Where is Everybody? The Fermi Paradox
One summer day in Los Alamos in 1950 a lunchtime discussion among a few theoretical physicists (Enrico Fermi, Edward Teller, Herbert York and Emil Konopinski) had covered flying saucers. Fermi did a few quick calculations and asked, jokingly "Don't you ever wonder where everybody is?". The paradox lies in the astrophysicist's central dogma of mediocrity: we live in a time and place which are ordinary in every way. If we exist, there should exist other civilizations, and we should not be the first; some should be older and much more technically advanced than we are. If space travel is possible, then some of them should have visited us by now. As we shall see, it should only take a few million years for an advanced civilization to explore our whole galaxy. With potentially thousands of such civilizations (see Drake equation earlier), we should be swimming in extraterrestrials. Why not? Fermi concluded that some assumptions must be wrong; either there is nobody else, or the rare civilizations are very far apart, or interstellar travel is impossible, or technological civilization lifetimes are too short.
Courtesy: Don Dixon
Reasonable galactic propagation times
Once interstellar travel is mastered, at what rate might you expect the galaxy to be explored thoroughly. How long before they find us? We have to make some assumptions. Let us assume that an average star-hop is 10 light-years distance and, at 1/10th the speed of light thus takes 100 years, ignoring acceleration and deceleration. Let us imagine that once a planet is reached, it takes only 100 years to produce another two exploratory craft, which are then launched outwards into unexplored space. At this rate a 50,000 light-year (ly) diameter galaxy would be crossed in one million years, with significant exploration by the 25000 exploratory craft from 5000 colonies!
You can imagine that the trip and next generation craft production might take longer, say 1000 years each. There are still only 5000 hops of 10 ly in a 50,000 ly galaxy, now taking 2000 years instead of 200. The total time only goes up to 10 million years. An optimized program, interleaving long and short hops for maximum exploration coverage would take less time. These exploration programs might be best left to robots in stripped-down craft, but one can also image follow-up ark-like expeditions.
Faster (robotic) exploration certainly seems feasible. Acceleration to relativistic speeds and then no stopping ensures a speedy journey, crossing a 50,000 ly galaxy in a bit over 50,000 years. A first craft might drop off light-weight sensors onto planets; these sensors would radio data to a follow-up craft trailing by two years or so, which would relay the message back home. Yes, slowing down the probes could be a problem, but stellar radiation pressure on an infalling solar sail might do the trick. You can doubtless think of other schemes.