By the time you read this, the invasion will be over. You're probably in an underground bunker somewhere, wondering what's going on. It all happened so fast, few people understood the reality of the situation at the time. Aliens came, that much is clear, but from where we don't know. How long they've been planning this is also unclear — years, decades, centuries? They keep demanding that Fred Astaire come out and dance for them, so it's safe to say they've been monitoring our TV broadcasts for some time now.
Our diplomats are trying to convince them that we've got some living dancers who are just as good, maybe even better, but they won't accept any substitutes, and when we explain that Astaire is dead, they just get mad. It seems mortality is a foreign concept for them. They seem genuinely baffled that we keep dying on them every time we catch one of their horrible alien diseases.
Another issue is that they live on sunlight and think eating food is vulgar. In fact, they find us disgusting overall — they're not cool with so much ingestion and excretion and illness and death. They don't seem to think much of our planet either — too damp, they complain, too many clouds. But this is all good news for us, because they're making noises about leaving, probably in a month or two, once they've completed repairs on their flying saucers. With any luck, we'll end up a minor footnote in their history of galactic exploration. So hang tight in that bunker; a good third of our population is still alive, and once the aliens leave, we can start rebuilding our cities.
OK, so obviously none of this has happened, but the question is, why not? That, in a nutshell, is the Fermi Paradox, which isn't technically a paradox at all, but rather a looming question: How come no aliens ever show up around here?
The story goes that in 1950, famous physicist Enrico Fermi was enjoying a pleasant luncheon with some fellow geniuses in the Los Alamos Jet Propulsion Lab cafeteria while idly flipping through a "New Yorker" magazine. Between bites of Waldorf salad (or possibly a fluffernutter sandwich), Fermi pointed to a cartoon of aliens unloading some New York City garbage bins they'd collected from a foray to earth. Casually, Fermi asked, "Where is everybody?"
What he was actually referring to, according to his colleagues, was the question of whether interstellar travel was at all possible [source: Gray]. At the time we hadn't even managed to leave our own atmosphere, and the moon landing was still 19 years off, so it was a fair question. Actually, it still is. We might be talking about sending a manned mission to Mars in the coming decades, but that's child's play next to visiting other solar systems.
Using current rocket technology, we should be able to get to Mars in about six months. By contrast, the nearest star, Proxima Centauri, is 4.25 light years away. That doesn't sound so bad, except that a light year is, as the name suggests, how far light can travel in a year, and even when we hit the turbo boost in our fastest rockets, we're crawling compared to that. Hustling at our top speed, it would take us 73,000 years to go next door, cosmically speaking [source: NASA].
Anyway, that's what Fermi was evidently getting at with his offhand lunchtime remark. But as the years elapsed, his question evolved as it was filtered through other scientists' ideas. In 1975, the astronomer Michael Hart alleged that the reason there aren't any aliens here is because they don't exist. If they did, he reasoned, they inevitably would have colonized the galaxy by now. Then, in 1977, an astrophysicist named David G. Stephenson said that Hart's statement could answer Fermi's question, which he officially dubbed "Fermi's Paradox." The Fermi Paradox as it's known today goes something like this: Our universe could, quite possibly, have billions of Earth-like planets teeming with intelligent life. If that's true, how come we haven't heard or seen a single, solitary speck of evidence of said life? [source: Gray]
Even if Enrico Fermi didn't actually pose this question, it's still an interesting one, and there are loads of possible answers. When the question is asked, usually something called the Drake equation is invoked. In the 1960s, an American astronomer named Frank Drake came up with an equation that would help us calculate how many alien civilizations there might be in our galaxy. The results of the equation can vary according to the numbers you plug in, but, by even the most skeptical estimates, our galaxy alone probably has at least 2 billion habitable planets. By "habitable," astronomers mean planets in the so-called "Goldilocks zone" — not too big, not too small, not too close to their star, not too far away from it, but juuuust right.
