A couple of possible reasons why we havent succeeded in finding other forms of life in space.

A couple of possible reasons why we havent succeeded in finding other forms of life in space.

I feel like "Far far away" and "A long road ahead of us" should be considered the primary reasons at the moment. We just don't have the ability to detect life at such great distances in such a short amount of time.

We just recently developed ways to detect planets. The fact that we can't see civilizations isn't even remotely surprising, which renders the other explanations very premature at the moment.

Once we've colonized a few thousand star systems, searched a healthy portion of the milky way, and still haven't found intelligent life, then it's time to invoke the other explanations. Right now it's just premature to try to explain why we haven't found other civilizations. Due to the ridiculous scale of the universe it seems pretty obvious why we haven't.

To say we've barely scratched the surface is a monumental understatement.

"To search expectantly for a radio signal from an extraterrestrial source is probably as culture-bound a presumption as to search the galaxy for a good Italian restaurant." Terence McKenna

To say we've barely scratched the surface is a monumental understatement.

One pixel in all of the pixels manufactured.

If aliens don't exist explain Alf

I personally consider "Early birds" to be very likely as well, though surely the opposite hypothesis is also credible? Considering we may well be paving the road to our own extinction, it's not inconceivable that many other sentient races have risen and fallen in a similar manner before life on our planet even learned to divide its own cells.

We may well be separated not just by space, but by time.

I like to use the comparison that if the sun is a single pixel wide, the closest star is 15 km away.

Cyborgs man, i keep telling people this. We're going to have to gradually assimilate ourselves into cyborgs so we can live much much longer. Then, since we cant really travel the speed of light, we will travel as light (aka data), somewhere else to be "rebuilt". I think that is more likely to happen before we can make ships move fast enough to be worth it.

And we've got the perfect place to practice this on. Mars! Just crash an ice comet into it, sprinkle some trees in it and then move our 3D printers n'shit on over.

"The Bistromathic Drive is a wonderful new method of crossing vast interstellar distances without all the dangerous mucking about with Improbability Factors. Bistromathics itself is simply a revolutionary new way of understanding the behaviour of numbers. Just as Einstein observed that time was not an absolute but depended on the observer's movement in space, and that space was not an absolute, but depended on the observer's movement in time, it is now realised that numbers are not absolute, but depended on the observer's movement in restaurants."

-- Douglas Adams

The last one, basically "they're too far away", is the only I sort of agree with except it demonstrates really narrow thinking. The distance factor should be measured in both space and time. We've only been looking for a very brief instant of time, other civilizations could simply be too far away from our own time period.

Also tied into this narrow thinking is the belief that once they come into existence, advanced civilizations (including ours) will stick around forever. The reality is probably the opposite - they flower and then die. Even if an advanced civilization existed for one million years, that's still an incredibly brief flash of time on the cosmic scale. How likely is it that we'd happen to catch them during such a brief flare up?

"Not Life as We Know It" - lets not draw the attention of the Reapers, okay?

At what ppi?

Or they're hiding. The galaxy is a Dark Forest.

Human lives are just too short. People live and die in the time it takes signals to travel. Generations would be born and die traveling between stars.

Light, the fastest thing in the universe, moves too slowly for us. We die in the blink of an eye.

It's almost ridiculously misleading IMO for anyone to assert that we haven't found anything in 50 years of "searching," because what constitutes "searching" is like trying to listen to one person talking across a football stadium filled with 80,000 people with a rolled up piece of paper to your ear.

Beyond that, there have been "interesting" signals received that are so far unexplained (not just the Wow! signal), but they're just that; unexplained. I highly doubt we're going to ever discover some sort of "Hi humans, we're your friendly local aliens" kind of signal. Truthfully, we don't even know what we're looking for nor do we really know how to interpret the things we do find that are of interest.

Yes, artificial transmissions should have an identifying pattern to it, but look at the history of pulsars for an example of the difficulty of even just extrapolating something interesting from natural cosmic noise.

