Now we have some important observational constraints: we do not see alien spaceships landing on Earth and we have not detected E.T. signals coming from outer-space. SETI? No signals. In recent news, a space survey of 100,000 galaxies didn’t find any clear sign of advanced alien civilizations. The observations could only rule out the presence of massive galactic colonization, however, with aliens using an amount of energy comparable to the total output of their galaxy \cite{Griffith_2015}. Advanced aliens might be more energy savvy, but 100,000 is also a lot of galaxies.

In the words of Italian Nobel Laureate Enrico Fermi: “Where is everybody?” What Fermi meant is it’s quite surprising we have seen no sign of extraterrestrial intelligence, despite the fact the Universe is so vast and long-lived. This is the essence of the Fermi Paradox.

At this point I often hear saying: “Wait, but what about Roswell, the WOW signal, all those UFO sightings...?” I am not going into that. I will just state that the most economical explanations for the aforementioned stories have nothing to do with E.T. and that at this time there is no clear evidence proving we had contact with alien life. Let’s use Occam’s Razor and throw away the conspiracy spoon.

Back to the main topic, the Fermi Paradox suggests that the number of communicative civilizations \(N\) in the Galaxy is small¹. Our revised version of the Drake Equation then implies two² interesting alternatives:

1. \(f_i\) is a small number. Life is common in the Universe, but intelligent life is not.

2. \(L\) is a small number. Intelligent life does not spend much time in the communicative phase.