refractive index arises from radiative reaction, rather than collisional damping. Davies used thermodynamic considerations and refractive index theory without complicated calculations involving Riemannian geometry to derive general conditions for determining the opaqueness of cosmological models[@Davies1; @Davies2].

Wave Function Reduction and the Reversal of the Arrow of Time
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Both retarded and advanced wave solutions are consistent with relativity, in the sense that the propagation of signals occurs at the speed of light and there is no superluminal communication of information. This is in agreement with the observational result that no speed of propagation exceeds the speed of light in vacuum. On the other hand, the idea of causality is based on the notion that some temporal event occurs [*before*]{} another event. If $O$ is a point in spacetime, a light cone determined by $ds^2=0$ belongs to it, where $ds^2$ is the square of the local Minkowski spacetime distance. We draw a timelike world line through $O$ and on this line observe the close spacetime points $X$ and $Y$, separated by $O$. If it is possible to send a signal from $Y$ to $X$, but not from $X$ to $Y$, then the one-sided, asymmetrical character of time is secured, and there 