To be clear, by "in a hundred years" I don't mean that with current technology it'd take the sail a hundred years to get there, I mean that by improving the tec

To be clear, by "in a hundred years" I don't mean that with current technology it'd take the sail a hundred years to get there, I mean that by improving the tech for a hundred years with an extrasolar destination in mind, we'd be able to send a probe that arrives there on human timescales. Currently there have only been low-orbit tests of solar sails, they need much more development to go interstellar, but not so much that it's completely infeasible to achieve in the near-ish future. Since we don't have much reason to want a closeup of planets in other star systems yet, we don't really put much effort into the associated propulsion technologies. But the detection of nearby life would change that by giving us a destination, similar to how the Moon and Mars give us a sort of 'ladder' of destinations for directing our development of space technology. Say we find that single-celled life is very common, then, considering our lack of detection of multi-cellular life, we'd have reason to put more effort into understanding the conditions which led to the emergence of multicellular life on Earth, since by the only statistics we would have, that must be a significant filter on the development of life. By having data on the environment they evolved in, we could understand our own evolution (eg if that planet is much younger than Earth, it could serve as a picture of what early Earth might have been like, if it's similarly old, we could try to understand what went differently on Earth). We could also figure out if they might be based on similar chemistry etc, all of which would feed into our understanding of the origins of life on Earth. Detecting intelligent life would have a different, more 'philosophical' effect, even if we couldn't interact with them. The galaxy is theoretically colonizable over millions of years. The technology to do so is probably only thousands of years away for us. But, since the galaxy has existed for billions of years, and has been hospitable to life for at least 4 billion years, it is unreasonable to assume that much of that intelligent life won't have been around for millions of years longer than ourselves. In which case, why aren't they already everywhere? It'd have to be some sort of nearly fundamental limitation all intelligent life runs into, in which case it'd also affect what we as a species can aspire to achieve. The more intelligent life we find, the more unsurmountable the limitation becomes.