Einstein explained · 1916–1917
Quantum Theory of Radiation
Einstein's quantum theory of radiation described how atoms absorb and emit light in discrete energy changes.
absorb · emit · stimulated emit
A · B · B
What it means
Atoms can absorb light, emit light on their own, or be stimulated by incoming radiation to emit additional light. Einstein described these processes using transition probabilities.
Einstein's contribution
In work from 1916–1917, Einstein introduced the ideas later described by the A and B coefficients. His theory required stimulated emission and also strengthened the particle-like picture of light.
Why it matters
Stimulated emission later became the key physical principle behind masers and lasers. Einstein's framework also became important in atomic physics and spectroscopy.
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Einstein’s key theories and contributions
1905Special RelativityHow motion changes measurements of space and time while the speed of light stays constant.Read explainer →1905Mass–Energy EquivalenceWhy mass and energy are connected by Einstein's famous relation E = mc².Read explainer →1907Equivalence PrincipleWhy free fall and acceleration gave Einstein the starting point for a new theory of gravity.Read explainer →1907Gravitational RedshiftHow gravity changes the frequency of light and the rate at which clocks run.Read explainer →1916–1918Gravitational WavesHow changing masses can send ripples through spacetime at the speed of light.Read explainer →1915General RelativityHow Einstein described gravity as curved spacetime shaped by matter and energy.Read explainer →1917Cosmological ConstantWhy Einstein added Λ to his gravity equations and how the term returned in modern cosmology.Read explainer →