If we’re willing to make assumptions about how likely it is that life arises on planets with certain similarities to a young Earth, we can indeed draw conclusions about the likelihood of intelligent life throughout the galaxy. The only problems are that our conclusions are only as good as our assumptions, which we have no reason to believe are very good. There may well be 36 alien civilizations in the Milky Way right now, but science has a long way to go before anyone — even the paper’s authors — are convinced of that conclusion.
As a police tool, the current deployment of tear gas reinforces the effect that made gases so powerful in World War I: fear. With reports showing tear gas being used on peaceful protesters, that fear is itself a deterrent by law enforcement on public demonstrations. Given legitimacy by the CWS in the interwar period, tear gas provides police with a chemical weapon that is no longer permitted in war.
Where are the alien ham radio operators beaming scientific secrets or extraterrestrial poetry? Why no mysterious engineering projects out among the stars? Where’s our invitation from the Galactic Council? As the great physicist Enrico Fermi once asked, “Where is everybody?”
Maybe the Great Filter got them, Dr. Hanson proposed. The Great Filter is a civilization-scale event or circumstance that would prevent a species from colonizing space or ever meeting other species — perhaps of even continuing to exist.
The filter could be a chemical bottleneck that prevents the formation of RNA that jump-started evolution, or a geophysical roadblock to the production of oxygen, which enabled multicellular creatures. But the filter could also be nuclear war, or a world-destroying asteroid, or global warming, or a malevolent artificial intelligence gone amok. Or, even, a vicious pandemic.