What does the acronym LASER stand for, in part?

Light Amplification by Stimulated Emission of Radiation - and the stimulated emission part is Einstein's, proposed theoretically in 1917, forty-three years before anyone built one. An excited atom can be triggered by a passing photon into releasing an identical photon, travelling the same way, in step - so one becomes two, two become four, and the light that comes out is all one wavelength, all in phase, and barely spreads.

When Theodore Maiman built the first working laser in 1960, it was widely described as a solution looking for a problem - a memorable phrase that has aged into a joke about how badly people predict what technology is for. There was no obvious use for a very pure beam of light.

It is now in essentially everything. Barcode scanners, fibre-optic cable carrying the entire internet, CD and DVD players, eye surgery, cutting steel, printing, measuring the distance to the Moon off mirrors left by Apollo, and the LIGO detectors that observed gravitational waves in 2015 by measuring a change in length a thousand times smaller than a proton. The maser, its microwave predecessor, also gave Charles Townes a Nobel Prize and a story about being told by senior colleagues to stop wasting money on it.