Mass/Energy

Under certain conditions, mass can be converted into energy à la E₀ = mc². Under certain restricted conditions, energy can materialize as mass. What is wrong in these statements?

Answer

Mass is energy. There is no distinction to be made. There is no conversion! In 1905, Einstein states explicitly: “The mass of a body is a measure of its energy content . . . ”. What Einstein was stating is that mass and energy are equivalent, that they are possibly two different aspects of the same physical quantity, only their units have been chosen to be different. One does not convert one to the other if they are equivalent.

We can imagine a conversation with a student: “Does a photon have mass?” asks the student. “Yes, because the photon has energy.” The student counters, “But for a photon E = pc, so the relation E² – p²c² = m2c4 becomes E² – p²c² = 0. Therefore m = 0 for the photon.” Can you complete this dialogue?