If you haven't heard of the Fermi Paradox, it goes something like this: given the vastness of the universe and the probability that implies of life evolving elsewhere, how come no alien civilization has ever gotten in touch? We have found many exoplanets in the brief time we've been looking. Surely there must be someone else out there who, like us, desperately wants to find others?
Since it was posed in 1950 by Enrico Fermi, there have been a range of answers, from the benign to the absolutely terrifying. One is that there simply hasn't been enough time yet. Alien civilizations may prioritize, as we do, searching for techno signatures, which we simply haven't been broadcasting for long enough. On the other end of the spectrum, it could be that the tendency throughout the universe is for civilizations to destroy themselves before they reach sufficient advancement to make contact.
But one answer, perhaps more unsettling than the others, is simpler. What if the reason why nobody has contacted us is because we're alone in the galaxy, or even the observable universe?
After the Fermi paradox came the Drake equation, which attempts to quantify the number of intelligent civilizations in our galaxy, or the universe. In it, we can place knowns or best guesses as to the number of stars that contain (for instance) planets in habitable zones, or best guesses as to how many of those will be able to sustain intelligent life.
One preprint study from 2018, which has not been peer-reviewed, argued that the Drake equation shouldn't use absolute values, however, but probability distributions given what we know. The Fermi paradox – that our estimates of alien civilizations in the galaxy do not match up with our zero telephone calls from them – arises only when we use absolute values, according to the team.
"The high confidence that causes this clash typically results from applying a Drake-like model using point estimates for the parameters. These estimates, however, make implicit knowledge claims about processes (especially those connected with the origin of life) which are untenable given the current state of scientific knowledge," the team wrote in their paper.
"When we take account of realistic uncertainty, replacing point estimates by probability distributions that reflect current scientific understanding, we find no reason to be highly confident that the galaxy (or observable universe) contains other civilizations, and thus no longer find our observations in conflict with our prior probabilities."
According to the team, which looked at the current scientific knowledge of parameters of the Drake equation and ranges of estimates on those parameters in the scientific literature, there is a significant chance we are alone.
"When we update this prior in light of the Fermi observation, we find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe [53 percent-99.6 percent and 39 percent-85 percent respectively]. ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable."
Though this may seem bleak, as we find out more (for example, finding more planets in habitable zones, or hints of life on exoplanets) this could change. But, given what we know at the moment, "we should not actually be all that surprised to see an empty galaxy", the team concluded.
The preprint has been posted to arXiv.