An electron has a mass of $m_e = \; 9.1094 × 10^{-31} kg$. A positron has the same mass. So during electron positron low-energy annihilation, won't the law of conservation of mass be violated?
Note: I am a high school student interested in physics and read about electron positron annihilation in a book. So if there is a flaw in a concept behind the question, please do point it out.
Best Answer
One has to understand that presently there is classical physics, to which belong Newtonian physics , thermodynamics and some other branches of mathematically modeling nature, and quantum physics, which models the data coming from the microcosm of particles : molecules atoms nuclei and elementary particles. Special relativity is much more important in the quantum mechanical framework. For ordinary velocities it reduces to Newtonian mechanics.
The law of conservation of mass belongs to classical macroscopic descriptions of nature. There mass is conserved.
The electron positron annihilation belongs to the quantum mechanical and special relativity framework.
In quantum mechanics and special relativity mass is not conserved; only energy and momentum are retained from the newtonian conservation laws. Mass is the "length" of the four vector describing a system of particles, called invariant mass, which is not the sum of individual masses. Invariant mass of single particle characterizes it uniquely.