In 1905, Albert Einstein published a series of papers that many consider a starting point
for the modern age of physics. But other than "E equals m c squared", most people only know
that Einstein was famous, not for what he was famous. Since this week marks the 133rd
anniversary of his birth, as is customary in western traditions we will celebrate by
taking a brief tour of his scientific publications.
Today, his paper from March, 1905 on why light is a particle.
Einstein didn't just pull this idea out of thin air – he first noted that the light
emitted from something hot (like a lightbulb filament), actually has the same energy distribution
as a gas, which is somewhat surprising if you're a nineteenth century physicist who
thinks that light is a continuous wave and very much not a gas composed of individual
molecules. And while the idea that "light behaves kind of like a gas" was already well
known before Einstein, no one had taken the logical but crazy next step to conclude that
light must then be made of individual particles, too! So Einstein proposed that these "light-quanta"
were in fact real particles that could account for a few recent and unexplained experiments
having to do with knocking electrons off of metals or gas molecules. He turned out to
be right on all counts and got a nobel prize for his work… but that's a story for another
day.