Re: how the laser happened

There was a thread somewhere above about photon wave/particle duality.
  This is worth reading:
 https://www.amazon.com/How-Laser-Happened-Adventures-Scientist/dp/0195153766
 Einstein, in one of his fits of genius, predicted in around 1916 that
under the right conditions, a photon could pass by an excited atom and
the atom would kick in another photon, or add to the wave amplitude,
depending on how you feel about these things. He called it stimulated
emission. He also declared that the laws of thermodynamics made this
effect impossible to use in practical situations.
 In 1951, Charles Townes invented a work-around trick and built the
maser, a gaseous microwave oscillator. His superiors thought he was
crazy to dispute Einstein and almost threw him out of grad school, but
it worked.
 In 1960, Theodore Maiman at HRL made the first ruby laser, and Bell
Labs soonafter made a HeNe.
 What's interesting is that any decent neon sign shop could have built
a HeNe laser in 1920.