Mass-Energy Equivalence
Albert Einstein · 1905
"Mass and energy are the same underlying quantity expressed in two forms — a small amount of mass corresponds to an enormous amount of energy, a relationship that later explained how stars generate light and how nuclear reactions release power."
In a three-page follow-up to special relativity, Einstein asked a small question — does a body's energy content affect its mass — and derived the most famous equation in physics: E=mc².
Einstein showed that when a body emits energy (as radiation), its mass decreases by an amount equal to that energy divided by the speed of light squared — meaning mass isn't just 'stuff,' it's a concentrated form of energy, related by an enormous conversion factor (c², the speed of light squared, a huge number). This meant even a tiny amount of mass, if fully converted, releases a staggering amount of energy — the principle later underlying both nuclear fission (splitting heavy atoms) and fusion (the process powering the sun and every other star, converting a small fraction of hydrogen's mass into the energy that lights the universe). The equation itself is simple, but the implication reshaped physics: matter and energy were no longer separate categories, just different forms of one underlying thing.
What does E=mc² mean physically, beyond being a formula?
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The explanation above is written with AI assistance. These are the originals — go to them to check it.
- Does the Inertia of a Body Depend Upon Its Energy Content? (1905, English translation)Annalen der Physik
General Relativity
"Gravity isn't a force pulling objects together — it's the curvature of spacetime itself caused by mass and energy, and objects simply follow the straightest possible path through that curved geometry."