# Are we alone in the Universe? The Fermi Paradox

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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