In general, solutions to Fermi’s paradox come down to:

  • Life is difficult to start and to evolve to an intelligent and technologically advanced stage and we’re the only one in the galaxy.
  • Advanced civilizations destroy themselves on short timescales.

As the revised Drake equation solution (N<<1) shows, it seems that hopefuly, the first solution is the answer.

Here is a list of the various assumptions along with notes pointing to related problems and contradictions.

They are here unobserved

They were here and they left material evidence:

UFO’s, alien artifacts, ancient astronauts. Problem: evidence for aliens is non-existent.

Zoo scenario:

The aliens are here and they are keeping us in a well designed zoo (cut off from all contact) or there is an treaty preventing contact with young races. Problem: this scenario can’t be tested. Only one ET could break the embargo.

They are us:

Humans are the descendents of previous alien civilizations. Problem: where are the original aliens? where are all the other alien civilizations?

They exist but…

They didn’t have enough time to reach us:

The speed of light slows communications and makes space travel too long. Extraterrestrial’s messages may not have reached us yet. Problem: the galaxy exist for billions of years, even if one ET civilization formed a few million years before us, the galaxy would be already filled with Bracewell-Von Neumann probes.

They are sending signals, but we do not know how to listen:

The galaxy is filled with killer robots looking for signals. ET is keeping low. Problem: no evidence of berserkers coming after us.

They have no desire to communicate:

ET has no interest in communicating with lesser intelligences. Problem: with millions of possible civilizations, some of the extraterrestrial ones would be interested in our species.


Civilizations only have a limited lifetime, they are all dead.
Dangerous particle physics
Nanobots -> the gray goo problem

They developed different mathematics:

Mathematics is the universal language, but humans may have a unique system of mathematics that ET cannot understand. Problem: then where are those incomprehensible signals?

They do not exist because…

We are the first in the galaxy:

Life is new to the galaxy and evolution takes time. We are the first intelligent civilization. Problem: the Sun being an average star, if other stars formed a million years ahead of us, then they would be a million years ahead of us technologically speaking.

Planets with the right conditions are rare:

Habitable zones, proper orbit for liquid water are rare
The galaxy is a dangerous place (asteroid impacts, gamma-ray bursters, etc)
The Earth/Moon system is unique (large tides needed for molecular evolution)
Life is rare

Life’s genesis is rare:

Intelligence and tool-making is rare
Language is unique mankind
Technology and science is not inevitable




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 35 reviews
 by Julio
Correlation between progress and volatility

There is one factor that is often overlooked in these discussions and that is the correlation between technological progress and war.

It is mentioned as fact amongst engineers that most of the technological advances that we enjoy today were initally developed during war. In the absence of war, hot or cold, we would not be nearly as technologically developed as we are. So, a "warrior spirit" is necessary to achieve the level of development necessary to communicate with alien civilizations.

However, this very "warrior spirit" has put human civilizacion in the brink of self annihilation mere decades after the first human flight. It is just a matter of time that we wipe ourselves off the surface of the planet. The same can apply to any advanced civilization.

I a nutshell, the same characteristic that will make a civilization capable of sending messages to the stars, will make it destroy itself relatively quickly.

 by jaswant gopaul

human beings are a most dispicable species.violent,hateful,greedy, selfish.human mantra is more.gimme more. screw the next guy.Human beings cannot be trusted. I could go on and on and on. I include myself. The question is! What intelligent species would want to come near us?

 by maikol
Vastness of Spacetime

The universe is vast in size and so is time. Human civilization has only been transmitting radio waves over the last 80 years or so which account to an instance in comparison to our presence here, the age of earth and the age of the universe so far.

Our civilization is a spark in the vastness of space and the odds to receive extraterrestrial signals in this very moment from a civilization far away is way too optimistic. I find it more likely that we will go extinct before we detect anything and so do other civilizations, apart from any that were fortunate enough to appear in close proximity and time

 by Eduardo
there are plenty of alternatives yet

Well if we can't even find a missing plane in a small part of our world.. if we can't map huge asteroids near bu, and just started to find exoplanets.. is it not possible that a) alien life doing this may be there but is not so common and b) is it not possible they are smart ai with enough cleverness to be careful and hide? Most arguments go from "they should be so many that is impossible not see them therefore there are none" to "we are so rare" .. what if they are rare but still plenty but we have not yet have the means to detect them? Our capabilities are a lot less than we think and as seen we have just started.. how long ago did we saw the first galaxy beyond ours? The first exo planet? The first black hole? And all of those are bigggg... won't those probes be small and hard to spot? Won't they also may be smart and capable of hiding? Would you not design them so?

