Review of Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life?

"On the way to lunch at Los Alamos Scientific Laboratory one day in 1950, Enrico Fermi and three other physicists--Emil Konopinski, Edward Teller and Herbert York--chatted about flying saucers. At lunch, when the talk had turned to other matters, Fermi suddenly said, "Where is everybody?" His companions realized that the talk of flying saucers had turned his mind to the possibility that there is intelligent life elsewhere in the universe and that he was asking why, if there is, we have seen no sign of it. The question encapsulates what is now known as the Fermi paradox."

Stephen Webb addresses this question in If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life.

The are three categories of answers: "they are here" (e.g. perhaps life on Earth was seeded from elsewhere, meaning that we are alien); "they exist but have not yet communicated," because they are too far away, have better things to do, or signalling is too hard or unwise; or "they do not exist," which means that life on Earth is very special and improbable.

The "they are here" idea is basically the plot of Prometheus: intelligent life from elsewhere "seeded" Earth with life. There is at least one little data point in support of this theory:

Crick and Orgel further suggested that directed panspermia might help resolve one or two anomalies in the biochemistry of life-forms on Earth. One of these was the puzzling dependence of biological systems on molybdenum. Many enzymes, for example, require this metal to act as a cofactor. Such a situation would be easier to understand if molybdenum were relatively abundant on Earth. However, its abundance is only 0.02% compared with 0.2% and 3.16%, respectively, for the metals chromium and nickel, which are chemically similar to molybdenum. Crick and Orgel commented: If it could be shown that the elements represented in terrestrial living organisms correlate with those abundant in some types of star-molybdenum stars, for example-we might look more sympathetically on "infective" theories.

Some of the most convincing answers are the "don't exist" ones. Also, if there are many reasons for them not to exist, then the expected distance to the ones that do exist rises, making communication harder and so less probable.

Other intelligent life first needs to be in a habitable galaxy. This can be an important constraint. For example, galaxies collide, which can be unpleasant for planetary systems in them. Our Milky Way galaxy has had a peaceful life so far, experiencing fewer collisions with other galaxies over the last 10 billion years. This would have made it more hospitable to life than galaxies which have suffered more collisions. It looks like our galaxy will collide with Andromeda - although this would not necessarily be harmful, and anyway it will happen after the end of the sun's useful life.

Within a galaxy, there is a "galactic habitable zone," which is the portion of a galaxy that a planetary system must reside in so that conditions will be suitable for life. This has to be close enough to the galactic center for heavy elements to form, because life seems to require what astronomers consider metals, i.e. elements heavier than hydrogen and helium. [Could you have just hydrogen and helium based life...?] But the further a planet lies from the galactic center, the less likely it is to be harmed by interference from other stars.

Then, within each planetary system (around each star), there is a circumstellar habitable zone within which it is theoretically possible for a planet with sufficient atmospheric pressure to maintain liquid water on its surface. This obviously matters if you believe that water (or some liquid solvent) is important for life. The habitable zone varies with the type and age of the central star. Stars like the sun have an attractive balance between habitable zone and other desirable qualities. This habitable zone constraint knocks out maybe 9 out of 10 potential planetary systems.

Another thought is that it is helpful to have another Jupiter-like planet within the same planetary system as the potentially habitable planet. The high mass attracts objects that might otherwise hit the habitable planet. For example, the Shoemaker–Levy 9 comet that hit Jupiter in 1994 had a fragment (G) that was ~1km in diameter and had left a scar on Jupiter the size of Earth. If Jupiter were not present, the frequency of impacts with our Solar System's inner planets would be higher.

Those are some of the "don't exist" explanations. The most interesting "do exist but haven't communicated" explanations are behavioral. I mentioned one a while back that was a Geoffrey Miller article,
Fermi's Paradox, Caused by Computer Games Not Atomic Weapons?

I suggest a different, even darker solution to the Paradox. Basically, I think the aliens don’t blow themselves up; they just get addicted to computer games. They forget to send radio signals or colonize space because they’re too busy with runaway consumerism and virtual-reality narcissism.

Of course, maybe they do blow themselves up. E.O. Wilson says,

"The foreign policy of ants can be summed up as follows: restless aggression, territorial conquest, and genocidal annihilation of neighboring colonies, wherever possible. If ants had nuclear weapons, they would probably end the world in a week."

This relates to the idea of the great filter, which essentially means: the easier it was for life to evolve to our stage, the bleaker our future chances probably are.

Anyway, this was thought-provoking and an easy read so I give it 4/5.