Mass conservation law
In chemics, it is well known that mass is conservated in chemical processes. In physics, the situation is different. There is no generally accepted mass conservation law. The example against is the annihilation. In this process, an electron and a positron are converted into pure energy, in particular a pair of photons. As with Einstein it is assumed that a photon has a rest mass of 0, it is considered that mass is lost in this process.
This is the typical german approach to this process. In America there are similiar false theories on this. As a photon has the rest mass 0, it is assumed that it has a mass of 0, too. Therefore it is concluded that there is no conservation of mass in a process of a closed system.
So let's go back to the basics: E = m * c²: Albert Einstein revealed this great equation and it made him famous. Nowadays there is a stream in modern physics that this equation is not always true, as in extreme situation the equation maybe false. This is really wrong. The equivalence of space and time in an absolute perspective proves that E = m * c² is true in all cases. There are no extreme situations thinkable, in that E = m * c² might be wrong. Speed or velocity is always the speed of light. So we see that there is an energy conservation law in physics. In a closed system, energy is constant. Let's take it to the equation: E = const. So we put that together with E = m * c². So we see that the product m * c² is always constant in a closed system. As the speed of light is always const., we devide something constant through a constant, so the result would be constant, too. An example: Let the energy in a closed system be 4. Let c be 1. So the mass of this system will always be 4. If the energy is not changing its value, so the mass won't change. This is true: m = const / 1² = const.
So the mass conservation law m = const. is valid in a closed system.
Let's go back to the annihilation, where one or several photon pairs are created. We see at the beginning of the reaction the particles have a mass, as electron and positron have a mass. As the mass must be conserved in this process, we can conclude: photons have a mass. Albert Einstein didn't answered this question, he only said, that photons have no rest mass.