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SUMMARY:Lifecycle of a constant: e - Alistair Isaac (University of Edinbur
 gh)
DTSTART:20231116T153000Z
DTEND:20231116T170000Z
UID:TALK206278@talks.cam.ac.uk
CONTACT:Lewis Bremner
DESCRIPTION:(Joint work with George Smith and Teru Miyake)\n\nI trace the 
 changing evidential status of the charge of the electron _e_ during the la
 st 100 years in order to tease out some distinctive features of the episte
 mology of high-precision measurement. Of special significance is the way f
 unctional interlinking between constants allows values of different consta
 nts to serve as crosschecks on each other. This holistic process of cross-
 checking established _e_ as an _anchor_ for other constants during the 191
 0s 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 determination
 s of _NA_\, however\, this changed\, as these methods could _derive_ value
 s for _e_ significantly more precise than Millikan's direct measurement. T
 he tension between oil-drop and x-ray values for _e_ ultimately exposed sy
 stematic 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_ wa
 s _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\, precisi
 on is given epistemic significance\, and taken as a corollary to accuracy.
  Second\, inconsistencies between high precision values of physical consta
 nts expose systematic error. Finally\, fixing constants such as _e_ as exa
 ct ensures that units are not a source of systematic error\, thereby enabl
 ing progress on measurements at the forefront of new physics\, such as tha
 t of the anomalous magnetic moment of the muon. I conclude with some contr
 oversial speculations of a realist bent.
LOCATION:Arts School Lecture Theatre A
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