TLDR: We've been "searching" for life for all of a cosmic blip, it's too early to draw any premature conclusions. Furthermore, our "search" techniques are archaic and we lack the knowledge to properly identify some of what we do find, anyway. It's very possible the Wow! signal was the one, but we never found it again.

Several. Also don't mix your units. That's hownwe lost Beagle 2.

The whole "paradox" seems to be based on the idea that aliens with only slightly better tech than we have could create self-sufficient replicator probes that could establish a presence in every system in the entire galaxy in just a few million years. And if they could, they would, and if they did, we'd see them.

While I have some quibbles with the feasibility of that plan from a distributed systems perspective, the more fundamental question is why. Why would aliens do that, even if they could? What is in it for them to say "we shall have a probe in every system in the galaxy! It will take many times longer than civilization has existed and we'll never know whether we even succeeded or not, but let's do it anyway!"

And why do we think we would surely detect such a probe? Hell, we just discovered two new moons of Jupiter.

There are lots more reasonable explanations than "there are no aliens." Mediocrity principle--the Earth is not unique.

Or Mr. Bean

I would disagree. There are only a limited number of ways to communicate across distance and they all involve transmission of matter or energy. Any species that wishes to communicate quickly will come to the realization that light is the best option because it is, by definition, the fastest.

I mean, I suppose you could have a species that doesn't mind communicating on years long time scales, or that simply doesn't communicate across distances, but that doesn't seem like a viable situation for an advanced species to be in as it traverses the galaxy. It would be like the U.S. being made of cities that only communicate through the use of foot messengers, but also managinh the same level of advancement it has in our reality.

The only other option is that faster than light communication is the standard, but thats not our own cultural presumptions causing problems, its our technological ignorance.

Well when you put it that way

Just to describe to people reading what this is, since subsequent comments annoyingly don't describe it either, it goes like this.

Basically the universe is a dark forest and we're all hunters. The forest is vast and resources are finite. So, any contact with other hunters means our first intuition is to destroy each other. Because space is so vast it makes communication the biggest hurdle to forming any sort of relationship or cooperate in any way. So, most civilizations keep quiet because they don't want to be destroyed.

I hope I described that correctly because I just looked it up.

My favorite explanation is this one: Time. There could have been huge interstellar civilizations in our cosmic neighborhood (and their radio signals and light reaching this planet) just when there was nobody looking at the stars on Earth. Humanity has been around for so little time that it's possible we simply missed a such a civilization nearby (by a few thousand years to Millions of years, who knows) or might miss one once we are gone.

In combination with the last two explanations from this inforgraphic, this might be the reason why the galaxy appears to be such a lonely place.

The Fermi paradox section lacks the important detail that the filter could be ahead of us before we reach near-c space travel.

I would disagree. There are only a limited number of ways to communicate across distance and they all involve transmission of matter or energy. Any species that wishes to communicate quickly will come to the realization that light is the best option because it is, by definition, the fastest.

Says the monkey who just graduated from using spears only a few thousand years ago...

I heard a comparison recently on Neil Tysons podcast that went approximately like this: Imagine the universe as all of the water of all of the oceans on Earth. In all of our exploration of the universe since the beginning of human spaceflight, we have searched the equivalent of one 12 ounce bottle. When you think about it that way, it's interesting that people claim that there aren't fish in the sea because there aren't any in your bottle.

I always found the Quarantine Theory interesting. The idea is that the intelligent civilizations of the galaxy have agreed to quarantine the human race and not contact us until we develop further, either because we're too violent or because we're too primitive and they want to preserve our developmental timeline (sort of like the policies of the Federation in Star Trek)

That's sort of explained by the great filter, but instead of something big stopping them before they get advanced it's something stopping them once they've already become advanced

Ah yes, Reapers

I agree. When you start doing the math on what constitutes a detectable signal and then apply that to the size of the universe, you realize that we've basically stood on a beach, looked straight down for a fraction of a second, and then wondered why we haven't seen any crabs.

Another possibility: we live in, as Liu Cixin calls it, a 'dark forest', and it's in every planet's best interest to stay silent to avoid the consequences of being heard.

