What is the Fermi Paradox?



Please literally explain it like I’m 5! TIA

Edit- thank you for all the comments and particularly for the links to videos and further info. I will enjoy trawling my way through it all! I’m so glad I asked this question i find it so mind blowingly interesting

In: 4425

Come up with an estimate for how many stars are likely to have habitable planets. Then of those, how many will form life. Then of those, in how many will that life become multicellular, then complex, then sentient, then sapient, then civilized, then industrial, then nuclear, then space-faring, and so on.

Take the combined percentages of all those things, multiply by an estimated number of stars in the entire universe, and according to those estimates, you should come up with something like hundreds of thousands of galactic-level civilizations popping up over time leaving evidence everywhere that alien life exists.

But we have no evidence of any of them existing. That’s the paradox.

edit – and sorry, this was a fairly quick reply that melded the answer with the Drake Equation and I could have gone into a lot better detail. Wasn’t expecting it to blow up

Our solar system is pretty young, and our galaxy is big, so some other intelligent life should have taken over the galaxy by now. We see no evidence of that happening. The most common response is that intelligent life is extremely rare, so it probably hasn’t happened in our galaxy before.

There are so many stars in the universe that we cannot count them. It’s a very very high number that is hard for us to contemplate! Because there are soooo many stars, some scientists think that there just HAS to be other life out there. Even though there are soooo many stars, and we are pretty sure there must be life out there, we have not been able to find evidence or proof other life exists outside of our planet.

The Fermi Paradox can be thought of like this: If there are so many stars, why haven’t we found other life out there in space?

I found Kurzgesagt’s series on the Fermi paradox explained things really well: https://youtu.be/sNhhvQGsMEc

Other responses have gotten the basic framing correct: Our galaxy is large, and much of it is much older than our Solar System. Taking basic wild-ass-guesses at various parameters that model the probability of intelligent life forming in the galaxy, we’re left in a position that it seems likely that it has developed. If the civilizations don’t die out, it ‘should’ be possible to have some form of probe/ship/exploration spread out over the galaxy in something on the order of 100’s of thousands of years, which really isn’t very long in comparison to the age of the galaxy.

We don’t see any evidence of this type of activity at all. This is the ‘paradox’ – it ‘should’ be there, but it isn’t.

Where the Fermi Paradox gets it’s popularity though is in the speculation around “Why don’t we any signs”. There is seemingly endless debate possible. To wit:

– We’re first. despite the age of the galaxy, we’re among the first intelligent civilizations, and nobody has been around long enough to spread.

– We’re rare. Variation on the above – intelligent life just isn’t as common as we might think.

– There is a ‘great filter’ that kills off civilizations before they can propagate across the galaxy.

– The Dark Forest: There is a ‘killer’ civilization that cloaks themselves from view but kills any nascent civilizations to avoid competition. (Or, an alternative version is that everyone is scared of this happening, so everyone is hiding)

i think the Fermi Paradox frequently seems to get more attention than it deserves, largely due to the assumption that spreading across the galaxy is an inevitable action for an advanced civilization. I’m not entirely convinced of this – if FTL travel isn’t possible (and I don’t think it is), then the payback for sending out probes/ships to destinations 1000’s of light years away seems to be effectively zero, and so I don’t see how it’s inevitable. But, there’s no question it generated a lot of lively debate.

For how quickly our technology has progressed and how long the universe has existed for, literally any civilisation that has survived has had the time to fully colonize a significant portion of the Galaxy. But we see nothing, not even a trace. We’ve had civilisation for maybe 4-12k years depending on your definition/sources which is an insanely small fraction of the time the universe has been around. So the paradox is if we got from monkey to space in that amount of time and the universe has been around for millions of times more time, why do we see nothing?

