Niels Bohr, objectivity, and the irreversibility of measurements
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Niels Bohr, objectivity, and the irreversibility of measurements Ulrich J. Mohrhoff
Received: 23 September 2019 / Accepted: 13 November 2019 © Chapman University 2019
Abstract The only acceptable reason why measurements are irreversible and outcomes definite is the intrinsic definiteness and irreversibility of human sensory experience. While QBists deserve credit for their spirited defense of this position, Niels Bohr urged it nearly a century ago, albeit in such elliptic ways that the core of his message has been lost or distorted beyond recognition. Then as now, the objectivity of empirical science was called into question. It was defended by Bohr along the lines of Kant’s (then) revolutionary theory of science, according to which the possibility of empirical science hinges on the possibility of thinking of experiences as experiences of a system of interacting, re-identifiable objects. What Bohr added to Kant’s theory was his insight that empirical knowledge was not necessarily limited to what is directly accessible to the senses, and that, therefore, it does not have to be solely a knowledge of objects of sensible intuition. It can also be a knowledge of phenomena that are not objects of sensible intuition but instead are constituted by experimental contexts, which are objects of sensible intuition. Bohr’s grounding of objectivity (or the objectivity consistent with quantum mechanics), however, is weaker than Kant’s (or the objectivity consistent with Newtonian physics). This conclusion is based on an examination of Bohr’s intentions in appealing to irreversible amplification effects or the sufficient size and weight of the measurement apparatus. Keywords Bohr · Experience · Irreversibility · Kant · Measurement · Objectivity · QBism 1 Introduction The beginning of the 21st Century saw the launch of a radically epistemic interpretation of quantum mechanics, by Carlton Caves, Chris Fuchs, and Ruediger Schack [1]. Initially conceived as a generalized personalist Bayesian theory of probability called “Quantum Bayesianism,” it has since been re-branded as “QBism,” the term David Mermin [2] prefers, considering it “as big a break with 20th century ways of thinking about science as Cubism was with 19th century ways of thinking about art.” The big break lies not in the emphasis that the mathematical apparatus of quantum mechanics is a probability calculus but in this plus a radically subjective Bayesian interpretation of probability plus a radically subjective interpretation of the events to which, and on the basis of which, probabilities are assigned by “users” (of quantum mechanics) or “agents” (in a quantum world). U. Mohrhoff (B) Sri Aurobindo International Centre of Education, Pondicherry 605002, India e-mail: [email protected]
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Mermin lists four stages of acceptance a radical new idea goes through: (1) It’s nonsense; (2) it’s well known; (3) it’s trivial; (4) I thought of it first. He holds that by now Stage (2) is w
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