Of course, just because they're habitable doesn't necessarily mean they're inhabited. Life might or might not be likely under the right conditions. We just don't know. Let's say it's not, let's say it's extremely rare. In fact, let's say only one-half of 1 percent of suitable orbs feature some kind of life form — that's still 100 million planets!
Of course, the next question is, how many of those potentially life-bearing planets evolve species capable of developing the technology necessary for communication and travel? This is a hotly contested question — are techno-capable species an inevitable outgrowth of evolution? Or are Earth's humans unique? Let's say, for the sake of argument, that the answer lies somewhere in between — species like our own are uncommon but not unlikely. Even if there's only one-half of 1 percent chance of life evolving technologically savvy populations, that would mean there should be 500,000 other civilizations in our galaxy alone. And if you multiply that number by the quantity of galaxies thought to be spinning around in the known universe (about 150 billion) you get a whole lot of smart aliens [source: BBC]. So, as Fermi said, where is everybody?
The Kardashev Scale
Another way of talking about this is to say that if you were to get a giant excavator to pile together every single bit of sand found on our entire planet, you would have to take each grain and multiply it by 10,000 to arrive at the number of stars in the universe. Next, factor in the age of said universe (13.8 billion years) and the relative youth of our planet (4.5 billion years), and it begins to seem extremely improbable that more than a few advanced civilizations haven't popped up here and there over the eons [source: Foley].
That said, as mentioned earlier, to achieve interstellar travel is no walk in the park. For starters, it would require access to vast quantities of power.
A Russian astronomer named Nicolai Kardashev came up with a handy rubric for different types of likely civilizations, catalogued in terms of power usage: types 1, 2 and 3. We're a century or two away from becoming a Type 1, which is a civilization that has advanced enough to be able make use of all of the available power on its planet.
A Type 2 civilization would be able to tap into the power output from its local star. Imagine if we could get an extension cord to the sun! All of our energy needs would solved. Naturally, you can't actually plug into the sun, but maybe we could use something like the Dyson sphere, a theoretical technology that wraps an energy-capture system around the sun and absorbs all of its output.
The astronomy world actually has been thrown into a tizzy by a sun known as Tabby's Star, located in the Cygnus constellation roughly 1,480 light years from here. It appears to have been dimming progressively and quite mysteriously over the years in a unique way. One theory is that an alien civilization is in the process of building a giant Dyson sphere around the star, slowly cutting it off from view [source: Swan]. If Dyson spheres really are popular with Type 2 civilizations, this might explain why we can't hear them; their radio signals never make it past the megastructures they've constructed around themselves.
Moving on, a Type 3 civilization on Kardashev's scale would be able to harness the energy output of an entire galaxy [source: Foley]. A species that has reached that level of sophistication and sheer power would have about as much time for us puny earthlings as we have for dung beetles. Their forms of communication might be completely unrecognizable to us, which would help explain why we can't detect their presence.
The Great Filter
Our solar system is middle-aged, which is an important factor in this discussion because it means that there should be many, many solar systems with Earth-like planets out there that are much older than ours. Many of them could be as much as a billion years older, in fact, implying a colossal head start in the process of evolving a technologically complex civilization. With a billion extra years to play with, you would think they could've harnessed those galactic levels of energy to facilitate interstellar travel. So, again, where are they?
One of the best-known explanations for the notable absence of aliens in these parts is something known as the "great filter." The thought is that there might be some inevitable phenomenon baked into the cosmos that prevents life forms from developing to the point where they can communicate or travel across interstellar distances.
The question then would be, where in the timeline of evolution does the great filter occur? More specifically, and selfishly, where are we in relation to this alleged filter? That depends entirely on the mysterious nature of the filter. If, for instance, the filter happens to be a kind of auto-destruct feature that dictates that civilizations destroy themselves before they reach the necessary point of technological development, then we're doomed.
Taking a rosier view, the filter could occur earlier on the timeline. For the first billion years of its existence life was content to take the form of super-simple prokaryote cells. Perhaps the leap to complex eukaryote cells is the great filter. This would mean that while there might be tons of life out there, it's all just a bunch of uncommunicative prokaryotes [source: Foley].
