Top critical review
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Thought provoking but not gripping
on 4 January 2015
As its title suggests, this 1999 book by Martin Rees, the UK's Astronomer Royal, addresses six numbers that determine whether a universe can support life as we know it. The first number Rees calls N, which is the ratio of the gravitational force to the electromagnetic force and is about 10^36. He explains how, if this ratio were less, and therefore gravity was relatively stronger, stars would be much smaller and would burn much quicker. There would not be sufficient time for life to evolve.
The second number, epsilon (ε), dictates the strength of the strong nuclear force. Rees refers to ε being equal to 0.007 and this denotes the proportion of energy that is released when hydrogen fuses into helium. If this force was weaker than it is (say, 0.006), then nuclear fusion wouldn't happen; we would have stars full of hydrogen but there would be no nuclear reactions. Conversely, a stronger force (say, 0.008) would have led to protons joining with other protons shortly after the Big Bang so that there would be no hydrogen left to fuel the stars. Either way, life as we know it would not have been possible.
The third number is omega (Ω), which Rees defines as the ratio of the actual density of the universe to the critical density needed for gravity to bring cosmic expansion to a halt. If the ratio were too high, the universe would collapse in a big crunch; too low and expansion would be too fast to allow matter time to condense into stars and galaxies. Discussion of Ω led into a discourse on dark matter, which accounts for about 90% of the universe but which could be made up of entities with masses ranged from 10^–33 g (neutrinos) up to 10^39 g (heavy black holes), an uncertainty of more than seventy powers of ten. As Rees points out, our ignorance on this subject is somewhat embarrassing.
Lambda (λ) is the fourth number, which concerns whether the expansion of the universe is increasing or decreasing. In fact, λ is almost, but not exactly, zero; it is a measure of dark energy although this phrase is not used by Rees. λ was first postulated by Einstein in his General Theory of Relativity as a form of anti-gravity to achieve a static universe but he later gave up this idea when it was shown by Hubble that the universe was actually expanding. Einstein later referred to his abandonment of this concept as his "greatest blunder".
Q is the fifth number and this refers to the amplitude of the irregularities in the density of the universe shortly after the Big Bang. If the universe had been completely homogenous then there would have been no areas where gravity was slightly greater than other areas, such areas seeding the formation of stars and galaxies. Q has a value of about 10^-5 which reflects the fact that gravity within galaxies is exceedingly weak. This value is crucial; were it much smaller, or much bigger, the texture of the universe would be quite different, and less conducive to the emergence of life forms. A much smaller value of Q would stop galaxies forming, whilst if much larger it would result in super large galaxies but with black holes rather than stars.
The last of the six numbers Rees calls D which is the number of spatial dimensions. In our universe D is equal to 3 and Rees argues that if D were not equal to 3 then life would not exist.
I can't say that I really enjoyed this book and, as a consequence, it took me a long time to finish it. Undoubtedly, Rees is an expert in his field but the excitement that he must feel for his subject didn't come across to me. His account didn't grip me and it seemed to me to be rather disjointed. For example, whilst the book is concerned with six numbers, it has eleven chapters, with three of the extra five being interspersed with the six on the numbers; I found these interruptions disrupted the flow of the book. Notwithstanding these criticisms, some parts of the book were thought provoking and prompt me to read more accounts on cosmology. Essentially, it is a book about the Anthropic Principle although this is another term that the author never uses.
Aside from the discussions on the six numbers, there were some interesting observations in places. For instance, I was surprised to read that it was as early as the late 14th century that it was postulated that planetary systems were probably commonplace around other stars and that some of these planets would undoubtedly hold intelligent life. This concept was the brain child of the far-sighted Giordano Bruno but, needless to say, his views were considered heretical by the Church and for this and other "crimes" he was burnt at the stake in 1600. It was also fascinating to learn that at the time Rees wrote his book there had only been one instance where the precursor star of a super nova was known. This was in 1987 and before it exploded the star in question had been a blue one of about twenty solar masses.