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Alien Civilizations, Asteroids, and the Fermi Paradox

by edlu | on Mar 20, 2012 | No Comments

A few weeks ago it was announced that most stars have orbiting planets, and that there are likely more than 100 billion habitable planets in our galaxy.  So life at least has the opportunity to be commonplace in our galaxy.  But this brings up a nagging question (first posed by Enrico Fermi): why don’t we see any signs of other advanced civilizations in our galaxy?  It is not for lack of looking.  The Search for Extraterrestrial Intelligence (SETI) project has for over 25 years been actively searching for radio and other signals that would presumably be emitted by intelligent civilizations.  There has to date been no such sign.

One possibility of course is that life itself is rare.  Maybe the Earth is just a special place, and life doesn’t even exist on other planets.  This runs counter to the Copernican revolution, which over the centuries has demoted the Earth from being the center of the universe to being an average planet orbiting an average star on the outskirts of an average galaxy.  If there are a multitude of places where life can thrive, why should life only have arisen on Earth?

Some have speculated that life is commonplace, but that advanced civilizations do not survive long because they also develop advanced weapons and they kill themselves off in great wars.  But there is another possibility.   Perhaps some natural process prevents civilizations from surviving long enough to be able to broadcast their presence.  Instead of civilizations knocking themselves out, maybe they simply lack the foresight to protect themselves from natural disasters.

It turns out that there is actually good reason to suspect this is the case.   The majority of these recently discovered planets orbit their respective stars as part of multi-planet systems, almost certainly along with leftover material that did not manage to coalesce into planets.  This debris, otherwise known as asteroids, will on occasion collide with the planets at great speed, just as it does here in our own solar system.

If a large enough asteroid hits a planet, nearly all life on that planet can be exterminated, just as happened when a 10km asteroid struck the Earth wiping out the dinosaurs.   But an asteroid does not need to be nearly that big to collapse a technically advanced civilization.  One paradox is that as societies become more advanced, they become more interdependent and in many ways more fragile.  Witness the worldwide economic and societal impacts of the recent Japan earthquake and of Hurricane Katrina in 2005 (both of which were relatively small events in the grand scheme of things).   On Earth, the impact of only a 1km asteroid is generally considered large enough to end human civilization.  That is because in addition to the direct devastation, it would throw up enough dust into the upper atmosphere to end growing seasons worldwide for a few years.  Given we only have a few months of food stockpiled worldwide, this clearly bodes badly for civilization as we know it.  After an impact like this, civilization would likely need to start over.  Impacts of 1km asteroids happen on Earth about once per million years.  Smaller impacts are even more frequent, and the global consequences are even harder to predict.  I would submit though that allowing even a 500 meter asteroid to hit the Earth, i.e. one we don’t believe to be large enough to end civilization, is not an experiment we want to conduct.

That means that between large asteroid impacts, there is a race of sorts.   Any civilization on any planet in our galaxy must develop the technology to prevent asteroid impacts and/or the ability to withstand them, or the civilization on that planet will be eventually be reset to a more primitive state.  We could consider this a test.  If a civilization advances fast enough, and has the foresight to confront this problem, it may ensure its long term survival.   If not, the consequences of orbital mechanics may wipe it out.  Perhaps this is why we don’t see any signs of other advanced civilizations.  This is also a warning to us.  Human civilization is only a few tens of thousands of years old, and the clock is ticking.  Will we pass the test?


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