I wonder if my thought experiment in proving the finite speed of field lines is valid. Say a spherical conductor has a cavity with positive charge Q inside as shown. say that the beginning of the universe starts at time 0, where there is the charge Q in the cavity and that the conductor is not polarized. We are speaking in a classical perspective, so it makes sense. Also, assume that the charge Q and the conductor are the only things in the entire universe. In the time at the beginning of the universe, there is a field due Q outwards, existing everywhere in the universe at once simultaneously, and no field at all due to the conductor. Then afterwards Q does work to polarize the conductor. If we consider the system as the cavity’s Q and the conductor, then before the induction, the total field energy in the universe is soley to Q. After that brief moment, the total field energy of the universe should still the same ,since the electric force is a conservative force. But we know that the energy is not conserved since the field lines are now terminated inside the conductor, leaving lesser length of field lines total in universe. Hence, less energy compared to time 0, since there was no gap. This concludes that the original assumption that the field lines are existing simulataneously every where at once at the inception of time is wrong. Therefore E has a finite speed. So at the beginning of time, as E field from the inner cavity charge moves outwards to the conductor, it does work on the conductor as the E field itself is propagating at the same time. Hence there would be no contradiction. As the E field flows into the conductor, it does positive work on the negative charges in the conductor and positive work to the positive charges. So energy was put into creating the speration and therefore, the loss of the red field lines(inside the picture) was the price to pay. And hence, energy is conserved.