  Given, in an arbitrary spacetime, a 2-dimensional timelike submanifold
(worldsheet) and an observer field on this worldsheet, we assign gravitational,
centrifugal, Coriolis and Euler forces to every particle worldline in the
worldsheet with respect to the observer field. We prove that centrifugal and
Coriolis forces vanish, for all particle worldlines with respect to any
observer field, if and only if the worldsheet is a photon 2-surface, i.e.,
generated by two families of lightlike geodesics. We further demonstrate that a
photon 2-surface can be characterized in terms of gyroscope transport and we
give several mathematical criteria for the existence of photon 2-surfaces.
Finally, examples of photon 2-surfaces in conformally flat spacetimes, in
Schwarzschild and Reissner-Nordstroem spacetimes, and in Goedel spacetime are
worked out.
