Physics and Philosophy
by Werner Heisenberg (1958)
Werner Heisenberg was one of the founders of quantum mechanics and an exponent of its Copenhagen interpretation. In this collection of essays, he tries to place the quantum revolution in a wider philosophical context. Mostly, it is a story of prior philosophies having been proven inadequate, although interestingly enough, Heisenberg explicitly connects aspects of quantum states to the Aristotelian concept of potency. The Copenhagen interpretation of quantum mechanics, to which Heisenberg subscribes in this book, has been subject to two well-known criticisms. First, we can only make sense of quantum mechanics as giving us probabilities for given observations by measuring devices assumed to be classical objects, and the existence of a non-quantum realm is an embarrassment if quantum theory is to be regarded as fundamental. Second, it can appear to eschew ontology altogether, to be not an interpretation of quantum mechanics but a positivistic reduction of it. To the first criticism, Heisenberg says that what distinguishes measuring devices is that they are not isolated, but interact with the outside world in countless messy ways, and these random environmental couplings somehow produce classical behavior. Physicists are still pursuing this idea; I suspect there’s something to it but am not yet sold on it.
More interesting is the quasi-Kantian framework in which Heisenberg places the Copenhagen interpretation. The math terminates on classical measuring devices primarily because classical physics describes core categories of the human mind that we need to make any sense of phenomena. Heisenberg is slightly more optimistic than Kant; we can get past the phenomena structured around our concepts to gain some knowledge of realms where deterministic causality, Euclidean geometry, etc break down, but this knowledge will always be mediated by the classical realm; the latter can never be entirely swept aside because of the kind of beings we are. We can learn that our classical ideas of space, time, matter, and causality break down–although a general lesson is that we can’t predict ahead of time where one of our concepts will break down–but we still need them. In the subatomic realm, we have the wave picture and the particle picture, each of which works in some regime and breaks down in others, but both of which are at least genuine ontologies. On the other hand, there is the full mathematical machinery of quantum mechanics, which never breaks down (so far as we know) but fails to provide an ontology.
This is something Heisenberg thinks we must learn to live with, but something which may actually be a blessing. As he understands it, the ultimate consequence of his great work has been to overthrow 19th century materialism. He sees alternate interpretations, such as that of Bohm, as desperate and mathematically unnatural attempts to rescue the old materialistic ontologies. No longer should we place blind trust in mathematically precise expressions of materialistic concepts. These have a range of validity, but it is limited and not to be categorically preferred to the natural intuitions of the human mind. As he writes
Furthermore, one of the most important features of the development and the analysis of modern physics is the experience that the concepts of natural language, vaguely defined as they are, seem to be more stable in the expansion of knowledge than the precise terms of scientific language, derived as an idealization from only limited groups of phenomena. This is in fact not surprising since the concepts of natural language are formed by the immediate connection with reality; they represent reality.
our attitude toward concepts like mind or the human soul or life or God will be different from that of the nineteenth century, because these concepts belong to the natural language and have therefore immediate connection with reality. It is true that we will also realize that these concepts are not well defined in the scientific sense and that their application may lead to various contradictions, for the time being we may have to take the concepts, unanalyzed as they are; but still we know that they touch reality. It may be useful in this connection to remember that even in the most precise part of science, in mathematics, we cannot avoid using concepts that involve contradictions.
The general trend of human thinking in the nineteenth century had been toward an increasing confidence in the scientific method and in precise rational terms, and had led to a general skepticism with regard to those concepts of natural language which do not fit into the closed frame of scientific thought–for instance, those of religion. Modern physics has in many ways increased this skepticism; but it has at the same time turned it against the overestimation of precise scientific concepts, against a too-optimistic view on progress in general, and finally against skepticism itself.
My copy has a fascinating afterward called “Science and Religion” in which Heisenberg reminisces on two conversations he had on religion with other physicists. It seems that Wolfgang Pauli and Niels Bohr also entertained hopes that the principle of complementarity would provide grounds for a rapprochement between religion and science, or at least an ease of tensions. Perhaps science does not provide a complete picture of reality because such a picture does not exist (at least for minds like ours), and perhaps religions speak in myths and parables not because they are false but because there are truths that can be expressed in no other way. Perhaps there are even resources here for wider rapprochements between rival religions and philosophies.
There is certainly some irony here. Heisenberg, a Lutheran Christian, thought he had dealt the death blow to 19th century materialism, just as Rene Descartes thought he had dealt the death blow to 17th century materialism. Needless to say, materialism is still going strong–stronger than ever–despite the brilliance of its opponents. In the case of Bohr and Heisenberg, even the memory that they ever saw their work in terms other than those of scientistic triumphalism has been largely forgotten.