Heisenberg was responsible for what has come to be called the indeterminacy or 'uncertainty' principle in quantum mechanics. This states that in determining, by measurement, the position of a particle such as an electron or photon we make its momentum indeterminate, and that in determining its momentum we make its position indeterminate. In the Bohr-Heisenberg-or Copenhagen-interpretation of quantum mechanics this principle is understood to mean not merely that we cannot simultaneously and exactly measure the position and momentum of an electron but rather that an electron does not at any time have an exact position and momentum. Positions and momenturns are, in effect, produced by the measuring process. The legitimacy of this 'ontological' or 'objective' interpretation of the indeterminacy principle has been questioned, not least because of its connection with the verificationism of logical positivism. It was rejected by Schrödinger and by Einstein, though it has been adopted as the standard interpretation by most physicists.
Heisenberg's view about the significance of the indeterminacy principle was that it signalled a revolution in our attempts to understand the physical world. It implied a radical dichotomy between, on the one hand, the experimental level at which measurements could be undertaken and where classical physics with its concept of causality was applicable, and on the other hand the submicroscopic level where causal concepts could not be applied and where, therefore, the future is not determined by the past nor the past by the future. The methods which led to the quantum theory are continuous with those used in classical physics, but they lead to an understanding of the concept of reality which is discontinuous with the past. We cannot know what 'really happens' at the quantum level between observations because, according to the Copenhagen interpretation of the quantum formalism, there is no way of describing what happens. The questions that this raises about the completeness of quantum theory have remained a prominent topic in the philosophy of physics.
American, b: 28 October 1929, New Jersey. Cat: Moral and social philosopher. Ints: Feminism; politics. Educ: Barnard College, NYC AB 1950; Columbia PhD 1968. Appts: Barnard College, 1964-6; Hunter College, CUNY, from 1965; Professor of Philosophy, CUNY College Graduate Centre, from 1977; Visiting Professor, Yale 1972, Dartmouth, NH, 1984, UCLA 1986; Truax Visiting Professor, Hamilton College, 1989; Director of NEH Summer Seminar, Stanford Law School, 1981; Visiting Scholar, Harvard Law School, 1981-2; Fulbright Fellow, 1950; Rockefeller Foundation Fellow, 1975-6; Fellow of Center for Advanced Study in the Behavioral Sciences, Stanford, 1984-5; President, American Section of International Association of Philosophy of Law and Social Philosophy, 1981-3; Executive Committee, Eastern Division, American Philosophical Association, 1979-81; Chair, Society for Philosophy and Public Affairs, 1972; Member of Society for Women in Philosophy; Member of editorial boards, Ethics, Hypatia, Philosophy and Phenomenological Research, Political Theory, Public Affairs Quarterly, Social Theory and Practice, 1982-91; Reporter on the The Reporter.