Lead article by John Polkinghorne: The consequences of quantum theory
The discovery of quantum theory in the first quarter of the twentieth century brought about the greatest revolution in understanding of the physical world since the discoveries of Isaac Newton. The Newtonian world of classical physics was clear and determinate; the quantum world is cloudy and fitful.
Our knowledge of it is limited by the uncertainty principle. If we know where an electron is, we cannot know how it is moving; if we know how it is moving, we cannot know where it is. In this strange world, light sometimes behaves like a particle (a little bullet) and sometimes like a wave (spread out and oscillating), a behaviour completely unintelligible in terms of everyday thinking.
These counterintuitive properties are due formally to what is called the superposition principle. In quantum physics one can add together in, a well-defined way, states that that classical physics would treat as totally immiscible. An electron can be in a state which is a mixture of both 'here' and 'there', in which it has no definite position. In consequence, quantum logic holds in the quantum world, different from the Aristotelian logic of everyday. The latter is based on the principle of the excluded middle: there is not state intermediate between A and not-A. In the quantum world there are many such intermediates, superpositions of A ('here') and not-A ('there'). This fact is linked to wave/particle duality. In classical physics there can only be states with a definite number of particles in them (look and count), but in the quantum world there are states with indefinite numbers of particles and it turns out that these are the states that manifest wavelike properties.
When a measurement of position is made on a state which is a superposition of 'here' and 'there', sometimes the result will be 'here' and sometimes it will be 'there'.
Interpretation of the phenomena
The relative probabilities of these results can be calculated from the proportions in the superposition, but it cannot be known which particular result will be obtained on a particular occasion. Quantum theory is irreducibly statistical and there are intrinsic unpredictabilities present in quantum physics. These unpredictabilities might be either epistemological or ontological in character. In the first case, they would be due to unavoidable ignorance of factors (called 'hidden variables') which, in fact, serve to determine precisely what happens. In the second case they would be due to an intrinsic indeterminacy in nature. It turns out that there are quantum interpretations of either kind which yield exactly the same empirical consequences. The choice between them, therefore, cannot be made on purely scientific grounds but it must be a matter for metascientific decision. Almost all physicists accept the indeterminacy interpretation, stemming from Niels Bohr, because they find the ignorance interpretation, stemming from David Bohm, to be clever but too contrived in character to be persuasive.
There is no universally accepted account of what it is that brings about a particular result on a particular occasion of measurement. This is called 'the measurement problem' and it is an illustration of the fact that scientists do not fully understand how the clear and reliable world of everyday experience emerges from its cloudy and fitful quantum substrate. After more than eighty years of exploitation, marked by the remarkable accuracy and success of quantum theory in its own domain, how it relates to a broader account of physical process stills remains problematic
A number of insights of general relevance and significance for theology can be culled from the history of quantum theory. The first is that there is no universal rationality. Aristotelian logic holds in the macroscopic world, but quantum logic in the quantum world. The second is that there is no universal epistemology either. Any attempt to know the quantum world with classical clarity is condemned to failure. That world can only be known in accordance with its Heisenbergian uncertainty. These insights certainly encourage theology to hold fast to what it has found to be to be the necessary character of its discourse about God.
The physical world has been found not to be merely mechanical in character, as many had thought following the deterministic discoveries of Newtonian physics. The world is something more subtle and more supple than a clockwork universe. The role of metascientific decision in interpreting quantum theory in terms of open process shows that we can take with due seriousness all that science has to say without being condemned to think of ourselves as automata or that God is confined to the role of an externally interfering Clockmaker. The Creator can be believed to be providentially active within the open grain of created nature. Science has not established the causal closure of the world on its reductionist terms alone.
If physics teaches us anything it is surely that reality is surprising, to a degree often beyond our power to anticipate without the actual nudge of nature. In consequence, the natural question for a scientist to ask about a proposition, within science and beyond it, is not 'Is it reasonable?', as if we thought we knew beforehand the shape that rationality had to take. No one in 1899 would have regarded wave/particle duality to be a rational possibility. Instead, the natural question for the scientist to ask is 'What makes you think that might be the case?', open to the unexpected but demanding evidential support for what is being alleged. I believe that theology can approach its truth-seeking task in exactly the same way.
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John Polkinghorne, KBE, FRS, is past President and now Fellow of Queens' College, Cambridge. Formerly Professor of Mathematical Physics at Cambridge University, he then turned priest and became the author of many books on science and religion, including "Quantum Theory. A very short introduction" (Oxford 2002) and "Science and Religion in Quest of Truth" (London 2011). In 2002, he recieved the Templeton Prize and became founding president of the International Society for Science and Religion (ISSR). More info here.