We live in times of extraordinary discovery. Exoplanets appear to be quite common in our galaxy. NASA’s Kepler Telescope has identified over 2,000 planetary candidates orbiting other stars. And yet the universe appears to be silent – at least when it comes to any detectable signs of alien civilizations, either at present in our galaxy or their remnants from the last couple of billion years.  

And let’s be clear: it isn’t just the failure of SETI  (the Search for ExtraTerrestrial Intelligence) to detect radio signals that constitutes “silence.” Indeed, there are strong reasons to believe that they have been looking in the worst possible way. No, the greatest SETI Observatory has been our own planet Earth, which had an oxygen atmosphere for up to two billion years but with no inhabitants higher than a slime mold to defend it against external colonization.  Had alien visitors ever flushed a toilet or dropped a sandwich wrapper into Earth’s seas, the bio changes would have been huge and visible in our rocks.

Physicist Enrico Fermi famously asked, “Where is everybody?” The Fermi Paradox or The Great Silence refers to this quandary of why we have never encountered extraterrestrial civilizations. I've written about all this extensively in scientific papers and in fiction, and my latest novel, Existence reveals dozens of scenarios about first contact.

Many experts have weighed in with explanations for the Fermi Paradox, and I’ve observed a strange phenomenon among smart fellows like Stephen Hawking, Michio Kaku,  Paul Davies and such.  They all-too often seem to leap upon just one  hypothesis – a bizarrely premature thing to do, especially in the only scientific field without any subject matter.  I have chosen instead to spend the last 30  years cataloging and categorizing up to a hundred theories for the Great Silence.  But I won’t list them here.

Instead, I recently crowd-sourced this question on the web; the top responses are presented here, ranging from the serious and thoughtful to the humorous and ironic.  Here are the top vote-getters… followed in each case by my comments.

#1 We don't have the capabilities to detect anything but a tightly beamed signal. And like detecting the sound of a jet in the sky, where you can see it, is not where you can detect signals from it. You have to point your microphone behind it. With tightly beamed signals over galactic distances, you have to know the proper motion of the planet and its sun and they have to know our proper motion to beam it to us. If they are ten light years away, they have to beam it to where we were ten years ago and we have to point our detectors to where they were ten years ago. All the SETI searches ignore this and hope a civilization is sending out a ridiculously powerful beam in all directions.  –Tony Farley

In fact, Tony, you are partly on-target with this one. But first, where you are wrong. SETI searches engaged in by the top group near Berkeley do compensate for motions and Doppler shifts and orbital variations to a degree that would amaze you.  They can detect a signal that is spectrum-varying with time and compensate for that as the source spins and rotates and revolves around a noisy star. These are clever folks.

But you are right that they still make untenable assumptions. They search the sky with narrow listening beams... looking for aliens who might be BROADcasting hello signals in all directions, or else leaking their own broadcast conversations, prodigiously, into the sky.  But our own noise leakage has declined fantastically as human communications grew more efficient and channeled, since the 1980s.  

As for those gigantor beacons, meant to teach newcomers? Well, there's no reason that even a beneficent race would do that, around the clock, for eons.  Horribly expensive.  They would, as you say, "ping" likely targets like our solar system, maybe once a century.  To detect such pings, we would need a system very different than the one that billionaire Paul Allen funded for the SETI Institute. Instead of one expensive SETI program in one place, aiming pencil-thin listening beams at one narrow patch of the heavens after another, we should have a thousand backyard receivers, networked, scanning the whole sky at once.  Look up Project Argus of the SETI League!

#2 The universe is big in space AND time. It would be a major accomplishment for a technological society to remain intact for a million years, yet that is just a blip on the scale of the universe. How many galactic empires came and went before the Earth was even capable of supporting life? –Thomas Nackid

A good question.  And yes, we might simply not overlap with the others in time!  But note, Thomas, your assumption is that the numbers of tech races must be very small (and that may be the case) in order for the statistical non-overlap idea to work.  But if there are numerous long-lived species, then we get the Fermi Paradox. And if they travel?  A lot?  Possibly colonizing as they go? Then all goes crazy in the numbers. Colonization changes everything!

