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

Option 1: Intelligent life is rare

On Earth, it took more than 3 billion years for life to evolve from single-celled bacteria to Homo Sapiens. This is a long time compared to the emergence of “simple” life-forms, an argument often used to conclude that \(f_i \ll 1\). Scientists P. Ward and D. Brownlee in their book “Rare Earth” claim that “Intelligent life on Earth relied on so many unlikely accidents that we are probably alone in the Universe”.

Is there anybody out there? – silence

Option 2: The Lifetime of Communicative Civilizations is short

The other option is intelligent life is common, but the time an advanced civilization spends reaching out to potential galactic neighbors is short. There could be all sorts of reasons for that, including transition to more efficient forms of communication beyond electromagnetic signals, singularity, and a loss of interest in exploration. However self-annihilation through nuclear war or the exhaustion of natural resources definitely come to mind as viable options. Therefore, in this scenario the absence of contact tells us something important about the future of our own civilization, i.e. that there should be an important transition for our species happening in a short timescale \(L\). We can’t say for sure what will happen, beside that radio silence will follow. But we can estimate when<