If the universe is teeming with aliens...where is everybody? [Repost]

FROM THE REVIEWS:
"Webb offers coherent, understandable, and sometimes humorous coverage of a diverse range of topics. He provides readers with non-trivial insights into research fields they may not have encountered previously . . . I think everyone who has ever considered the possibility that other intelligent civilizations exist elsewhere within our galaxy will enjoy Where Is Everybody? They will find much to agree with, and much to argue about, in this very accessible volume." -SCIENCE
"WHERE IS EVERYBODY? is a delightful mental romp. With a light-hearted, enthusiastic tone, Webb offers lively coverage of UFOs, crop circles, and the books of Erich von D?niken, the infamous proponent of the idea that aliens visited the Earth in the distant past. Science-fiction fans will enjoy the frequent references to Star Trek, and science buffs will appreciate mention of the ideas of Carl Sagan, Fred Hoyle, Frank Drake, and Freeman Dyson. This book is a must-read for anyone who has ever pondered the question, "Are we alone?" -ASTRONOMY
"[Webb] is a polymath, able to write informatively - even authoritatively - on an exceedingly wide range of subjects, including physics, astronomy, biology, and neurobiology. His writing is encyclopedic in scope, lucid, often poetic - and in the end it is both enormously inspiring and a little sad if he's right, as I'm afraid he might be, in concluding that we are the only advanced civilization in the Galaxy. Readers are free to differ with Webb's conclusion, but they will be surprised to learn how convincing it is. I have read a good number of astronomy books this past year; but this is the one I regard as indispensable. If I were Robinson Crusoe - shipwrecked and lonely on an island in space - I would want this book with me." -MERCURY
During a Los Alamos lunchtime conversation that took place more than 50 years ago, four world-class scientists agreed, given the size and age of the Universe, that advanced extraterrestrial civilizations simply had to exist. The sheer numbers demanded it. But one of the four, the renowned physicist and back-of-the-envelope calculator Enrico Fermi, asked the telling question: If the extraterrestrial life proposition is true, he wondered, "Where IS everybody?"
In this lively and thought-provoking book, Stephen Webb presents a detailed discussion of the 50 most cogent and intriguing answers to Fermi's famous question, divided into three distinct groups:
- Aliens are already here among us. Here are answers ranging from Leo Szilard's suggestion that they are already here, and we know them as Hungarians, to the theorists who claim that aliens built Stonehenge and the Easter Island statues.
- Aliens exist, but have not yet communicated. The theories in this camp range widely, from those who believe we simply don't have the technologies to receive their signals, to those who believe the enormities of space and time work against communication, to those who believe they're hiding from us.
- Aliens do not exist. Here are the doubters' arguments, from the Rare Earth theory to the author's own closely argued and cogently stated skepticism.
The proposed solutions run the gamut from the crackpot to the highly serious, but all deserve our consideration. The varieties of arguments – from first-rate scientists, philosophers and historians, and science fiction authors – turn out to be astonishing, entertaining, and vigorous intellectual exercises for any reader interested in science and the sheer pleasure of speculative thinking.
Stephen Webb is a physicist working at the Open University in England and the author of Measuring the Universe.