 by Dylan C. Holtz
Mass Suicide

Let's assume that science is inevitable for sentient life. Given it's exponential growth rate, perhaps the end of physics is almost always reached within a blink of an eye - around 1k to 100k years. What if heat death, proton decay, and all else that spells doom and total information loss is proven to be inevitable? I imagine some kill themselves out right, while others choose not to reproduce and wait to fade. If we can prove that life is meaningless beyond reasonable doubt, how would our society respond. That is of course assuming that we take science seriously enough at that point to reach the ends of it.

 by Lane Harvey
Solution. Earth is super rare Earth x1.4

The Earth, that is able to support and nurture diverse life, may be extremely, prohibitively rare. We just take it for granted- as a normal world. It is not. Earth was hit by a mars sized planet in its formation, the after effect created the moon with leftover orbital debris. But what about the mars sized iron core? Yup- added to Mother Earths'. This is why I think of Earth as a "world x1.4".

Earth has an active core and magnetosphere to this day. Look around at the other terrestrial worlds neighboring us (Mars and Venus). Active core? nope. My theory is that in normal planetary formation any given terrestrial worlds' amount of core constituents results in it freezing solid and inert in only a few hundred million years. It takes a super rare collision and donation of that extra bit of iron and radioactives from another world to make a world with a continual active core and magnetosphere in order to support long term billion years of evolution of life.

So Earth may in fact be very, very very special after all.

 by Chris
If there is xistence of Carbon else where...there must be.

The universe (just our universe) is so vast, to say that there is no other existence of some form of communicable life is not sensible. There are so many 'other' universes. Our searchs are kinda like useing a cup, scooping a cup of ocean water, looking in the cup and saying...oh well no life in here, so there must not be any life an all of the oceans.

 by Boiboi
My life is a lie

My life is a lie! Aliens exist! The earth is flat. The government is hiding them from us.

 by Donald Wood
Finite lifespan of civilizations and limitations of light speed communication

Distances are huge in interstellar space. In the space time scales on a galactic/universal scales, ET may have showered Earth with signals for thousands, if not millions of years. But that civilization may be long extinct. And their signals have long since passed us by and are heading away from our planet... Irretrievable and getting more and more red-shifted everyday.

 by Shaun

The more I read about the Fermi paradox and the possible reasons the more I'm drawn towards simulation theory

 by Jordan

the whole universe is a human body and we are tiny cells

 by mariah weissman
cant tell

The aliens might have already sent messages. it is probable that they would be quite diffrent from us, and messages could look like something entirely diffrent and ordinary to us. they might say hello, and we dont hear them talking to us.

 by Craig
Age of the Universe

Maybe the fundamental physics our universe did not, as the current Big Bang Theory suggest, solidify in an infinitesimal moment but were variable and became unchanging much later (BBT itself already indicates everything traveled faster than light for a bit). Or, physics varied in different regions. The age of the universe could be vastly overestimated. If the universe is very young, having only one space exploring civilization (which we barely are, or not even by some definitions) in it makes a bit more sense.

 by Earthling Number 6 billion and 1
"Sorry, didn't think to look there."

It's possible that when we receive that first message from a nonterrestrial intelligence, it will begin with an apology. "Sorry, we never thought to look so close to a star." If there is a union of intelligent and communicative creatures, it's quite possible they've only been able to form because they avoid the radiation from stars. Perhaps they exist only between galaxies or inside galaxies but far from the radiation starts put out. The Drake equations asks us to assume that intelligence would require a moltent ball of rock with a thin crust on which DNA dependant life evolved in liquid water to become intellegent. But, what if that was the last place an intelligence normally would develop? Isn't it possible that even without the energy from a star and give billions of years to form, intelligence could exists far from the damage that stellar radiation does? We might be very odd.

 by Stan
What if we are the Von Neumann prob

I've got a thrilling thought. What if we or life itself is the prob that a hyper advanced civilization sent. They have dominant power in the university. And we, as a bug wouldn't become a threat to them at all.

Our job is just to terraform the planets, make it habitable for their later use.

