Albert Einstein is a name that most people recognise, but did you know that Albert was a big fan of a fellow scientist called Max Planck? The winner of the 1918 Nobel Prize in Physics, Planck discovered some fascinating things about science and the workings of the universe. In particular, he came to realise that there is a distinct relationship between energy and the wavelength (the frequency) of light. What does this really mean? Read on to find out!

Yale University Press’ *Little Histories *collection is a family of books that takes a closer look at some of the most significant events, ideas, discoveries and people throughout history. As part of our ongoing coverage of the collection, here’s an excerpt from William Bynum’s *A Little History of Science*, a book that examines the scientific discoveries that radically altered our understanding of the world. This post turns the spotlight on Max Planck, the originator of the quantum theory.

Today we celebrate the birthday of Max Planck, a German scientist who studied physics, the science of matter and energy. Planck’s work changed the way that scientists understood the world; in fact, Albert Einstein relied heavily upon Planck’s findings for his own theories. For his achievements, Planck was awarded the Nobel Prize in Physics in 1918. In *A Little History of Science*, William Bynum explains why Planck’s findings were so important:

‘Planck was a thinker and an experimenter. He was about forty years old when he made his most fundamental discovery, at the University of Berlin […] Planck knew that the amount of energy absorbed depends on the particular wavelength (the frequency) of the light. He took his very careful measurements of the energy and wavelength and put them into the mathematical equation *E = hv*. The energy (*E*) is equal to the frequency of the wavelength (*v*) multiplied by a fixed number (a ‘constant’ – *h*). In this equation the energy output Planck measured was always a whole number, not a fraction. This was important because being a fixed number meant that the energy came in individual little packets. He called each of these little packets a ‘quantum’, which just meant a quantity. He published his work in 1900, introducing the idea of the quantum to the new century. Physics, and how we understand our world, have never been quite the same since. The fixed number (*h*) was called ‘Planck’s constant’ in his honour. His equation would prove just as important as Einstein’s more famous *E* = *mc*2.’

As is often the case in science, one scientist’s findings led to another’s discoveries; Planck’s work became the foundation for Einstein’s game-changing theories:

‘It took some physicists a while to appreciate the real significance of Planck’s experiments. Einstein was one who saw what it meant straight away. In 1905, he was working in the Zurich Patent Office as a clerk, and doing physics in his spare time. That year, he published three papers that made his name. The first, which won him a Nobel Prize in 1921, took Planck’s work to a new level. Einstein thought more about Planck’s black body radiation, and drew on the still-new quantum approach. After much though, he showed—by some brilliant calculations—that the light was indeed being transmitted in small packages of energy. These packets moved independently of each other even thought together they made up a wave. This was a startling claim, for physicists since Thomas Young a century before had analysed light in many experimental situations as if it were a continuous wave.’

Happy birthday, Max Planck!