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It was on this day in 1900 that the physicist Max Planck published his theory of quantum mechanics, which is often considered one of the most radical scientific discoveries of the 20th century.

…a person who was not shocked by quantum theory did not understand it…

Max Planck was working in a laboratory in 1900, heating up various substances and examining the color of light they emitted when they reached certain temperatures. He wanted to describe his results in mathematical terms, but no matter how hard he tried, his mathematical calculations didn’t make sense. The only way he could fix the problem was to assume that light travels in little packets, like bullets, even though this seemed impossible.

But five years later, Albert Einstein took Planck’s theory of light seriously, and wrote his first major paper exploring the idea of light traveling in packets, which he called photons. Even though he became better known for his theory of relativity, it was Einstein’s work expanding on Planck’s original ideas about light that won him a Nobel Prize. Einstein later said, “I use up more brain grease on quantum theory than on relativity.”

With the discovery of quantum mechanics, physicists found that subatomic particles were by nature unpredictable. If you shot one across the room, you could guess where it might end up, but you could never be sure. This idea made Einstein miserable. He famously said, “I am at all events convinced God does not play dice.”

Today quantum mechanics remains one of the most mysterious and difficult scientific theories ever. The Danish physicist Niels Bohr once said that a person who was not shocked by quantum theory did not understand it, and the physicist Richard Feynman once said that while only a modest number of people truly understand the theory of relativity, no one understands quantum mechanics.

Max Planck himself died in 1947, and he never came to fully accept the theory he discovered. But even if few people really understand it, quantum mechanics led to the development of modern electronics, including the transistor, the laser, and the computer.