3,450 times, or jogging around the track at your local high school 100 trillion times. It would take a spacecraft about 75,000 years to get to Proxima Centauri using today's rockets. 

There aren't enough honey-roasted peanuts and Sudoku books on Earth to make that trip bearable. Even if we assume that aliens, with their pulsating and extravagantly veined brains, have developed super-fast propulsion tech that puts our puny human gear to shame, there's still a big problem: energy. Say the aliens, like Starfleet engineers, know how to build matter-antimatter engines that can accelerate a ship to 75 percent the speed of light. Just making an Earth-Proxima Centauri round trip with this craft would require 100,000 times more energy than the United States uses in an entire year, physicist Lawrence Krauss wrote in his book The Physics of Star Trek. Is the aliens' desire to probe us, or to give the ancient Egyptians some killer pyramid blueprints, really that strong? 

Or maybe ET just doesn't want to interfere with the development of life on other worlds — and has hewed to this noble "prime directive" far more successfully than Captain Kirk and his crew have managed to do in the Star Trek universe. (Remember when the Enterprise gang took it upon themselves to destroy the machine-god Vaal in an original series episode? Vaal seemed like a jerk, but still.) It's even possible that aliens are watching us right now, to monitor our technological progress, figure out how we tick, or keep their bratty kids occupied for a few hours. 

Some thinkers take such reasoning a step further, suggesting that we and everything else in the observable universe — yes, even love — may be part of a simulation run on a very fancy alien computer. Before laughing this off, consider how much cooler Fortniteis than Burger Time. Those two games were released just 35 years apart, and the hypothetical aliens have had billions of years to come up with amazing graphics and compelling yet believable storylines. Indeed, philosopher Nick Bostrom has argued that the odds we're trapped in a Matrix-style pseudo-existence are actually quite high — provided there are a decent number of super-advanced civilizations out there and at least some of them are keen to create convincing virtual worlds, for fun or profit. Given these two assumptions, the number of artificially created universes, or patches of universe, will far outstrip the number of real ones, according to this line of thinking. 

Along similar lines, perhaps ET's technical mastery has driven its focus away from the real world and into the virtual, sapping its desire to explore the cosmos or meet any potential neighbors. (Humanity may well succumb to this fate when high-quality virtual reality porn hits the marketplace.) 

There are other reasons why advanced aliens may be keeping their heads down as well. Self-preservation springs to mind: what if they're trying to avoid being destroyed or enslaved by big-time cosmic jerks, like the Borg from Star Trek or the Galactic Empire in Star Wars? Scientists have even suggested that evil aliens may have sent fleets of intelligent, self-replicating "berserker" probes out into the galaxy to hunt for radio transmissions and other signs of intelligent life — and to exterminate any civilizations they find. [The Evolution of 'Star Trek' (Infographic)]

Extinction is another possibility. Maybe those berserkers have done a lot of exterminating over the eons. Or perhaps alien civilizations tend to off themselves in relatively short order. Humanity has come perilously close to a nuclear holocaust several times, after all, and we've recently spurred a global mass extinction that may end up claiming our species as well. And yet, with all that, we've been capable of sending signals to other stars for only a century or so. 

If a 100-year messaging life span is the norm for civilizations, "then it's as if there are two fireflies that each flick on once during the course of a long night," said Douglas Vakoch, president of METI International, a San Francisco-based nonprofit dedicated to astrobiology and SETI research. (METI stands for messaging extraterrestrial intelligence — the controversial notion that humanity should reach out to potential alien civilizations, rather than just passively listen.) 

The odds that these cosmic fireflies will flash at the same time are, of course, not good. That's sad for them, and sad for any giant space monsters that want to catch them and put them in jars. 

It's also possible that ET is trying to get our attention, and we just haven't noticed yet. After all, humanity has been searching for alien transmissions for less than 60 years — the last 0.000001 percent of Earth's history — and always on a shoestring budget. 

