Are Earths Rare?
When Enrico Fermi at a lunch in 1950 asked the famous question "Where are they?", his colleagues immediately understood what he meant. Almost everything favoured that life should be very prevalent in our galaxy, the Milky Way. It was known the galaxy contained at least 100 billion stars, some of them very similar to our Sun. And life on Earth had not developed to higher levels until the last few million years.
Since the universe and the galaxy is several thousands of million years old, should not life already have been created and developing far earlier somewhere else? Then one fascinating question becomes: where and when does the evolution of intelligence terminate? Should not someone already have colonised the entire galaxy, in a way that we on Earth should have noticed long ago?
The question relates to the notion of the "Great Filter", something which in a mysterious way stops the galaxy from being colonised. This topic is thoroughly discussed among others by Nick Bostrom, Swedish philosopher, who argues that this filter exists, either in the past or in the future. If it existed in the past, in the form of a very narrow threshold, it could be creation of life itself, or it could be its advancement to intelligence. In that case humanity has been lucky enough to pass it and face a very bright future.
However, If it lies looming in the future, humanity can expect to become extinct before conquering of the galaxy becomes possible.