How did Albert Einstein’s 1916 idea help lead to the development of lasers?

In 1916, Albert Einstein proposed that under specific conditions, atoms could release excess energy as light, either spontaneously or when triggered by incoming light. This concept, known as stimulated emission, established that an excited atom could be forced to emit a photon identical in energy, direction, and phase to the stimulating photon.123

Einstein’s theoretical work provided the fundamental physical basis for the laser—an acronym for "light amplification by stimulated emission of radiation". By mathematically describing the statistical nature of radiative transitions, he showed that this process could create coherent, monochromatic light.32

While Einstein’s idea was initially theoretical, it eventually led to the development of the maser in the 1950s, which generated stimulated emission at microwave frequencies. Researchers subsequently extended this principle to shorter wavelengths of visible and infrared light, resulting in the invention of the first working laser in 1960.13