Werner Heisenberg

True revolutions in science :

• involve spectacular discoveries and rapid advances in understanding
• change the concepts on which the subject is based.

Such a fundamental transformation took place in physics during the first 30 years of this century, culminating in what has been called the Golden Age of Physics.

The developments that triggered this monumental convulsion involved the formulation of 2 dramatically new theories.

1. A theory of space, time and motion, called relativity.

2. A theory of the nature of matter and of the forces that act on it.

This came from 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.

Heisenberg played a leading role in the early formulation of quantum mechanics and in the subsequent clarification of its revolutionary implications.

The famous ‘uncertainty principle’, a key component in quantum physics, is named after Heisenberg.

Right up to his death in 1976, he retained a deep interest in the nature of the quantum universe and the profound philosophical implications that flow from it. The exposition that follows is a sweeping survey of these ideas, together with an appraisal of relativity and some aspects of nuclear and particle physics.

It is a model of clarity and one of the most lucid accounts of the so-called Copenhagen interpretation of quantum mechanics that has become the standard viewpoint.

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.

In many ways the conceptual upheaval demanded by the theory of relativity is more easily accommodated than that due to quantum mechanics. True, relativity contains some strange ideas, such as time dilation and length contraction, curved space and black holes.

It also asserts that certain types of question, which sound perfectly reasonable and meaningful, have no unambiguous answer.

To ask, for example, at what time an event occurs, or whether two events that are separated in space occur at the same moment, may not be answerable as the questions stand because the theory tells us that there is no absolute universal time, nor is there a universal concept of simultaneity. Such things are relative and have therefore to be referred to a specific reference frame before the question has meaning.

But although these ideas are strange and unfamiliar, they are not obviously absurd. Nor do they present any real interpretational problems. For this reason the theory of relativity, in both its special and its general forms, must be considered uncontroversial.

The deepest philosophical problem presented by relativity is the possibility that:

• the universe may have had its origin at a finite moment in the past
• this origin represented the abrupt coming into being not only of matter and energy but of space and time as well.

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 2 are 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. But the violence done to our concept of physical causation is pro-found, and it is only very recently, within the context of modern quantum cosmology (developed after Heisenberg’s death), that a satisfactory picture of the origin of space-time has been forthcoming.

By contrast with the theory of relativity, quantum mechanics presents us with much greater conceptual and philosophical problems, and it is these problems that Heisenberg addresses so clearly. It should be stressed at the outset that most students learn quantum mechanics prescriptively and apply it without ever having to become embroiled in philosophical issues.

The practical application of quantum mechanics is extraordinarily successful and has penetrated many areas of modern science and technology. Nobody questions what the theory predicts, only what it means.