But even at low temperatures, electrons can radiate and create new particles of light—photons. Thus a box that starts out containing only electrons will, over time, evolve into a box of both electrons and photons. The reason that the number of electrons stays fixed in this case is that each possesses electric charge, and, as far as we know, electric charge can never be created or destroyed. If the number of electrons in the box were to increase or decrease, the total amount of electric charge would change too. Photons, in contrast, carry no such electric charge, and thus can be freely created
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