# Are we alone in the Universe? The Fermi Paradox

Previous “Astrobiology” – Next “Interactive Drake Equation”
With an estimated diameter of 93 billion light years and age of 13.7 billion years, our Universe is an astonishingly big place that’s been around for a very long time. When you look up, you only get a short glimpse at a fraction of the hundreds of billions of stars that populate our Galaxy (which in turn is one of hundreds of billions in the cosmos), but it’s enough to make you wonder: “Are we alone?” In the previous post we discussed the likelihood of the emergence of (intelligent) extraterrestrial life. Starting from the famous Drake Equation and using recent findings in astrophysics and some astrobiology arguments, we obtained a simple way to estimate $$N$$, the number of communicative civilizations in our Galaxy. This reduces to the product of the chance of emergence of intelligent life $$f_i$$ and the longevity $$L$$ (in years) of a civilization’s communicative phase:

$\label{eq:Drake_simplified} N \approx \, \frac{1}{4}\, f_i \, L \,.$

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 (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.

1. Some peculiar solutions to the Fermi Paradox do not require $$N$$ to be small, see this post for a nice discussion

2. Of course the third option is that both $$f_i$$ and $$L$$ are small