In 1905, Albert Einstein published E = mc² — energy equals mass times the speed of light squared. The speed of light c is enormous, so even a tiny amount of mass contains staggering energy.
The breakthrough
Mass is not separate from energy. Convert mass to energy (nuclear reactions) or energy to mass (particle colliders) — the accounting always balances.
This explained why the sun burns for billions of years and why atomic bombs release city-level energy from grams of material.
Modern applications
Nuclear power plants, PET scans in hospitals, particle accelerators at CERN, and GPS satellite corrections all depend on relativistic physics rooted in E = mc².
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