Galaxy Simulations Offer a New Solution to the Fermi Paradox
I have a boring solution to the Fermi paradox.  It is old, but really, really boring.Start looking at things that are weird about our Solar System and the Earth.  Figure out which ones are probably necessary for intelligence.  Add those into the Fermi calculation.  And see that it is likely that we're the first intelligent life in this galaxy.What are some of those special things?  It is not enough to have a planet that has the right ingredients to start life.  It is necessary that it's progress not be constantly wiped out by major disasters over a very long time scale.  What kinds of disasters?You don't want a supernova going off too close.  We have been lucky to avoid that.  Our odds were greatly improved by the fact that we spend most of our time out of the galactic plane, away from other stars that could be about to go boom.  This is an unusual orbit.You don't want to be hit by too many comets.  We've been hit by some, but far, far less than we would have without Jupiter acting like a sweeper to clean up dangerous stuff in our neighborhood.  How rare is that?  We have cataloged planets in hundreds of other solar systems.  We see ones with only rocky planets.  Ones with only gas giants.  But we're the only one we know of with both gas giants and rocky planets.  The only one where the rocky planets wind up protected from most of dinosaur killer kinds of impacts.Oh, and get this one.  Without the Moon, the tilt of the Earth's spin is unstable.  Without it, in simulations we would wind up with one pole aimed at the Sun and the other not once every few tens of millions of years.  Probably not good for the development of intelligent life.  Again we don't know how rare this is, but we suspect that it is rather uncommon.Suppose that each of these only happens to one star out of a thousand.  Suppose further that there is one more, as yet unidentified, special factor about us that is also required.  Again make that a 1/1000 coincidence.  That would make the odds good that of the ~250 billion stars in our galaxy, we're the only ones with intelligent life.Or if you do the back of the envelope just with the three that I named, and made them 1/10,000 coincidences, you get the same result.As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely.  And given Fermi's argument, it is probably unlikely at least on the scale of 100 billion to one against.