  Guided primarily by versions of a theoretical framework called Doubly Special
Relativity, or DSR, that are supposed to entail speeds of light that vary with
energy while preserving the relativity of inertial frames, quantum-gravity
phenomenologists have recently been seeking clues to quantum gravity, in
hoped-for differing times of arrival, for light of differing energies, from
cosmologically distant sources. However, it has long been known that signals,
of arbitrarily high speed in opposing directions, could be used to observe the
translational state of (absolute) rest, as could signals of a fixed speed
different from c. Consequently, the above versions of DSR are nonviable.
