My latest book review:
Nicolaus Copernicus is credited with the realisation that the Earth is not at the centre of the Universe, but orbits around the Sun. This was a key step in the development of the idea that we do not occupy a special place in the Universe, and that, by implication, there may be nothing special about us, cosmically speaking. In the late twentieth century, this led to the so-called “principle of terrestrial mediocrity”, which says that our place in the Universe is so ordinary as to be typical — that we live on an ordinary planet, orbiting an ordinary star, in an ordinary galaxy. Caleb Scharf argues that this approach, what he calls the “Copernicus complex”, has gone too far. The Earth, he says, is a rather unusual planet situated in a rather unusual location; this gives a different perspective on the likelihood of other life forms like us existing across the Universe, which he puts in context by comparing our Solar System with the hundreds of other planetary systems that have now been discovered.
In order to get to the meat of his argument, Scharf runs through a breezy historical introduction, name-checking all the usual suspects from Aristarchus to Newton, via Copernicus, Tycho, Kepler and Galileo. The story of the discovery of our place in the Universe is a familiar one, but neatly summed up in a sentence: “The Sun with all its worlds is like a single raindrop on a particular hour of a particular day in a specific cloud somewhere in the skies of Earth.” [ED: Quote from p 52, please check that it is in final book] It is by making a comparison with other “raindrops” — other planetary systems — that Scharf reaches a conclusion that would have surprised previous generations of astronomers.
Our Solar System is a relatively orderly place, with widely spaced planets following roughly circular orbits around the Sun. This has allowed the Earth to be undisturbed for billions of years while life has evolved on its surface. While ours was the only planetary system known, it was natural to think that this is a typical example. But with many other planetary systems now known, it is clear that this is not the case. In most other systems, orbits are more elliptical and planets are closer together, allowing interactions which make chaotic disorder common and make it impossible for a planet to stay in a stable orbit with the right conditions for life for billions of years. Scharf calculates that we are in a 2 or 3 per cent “club”. In other words, that 97 planetary systems out of every hundred do not allow for the existence of Earth-like planets in stable orbits, providing suitable homes for life forms like us. “Our solar system is at least somewhat unusual, and we have the numbers to back that up.” [ED quote page 125]
He then goes on to consider the chances of complex life forms like us evolving even on those planets in the 2 or 3 per cent club. This is a much tricker proposition, since, as with the case of planetary systems a few decades ago, we only have one example to guide us. But in explaining why this is such a tricky problem, Scharf provides the best explanation that I have ever seen for the non-specialist of the statistical technique known as Bayes’ Theorem. It is almost worth reading the book for this alone, for Bayesian techniques underpin much of our everyday lives, including the spell-checker that is correcting my words as I write, and the number plate recognition systems that identify cars caught in speed trtaps.
The key features of life, as other people have observed, are that it involves self-sustaining cycles of activity, feeding off a flow of energy (for example, sunlight) and that it exists on the border between orderly and disorderly extremes — on the “edge of chaos”, as it is sometimes referred to. One consequence of this is that life drives systems away from chemical equilibrium. The classic example is the difference between the atmosphere of the Earth, rich in highly reactive oxygen, and the atmosphere of Mars, composed of stable, unreactive carbon dioxide. This alone tells us that Mars is a dead planet today, whatever may have happened on its surface in its youth. Scharf discusses these ideas clearly, with a particularly informative account of the role played by bacterial organisms in the story of life on Earth, but, curiously, without mentioning Gaia theory, which is the most powerful presentation of this kind of argument.
Finally, he looks at the Universe at large, which emerged from a Big Bang just under 14 billion years ago and is now expanding ever more rapidly, so that in billions of years time no other galaxies will be visible from the confines of our Milky Way. About 95 per cent of all the stars that will ever exist have already come into being, and for the rest of eternity galaxies will fade away as the stars age. “We exist during what may be the only cosmic period when the universe’s nature can be correctly inferred by observing what is around us.” [ED page 211]
The bottom line of the book is that planets like Earth in systems like our Solar System are rare, but not unique. That the particular kind of complex life forms that we represent may be unique, but that other forms of complex life may have evolved elsewhere along different pathways. “We end up with this: Our place in the universe is special but not significant, unique but not exceptional.” We could, says Scharf, “be special yet surrounded by a universe of other equally complex, equally special life forms that just took a different trajectory.” [ED both from page 221]
Which leaves us with the biggest question of all. If intelligent life is common in the Universe, why can we see no trace of it? In particular, why hasn’t it visited us? Dubbed the “Fermi paradox”, after the physicist Enrico Fermi who first pointed out how easy it would be, given the age of the Milky Way galaxy, for spacefarers to send probes to every Sun-like star, this is still the most powerful argument against the existence of extraterrestrial civilizations. On balance, it seems to me that Scharf is wrong; but I hope he is right!
is a Visiting Fellow in Astronomy
at the University of Sussex
and author of
Deep Simplicity: Chaos, complexity and the emergence of life (Random House)
A version of this review appeared in the Wall Street Journal