Given the complete lack of proof, it's not a dumb question. It's not necessarily correct or even probable, but not a dumb question.

It was the Mars Climate Orbiter: https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_failure

Just finished The Dark Forest by Liu Cixin. Haven't heard of the concept before then but boy it sure seems plausible and terrifying..

"The first nonabsolute number is the number of people for whom the table is reserved. This will vary during the course of the first three telephone calls to the restaurant, and then bear no apparent relation to the number of people who actually turn up, or the number of people who subsequently join them after the show/match/party/gig, or to the number of people who leave when they see who else has shown up.

"The second nonabsolute number is the given time of arrival, which is now known to be one of those most bizarre mathematical concepts, a recipriversexcluson, a number whose existence can only be defined as being anything other than itself. In other words, the given time of arrival is the one moment of time at which it is impossible that any member of the party will arrive. Recipriversexclusons now play a vital part in many branches of mathematics, including statistics and accountancy, and also form the basic equations used to engineer the Somebody Else's Problem field.

"The third and most mysterious piece of nonabsoluteness of all lies in the relationship between the number of items on the bill, the cost of each item, the number of people at the table and what they are each prepared to pay for. (The number of people who have actually brought any money is only a sub-phenomenon in this field.)"

Exactly! Hasn't anyone seen the documentary X-files?

Can a group of lifeforms evolve to the point of developing interstellar travel without eradicating itself or its habitat?

That's a strong question with no definitive answer.

We are the aliens.

I don't think there are any "theories" worth their name about made-up / hallucinated aliens.

We'll bang, okay?

"This theory is explained very well near the end of the science fiction novel, The Dark Forest by Liu Cixin. The first axiom is that survival is the primary need of civilization. Therefore, civilizations will do whatever it takes to ensure their own survival. The second axiom is that civilizations always grow and expand, but the amount of matter and resources in the universe are finite.

So every civilization other than your own is a likely threat. At the very least, they are occupying a planet that you could use to expand your civilization. At worst, they are more technologically advanced and will wipe out your civilization to expand their own.

When two civilizations meet, they will want to know if the other is going to be friendly or hostile. One side might act friendly, but the other side won't know if they are just faking it to put them at ease while armies are built in secret. This is called chains of suspicion. You don't know for sure what the other side's intentions are. On Earth this is resolved through communication and diplomacy. But for civilizations in different solar systems, that's not possible due to the vast distances and time between message sent and received. Bottom line is, every civilization could be a threat and it's impossible to know for sure, therefore they must be destroyed to ensure your survival.

You might be thinking that if an advanced civilization detects the radio signals from Earth then they would know that we are less advanced and therefore not a threat. But again you have to consider the vast distance and time it takes for those signals to travel. Even if a nearby civilization (only 10 or 20 light years away) detects us, it would take hundreds or even thousands of years for them to reach us and that is plenty of time for a technological explosion. If they don't attack us at once, then we might develop technology fast enough to catch up and threaten them.

It won't be like Star Trek. Without faster than light travel, there won't be any communication, diplomacy or trade with alien races. It's kill or be killed.

So that's why we haven't heard a peep from other civilizations. The universe is a dark forest where every civilization is a silent hunter. They desperately try to stay undetectable while hunting for other planets to colonize and threats to destroy."

2.2 mm? that is a light bright.

Yes, the interpretation here is much more optimistic than when I first leaned it. If Earth is not special, then intelligent life evolved elsewhere, encountered the filter, and perished. Intelligent life on Earth is still here because we have not hit the filter yet.

Of course, due to the "not special" premise, this implies we are also doomed.

A) No such thing as a dumb question

B) Literally no proof either way, perfect time to ask a question

C) Fuck You

D) Fuck Dallas

Im serious, lol. Its a vast oversimplification but I think sometimes you need that at a cosmic scale.

They're already here

They didn't mention the most likely reason...

We're super boring!

Seriously, lets assume that there are inter-galactic civilizations in the universe. They could be operating on much different timelines than we are. Them being at the inter-galactic travel stage of development would be just as likely right now as it would be 100 million years ago.

