The observable universe is about 90,000,000,000 light years in diameter.

There are at least 1,000,000,000 galaxies

Each with 100,000,000,000 to 1,000,000,000,000 stars.

Recently, we've learned that planets are very common too

And there are probably trillions and trillions of habitable planets in the universe

Which means there should be lot of opportunities for life to develop and exist, right?

But where is it?

Shouldn't the universe be teeming with spaceships?

Let's take a step back.

Even if there are aliens civilisations in other galaxies,

there is no way we'll ever know about them.

Basically, everything outside of our direct galactic neighborhood,

the so called, "Local Group" is pretty much out of our reach forever,

because of the expension of the universe.

Even if we had really fast spaceships

it would literally take billions of years to reach these places,

travelling throught the emptiest areas in the universe.

So, let's focus on the Milky Way.

The Milky Way is our home galaxy,

it consists of up to 4 hundred billions stars.

That's a lot of stars, roughly 10 thousands for every grain of sand on earth.

There are about 20 billions sun-like stars in the Milky Way

and estimates suggest that a fivth of them have an earth-sized planet in its habitable zone,

the area with conditions that enable life to exist.

If only 0.1% of those planets harbored life, there would be 1 million planets with life in the Milky Way.

But wait, there's more.

The Milky Way is about 13 billion years old. In the beginning, it would not have been a good place for life

because things exploded a lot, but after 1 to 2 billion years, the first habitable planets were born.

Earth is only 4 billions years old, so there have probably been trillions of chances for life to develop

on other planets in the past.

If only a single one of them had developed into a space travelling super civilization we would have noticed by now.

What would such a civilization look like?

There are 3 categories.

A Type 1 civilization would be able to access the whole energy available on its planet.

In case you are wondering, we are currently around 0.73 on the scale

and we should reach Type 1 sometime in the couple hundred of years.

Type II would be a civilization capable of harnessing all of the energy of its home star.

This would require some serious science fiction, but it is doable in principle.

Concepts like the Dyson sphere, a giant complex surrounding the Sun would be conceivable.

Type III is the civilization that basically controls its whole galaxy and its energy

an alien race this advanced would probably be godlike to us.

But why should we be able to see such an alien civilization in the first place?

If we were to build generations of spaceships that could sustain a population for around one thousand years

we could colonize the galaxy in 2 million years.

Sounds like a long time, but remember, the Milky Way is huge.

So, if it takes a couple of million years to colonize the entire galaxy

and there are possibly millions if not billions of planets that sustain life in the Milky Way

and these other life forms have had considerably more time than we've had, then

where are all the aliens?

This is the Fermi Paradox, and nobody has an answer to it

But we do have some ideas.

Let's talk about filters.

A filter in this context represents a barrier that is really hard for life to overcome.

They come in various degrees of scary.

One: There are Great Filters and we've passed them.

Maybe it is way harder for complex life to develop than we think.

The process allowing life to begin hasn't yet been completely figured out

and the conditions required may be really complicated.

Maybe in the past the Universe was way more hostile, and only recently things have cooled down to make complex life possible

This would also mean that we may be unique, or at least one of the first, if not the first civilization in the entire Universe.

Two: There are Great Filters and they are ahead of us.

This one would be really really bad.

Maybe life on our level exists everywhere in the Universe but it gets destroyed when it reaches a certain point, a point that lies ahead of us.

For example, awesome future technology exists, but when activated, it destroys the planet.

The last words of every advanced civilization would be

"This new device will solve all of our problems once I push this button."

If this is true, then we are closer to the end than to the beginning of human existence.

Or maybe there is an ancient Type III civilization that monitors the Universe

and once a civilization is advanced enough it gets eliminated, in an instant.

Maybe there is something out there that it would be better not to discover.

There is no way for us to know.

One final thought: maybe we are alone.

Right now, we have no evidence that there's any life besides us.

Nothing. The Universe appears to be empty and dead.

No one sending us messages no one answering our calls.

We may be completely alone, trapped on a tiny moist mud ball in an eternal Universe.

Does that thought scare you? If it does, you are having the correct emotional reaction.

If we let life on this planet die, perhaps there would be no life left in the Universe. Life would be gone, maybe forever.

If this is the case, we just have to venture to the stars and become the first Type III civilization

to keep the delicate flame of life existing and to spread it until the Universe breathes its final breath

and vanishes into oblivion.

The Universe is too beautiful not to be experienced by someone.

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