Instantaneous Inertial Frame but Retarded Electromagnetism in Rotating   Relativistic Collapse
  Slowly rotating collapsing spherical shells have flat spaces inside and the inertial frames there rotate at omega_s(t) relative to infinity. As first shown by Lindblom & Brill the inertial axes within the shell rotate rigidly without time delays from one point to another. Although the rotation rate of the inertial axes is changing the axes are inertial so, relative to them, there is neither an omega dot_s times r (Euler) fictitious force nor any other. However, Euler and other fictitious forces arise in the frame which is at rest with respect to infinity. An observer at the center who looks in one direction (phi = 0, theta = pi/2 say) fixed to infinity will see that the sky appears to rotate and can compare its apparent rotation with those of the local inertial frame and of the shell itself. By contrast in the electromagnetic analogue there is a time delay in the propagation of the magnetic field inside a rotating collapsing charged shell in flat space. We demonstrate this time delay by devising a null experiment in which the Larmor precession of a charged oscillator would be exactly cancelled by the rotation of the inertial frame but for the delay. In the ``combined'' problem of a collapsing charged shell we show that, due to the coupling of electromagnetic and gravitational perturbations, the instantaneous rotation of inertial frames inside the shell can be caused by pure electric currents in a non-rotating shell. 
J. Katz, D. Lynden-Bell, J. Bicak