So since the time element could be randomized to the history of our planet, what are the chances they would observe Earth in the last 50 years?


4.5 Billion years ago : earth formed.

2-3 Billion years ago : plants started forming

500 Million years ago : more complex life, animals

240 Million years ago -> 60 Million Years ago : Dinosaurs

1-2 Million years ago -> humans

55 Years ago -> humans in space


Based on these numbers, here are the chances of what aliens would find if they visited Earth:


50% chance : Big rock

40% chance : super primitive plants/organisms

7% chance : tiny primitive animals

2.5% chance : Big ass lizards

0.4% chance : Primitive humans

0.0014% chance : spacefaring humans

Given our rapid merger with technology, it's far more likely all "intelligent life" merges into one big cosmic intelligent computer.

Like in Isaac Asimov's short story The Last Question.

At immense risk. Unless these hypothetical explorers don't care at all whether they alter the systems they observe and skew their data, they'd want to be careful to make sure no such probe could ever be found by less advanced species. Indiscriminately jump-starting technology by throwing advanced self-replicating probes all over the place seems like not the best way to study your environment.

Why would aliens do that, even if they could? What is in it for them to say "we shall have a probe in every system in the galaxy!

Exploration. Finding out more about the galaxy and possible alien life without having to deal with limited lifespans and maintaining lifesupport etc.

It will take many times longer than civilization has existed and we'll never know whether we even succeeded or not, but let's do it anyway!"

It will not take that long to know if you succeeded at nearby stars, and from there on the process will probably repeat itself automatically.

Although, even in our current biological form, we have expanded our lifespan notably in our species' history

But we haven't. The reason the average lifespan used to be so short is because it's the average lifespan. The infant mortality rate used to be much higher than it is now. Add in better survival rates for women in childbirth, and just plain old better food, medicine, and sanitation, and that explains our greater average lifespan now.

We don't have any life extension technology that's slowed the natural aging process -- our actual lifespan hasn't changed. We mostly just stopped letting as many of our species die young, so more of us can get closer to reaching that same span.

The idea is that it doesn't even need to be a single dedicated project. The time-scales are so large that a haphazard and intermittent exploration of the galaxy should still be easily completed within ten to a hundred million years - about 0.1% to 1% of the age of the universe. I mean, the amount of technological progression we've had in the last thousand years is enormous, but compared to a million years, that's a tiny amount of time.

As another example, suppose that each colony decides to found a new unique colony around a new star only once every thousand years. Compared to how many civilizations and cultures have risen and fallen in that time on Earth, and how advanced their technology would be, that's a very slow rate. At this rate - doubling the number of colonies every thousand years - you end up with a colony around every star in the galaxy after less than 40,000 years. Even if you build a colony once every million years (enough time that colonies will start to diverge into different species!), you still end up colonising the whole galaxy after less than 40 million years, which is still only a fraction of the time that life has been around on Earth - I think 40 million years ago you already have the beginnings of monkeys & apes appearing, for instance. So if, once every million years, one space-ship per colony gets marooned in space and starts a new colony, that would be enough to have completely colonised the entire galaxy in a comparatively short period of time.

So unless a space-faring civilization has only emerged very recently - within the past tens of millions of years - it just seems incredibly unlikely that they won't have spread across the entire galaxy by now, even if they didn't really intend to. It's not like prehistoric mankind set out with a plan to settle every piece of land in the world, but by the time recorded history begins, we have people spread out across almost every major land mass on the entire planet, and that's using extremely primitive technology.

Also explain cats.

Spawn kill. Snipers are hidden in the dark. They kill anything they see before it gets powerful enough to challenge them. Any species with any sense reliazes this and hides. The killer nano-bot carrying particle beams that are going to destroy us have already been sent. They will arrive in a few hundred thousand years. Our only hope is to scatter like spores in the vast emptiness of intergalactic space. We had better hurry up and get off this disco ball.

