Physics, philosophy of

2. The philosophy of physics today: convergence

Nowadays, the separation between physics and philosophy is, fortunately, much lessened. There are two main reasons for this. The first reason relates to the formative years of analytic philosophy of science, the second to developments within physics, especially since the 1960s.

First, physics had an enormous influence on the early phase of the analytic movement in philosophy. This influence does not just reflect the fact that all philosophers, and intellectual culture at large, had to respond to the explosive growth after 1850 in science and technology, which is now recognized as amounting to a second Scientific Revolution. Nor is it just that for the logical positivists and logical empiricists, and for important contemporaries such as Russell, physics represented a paradigm of empirical knowledge. Agreed, these facts are both important in themselves and historically influential: they mean that in contemporary philosophy of science, discussions of epistemology, or scientific method, and of the unity of science, are much influenced by the practice, and successes, of physics; (see Models in physics; Unity of science).

But physics also wielded much more specific influences on analytic philosophy. Each of the three main pillars of modern physics – thermal physics, quantum theory and relativity – contributed specific ideas and arguments to philosophical debate. Among the more obvious influences are the following:

Thermal physics. This divides, broadly, into two parts: thermodynamics, which relates concepts such as heat, work, energy and entropy, without regard to the physical system’s microscopic constituents; and statistical mechanics, which aims to explain macroscopic phenomena about heat etc. in terms of a postulated realm of atoms and molecules, interacting according to the laws of mechanics, and described using probability theory. Accordingly, the scientific controversy in the second half of the nineteenth century about the existence of atoms bore upon the philosophical debate between realism and instrumentalism. Also the rise of statistical mechanics fuelled the philosophy of probability (see Boltzmann, L.E.; Matter; Maxwell, J.C.; Probability, interpretations of; Scientific realism and antirealism; Thermodynamics; Time, direction of).

Quantum theory. The most pervasive influence of quantum theory in philosophy has undoubtedly been to make philosophers accept that a fundamental physical theory could be indeterministic. But this influence is questionable since, as every philosopher of science should know, determinism only enters at the most controversial point of quantum theory: viz., the alleged ‘collapse of the wave packet’ (see Determinism and indeterminism; Quantum measurement problem). In any case, the obscurity of the interpretation of quantum theory threw not only philosophers, but also the giants of physics, such as Einstein, Bohr and Heisenberg, into vigorous debate: and not only about determinism, but also about other philosophical fundamentals, such as the nature of objectivity, locality, and even the nature of logic (see Bell’s theorem; Quantum logic; Quantum mechanics, interpretation of).

Relativity theory, both special and general, revolutionized the philosophy of space and time, in particular by threatening both neo-Kantian doctrines that geometry must be Euclidean, and Poincaré’s conventionalist doctrine that, however physics turned out to develop, physicists could and would choose to maintain Euclidean geometry on account of its simplicity. Among the logical empiricists, the author most influenced by the rise of relativity, and most influential in his response to it, was Reichenbach (see Conventionalism; General relativity, philosophical responses to; Relativity theory, philosophical significance of; Spacetime).

These influences meant that when the analytic movement became dominant in anglophone philosophy, the interpretation of modern physics was established as a prominent theme in its sub-discipline, philosophy of science (see Science, philosophy of). But from the 1960s onwards, philosophy of physics has also grown for a reason external to philosophy. Namely, within physics itself there has been considerable interest in foundational issues, with results that have many suggestive repercussions for philosophy; so that nowadays these foundational issues provide some of the most interesting and important problems in the philosophy of physics. Again, there have been various developments within physics, and thereby various influences on philosophy. To convey a sense of this, without citing again some of the topics, and corresponding entries, just mentioned, it will suffice to give three further examples.

First, there is the enduring fertility of classical mechanics, both in itself and as a guide to understanding quantum theory, and the philosophical issues thereby raised (see Chaos theory; Conservation principles; Mechanics, classical). Second, the rise since the mid twentieth century of scientific cosmology raises many philosophical issues, among them the assessment of general relativity’s apparent allowance of time travel (see Cosmology; Time; Time travel). Finally, there is the fact that quantum field theory has for some 70 years proved itself to be our most successful framework for understanding matter. Its basic structure and concepts form a rich field for philosophical interpretation, quite apart from the interpretative problems facing quantum theory in general (see Field theory, quantum). Besides, the details of its application to describe and explain the behaviour of matter, especially macroscopic matter, raise philosophical issues, in particular about emergence (see Emergence in physics).