Even on the clearest, darkest night far from city lights, you can see only about 1% of the Milky Way galaxy's 100 billion to 400 billion stars.
Here's the real trip though: For every star in the Milky Way, there's a unique galaxy drifting through the universe, each with its own billions of stars, and approximately one planet orbiting each of those stars. That's billions and billions and billions of worlds.
And yet decades' worth of missions by Search for Extraterrestrial Intelligence (SETI), an organization which listens for signs of life in space, have come up completely empty handed. Every. Single. One.
Physicist Enrico Fermi is famous for posing the natural question that follows: Where is everybody? The scale of the universe and basic math tell us alien life must be common, yet there's no evidence for it.
Welcome to the Fermi paradox.
Philosophers, physicists, and astronomers have tried to answer the Fermi paradox since its unofficial inception in 1950. Even Edward Snowden, a digital surveillance expert and former NSA contractor, recently shared his best explanation on StarTalk, astrophysicist Neil deGrasse Tyson's podcast.
These and other answers proposed by experts are deeply unsettling — especially if you spend too much time researching them, like I did.
Keep scrolling down to get a little background on why it's so inconceivable we are utterly alone in the universe, and why it's so spooky we have yet to hear from anyone.
Aliens 101: The Kardashev Scale & Fermi Paradox
Think about how far humanity has progressed in its short 200,000 years of existence. Now consider that our galaxy is roughly 10 billion years old.
If we can go from cave-dwelling hominids to an internet-using and robot-building society in 200,000 years, what could an alien race achieve in 10 billion years?
That's more than enough time for a civilization to develop sophisticated rockets — possibly faster-than-light travel, wormhole technology, or some other kind of cosmic shortcut that would allow them to rapidly colonize the galaxy and beyond.
The Kardashev Scale, created by astrophysicist Nikolai Kardashev, is helpful when considering such technological advancement by a developing civilization. It has three types:
- A Type-I civilization has figured out how to harness all the energy on its planet. Humans are getting close to achieving this, but that's just the first tier.
Type-II civilizations are so intelligent they've figured out how to harness all the energy of their own star — an incomprehensibly larger amount of energy than what's available on one puny planet.
- That's nothing compared to the Type-III civilizations, though. Those have harnessed all the energy available in their galaxy.
You can group the best explanations for the paradox into two distinct categories: one in which aliens don't exist, and we're completely alone in the universe, and one in which aliens do exist, but for some reason we haven't made contact.
Aliens don't exist.
Life on Earth might simply be a freak accident of nature, and it may not exist anywhere else in the entire universe.
This idea is called the rare Earth hypothesis. It suggests a perfect storm of things like Earth's protective magnetosphere, temperature, size, axis tilt, etc., all came together to create a very precise cradle for life to arise. These are the only conditions that life can exist in, and they don't exist anywhere else.
While it's certainly possible, the odds aren't in this idea's favor — the universe is far too vast.
For example, the European Space Agency estimates there's about 1,000,000,000,000,000,000,000,000 (or 1024) stars in the observable universe.
Put another way, roughly 100 Earth-like planets exist for every grain of sand on Earth. Surely one of those would have life on it?
The Great Filter
If Earth isn't the only planet capable of supporting life, and there are definitely no aliens out there, then something grim is going on.
That something is called the great filter, and it's kind of terrifying.
The idea is that before a civilization can make it very far along the Kardashev scale of intelligence, it hits a wall — a filter — and it dies. That's why we haven't heard from anyone: Life regularly evolves to where we're at here on Earth, then some powerful, inevitable force snuffs it out. (Nuclear weapons? Overpopulation? Asteroids? Disease?)
The problem is that there's no way to know where on the timeline of life that the great filter sits. Did we already make it past the filter, or are we are on our way to inevitable doom?
...Or maybe we just haven't heard from anyone yet.
We can also consider the possibility that extraterrestrial life is abundant — but for a number of reasons, we haven't been able to get in touch with any of it.
Aliens do exist, they just visited Earth before intelligent life existed here
Or perhaps aliens seeded life on Earth?Twentieth Century Fox
Humans have only existed for about 200,000 years out of the Earth's 4.5-billion-year existence. That's just a tiny sliver of our planet's timeline.
Aliens might have visited the Earth when is was still a molten ball of lava or a stew of primordial soup.
Or, for all we know, the dinosaurs saw aliens that left long before humans arrived on the scene.
Aliens do exist and the galaxy is colonized, but we live out in the boonies
Whatever happened to "Firefly"?"Firefly"
When Europeans landed in the Americas, it took a long time before they reached some distant Native Americans living far out on the West Coast.
It might be possible that alien civilizations just haven't traveled far enough to stumble upon us yet. Sort of like the remote ice planet Hoth in "Star Wars," or the wild west of space travel, as is portrayed in the TV show "Firefly."
Aliens do exist and they're broadcasting all kinds of signals, but we're so laughably primitive we can't pick them up
Aliens might be actively broadcasting signals and trying to communicate with us, we just might not have the means to detect any of it.
Maybe we can't access the right frequencies. Maybe we don't have the right technologies yet. Maybe other life forms only communicate via telepathy.
Or perhaps super-advanced aliens might exist, and they don't want to blow our minds by visiting us with their incomprehensible technology and intelligence. They're simply waiting until we catch up.
Aliens do exist but they're super-intelligent civilizations prey on lesser civilizations like ours
It's possible we haven't heard from anyone else because they know better to broadcast any signals, or at least encrypt them (as Snowden suggests) — lest one of these killer civilizations pick it up. That's why some experts are vehemently opposed to a Messaging to Extraterrestrial Intelligence (METI) mission: the controversial next step to SETI, where instead of just passively listening, we actively send messages out into the cosmos.
You can take this creepy idea even farther. Maybe the first civilization that managed to achieve super intelligence is the ruler of the universe and it's out there destroying anyone else that gets close to its level.
So unfortunately, once again, we might be on a dangerous path — this time by blasting any and all communications into space.
Aliens are everywhere, but we can't wrap our puny Type-I minds around what they are
If you've seen Interstellar, think about the aliens that exist in a fifth dimension and built a wormhole.
Like a person trying to communicate with a bug, we may not be able to comprehend what aliens are at all. In this scenario we're completely irrelevant to them.
...Or we're completely wrong about all of this, and reality is nothing like what we perceive
We could be living in the Matrix, and we were put here by another super-intelligent race as an experiment. Or maybe we're all just one big computer simulation.
The bottom line is we don't know. The most intelligent people in the world have wildly different opinions about the answer to the Fermi paradox.
Humanity could be completely alone. Or other Type II and Type III lifeforms exist and we're a tiny, insignificant piece of a vast universe full of life. It's humbling, but also oddly comforting to consider that there could be far more to existence than we think there is.