It was on this day (December 14) 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. At that time, physicists accepted the work of Isaac Newton without any criticism. They believed that the interactions between all physical objects, from atoms to planets, would be predicable and logical. But one thing that physicists couldn't quite understand was the way light worked.

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. He published his calculations on this day in 1900, calling his theory about light "an act of desperation." He assumed that some future physicist would figure out what he had done wrong.

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."