From Chris Oldman
Brian Bowsher, celebrating SI units’ anniversary, states that all bar one “have been defined by universal, natural constants” (30 October, p 29). But one SI base unit, the metre, is defined in terms of light travel based on another SI unit, the second. While the speed of light might be a universal, natural constant, the second is not.
While two units of mass, length or energy levels may be observed and compared directly in a single event, two seconds of time can never be set one against the other: 9 billion periods of radiation in a caesium atom may currently define a second, but how can we know whether any such batch of periods takes as long as another?
Cheltenham, Gloucestershire, UK