Even if they just explore and don't colonize, then the Earth would likely have been visited.  But even one toilet flush during the Archaean would have changed life on Earth in ways we'd detect in the rocks.

#3 Life, even intelligent life, is common in the universe, but advanced civilizations are rare, and hard to find in the small window of time that we have been looking, and not all advanced civilizations are nice. Getting between stars and communicating between stars is hard, and having someone close enough to communicate with at the same time you're communicating is rare, and sometimes perilous. We have not found anyone yet because we can only shout at our nearest neighbors, and our local neighborhood is currently empty, probably by chance and possibly by malice. –Ilithi Dragon

I am one of the SETI experts who has been arguing that the Great Silence may be telling us something.  "If all the races more advanced than us are being quiet... maybe they know something we don't know?"

Several major voices in the field, Like former NASA SETI chief John Billingham, have joined me in resigning from major committees in protest over the SETI Institute's high-handed role in helping clear a path for METI or "MESSAGE to ETI" – actively beaming messages to space. See our complaint: Shouting at the Cosmos -- or How SETI has taken a Worrisome Turn into Dangerous Territory.

#4 They won't unscramble the signal until we put a deposit down.  –Lone Hanks

Along those same lines: We haven't yet chosen a intergalactic long distance carrier. --Christopher R. Vesely.

Heh, you two may be saying this tongue in cheek.  But read Existence!  These thoughts can be re-expressed as real hypotheses that have a chance of explaining the Great Silence.  (I hope those passages will both make you laugh and make you think.)

#5 The "Do Not Feed the Humans" sign just past Pluto deters all but delinquents making crop circles.  –Kevin King

Uh-huh.  See my answer to #4.  Also a short story about alien “teasers” I wrote, called Those Eyes.

#6 Civilized people do not just drop in uninvited. –Eli Roth

We've been inviting!

Along those same lines: There may be a "Prime Directive" ethos that they stick to. --Glenn Brockett


That's the "Zoo Hypothesis" that comes in dozens of variations... all of which assume either that the ETIS are few and share the same value system, or else they have one heckuva police force...

And here’s an intriguing variant on the same idea:

As society gets rich enough and technologically sophisticated enough, eventually everyone is able to live in their own personal Matrix, customized to provide them with their ideal life. Soon after the civilization stops bothering to expand any further, as the perfect existence can already be found on their home planet and nothing more could be wanted. Humans have a rare neurological structure that prevents them from being satisfied with this sort of simulation. –Eneasz Brodski

See also a discussion of The Great Filter: Does a Galaxy Filled with Habitable Planets Mean Humanity is Doomed? on io9 -- Robin Hanson’s concept that there may be some obstacle that consistently prevents species from reaching the technological stage where they can traverse interstellar distances.  Also, if you really hunger for more deep mind games, try the Transcension Hypothesis of John Smart.  Both very very very brainy guys.

#7 We're an evolutionary simulation coded into some incredibly complex computer, and while there's enough computing power to model the behavioral and biological processes and interactions of all the life on planet Earth, there isn't enough to model intelligent (or otherwise) life for the rest of the universe, so they have to rely on simpler astrophysics algorithms. Maybe if that next grant gets approved, they'll be able to add in another few clusters and work on a "First Contact" situation... —Carter Boe


A big concept, but also a bit of a Giant Waffle.  I have put some creative thought into it.  We’re all in a simulation is becoming clichéd… like all great ideas… even as it draws a growing following.  Speaking of which --


