On thermal waves' velocity: Some open questions in thermal waves' physics

This paper presents some considerations about the truly character of thermal waves. Starting from the comparison of typical characteristic velocities, it is shown why a limiting frequency must exists, above which the parabolic treatment of the heat transfer in presence of time varying periodical heat sources is no longer valid due to the constancy of the speed of light in vacuum. Although it is demonstrated that this frequency is much smaller than that at which the thermal wave velocity can become the speed of light, many questions remain open, such as the behavior of thermal waves at intermediary frequencies. The discussion presented here can be useful to stimulate further discussion on this theme among students, teachers and scientists dealing with heat transport under non-stationary conditions.