An object does not have to move to have energy. Einstein’s equation connects its mass, m, with its rest energy, E₀. The factor c², the speed of light squared, gives the relationship between them. Even when an object is completely at rest, its mass corresponds to an enormous amount of energy.

This connection also changes how we understand the mass of a whole system. Heat a sealed container of gas, and the energy added increases its mass by a tiny amount, even though no particles have been added. The mass of a system reflects its total energy in its rest frame, including the motion and interactions of its constituents.

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