And one day the god-like alien would come and say, you've done a good job probs, Let's gather you into the trash bin..

 by Habib Malik
Quantum Entanglement

Nonsense! Before such advanced aliens reached the level where they can use sophisticated communications to conceal themselves totally from us they had to have passed through stages of development when they in fact used communications that we can detect, and those waves are still out there for us to detect. The fact that we haven't detected the traces of past communications by aliens when such communications were detectable by us might mean such aliens do not exist.

 by Mike
Quantum entangtlement

It's quite simple. ET uses entangled quantum communications such that the signal photons are indistinguishable from the background noise without the entanglement correlations.

 by Ludger e (Ed)
Heroic and self-sacrificing causes are our

"Upbringing", notice how there is no "god" or religious references here....seems making ourselves feel good about ourselves some would say " masturbatory " references to us related superbeings is just that masturbatory. Regardless,. All those coping behaviors that got us here can barely possibly be turned off to make survival possible on this planet.

 by Mark
Other unobserved scenarios

Perhaps it is a prime directive thing, and they are waiting for us to say we're ready to accept them.

Perhaps they exist in other dimensions than our own, and can observe us as unnoticed as ghosts.

Perhaps a few contacts have happened, and conspiracy theorists are correct that world governments are conspiring to keep it a secret while funneling alien technology to big corporations.

 by Del Arno
RF advertising

I think it is disadvantageous for an advanced civilization to advertise its presence. the reason we are is because we are nascent and nieve. I believe the universe is imperialist for resources we don't yet understand.

I loosely hypothesize genetic information or life itself as a source of information and alliances/cooperation once we become a player.

just as a babies immune system begins to develop in the womb, a civilization must learn to become stealth to curb external interference. What advantage would a subordinate intelligence have in advertising? what civilization isn't subordinate in a universe of infinite possibilities?

 by Sreehari S

I think the advanced civilizations gradually loose interest in exploring new worlds and even their urge to survive and reproduce. They might have just died off or they put themselves into a state of eternal pleasure.

 by Marty Johncox
Not everything has to happen right now

People discussing this issue commonly fail to appreciate the immensity of time. It's possible aliens recently existed and even visited us, or will find us very soon (say, 100 million years either way). Earth has had sentient life on it for just the last 1/30,000th of its existence, so the chance of aliens coexisting with us and then visiting us is vanishingly small. It's entirely possible that life is so rare - and intelligent life rarer still - that it never coexists and there may be vast periods of time where there is no intelligent life anywhere in the universe. Look at it this way: The expected lifespan of the universe is 100 trillion years (100,000 billion). Just 14/100,000ths of the way through that and we know intelligent life emerged (us) - holy cow, that was fast! Suppose 1 million intelligent species came and went during those 100 trillion years. Sounds like a lot, right? That would come out to 1 intelligent species every 100 million years. By comparison, dinosaurs died out about 65 million years ago. Life and intelligent life may be very common in the timescale of the universe, but nonexistent during the fleeting existence of any single solar system. It's arrogant to expect the universe to operate on our teensy-tiny timescale.

 by Dave
We are they

If an alien civilization wanted to go to other planets, they wouldnt send space ships, they would send dna that was robust enough to germinate on other worlds. Eventually this dna would create sentient beings if conditions permitted. These sentient beings would want to connect with the alien worlds.. just as we do. And we will send out dna, in some robust form, perhaps 10000 moles, in all directions in the sky, to settle on t0 other worlds, and create environments suitable to advanced life again.

If we were motivated, we could do this now and fulfill our destiny.

 by Okay

"ET has no interest in communicating with lesser intelligences. Problem: with millions of possible civilizations, some of the extraterrestrial ones would be interested in our species."

This "problem" is an example of anthropocentrism. Shocking for some to consider, but even if there are millions of interstellar alien civilizations, they would have within their power ways to observe and learn about our planet without interfering with the course life.

 by Shannon
Time Distance Commitment

Nothing travels faster than the speed of light, except the universe's expansion. The time it would take to physically travel to Earth would be an impossibility, even if travelers would be willing to commit to it and their offspring for generations. Unless there is a way to travel faster than light, radio waves would have too far to travel to reach us.

Additionally, technology took millions of years to be developed from the origin of hominids. I imagine over prototypes still evolving as well. Sadly, I think technology ultimately destabilizes civilizations and anyone that was advanced enough to contact us, probably no longer exists.

 by Torbjorn Berglund

If we assume that the time a "Tech civilisation" uses electromagnetic radiation for communícation to be shorter than 100 years, then we definately will have problem finding them.

Then, it is most likely that we are listening through the wrong channels.

Using accelerated electrons for Communication might be a relative ancient phenomena in the universe.

Still we have not seen evidence for quantum entanglement Communication.

