In Book 1, Millie is intrigued by the electrons’ pointy "hats" and curiously asks about them. She soon discovers that these aren’t hats at all, but a built-in feature of electrons known as spin.
Spin is an intrinsic property of electrons and all quantum particles. An intrinsic property is a characteristic that belongs to an object and doesn’t change, no matter where it is or what happens to it. It’s like a built-in part of what makes that object unique. Think of a rainbow: it always displays the same colours in the same order. No matter where we see a rainbow, those colours and their sequence are always the same because that’s just what a rainbow is. In a similar way, the order and colours of a rainbow are intrinsic properties.
Spin, however, is something that only exists in the world of quantum particles, making it difficult to picture in everyday terms.
The electron's spin was discovered in 1925 by George Uhlenbeck and Samuel Goudsmit. It was called spin because, initially, they suggested that electrons might be like tiny spinning spheres, rotating on an axis, with their spin acting as a kind of angular momentum. If you’re unfamiliar with angular momentum, you can learn more in the article Angular momentum. According to this early idea, if an electron were "spinning" clockwise, its spin would point "up," while if it were "spinning" counterclockwise, its spin would point "down."
Later, the same scientists found that electrons don’t spin in the same way as objects in everyday life. So, while “spin” behaves like angular momentum, there isn't a real rotation.
The term “intrinsic angular momentum” stuck, and even now, spin is often called the intrinsic angular momentum of a particle. This description helps us imagine the particle with an “up” or “down” property, making it easier to visualize something that exists only in the quantum world. In Millie's books the two vectors representing the spin of electrons are inside the pointy hats that they can't remove.
Millie will learn more about electron's spin in Book 2 and we will discuss it further in the articles Discovery of electron spin and Electron spin - Part2.
