One way in which we encounter questions about symmetry in everyday life is when we look at ourselves in a mirror. Why is it that our reflection is reversed from left to right but not from top to bottom? The short answer, Close says, is that mirrors don't in fact, reverse left and right. The impression we have that they do is an illusion, he thinks, and he suggests some experiments we can do to convince us that this is the case. (One of these involves standing with your back to the mirror and bending backwards until you can see yourself upside-down in the mirror; Close says that you may then find that the mirror has reversed up and down, but don't try this unless you are young and flexible.)
Among the many discoveries made by Louis Pasteur was the existence of left-handed and right-handed molecules, which are handled differently by the body even though they are chemically identical apart from their shapes. Our bodies contain hundreds of different proteins but these are made up of only twenty-odd aminoacids. These exist in two forms, left-handed and right-handed. Almost all the aminoacids used by organisms in our world to build proteins are left-handed. (Sugars, in contrast, are right-handed.) There seem to be two possible explanations for this. It could be chance: perhaps the first life to have originated on Earth just happened to use a left-handed molecule and all its descendants followed suit. Alternatively, the aminoacids on Earth may have come from space via comets or meteors and these may have been mainly of one kind.
There is a little evidence to favour the out-of-space hypothesis. In 1969 a meteorite fell near Murchison in Australia. It contained a large number of organic molecules, including aminoacids, and these were predominantly left-handed. If this meteor is typical of those that reached Earth early in its history it would suggest that left-handed aminoacids predominated then, but we still have to wonder what determined the bias in favour of left-handedness in space.
A possible explanation for this bias has come from the discovery that there is a lot of circularly polarised light in the Orion nebula, where new stars are forming and there are also many organic molecules. Similar conditions are thought to have existed when our own solar system was forming. Circularly polarised light can selectively destroy either right-handed or left-handed molecules, so it is plausible that left-handed organic molecules were the survivors of intense bombardment by circularly polarised light at this early time. Yet more profound questions arise at this point: was the choice of polarisation itself a matter of chance, in which case there may be parts of the universe where right-handed organic molecules predominate, or are there reasons for a universal preference for left-handedness?
The second half of the book is concerned with physics. After something of a digression into the history of physics in the late nineteenth and early twentieth centuries, we come back to symmetry as it affects the structure of atoms. Radioactive decay under the influence of the "weak force" is an asymmetric process; our universe has a preference for left-handedness here.
In our world protons are electrically positive and electrons negative, but it is possible to have a world of reversed polarity, with negative protons and positive electrons (positrons). This is not just a theoretical possibility; antimatter can be produced experimentally, although positrons only exist for a very short time because they quickly encounter an electron and are destroyed. The mutual incompatibility of matter and antimatter means that our universe probably consists entirely of matter. (It is possible though unlikely that there are regions of antimatter remote from us.)
It is thought that matter and antimatter were created in equal amounts at the Big Bang, so why did the universe not simply dissolve into pure energy? This "is perhaps the greatest of the outstanding mysteries." As Close explains in his penultimate chapter, with the help of prints by M.C. Escher, it is possible that there is a slight but significant bias in the structure of the universe that created a predominance of matter over antimatter.
At the end we are left with many unanswered questions, which is of course good in a book about science. Many physicists still long for a resolution of asymmetry, often in the form of a theory known as supersymmetry (SUSY), but whether that is possible is still unknown. Close concludes his book with a quotation from Pasteur: "I can even imagine that all living species are primordially in their structure and in their external forms, a function of cosmic asymmetry."
The book is, I think, somewhat uneven, and not all of it is equally interesting. It is also a little dated; it was published in 2000 and a lot has happened in science since then, including the discovery of the Higgs boson, which Close talks about in relation to the Large Hadron Collider at CERN. I liked his suggestion that the facts of biology may, as Pasteur speculated, be a consequence of the way the universe works as a whole. But it is still a speculation.
16-09-2015