However, due to the nature of new findings, let´s not be to sure that it´s impossible yet.

Sooner or later some hard working people find out that quantum networks for Communication already exist interplanetary?

I like the ideat that what we regard as for sure totally impossible, might turn against us one day.

 by Gavin
They have left the universe.

What if the solution is that they all leave this universe. Here are some possibilities.

1. Perhaps this universe is filled with chaos that tends to spawn new life. As such it is dangerous compared to other universes (filled with radiation, supernovae, solar flares etc. There is an alternative universe nearby.

2. They have discovered heaven - it is nicer!

3. We are in a game-world/virtual-world. To make an analogy Morpheus has shown them the red and blue pills. Can you live a lie or do you leave?

4. There is the possibility of war here. All races eventually learn how to make their own private universe. Problem solved - instant rulers of everything. Why would you stay?

5. They discover a way to create a bubble of time. If the universe outside your bubble stays at one moment you have locked your world away from all change in the universe. Your survival is almost guaranteed. Discovery of your world becomes unlikely.

 by Cashme Morz
engaged in physics

Given all of the pros and cons in the main body of the article, the resulting environment where humans are contacted by aliens is so narrow that those aliens that could potentially be in contact with us humans is a very rare occurrence. Some of the factors that would have to be satisfied to allow for meaningful meeting to take place are: 1)sufficient similarities in type of organism. If the differences between us and them are too great, then they would have too little reason to contact us. 2) They and we would have to have the sufficiently similar characteristics to exist within a very narrow time frame. If they were similar to us a million years ago, but are now extremely advanced, to make them too different from us currently, see 1). 3) They may be similar to us in terms of organism type and technology, but have any number of other cons such as them having paranoia or fear of anyone they come across. This tends to indicate a myriad of factors that could go against them contacting us, even if they are here.

We just don't know what all the other factors are that add up to a meeting. A being anywhere near to our complexity could have just that number of factors that add up to allowing or precluding if we meet or not.

So we don't know much of anything as far as meeting up with aliens, whether now, in the near or far future, here on earth or anywhere else, even if we find someone on some other planet. Will we have qualms about going up to them and saying "Hi", or "take me to your leader" or anything that looks like a meeting. To know anything about meeting them we would first have to understand them sufficiently to know if we even want to meet them, observe them, keep out of their way or anything to do with them.

 by Will
The Solution

Our galaxy alone has over 400 billion stars with many of those stars containing planets. If we assume we are the only life that is being naive, look at Earth where there are and have been millions of species throughout this planet's existence all from a mix of habitable conditions for life as we know it on Earth could form. Tool making and technology does not take very much to advance when sentient species evolve and adapt. For example think of how Humans first used tools then used them to make fire then how to use that fire. We had limited intelligence then, but yet we figured out how to use things around us to forge weapons to kill better and eventually make fire ourselves instead of seeing it in nature. All of that is because Humans are a unique, one of a kind creation of Earth, we are sentient. If it can happen on one planet among billions, sentience will arise elsewhere. The answer: Sentient life is a very unique creation within the universe. Yet for that life to achieve the ability to travel among the stars in mere fractions of their lifespans, is an even more rare occasion. While there could millions of species at this very moment at the technological and social state of Humans, there may be as little as 50 to 100 or as many as several hundred. All scattered throughout the galaxy. Yet for any of them to use the same communication techniques and even the same technologies could be a very slim chance. Sentient life adapts depending on where they are and what natural things occur around them. They could develop electricity and use it in all sorts of ways, far before they even discover fire. It all depends on their upbringing.

 by Rubellite Fae
They left?

Perhaps the great filter isn't so much something that destroys civilizations. Maybe after a certain point they "ascend the need for bodies." Like, they can upload/transfer their consciousness technologically. Or, there's some sort of dimensional/"spiritual" ascension we don't know about yet. Or, maybe they discovered a way to travel to a different era. Or, maybe there's a way to survive a journey through black hole's event horizon. Or, maybe they create their own Universes to play in, virtual or real.

The point is, there may be a threshold of advancement after which beings simply "move on."

The other thing I'd like to add, is there are always unknown unknowns. So, we may later discover other variables that should have been entered into the equation and that intelligent life is rarer than expected.

 by Nick Mott
wrong number

Function follows form - means:

Any structure is defined.

The definition of a structure defines - by the excluding of incompatible functions to the structure - a reducing of possibilities of the defined structure.

Our brain is such a structure whose defined functionality excludes an unknown scale of possibilities which are literally unthinkable for it.

