In 1938 Orson Welles put on a radio show in New York City that dramatized the famous novel by H.G. Wells, _The War of the Worlds_. This novel is about an invasion of Planet Earth by intelligent beings from Planet Mars, with this invasion just barely being defeated. Several movies have been made of this famous novel, probably the first to present this now long-running sc-fi theme of our planet being invaded by aliens from outer space. However, whar was especially important about this particular radio performance 80 years ago is that many people turning on their radios and hearing ongoing reports of a Martian invasion of New Jersey is that many people believed it and a temporary panic ensued. Lots of people thought it highly likely that Mars was inhabited by intelligent beings who were a threat to us.
How things change. Now we have sent several vehicles to Mars, where not only are there not dangerously threatening intelligent beings, but we have yet to find any signs of even single-cell life, although the recent discovery of some actual water there may yet possess the possibility that some simple form of life is there, or perhaps was, although increasingly the search for probably only single-cell life to moons of Jupiter and Saturn, with no luck so far. If there are intelligent space aliens, they are on planets in other star systems. However, until recently, given over 100 billion stars in our Milky Way galaxy, various estimates of the probabilities involved had it as near certain there is life elsewhere in the galaxy, and also highly likely that there is intelligent life of some source, probably on multiple planets. And we have indeed discovered "exo-planets" around many stars (I note that my brother-in-law, Michael Werner, has long been a major leader of the search for these exo-planets using the Spitzer infrared telescope).
These calculations suggesting a high probability of intelligent life elsewhere triggered the initiation of the Search for Extra-Terrestial Intelligence (SETI) about a half century ago, sending out various messages hoping to get a reply from somebody out there. So far there have been no replies. This has begun to shift views to the point that now we have flipped to the opposite view of that held in 1938: now we have commentators suggesting that the probability of life is much lower than previously thought, that we arose from a very curious and special set of circumstances, these so weird we may in fact be alone in the galaxy, and even possibly in the entire observable universe.
A recent example is an article in the latest Scientific American (September, 2018): "Alone in the Milky Way" by John Gribbin (subtitle, "Why We Are Probably the Only Intelligent Life in the Galaxy"). He notes a sequence of supposedly "improbable coincidences" necessary for even simple life to arise, much less technologically advanced intelligent life like we are. Supposedly we are lucky that earth was formed late in the life of the galaxy, allowing us to accumulate lots of metals. Also, supposedly only a narrow band of the galaxy will support life: too close to the center too many "accidents" and too far not enough metals for building rocky planets. Then there is the matter of having a rocky planet just the right distance from a star in a Goldilocks position: not too hot and not too far. Gribbin notes that it only took a billion years from earth's beginning to get single-cell lift, but much longer periods to get to more complicated forms, with the jump to multiple-cell organisms onl coming 700 million years ago. Then there is the matter of actually evolving beings as smart as us, with Gribbin noting that our ancestors nearly went extinct about 70,000 years ago. He says this string of events is so improbable that probably we are "alone in the Milky Way." Many are now taking such arguments seriously, with Tyler Cowen at Marginal Revolution just a few days ago posting an estimatte of "the economic value of the [observable] universe" (about 60 septillion dollars, supposedly), with this estimate being based on the assumption of one planet per galaxy like ours with beings like us and economies like ours.
All this may be correct, but I have a suspicion that this is one of those swings that is going too far in the opposite direction of what was previously widely believed,in short, an intellectual fad. For starters, SETI has only been at it for about half a century. If some intelligent beings have heard us and responded, their message will need to get back to us, which means that the effective distance of SETI is only about 25 light years. The Milky Way is 100,000 light years across, so SETI has effectively reached only a tiny portion of our galaxy, tiny to the point of miniscule. I could poke at some other of the pieces of the argument, but will not go beyond this point here. My guess is that the probability of intelligent life in our galaxy is higher than Gribben estimates, although maybe not as high as thought several decades ago.
This said, I came to the bssic conclusion that we may be rarer than previously thought some years ago, indeed due to thinking about the non-response to our SETI transmissions. This led me to come to a similar ultimate conclusion as Gribbin does in his article: that if we are so alone and special, then this puts on us as a species a special responsibility to manage our planet and ourselves especially carefully. I definitely think this. Indeed, I came to this point at the very end of my 2011 book, Complex Evolutionary Dynamics in Urban-Regional and Ecologic-Economic Systems: From Catastrophe to Chaos and Beyond, at the end of a chapter contemplating the problem of climate change. I closed the main text of the book by quoting Nasssim Taleb from his book, The Black Swan:
"Imagine a speck of dust next to a planet a billion times the size of the earth. The speck of dust represents the odds in favor of your being born, the huge planet would be the odds against it. So stop sweating the small stuff. Son't be like the ingrate who got a castle as a present and worried about the mildew in the bathroom. Stop looking the gift horse in the mouth - remember that you are a black swan."
To this I attached a footnote, the actual last thing in the book (except for a mathematical appendix), which says (p. 211),
"A broader perspective of this involving planets is the absence so far of any signals indicating life on other planets by [SETI] that has going on for decades now...The apparent rarity of life such as ours, at least in our region of the galaxy, suggests that we may face a greater responsibility than we thought for the proper care and stewardship of our planet and its global ecosystem and noosphere."
Gribbin concluses (p. 99) that, "And if our planet is so special, it becomes all the more important to preserve this unique world for ourselves, our descendants, and the many creatures that call Earth home."