Foreword

We live in heady and exciting times. Today scientists seriously consider whether they may soon have a sample of an alien biology to study: life from another world. Some of these scientists are old enough to remember that when the famous biologist Joshua Lederberg coined the term “exobiology,” it was ridiculed as “a science without a subject.” The scientific tide has turned, and today there is growing enthusiasm for trying to find out 
whether life exists elsewhere in the universe. 

 
Life as we know it is a planetary phenomenon. The Earth has hosted life and has strongly influenced its evolution for the past 3.8 billion years. Life in its turn has significantly modified its host planet. At one time, life may have called the surface of the planet Mars 
home, and some scientists speculate that Mars may still harbor life, deep beneath its surface, where liquid water might persist today. Europa and Callisto, two of the planet-sized moons of Jupiter, may have oceans of liquid water, and possibly even life, beneath their icy exteriors. With only our single example of terrestrial biology to guide us, our search for life beyond the Earth must start with searching for “habitable,” 
planetlike places.  As we learn more about life on Earth, and as we begin to appreciate how tough and opportunistic it is–living around the scalding-hot vents of the deep ocean floor, in sulfurous hot springs, in the radioactive cooling water of nuclear reactors, and within rock miles beneath the Earth’s surface–our definition of “habitable” expands. 

During the past five years, the number of planets known to be orbiting other stars like our own sun has grown from zero to more than 50! The biases imposed by our instruments have thus far excluded detection of other solar systems like our own. However, before another decade passes, we should know whether other worlds similar to the Earth are common or rare in our Milky Way galaxy.  This is a key piece of information.                                . 
 
The universe is vast and old. Humans are newcomers on the scene. Throughout our short history, we have looked to the heavens and wondered whether we are alone. Over the millennia there have been many religious and secular belief systems whose leaders have offered their own answers to this question. Increasingly throughout the 20th century and into the 21st, partial answers of a different kind have been pieced together.  Answers based on observation, experiment, and rigorous scientific study have emerged in fields as diverse as molecular biology and high energy astrophysics. 

 The authors of this text, Donald Goldsmith and Tobias Owen, introduce you to our current understanding of humankind’s place in the cosmos, and provide perspective by showing how our ideas have changed over time, and where they are likely to change again in the future. To do this it is necessary to consider scales of space and time so vast that they are measured in the billions, as well as scales so tiny that we measure them with billionths; we must consider both the universe and the world of viruses. Some of the claims in this text may seem quite incredible, but the authors provide the evidence to back up their claims. When they speculate, they tell you so, and illustrate the difference between scientific speculation and random wild ideas. If you take nothing more away from your interaction with this text and these authors than an enhanced capacity for critical thinking, you will be well served.                                                                . 
 

I have been fortunate enough to spend my scientific career in a search for intelligent life elsewhere. All of us working on SETI (the search for extraterrestrial intelligence) search by looking for some evidence of another technology; if we find it, we can infer the existence of intelligent technologists. Although we can and do make guesses (such as looking for radio signals), we don’t necessarily have to know what that technology is. In fact, if another technology were sufficiently advanced, we would be unable to conceptualize it! But by observing the universe, it may be possible to detect anomalies for which the simplest explanation is astro-engineering, rather than astrophysics. That approach requires that we start with a good understanding of the observed universe, such as the one provided by this textbook.  The fact that radio SETI searches have been going on for over forty years, and searches for short optical pulses for a few years, so far without  success, might indicate that other intelligent civilizations do not exist, but it is far too soon to draw that monumental conclusion. It also might mean we haven’t searched well enough or in the right way. We are a very young technology in a very old galaxy. 
Our ability to search for other technologies is quite primitive. In fact, we have yet to explore our own cosmic neighborhood systematically. Our tools are getting better, and as Guissepe Cocconi and Phillip Morrison stated in the first SETI paper to appear in a refereed journal, “The probability of success is difficult to estimate, but if we never search the chance of success is zero.” I’m enthusiastic about continuing the search.       . 
 

Scientists and engineers may soon be able to answer this old and fundamental question, Are we alone in the universe?   So, welcome aboard! Fasten your seat belts. This textbook will take you on a fascinating journey. Along the way you will discover that supernovae, carbonaceous chondrites, nuclear fusion, “inflationary” cosmology, greenhouse gases, comets, killer asteroids, gamma-ray bursters, bipolar outflows, continental drift, orbital eccentricity, and many, many other exotic sounding things really do have something to say about how you came to be here and whether there may be microbes or minds elsewhere in the universe. Today there are many unanswered questions, but our tools to search for answers are getting better and better. Your generation will not only answer many of today’s questions, but will undoubtedly raise new ones we can’t begin to imagine. Indeed, the truth is out there–but the real science behind searching for life in the universe is far more compelling than any movie or television drama. Happy searching! 

–Jill Tarter