Free Astronomy Magazine March-April 2023

36 Considering the vastness of the Milky Way, even a thousand uniformly distributed techno- logical civilizations would be considered “relatively few” be- cause it would mean that there are, on average, one for every 200-300 million stars and, on average, they are far enough away from each other as to make any attempt at “physical contact” impossible. Perhaps, however, electromagnetic sig- nals are not too distant to ex- change, either intentionally or casually. Even if only a tenth of those civilizations routinely sent signals of various kinds into space powerful enough to be discernible hundreds or thousands of light-years away, the chances of registering them with SETI programs would not be entirely negligible. But that has never happened. Why? Several solutions have been proposed to this aspect of the Fermi paradox: perhaps we don’t know which specific wavelengths to tune into, or we don’t “listen” for long enough, or we still don’t know where to point our instru- ments. There could also be forms of communication that we do not know of, or there simply are no alien signals trav- eling in the galaxy, at least not in our direction. Regardless of assessments made by any aliens, intentionally sending messages to specific targets requires some energy, and unless this is available in unlimited quantities, it is likely that any aliens will also have to select the most promising targets. From anywhere in our galaxy, a civilization analyzing our planet’s atmosphere could at best establish that there is life here, although without un- How likely is extraterrestrial life? What does existing scientific knowledge about physics, chem- istry, meteorology and biology tell us about the likelihood of extraterrestrial life and civilizations? And what does the fact that there is currently no credible scientific evidence for the existence of extraterrestrial biospheres or civilizations teach us? This book reviews the various scientific issues that arise in considering the question of how common extraterrestrial life is likely to be in our galaxy and whether humans are likely to detect it. Life as we do not know it For the past twenty years, Peter Ward has been at the forefront of popular science writing, with books such as the influential and controversial Rare Earth. In Life as We Do Not Know It, Ward, with his signature blend of eloquence, humor, and learned insight, vividly details the latest scientific findings, cut- ting-edge research, and intrepid new theories on the subject of alien life and the possible extraterrestrial origins of life on Earth. In lucid, entertaining, and bold prose, Peter Ward once again challenges our notions of life on earth (and beyond). Are we being watched? — The search for life in the cosmos There is no more fascinating question than whether or not we are alone in a vast universe. Here, Paul Murdin applies the lat- est scientific discoveries and theories to inquire whether life exists on other planets and, if so, what forms it might take. Could there be somewhere life as advanced as here on Earth, or are we more likely to find primitive life-forms? Or are we the sole living organisms in a desolate and boundless cosmos? Professor Murdin invites us to join him in exploring an extraor- dinary array of evidence to determine if there is life elsewhere in the cosmos. Signatures of life — Science searches the Universe Are we alone in the universe, or is life a universal phenomenon? For decades, astronomers in SETI have scanned the Universe for intelligent signals, but with no success. In this intriguing book, Edward Ashpole explains the probable reasons for this and dis- cusses other avenues of investigation more in line with the nature of science and technology. The author examines the problems in- herent in scanning the universe for radio or optical signals from alien intelligence. These include the difficulty of trying to commu- nicate with another species possessing a completely unknown form of technology, and the vast distances that alien communica- tions would have to travel to reach us.