It goes like this

1. Earth has intelligent life on it.
2. Even if intelligent life is extremely rare, the galaxy has hundreds of billions of stars, so there should be other intelligent life elsewhere. Even one other planet with interstellar travel technology is enough.
3. Even at slow speeds, an technological civilization can visit every star in the galaxy in less 50 million years. (The 50 million is actually a bit of an *over*estimate)
4. The galaxy is old enough that there were planets as old as Earth is now at the time the Earth first formed.
5. That’s plenty of time for a technological civilization to get to the solar system and leave evidence of that.
6. There is absolutely no evidence that this has ever happened, so where the hell are they?

There are plenty of answers to the paradox, but they generally fall into assuming that aliens choose to not come to the solar system. Remember that they can visit literally every star in the galaxy, so them not coming here makes us a special case that needs explanation. There’s an idea called the Copernican principle that we should assume we’re average without evidence otherwise.

Alternately, there could be no other life in the galaxy which is odd for two reasons. The first is that life isn’t made out of anything special. You’re pretty much made of methane, ammonia, water, and carbon dioxide linked together in complicated ways, and the ancient Earth was covered in those chemicals. Life also appears at pretty much the earliest time it could, so it seems reasonable to assume that any planet like the early Earth will end up with life like the Earth.

Other folks have explained the *what* question really well. My answer will include my personal preferred *solution*. Other folks will have theirs, or they might certainly be able to provide counterpoint to what I’m about to write.

My personal educated opinion is that life in some form is abundant throughout the galaxy. Intelligent life is rare, but my optimistic side says it’s greater than zero (in addition to us humans).

Assuming that much, my brain has chosen to divide those potential alien civilizations into three logical groups, depending on how their advancement level compares to ours.

The first group are the normal Star Trek-style aliens who are roughly on par with humans technologically (maybe within a century or two). Those aliens would be exceedingly hard to find — our solar system is about 4.5 billion years old, but humans have been using radio for about a century. To find some other civilization in the middle of that equivalent microscopic snapshot would be extremely unlikely. So they can be logically disregarded in any traditional SETI radio search.

The second group are the aliens who are less advanced than we are. They’re the ones who haven’t discovered radio yet. We can also disregard them — if they exist, they’re not talking in ways that we can hear.

That leaves the third group of aliens, who are *more* advanced than we are. The question then becomes, how *much* more advanced? At least on the order of thousands, probably on the order of *millions* of years more advanced. Their data requirements in communication are probably so large, and their data compression needs so extreme, that any transmissions we overhear are probably indistinguishable from background noise if we’re limited to Earth-modern technology.

To find a civilization communicating at that level (assuming they’re even using radio in the first place, as opposed to some more advanced kind of physics we haven’t yet discovered) would be a lot like tapping into a copper wire, looking for Morse Code pulses, and finding Modem static instead.

If all you knew was Morse, would you even recognize the Modem static as intelligent, let alone have any way of deciphering it? Probably “no”, either way.

On Earth it took about a century to graduate from telegraphs to Modems, and Modems themselves are already obsolete even within our lifetime. Add another million years to that development rate and you can start to see the problem.

TLDR: if aliens exist, either they’re not talking, or we haven’t learned how to listen.

In [1950](https://youtu.be/zqzMAnPKa_s) at Los Alamos a scientist was talking with his friends

The conversation had them all fully engrossed about this universe that almost has no end

And as they were all about to return to their study

[Enrico](https://en.m.wikipedia.org/wiki/Enrico_Fermi) suddenly shouted: *Where is everybody?!*

And he sat down and did a few simple calculations

That indicated we should’ve been visited thousands of times

At least based on his estimations

Well, that’s the Fermi paradox, if they’re out there why don’t we hear them talk

And the galaxy just keeps on spinning, with 400 billion stars in it

And I just can’t believe that we could be unique

When there’s so much space in this galaxy

I want Pandora’s box-

-to be open but instead we’re stuck in Fermi’s paradox

That’s a bit “for fun”, but it gets at the idea. For the amount of *stuff*, there *should* be intelligent life, but we’ve never seen any indication of its presence.

1. There are lots of stars
2. Lots of those stars have habitable planets
3. Lots of those planets are much older than our own
4. Why has no one contacted us?