Alternatively, the filter might be something like gamma-ray bursts, gigantic electromagnetic explosions that might periodically wipe out life forms before they grow into anything interesting. According to this theory, we're fortunate enough to be in period of relative astronomical stability allowing for long-term evolution. And if that's the case, we might be one of many species co-evolving at about the same pace, and we'll all burst onto the interstellar scene around the same time (in roughly 200 years) [source: MIT Technology Review].
Turning gloomy again, it could also be next to impossible for life to occur, in which case we're just an outrageous fluke, a blue-green miracle lost in a vast, otherwise lifeless universe. That, of course, would mean that we're utterly and completely alone and always will be.
Veni, Vidi, Yawn
There's another way to answer the question about the whereabouts of all the aliens, which is to say, they're already here, we just don't realize it. If there really is a civilization, or many civilizations, out there that are a billion or so years older than us, they could very likely move around undetected. Assuming that just because we can't see or hear them with our feeble technology it means that they're not around could be akin to standing on a hillside using semaphore while everybody around us is snapchatting on their smart electronic devices. Just because nobody waves their arms back at us doesn't mean they're not here; it means they're too busy staring at their devices to see us.
Or maybe they do see us, but they're observing us without revealing themselves. We might be, for them, zoological curiosities worthy of scrutiny but not intervention. If they're in "Star Trek" mode, they could be following a Prime Directive protocol, which prohibits them from interfering in the affairs of primitive civilizations such as our own.
Alternatively, they just don't care. We're an inconsequential nothing in a remote corner of an ordinary galaxy. Theoretical physicist Michio Kaku has suggested that we might be akin to an anthill in Peru at the time Pizarro tramped by on his way to subjugate the Incas. In other words? Irrelevant. Along the same lines, they could have already come, seen, gotten a look at our sorry simian state and left. Maybe they showed up a few million years ago, checked out the tortoises and giant ferns and decided to move on?
Or maybe these advanced aliens have transcended mortality and even material existence. They might dwell in some numinous Shangri-La so far removed from the sweaty concerns of our sphere that bothering to communicate with us would be a waste of time so laughably pointless it wouldn't even cross their celestial minds.
Another less benign scenario is that we're nothing more than a holographic simulation, or possibly a game designed by an alien super-intelligence who is currently roaring with laughter at our foibles, or has long since grown bored and walked away, leaving the simulation running. In which case, it's just a matter of time before the off switch cancels us or the batteries run out. Maybe there used to be multiple intelligent life forms in the game, but the other species figured it out and got pulled, which would explain our current solitude.
Lots More Information
Author's Note: How the Fermi Paradox Works
Some people are bent on sending out signals to let other intelligent life forms know we're here. Skeptics liken this to walking through a jungle full of potential predators all the while shouting, "I'm over here!" Maybe we should keep a low profile until we get a better sense of who is out there and whether they're into pillaging and/or annihilation. But we're already emitting signals at a constant rate, so we might as well let the universe know that we've got more on our minds than what our sitcoms suggest. Otherwise, annihilation might be the obvious response.
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Becker, Kate. "Holograms, Black Holes, and the Nature of the Universe." PBS. Nov. 15, 2011. (Dec. 7, 2016) http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/holograms-black-holes-and-the-nature-of-the-universe/
Circovic, M.M. and Vukotic, B. "Astrobiological phase transition: towards resolution of Fermi's paradox." Origins of life and evolution of the biosphere. Vol. 38, No. 6. Pages 535-47. December 2008. (Dec. 9, 2016) https://www.ncbi.nlm.nih.gov/pubmed/18855114
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Gray, Robert H. "The Fermi Paradox Is Not Fermi's, and It Is Not a Paradox." Scientific American. Jan. 29, 2016. (Dec. 7, 2016) https://blogs.scientificamerican.com/guest-blog/the-fermi-paradox-is-not-fermi-s-and-it-is-not-a-paradox/
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