  In this paper we consider a thought experiment involving the effect of
gravitation on an ideal scale containing a photon. If the tidal forces inherent
to a gravitational field are neglected, then one is led to scenario which seems
to bring about perpetual motion violating the first and second principle of
thermodynamics. The tidal effects of gravity must neccessarily be included in
order to obtain a consistent physical theory. As a result, Albert Einstein's
thought experiments according to which the physical effects of inertia in an
accelerated reference frame are equivalent to the effects of gravity in a frame
at rest on the surface of a massive body must be reconsidered, since linearly
accelerated frames do not produce tidal effects. We argue that the equivalence
between inertial effects and gravitation can be restored for rotating frames
and in this context a relation with the possible nature of quantum gravity is
conjectured.
