  We investigate in detail the special case of an infinitely thin static
cylindrical shell composed of counter-rotating photons on circular geodetical
paths separating two distinct parts of Minkowski spacetimes--one inside and the
other outside the shell--and compare it to a static disk shell formed by null
particles counter-rotating on circular geodesics within the shell located
between two sections of flat spacetime. One might ask whether the two cases are
not, in fact, merely one.

General Relativity and Quantum Cosmology