Based on what was written in the previous two chapters, we can write a system of Maxwell equations for a universal gravitational-electromagnetic field in the following form
indicates the relative permittivity of the vacuum,
which is changed by gravity.
The consequence of this system of equations is that the speed of propagation of gravitational waves must be higher than the speed of light,
because gravity slows the propagation of electromagnetic waves, but does not slow the propagation of gravitational waves.
At the limit, when gravity tends to zero, they must be equal.
That gravity reduces the propagation speed of electromagnetic waves, but does not reduce the propagation speed of gravitational waves,
explains why gravitational waves come out of the event horizon of black holes and electromagnetic waves can't come out of it.
It may seem that this system of equations contradicts the theory of Relativity, because in it the speed of light in a vacuum is not constant,
but variable. But the fact that the speed of light in a material medium is less than the speed of light in the void does not contradict the theory of Relativity.
And this system of equations assumes that a vacuum is not a void, but a material medium.