Without a possibility even to guess the loss of thinkables due to the determining structure of our brain we can only be aware of what we can think.

There is the capability of existing 'life' in a structural way we will never be able to discern out of the tremendous 'sough' of unseen, unknown potentials of the given primal ground in this universe?!

We don´t even KNOW what electrons make move for ages!

We don´t even KNOW what gravity really IS.

We are slicing the universe like a dead frog to understand what makes him live and jump.

Slicing life means to find none.

There is life in this universe for sure, but the chances are great that life can be so much more than we represent ourselves that we will never get it, because a toe is not an eye.

We share a considerable percentage of paradigms with mammals.

But we do not understand an horse or even an ape REALLY!

We just draw analogies between them and us, we try to find pieces of concepts we are familiar with to get an idea of their worldview.

But never ever we will reach them.

So: we can´t even reach our cousins in paradigm - how, the hell, should we reach THEM out there???

 by Greg
Disinterest from ETI's - we would be like ants to them

This pertains to the notion of disinterest from advanced ETI's - ”why would an advanced extraterrestrial intelligence have any interest in us, we would be like ants to them.”

My issue with this argument is that it is not really an effective comparison. NDT has made a similar comparison with chimps, and paraphrasing him here, calls out that our DNA is only 3% different and yet look at the difference in the capabilities between the species. While the difference in DNA is notably more narrow than the capabilities of each species, there is one key aspect involved that makes the comparisons invalid in the context of the supporting argument – as far as we know, ants and chimps are not sentient beings able to communicate in a sophisticated manner (certainly not in mathematics) and question the world around them. Last I checked we can, and think it is fair to assume an advanced intelligent species could recognize that in us. If we recognized even the slightest, primitive ability of ants or chimps, or any species, to demonstrate sentient thoughts and communicate them, we would have legions of scientists and researchers that would be highly interested in them.

Another part of the argument I challenge is the built-in assumption that an advanced ETI would necessarily have been in contact with or able to observe other ETI’s less advanced than them, and so adding to the notion of how uninteresting us ants would be – been there, done that, nothing to see here. It is very likely, if you prescribe to some of the other Fermi Paradox hypotheses, that ETI’s are more rare than we may think and so where they do exist, are probably spread very far apart within the universe. It is very possible that even a very advanced ETI still deals with limits created by the law of physics and distances involved, or otherwise for some reason, not interested in the mass exploration or colonization beyond a galactic scale. So they could explore and colonize an entire galaxy, and end up being the only intelligent species in it having never discovered any other ETI along the way. If at some point an ETI arises in their galaxy, it would be hard to believe no matter how primitive it may be initially or over time that there would not be a great interest in that species, the first of which they had ever seen capable of sentient thought and communication.

We have 1000’s of scientists studying ants and chimps today without these capabilities. Imagine how many there would be, how different today’s science would be, if they could exhibit them or we discovered a species other than us that could.

 by Sean

My theory is that the nearest civilizations are under a non-interference accord that prevents them from making contact with covilizations that are still developing spacefaring technology. In all likelyhood they probably just enjoy watching us put the pieces together, kind of like how we enjoy watching babies and young children solve puzzles that challenge them but are easy for us. Who knows?

 by Laurentiu Motos

Whatever the conditions needed for life may be, whatever the technological capabilities, there is one fundamental difference that i did not see posted anywhere in regards to the Fermi Paradox. Curiosity.

Taking into account the diverse carnivorous species on earth, the ways hunting has evolved in animals, all of which we as humans had to keep our eyes on and in time our brain evolved to think in such a manner to envision potential ways that we could be attacked and killed. This is not necessary to be true on other planets. Another difference would be the basic way our brain categorizes things, fits them into categories so that they may be easily predicted and analyzed. Pattern recognition has no logical reason to be developed outside our planet and it's diverse environment. As such, even if life has evolved on other planets, it may just not be fundamentally curious, and, as such, never have evolved technological capabilities which require patterns in order for any kind of technology to work.

 by Pete

An answer to the Fermi paradox could be that we haven`t developed the technology to receive messages from other civilizations. The idea that we will detect signals that crawl across space at the speed of light is the issue: a more advanced civilization *might* have a faster than light communication system that we have yet to dream off. Faster than light / warp type drive system seem to be a requirement for meaningful travel between stars. Of course these things are beyond our current understanding. Einstein`s rules preclude FTL- yet we can envisage possible ways around the limits - albeit requiring huge energy and technology beyond our current abilities.