Spherically symmetric infinitesimally thin shells can be described via the well known junction formalism of Israel. The equation of motion of thin shells is (G=c=1): (dr/dtau)^2 = (mg/mr)^2 - 1 + (2mc+mg)/r + (mr/2r)^2 This is an energy balance equation, where r is the circumferential radius, tau is the proper time, measured by co-moving observer, mc > 0 is the central mass (in case of Schwarzschild spacetime, this is the mass parameter of the inner Schwarzschild spacetime) mr > 0 is the rest mas of the shell = 4*pi*r^2*sigma, where sigma is the surface energy density, mg is the gravitational mass of the shell, mg = M - mc by definition, where M is the total mass of the system, if the outer spacetime is Schwarzschild vacuum, then M is the outer Schwarzschild mass parameter. mc and M has to be non-negative. There are lots of papers studying for example the dust case, when mr is constant during the motion. My question: Usually (dr/dtau) > 1 (speed of light), how is it possible? If this is just a coordinate velocity, how can i express a velocity formula, which has to satisfy the casuality condition |v| < 1?