CLOCK RATE SYNCHRONIZABLE REFERENCE FRAMES IN CURVED SPACE-TIMES

It is well known that all reference frames that are clock synchronizab le are hypersurface orthogonal. A weaker requirement for clock synchro nization was first suggested by Zhao [Z. Zhao, Sci. China 34(7), 835 ( 1991)] using coordinate language. Reference frames satisfying this req uirement are hence referred to as class Z in the present paper. We giv e a geometrical definition of class Z reference frames and investigate their physical meaning as well as geometrical properties in detail. F inally, another kind of reference frame referred to as class (z) over tilde is introduced and its possible relation to another of Zhao's cla ims is pointed out. (C) 1998 American Institute of Physics.