The paradox is that it should be impossible that we have not seen evidence of life in space based on how much we can see.

Additionally, consider that our planet is only 4.5 billion years old, and humans developed within 200,000 years. Consider how our technology has advanced in 100 years. Now ask yourself why planets 10 billion years old or older havent managed to produce a species we can see evidence of in its 25million + more attempts at 200,000 years.

Honestly the
[Kurzgesagt](https://www.youtube.com/watch?v=sNhhvQGsMEc) video will eli5 better than any post probably. Also watch/ read about the great filter for great info on the same topic.

https://waitbutwhy.com/2014/05/fermi-paradox.html This is the best article I’ve come across, and it has really easy to understand pictures (also I just love this site and everything he writes 😆)

I’m surprised that all the answers are talking about size and not time.

The Fermi paradox is that the universe is really old and we’ve only been around for a tiny fraction of it. So there should have been at least one intelligent space going civilization what showed up before us and colonized the whole galaxy in less than 100 million years. But we don’t see any. So why is that?

No-one here is actually explaining it like you’re five, so I’ll try.

Space is very big. There should be aliens everywhere. But we can’t see any. Is it because

-We’re the only ones here?

-We’re the only ones who lived long enough to get smart.

-We haven’t killed ourselves like everyone else yet but we will soon (scary)

-Something else is killing all the aliens and we’re next. (Scarier)

There is a really in depth podcast about this called The End of the World with Josh Clark. I know this is ELI5 but it’s such a good series and definite worth the listen.

Imagine it this way: you wake in your house with your pets. The bird is chirping in its cage, fish are swimming, dog is scratching fleas, cat is stalking a bug, hamster is hamstering, ferrets are getting into your large hydron coll-…. Roomba.There’s even a goat and chicken in the backyard. Life is good. But as you’re standing outside watering your flowers, watching aphids destroy you vegetables, and avoiding a bee you stop and look around, beyond your yard, into the surrounding neighborhood. It’s quiet. There’s no people out. More than that, there’s no animals! No birds or bugs. There’s not even trees or plants or flowers. While your yard and house is teeming with life…. No one is in your neighborhood. Nothing living, exists beyond your lush green lawn. Odd, right?

You can’t really leave your house to explore so you climb to the roof and start looking around. You see more of the same. Lots of rocks. Plenty of sand. But no life… not even decaying houses to show life was once there. It’s untouched barren land as far as you can see. So you build a some robot friends and send them out to explore for you. They head out of your yard… past your block… beyond your neighborhood… far into the city…. And further still. They travel into the country side beyond the city, into the land that borders your city area. And all they send back is more of the same. Cold rocks with no sign of neighbors, no sign of vegetation, no sign it was ever even there. You’ve figured out approximately how large earth is (theoretically) and you know you still have lots of land to cover …. But you really should have come across even a *sign* of life. A foot print. A dry leaf. Animal bones. Feathers. Soil! water! Fossils! Sea shells! An old Nokia! SOMETHING!…

But your farthest reaching robot friends have reported back from beyond your borders…. Nothing.

So you’re standing in your lush green yard with abundant water and animals everywhere and food growing like crazy and this chaotically diverse buzzing-with-life plot of land that you simply woke up on… and it appears to be the only even remotely living thing for miles all around you… even when you find a chunk of land that has all the same variables as your yard (not too warm or cold, weather just right, etc) nothing appears to grow there, not even weeds.


Why is your yard the only speck of green in a world of cold non living rock? Your yard can’t be the only thing on the entire gigantic earth with life on it… can it?

Where are all the neighbors?

So many planet. So many planet that may hold life. But where life?

Enrico Fermi was a Professionally Smart Dude. He was a physicist and one of the dudes who worked on the Manhattan Project, the top secret American program to build the world’s first Atomic Bomb.

