Werner Heisenberg

The developments that triggered this monumental convulsion involved the formulation of two dramatically new theories. The first was a theory of space, time and motion, called relativity. The second was a theory of the nature of matter and of the forces that act upon it. The latter had its origins in Max Planck's observation that electromagnetic radiation is emitted in discrete packets, or quanta. In the 1920S this ‘quantum theory' was elaborated into a general quantum mechanics.

The central theme of Heisenberg's exposition, which is based on his 1955—6 Gifford lectures at the University of St Andrews, is that words and concepts familiar in daily life can lose their meaning in the world of relativity and quantum physics. Thus questions about space and time, or the qualities of material objects such as their positions, which seem entirely reasonable in everyday discourse, cannot always be meaningfully answered. This in turn has profound implications for the nature of reality and for our total world view.

the theory tells us that there is no absolute universal time, nor is there a universal concept of simultaneity

Probably the deepest philosophical problem presented by the theory of relativity is the possibility that the universe may have had its origin at a finite moment in the past and that this origin represented the abrupt coming into being not only of matter and energy but of space and time as well. Indeed, the central lesson of the theory of relativity is that space and time are not merely the arena in which the drama of the universe is acted out but part of the cast. That is, space-time is as much a part of the physical universe as matter; in fact, the two are ix

intimately interwoven. As Heisenberg remarks, the idea that time does not stretch back for all eternity but was created with the universe was anticipated in the fifth century by St Augustine.

There is thus a scientific counterpart to the creation ex nihilo of Christian tradition

At the heart o f the quantum revolution is Heisenberg's uncertainty principle. This tells us, roughly speaking, that all physical quantities that can be observed are subject to unpredictable fluctuations, so that their values are not precisely defined. Consider, for example, the position x and the momentum p o f a quantum particle such as an electron. The experimenter is free to measure either of these quantities to arbitrary precision, but they cannot possess precise values simultaneously. The spread, or uncertainty, in their values, denoted by Ax and Ap respectively, are such that the product of the two AxLp, cannot be less than a certain constant number. Thus more accuracy in position must be traded for less in momentum, and vice versa. The constant that enters here (called Planck's constant after Max Planck) is numerically very small, so that quantum effects are generally only important in the atomic domain. We do not notice them in daily life.

It is essential to appreciate that this uncertainty is inherent in nature and not merely the result of technological limitations in measurement. I t is not that the experimenter is merely too clumsy to measure position and momentum simultaneously. The particle simply does not possess

simultaneously precise values of these two attributes.

Thus the popular model of the atom, with electrons circling the nucleus along distinct orbits, is badly misleading. Heisenberg tells us that such a model can be useful in producing a certain picture in our minds, but it is a picture that has only a vague attachment to reality.

This unpredictability of quantum systems does not imply anarchy, however. Quantum

mechanics still enables the relative probabilities of the alternatives to be specified precisely. Thus quantum mechanics is a statistical theory.

The classical world view, so passionately espoused by Einstein, accords well with

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common sense by asserting the objective reality of the external world. It recognizes that our observations inevitably intrude into and disturb that world but that this disturbance is merely incidental and can be made arbitrarily small. In particular, the microworld of atoms and particles is considered to differ in scale, but not in ontological status, from the macroworld of experience.

Heisenberg's thesis. How, he asks, can we speak about atoms and the like if their existence is so shadowy? What meaning are to we attach to words that refer to their qualities?

The question Ìs an electron a wave or a particle?' has the same status as the question Ìs Australia above or below Britain?' The answer is `Neither and both.'