We consider Weyl gauge theories of gravity (WGTs), which are invariant both
under local Poincar’e transformations and local changes of scale. Such
theories may be interpreted as gauge theories in Minkowski spacetime, but their
gravitational interactions are most often reinterpreted geometrically in terms
of a Weyl–Cartan spacetime, in which any matter fields then reside. Such a
spacetime is a straightforward generalisation of Weyl spacetime to include
torsion. As first suggested by Einstein, Weyl spacetime is believed to exhibit
a so-called second clock effect, which prevents the existence of experimentally
observed sharp spectral lines, since the rates of (atomic) clocks depend on
their past history. The prevailing view in the literature is that this rules
out WGTs as unphysical. Contrary to this viewpoint, we show that if one adopts
the natural covariant derivative identified in the geometric interpretation of
WGTs, properly takes into account the scaling dimension of physical quantities,
and recognises that Einstein’s original objection requires the presence of
massive matter fields to represent atoms, observers and clocks, then WGTs do
not predict a second clock effect.
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