#8 Our universe is part of a very advanced simulation in another universes cutting edge computer system. The system is designed to test out various theories of creation i.e. a big bang based on whatever the prevailing theories are - they then watch it all unfold and see how closely the results are to what these beings perceive in their "real" universe. this simulation has been tweaked and rerun many times because the results didn't quite match - the last time has been amazingly successful so they let it keep going and add memory and processing power as time goes by and as the simulated cosmos coalesced into our universe. They kept it running but tweaked it here and there and eventually decide to help form a world that can contain life similar to their own. Most of the computers' processing power in concentrated on resolving the detail and simulated life on that single simulated world. Every individual being, their thoughts and dreams, every bird that falls from the sky... They simply don't have enough memory and drive space yet to create "aliens" for us. (spoiler alert) in the "real" universe they never generated any speculative fiction so they haven't wondered in any important way at the coincidence... why are there no radio signals coming from intelligent life in their space? Until they read some sci fi created by the folks in their simulation...—Jim Simbrel


Hmmmm you folks certainly glom onto a fashionable idea!  It was pretty fresh a decade a go!


#9 We are, in fact, alone in the Universe. We are the first, We are the Progenitors of the great galactic civilizations yet to come. It's lonely at the top. –Tom Owoc


For a related scenario, take a look at my short story:  "The Crystal Spheres."  And I don’t say it isn’t so.  A variant is this: we may be the first to survive our adolescence and move onward.  That is… supposing that we do.


#10 Most societies evolved real-time communications using a fundamental principle or particle of physics we never discovered and thus never had to leverage the electromagnetic spectrum in this way. Radio is our solution to a problem no one else has and thus unique in the universe.  –Adam Maxwell


Hm... well, maybe.  And yet when we found  out about and started using radio, did we completely abandon drums?  Completely?  Or even at all? New Guinea natives might not notice the radio waves all around them, but they'd recognize the thumping on a passing ocean liner as having human origins!


#11 There are one or more paranoid, raptorial spacefaring species who attack, pillage, and destroy any civilizations whose electromagnetic radiation they detect. The only civilizations to escape destruction are those who have shielded their EM radiation sources from detection, by virtue of natural, innate caution, or from having learned of the dangerous aliens prior to developing electronic technology. For all other civilizations, they are detectable only in a narrow time window, until they are discovered and annihilated by the aggressors. This produces a relatively silent galaxy that may in fact harbor hundreds of sentient species. –Ed Uthman


Very much a theme in Existence.  But also, again, have a look at this missive against METI: Shouting at the Cosmos…or How SETI has taken a worrisome turn into dangerous territory. We aren't saying this is likely.  We are saying that sensible people should discuss it before arrogant fools scream into the cosmos "Yoohoo!" on our behalf.


#12 Given the scale of just our own galaxy, much less the vastness of the universe, the likelihood of anyone being in our celestial "neck of the woods" is slim at best. I'd propose that there's no paradox...if they're out there, they're just too far away.  –Jared Freeman


True, we might simply not overlap with the others! But this assumes that the number of advanced races is very small in order for the statistical non-overlap idea to work.  But if there are numerous long-lived species, then we get the Fermi Paradox. And if they travel?  A lot?  Colonization changes all the numbers!


#13 The civilizations that are advanced enough to communicate with us are too advanced to want to communicate with us. —Derek Whittom


Hrm. So we're like ants to them?  Well there are still plenty of human scientists who are interested in ants.  You neglect how inherently interesting we are!  The number of new tech races appearing in the galaxy at any time is not comparable to ant colonies on Earth.  At absolute maximum it might be one or two a year. Any truly advanced race would deputize specialists, or robots, or lesser selves to look into and see what such newbies might have that's interesting or entertaining to offer.  And to inspect them for potential danger.


Of course, they might do that in secret… 


Good stuff.  What impresses me most among those of you who answered – and those of you who will continue the discussion here - is your mental agility and verve. Keep at it! Stay interested and lively.  And make sure that our politicians are forced to discuss issues of science and the future.  


Never let us stop being a vigorously future-facing and scientific civilization.