PROPOSITION. “Consider a system of two photons. If the photons fly in one direction, then the mass of this system is zero. Each of the photons will therefore also have zero mass. However, let the photons fly in opposite directions with the same energies E. The total momentum of such a system is zero, and the total energy is 2E. Then the mass of this system is equal to 2E/c2, and the mass of each of the photons is equal to E/c2. Such a formal consideration of the concept of mass requires an additional more detailed informal analysis.”

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