How shoestring? Well, the US government hasn't bankrolled a SETI operation for a quarter-century. NASA began an ambitious observing project in 1992 but had to stop a year later when Congress cut off the money. (The leader of the defunding push, Nevada senator Richard Bryan, painted the SETI effort as a Mars safari for some reason. "The Great Martian Chase may finally come to an end," Bryan said in 1993. "As of today, millions have been spent and we have yet to bag a single little green fellow. Not a single Martian has said, ‘Take me to your leader,' and not a single flying saucer has applied for FAA approval.") 

The SETI Institute and other such groups generally rely on private donations to keep the lights on and the telescopes listening. These donations don't always come through. The SETI Institute had to idle its main ear to the universe, the forty-two-dish Allen Telescope Array in Northern California, for four months in 2011, and the original plan called for the ATA to consist of 350 telescopes, but there hasn't been enough cash to complete the build. 

Given this situation and the huge scale of the Milky Way galaxy, scientists have not yet been able to mount a comprehensive SETI survey. They haven't even come close. 

Tarter often relies on an analogy to get this point across: imagine that you're searching for fish across the entirety of Earth's oceans, and you wade into the surf and scoop up a single glass of seawater. "If you did that experiment and your glass didn't contain a fish, you probably would not conclude that there aren't any fish," Tarter said. "Well, numerically, the amount of searching that we've done versus the amount that we might have to do is equivalent to that one glass of ocean." 

We may not even be looking for the right kinds of signals. The SETI search to date has focused heavily on radio waves and to a lesser extent laser-light pulses, because those are technologies that humanity has mastered. But we're already weaning ourselves off radio-wave transmission just a century after inventing it; when's the last time you sharpened your TV's picture by crumpling some tinfoil onto rabbit ears? Would a billion-year-old alien civilization really still be communicating like this, or in any way we could understand? Maybe ET sends messages via neutrinos, the bizarre and unfathomably numerous particles that zoom through planets unimpeded like subatomic Houdinis. (Trillions of solar neutrinos passed through your body in the time it took to read that last sentence.) Maybe the aliens are telepathic. Who knows? 

Our current strategy may be akin to trying to eavesdrop on people via walkie-talkie, according to astrobiologist Dirk Schulze-Makuch, who's a professor at the Technical University of Berlin in Germany and an adjunct professor at Arizona State University and Washington State University. 

"You probably won't get anything, because everyone is on Facebook," Schulze-Makuch said. 

As this discussion shows, many of the ideas bandied about to explain Fermi's paradox basically amount to ET psychology. And that's not the most promising path for a breakthrough: getting inside the heads of super-advanced aliens is beyond us, at least until we stop devoting most of our creative energies to meme generation. (Thank you for indulging this "get off my lawn" moment.) 

Possibility 3: We are alone 

The last alternative is the most depressing: the cosmic silence speaks volumes. 

Maybe Earth is the only inhabited world in the entire galaxy. God loves us that much! Or, if you want to get all science-y about it, the jump from complex organic chemicals to wriggling microbe may be so improbable that it occurred just once, and we hit the jackpot. 

This could be a stretch, given how quickly life got a foothold on Earth. Microbes were here by at least 3.8 billion years ago, and perhaps even earlier; some evidence pushes life's emergence back to 4.1 billion years ago, pretty much as soon as Earth had cooled down enough to be habitable. But even if microbes are common throughout the cosmos, intelligent life could still be vanishingly rare. (Astronomers and astrobiologists do actually crack the obligatory joke from time to time: "Hey, we're still searching for intelligent life on Earth!" or "You won't find it on Capitol Hill!") Why? Well, maybe not many planets can offer the long-term TLC required for complexity and smarts to evolve. For example, Earth boasts a large moon that stabilizes its tilt (and thus its climate), and it enjoys the protection of a giant outer planet (Jupiter) whose powerful gravity nudges some dangerous comets away. Perhaps such characteristics are rare for rocky worlds in the habitable zone. 

