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How did relativity change the classical idea that mass and energy are separate?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In classical physics, mass and energy were viewed as distinct, independent entities governed by separate conservation laws. Mass was considered impossible to create or destroy, while energy was treated as a separate quantity.12

Albert Einstein’s special theory of relativity fundamentally changed this by demonstrating that mass and energy are equivalent and interchangeable. This relationship is expressed by the famous equation $E = m c^{2}$ , where $E$ represents energy, $m$ represents mass, and $c$ is the speed of light.134

Rather than two separate conservation laws, relativity establishes a single conservation law for mass-energy. This means that mass can be converted into energy and vice versa, as seen in phenomena like the annihilation of subatomic particles or the release of energy in nuclear reactions. Consequently, mass is now understood to be associated with every form of energy, including the kinetic energy of a body in motion.243