In 1927 he found out that the act of measuring an electron's properties by hitting it with gamma rays would alter the electron's behavior. He could measure the position of an electron (or other particle) OR he could measure its momentum. But the more precisely you measerue the one property, the more you throw the other one off. This became accepted as a fundamental law of nature, called the uncertainty principle. He also invented matrix mechanics, the first version of quantum mechanics, in 1925. They were later used by Erwin Schrödinger.

In 1927, the world population reaches 2 billion.