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The likeliest reasons why we haven't contacted aliens are deeply unsettling

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 it’s 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 organisation 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 maths 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

Universal Pictures

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 civilisation 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 civilisation. It has three types:

  1. A Type-I civilisation 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.
  2. Type-II civilizations are so intelligent they have 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.
  3. That's nothing compared to the Type-III civilizations, though. Those have harnessed all the energy available in their galaxy.

Any type of civilisation on the Kardashev Scale would be more than capable of colonizing the universe. But we haven't detected any of these civilizations -- and that's the heart of the Fermi Paradox.

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.

Let's start with leading ideas in the former category.

Aliens don't exist #1: The Rare Earth Hypothesis

NASA

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 favour -- 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.

There's no scientific consensus as to how many of those stars might be like our own Sun, and how many may have Earth-like planets orbiting around them. But if you take even the most conservative estimate out there, then about 5% of those 1024 stars are like the Sun. That means there are 500 billion billion other Sun-like stars.

Next, if you take the lowest estimate of how many of those Sun-like stars have an Earth-like planet orbiting it (22%), that means about 100 billion billion other Earth-like planets are out there.

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?

Aliens don't exist #2: The Great Filter

Paramount screencap

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 civilisation 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?

There are three possible scenarios to explain why we're still here.

a) We made it past the great filter.

20th Century Fox

This is the most optimistic interpretation. Maybe we're so clever and rare that we somehow found our way past the Great Filter while all other lifeforms have failed.

There's no way to tell what critical moment in our history may have gotten us past the filter though. It may have happened at very beginning of life. Maybe the great filter is the likelihood that life arises at all in the first place.

Or it could be the jump from simple cells to complex ones. Or the jump from semi-intelligent animals (apes) to intelligent hominids.

c) The great filter is still ahead of us and we're all screwed.

Being in the right place at the right time would take a lot of luck. So if you don't think we're past the great filter, and you don't think we're the first life forms with a shot at making it past the great filter, there's only one remaining conclusion: Humans are toast.

A cataclysmic natural disaster, such as a huge asteroid impact or gamma ray burst, could obliterate the planet. Or we could invent some kind of super-advanced tech that will destroy us all.

That's why some scientists and philosophers are praying that we don't discover even simple microbial life somewhere like Mars or Europa. If we find life elsewhere, that will mean that it's much more likely that the great filter is still ahead of us and we're headed for disaster.

'So this is why I conclude that the silence in the night sky is golden, and why, in the search for extraterrestrial life, no news is good news,' Oxford philosophy professor Nick Bostrom writes on his website. 'It promises a potentially great future for humanity.'

...Or maybe we just haven't heard from anyone yet.

Warner Bros.

Let's now move past that depressing 'great filter' idea of inevitable doom.

Instead, we can 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 #1: They just visited Earth before intelligent life existed here

Twentieth Century Fox
Or perhaps aliens seeded life on Earth?

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 might have seen some crazy aliens that left long before humans arrived on the scene.

Aliens do exist #2: The galaxy is colonised but we live out in the boonies

Whatever happened to '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 travelled 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 #3: Other civilizations live in a utopia and can't be bothered by us

http://www.koryogroup.com/

Intelligent alien life may have discovered a way to upload their minds into a completely self-sustaining virtual reality world where they don't need access to outside resources or energy.

They're perfectly content in the world they created, and not only completely unconcerned about getting in touch with us but reticent to do so.

Aliens do exist #4: Super-intelligent civilizations prey on lesser civilizations like ours

Runwayjam

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 civilisation 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 do exist #5: Aliens are 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, treating us as an exhibit at a zoo -- look but don't touch...

Aliens do exist #6: They're everywhere, but we can't wrap our puny Type-I minds around what they are

Spinning black hole in 'Interstellar'

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.

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