Also, forget what those cartoons showing apes marching toward a proud, pants-wearing future may suggest; there's no "arrow of progress" inherent in evolution. Natural selection favors whatever works, so if simple is successful, simple stays simple. Indeed, that was the story for most of Earth's history. Multicellular organisms don't show up in the fossil record until nearly 600 million years ago — meaning single-celled microbes had the planet to themselves for at least 3 billion years. And there was another long gap before super-smart animals — modern humans — came along. 

So it might take a really special set of circumstances to jolt life out of its simple, slimy origins and eventually reach the point where it can invent radio transmitters, spaceships, wheely shoes, and other cool stuff. After all, Earth might still have reptilian overlords if not for that asteroid strike 66 million years ago, which allowed our mammalian ancestors to scurry out from the shadows. [Images: Potentially Dangerous Asteroids]

There are some other important things to keep in mind as well. For example, not all intelligence is the same, as the diversity of life on Earth clearly shows. Chimps, ravens, dolphins, sea otters, octopi, and a number of other species are smart enough to use tools, but only humans have built radio transmitters, spaceships, and wheely shoes. (As far as we know. But if chimps had wheely shoes, you'd think Jane Goodall would've said something.) We can't assume that every intelligent alien species would be technologically smart or able to communicate with us. 

The circumstances of their birth may cut many smart aliens off from the rest of the universe. If our own solar system is any guide, the most common life-supporting worlds in the galaxy may be frigid moons and planets with liquid-water oceans beneath their icy shells — places like Saturn's moon Enceladus and the Jupiter satellite Europa. If complex, intelligent life has evolved in such environments — and that's far from a sure thing, given the likely dearth of energy in those dark depths — we might never hear from it. 

"How long would it take sentient beings, confined to their pitch-dark liquid habitat by a solid sky hundreds of kilometers thick, to discover that there was a vast universe beyond their world's apparently impenetrable roof?" theoretical physicist Paul Davies wrote in The Eerie Silence, his 2010 book about the Fermi paradox. "It is hard to imagine that they would ever ‘break out' of their ice prison and beam radio messages across space." 

Getting an answer 

You've made it through the Fermi Paradox Hypothesis Sampler Platter! Did any of the ideas jump out at you? Perhaps the berserkers, for violence and action, or the buried-ocean dwellers, for poignancy? (I picture sallow, eyeless mercreatures sadly strumming lutes.) If so, that's nice, but you probably shouldn't get too attached. We just don't have enough information at the moment to know what's actually going on. 

"I find it silly that so many people leap to shout, ‘Aha! I know the answer!'" Brin said. "All we can do is catalog them for now, and maybe rank a ‘Top Ten.'" 

But we could start getting at that answer, and soon. Say scientists discover a "second genesis" of microbes — tiny organisms completely unrelated to any kind of life as we know it — on Mars, Enceladus, or another solar system body. We would then know that life is not a super-lucky one-off affair, and we'd strongly suspect that it's widespread throughout the galaxy. This news, combined with a continued SETI silence, would also be troubling for anyone who cares about humanity's future, for it would suggest that the bottleneck limiting the number of intelligent civilizations still lies ahead of us. (There could be a benefit, though: if we think we're the only technologically smart creatures in the galaxy, the resulting sense of responsibility may provide the nudge we need not to destroy ourselves.) 

By the same reasoning, getting even a single SETI ping would be a real pick-me-up. 

"The detection of a signal—even a cosmic dial tone, with no information — goes on to tell us that we can have a long future," Tarter said. "If somebody else made it through, we can, too." 

• You can buy "Out There" at Amazon.com. 
• Excerpted from "OUT THERE: A Scientific Guide to Alien Life, Antimatter, and Human Space Travel (for the Cosmically Curious)." Copyright © 2018 by Michael Wall, PhD. Illustrations by Karl Tate. Reprinted with permission of Grand Central Publishing. All rights reserved. Follow us @Spacedotcom or Facebook. Published on Space.com.