This is an excellent point. The light fluctuations of Tabby's Star, for example, probably aren't the result of a Type II civilization, but what if they are? That would mean we've already got evidence of life with an incredible level of technological advancement but that we simply can't recognize it. Unfortunately, something like that won't give us any clarity whatsoever until we already have our answers.


Andromeda Strain brought up a statistic that said that theres effectively a 90+% chance that our First contact with alien life will be with bacteria.

Of course they could, this is a dumb question.

If you're talking about undirected actions like that, where civilizations colonize nearby systems out of necessity rather than pure exploration, then you get into a "percolation" model like what was described by Geoffrey Landis. Depending on your assumptions, there will either be small voids of uncolonized space, arbitrarily large areas of colonized and uncolonized space, or mostly void with pockets of colonization in places.

The other thing that you can't assume is that if civ A colonizes star B and thus creates civ B, that civ B is then going to continue on to star C for its next colonization project. It may well turn right back around and go back to star A. Especially if you're talking about timescales closer to a million years -- civ B might not even remember that civ A ever existed, and think that star A is a completely empty system.

This is my main complaint with the Von Neumann probe idea too. Distributed command and control with no communication over such vast distances and time scales is a hard problem. Once you've got a reasonable density of colonies, it's really hard to actually explore new stars, rather than accidentally re-exploring already-visited stars over and over again. You're much more likely to step on your own toes. So you can't just calculate log_2 of 400 billion and be done with it.

Or perhaps life is abundant, but the technology to physically travel and explore the universe is simply impossible, explaining why aliens have not knocked on our door.

The data would be sent back continuously when the first probes reach the nearby stars, where they will self-replicate and send probes to other nearby stars. It's a well thought out idea, and perfectly possible for an advanced civilization, and imo the logical way to explore the galaxy.

After reading that book I'm entirely convinced that's the answer. Doesn't Hawking also say this is the most likely answer?

I don't see any reason why that would not be the case.

the opposite hypothesis is also credible?

That doesn't touch the base of the "early bird" theory. We know how old the universe is, we roughly know how old it will become. And thus, we know our Universe is very very young.

If we also consider the time it took to form stars and that life without stars may not be possible, the time the universe has been "fertile" is even shorter.

Considering we may well be paving the road to our own extinction, it's not inconceivable that many other sentient races have risen and fallen in a similar manner

If that is the case, then many many more sentient races will still come after we've died out and we will still belong to the early birds on a scale that encompasses the lifespan of the universe. Simply because how long the universe will still be around.

So I wouldn't consider the "we go extinct before we make it" idea opposite to the "early bird". It's just a different explanation alltogether.

I'm sure as shit convinced

Not exactly the answer but should get you started https://www.cloudynights.com/topic/114077-if-the-sun-was-one-inch-in-diameter/

Ok what ppcm?

We're already in the process of trying to send probes to the nearest star.

It seems pretty obvious to me that while this universe may be prone to developing self replicating systems (AKA life) there's no specific evolutionary pressure for it to be "intelligent" (i.e. interstellar signal sending / space faring). I mean how many billions of years did life exist on this planet without interest or means to be "intelligent"?

So do you think it was a bad idea we sent out that probe with coordinates to earth and pictures of humans on it?

A light year specifically is only an absolute measurement of distance, ~9.461 x 1012 km. Distance in general, however, can be used to describe observations in terms of time, especially larger distances.

For example, if something is 10 light years away, the distance between the observer and the object is still just 10 ly (9.461 x 1013 km). However, the observation has a ~10 year latency, meaning what is observed is actually from 10 years ago. Another example for smaller scale purposes would be the Sun. If we observed the Sun right now, what we are observing is the Sun roughly 8 minutes ago.

So I think you were on the right track but perhaps your wording or interpretation was a bit off. If something is too far away but is potentially developed with ET life, we may not know simply due to the latency of observation dependent on distance. It works both ways too: if there was a civilization 200 million ly away and observed us right now, they would see an evolving planet with wild flora and dinosaurs.

We barely made it through the 20th century

Really? Our numbers are increasing fast. There's no barely about it. We survived the 20th Century with ease.