Lifecycle of a constant: e

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• Alistair Isaac (University of Edinburgh)
• Thursday 16 November 2023, 15:30-17:00
• Arts School Lecture Theatre A.

If you have a question about this talk, please contact Lewis Bremner.

(Joint work with George Smith and Teru Miyake)

I trace the changing evidential status of the charge of the electron e during the last 100 years in order to tease out some distinctive features of the epistemology of high-precision measurement. Of special significance is the way functional interlinking between constants allows values of different constants to serve as crosschecks on each other. This holistic process of cross-checking established e as an anchor for other constants during the 1910s and ‘20s, when recommended values for constants such Avogadro’s number and Planck’s constant, which could only be directly determined at lower levels of precision, were adjusted to maintain consistency with Millikan’s oil drop value for e. With the rise of x-ray diffraction determinations of NA, however, this changed, as these methods could derive values for e significantly more precise than Millikan’s direct measurement. The tension between oil-drop and x-ray values for e ultimately exposed systematic error in Millikan’s method due to uncertainty in the viscosity of air. After a long period as a derived value, in 2019 the value of e was fixed in order to define the ampere in the new SI. The changing status of e from anchor, to derived, to fixed illustrates three unique features of evidential reasoning in physical measurement. First, precision is given epistemic significance, and taken as a corollary to accuracy. Second, inconsistencies between high precision values of physical constants expose systematic error. Finally, fixing constants such as e as exact ensures that units are not a source of systematic error, thereby enabling progress on measurements at the forefront of new physics, such as that of the anomalous magnetic moment of the muon. I conclude with some controversial speculations of a realist bent.

This talk is part of the Departmental Seminars in History and Philosophy of Science series.

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