Quantum Thinking

P C W Davies and J R Brown (editors ) The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics, Cambridge University Press, 1986.
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In the eighteenth century George Berkeley argued that there are only two kinds of things in the universe - ­minds and ideas, The concept of the latter is derived from our perceptions which are generally much clearer and more coherent than our own thoughts. He said that the concept of matter is redundant because we have no reason to suppose that there has to exist some thing which underlies our perceptions and somehow causes them. This line of argument raised the famous question: How do we know that an object exists when nobody is perceiving it? Berkeley's answer was to the effect that everything is at all times perceived by God: God creates and maintains the universe by keeping the idea of it in mind.

Many philosophers of Berkeley's time, and the overwhelming majority of them today, wish to consider the epistemological and ontological problems concerning the nature of space, time and matter without having to bring God into their arguments, However, they began to encounter serious difficulties when quantum theory was developed and its philosophical implications began to be discussed. In discussing the question as to whether the external world exists independently of our observations of it, the physicist David Bohm (one of the contributors to this book) says:

“Every physicist really believes that. For example, he talks about the universe having evolved before there was anyone around to look at it, except possibly God. Now unless you want to attribute it to God, as Bishop Berkeley did (and most physicists don't want to do that) you're unable to solve the problem of how the universe exists without physicists to look at it -- or without somebody else to look at it”

The problem arises from the unreality of unobserved quantum events. One example given is that of light passing through polarising filters arranged so that half of the light will pass through. If a photon approaches the filter it has a 50-50 chance of passing through, depending on its polarisation state which, according to quantum theory, is unpredictable. There is no way, even in principle, of determining what might happen except by making an observation of the photon either passing through or failing to pass through the polariser. It is the observation of the experimenter which causes one of the two possibilities to 'collapse' into reality. There are many ways of trying to solve the problems of interpretation posed by quantum theory and several of them are discussed in this book.

The competence of the editing and the clarity of the writing make it an excellent introduction to a difficult but fascinating subject – John Harney, Magonia 27, September 1987.