Interestingly, an atomic bomb is an **uncontrolled** nuclear reaction. Well, they all thought it would be a good idea, before they produced an UNcontrolled reaction, maybe they should try making a CONTROLLED reaction first, and Fermi was the dude they put in charge of that. He created the Chicago Pile, the first self-sustaining nuclear reactor. He called it a Pile because he was Italian by birth, spoke Italian as his native language, and he thought English was adorable. Like, what is it? I dunno man it’s just a bunch of nuclear stuff in a heap. A pile. A pile of nuclear stuff. He loved English because it could be direct and pragmatic like that.

Anyway Fermi was rare because he was good at making stuff (like the Chicago Pile) but he was also very good at thinking about stuff. There’s an entire class of subatomic particle named after him. The Fermion. Because he was the first dude to figure out how to statistically model the behavior of certain particles.

So Fermi is a professionally smart dude and he’s working at the desert lab where they originally developed the Atomic Bomb. He worked on the bomb in the 40s, they drop the bomb(s) in 1945, a lot of the people working there feel like “job done!” and go home, but Fermi wasn’t ever really bomb-oriented. He was the dude in charge of the first reactor and he, and a lot of the guys who kept working there, were interested in non-destructive uses of nuclear power.

The point is, when dude went to lunch in the cafeteria every day, he was sitting around eating egg salad sandwiches with some of the smartest guys on the planet. They talk about all sorts of stuff, baseball, movies. But something interesting has happened every since they dropped the bomb. People start talking about UFOs.

This wasn’t really a thing before 1945, but now suddenly people think there might be Space Aliens visiting Earth for no obvious reason. And the guys in the cafeteria talk about this.

Nowadays, we know a LOT about how the universe works, but back then, not so much. Still figuring it out. Fermi and those folks were among the first generation to **really** start figuring out the nuts and bolts of how reality works, and we owe much of our current understanding to the scientists of the 20th Century.

For instance, before about 1930, everyone assumed that all the lights you see in the sky are stars, and that “our universe” and “our galaxy” were basically the same thing. One galaxy, lots of stars.

Well Fermi is in the cafeteria with his buddies and it’s been about 20 years since humans have discovered that actually some of those lights in the sky are OTHER GALAXIES WTF?! Like, our galaxy has BILLIONS of stars in it, and it turns out there are literally BILLIONS of other galaxies!

Hang in there, we’re almost done.

One of the foundational assumptions of physics is “The universe is basically the same all over.” Whatever magnets do here on Earth, probably the same thing they’d do on any other planet anywhere else in the universe. Unless we have a reason to believe something’s unusual, we assume it’s not unusual.

Now, we are well aware that something MAY be unusual, but unless we have a reason to believe it IS unusual, we assume it’s not.

So, our solar system. Nine planets. One of them has people on it. Back in the 1950s, we had no evidence of planets orbiting other stars. But that was only because our telescopes weren’t good enough to see things like planets around other stars. Even the closest star is way too far away. And they knew that. They knew their telescopes were shit back then.

These guys knew A: we’ve never seen any planets orbiting other stars but B: that’s because our telescopes are shit. There doesn’t appear to be anything special about our solar system. There are *probably* lots of solar systems out there.” (spoilers, there are!)

So. Our solar system, not unusual. Universe, turns out to be massive with billions of galaxies each with billions of stars. And everyone’s talking about UFOs now because it’s the 50s and they don’t have Fortnite yet.

And one day at lunch, Fermi says… “So where IS everybody??”

That’s it. That’s the paradox. IF there’s nothing unusual about our solar system AND there are literally tens of trillions of solar systems out there…where are all the other people? Why isn’t the sky bursting with radio communication between interstellar civilizations? You know, maybe an actual visit would be a pain in the ass and there’s no reason for an interstellar civilization to know we’re here in the first place, but wouldn’t we be able to pick up there communications??

And it’s been SEVENTY YEARS and we have WAY BETTER tech than anyone back then and still…nothing. Zero.

So, that’s it. That’s the Fermi Paradox. Everyone has an idea about WHY we’ve never heard from anyone, or ever seen anyone. But no one…knows WHY we appear to be alone. Every IDEA you read about why we’ve never heard from anyone…is just a guess. And your guess is as good as theirs.