  In a Weyl geometry there are no ideal absolute “meter  sticks” or “clocks”. For example, the rate at which any clock measures time is a function of its history. However, as  Einstein pointed out in a Nachtrag (addendum) to Weyl  (1918a), it is precisely this situation which suggests that Weyl’s  geometry conflicts with experience. In Weyl’s geometry, the frequency of the spectral lines of atomic clocks would depend on the location  and past histories of the atoms. But experience indicates otherwise.  The spectral lines are well-defined and sharp; they appear to be  independent of an atom’s history. Atomic clocks define units of time, and experience shows they are integrably transported. Thus, if we  assume that the atomic time and the gravitational standard time are  identical, and that the gravitational standard time is determined by  the Weyl geometry, then the electromagnetic field tensor is zero. But if that is the case, then a Weyl geometry reduces to the standard  Riemannian geometry that underlies general relativity, since the  vanishing of Weyl’s Streckenkrümmung  (length curvature) is necessary and sufficient for the  existence of a Riemannian metric \(g_{ij}\). 