In the article on wave-particle duality, we explained that the world of particles is filled with strange phenomena that differ significantly from our everyday experiences. For instance, in our daily lives, it's normal to measure the speed of a car while simultaneously knowing its precise position at every moment. But it's impossible to do the same thing for a particle.

Imagine holding a bouncing ball in your hand. If it’s still, you know exactly where it is (in your hand) and how fast it’s moving (not at all). But if you throw it across the room, it gets harder to say exactly where the ball is at every moment because it’s moving so fast.

With really small things, like electrons, this is even trickier! The more we try to figure out where the electron is, the harder it becomes to know how fast it’s going—and if we focus on how fast it’s moving, we lose track of where it is. It’s like the electron is always playing a game, where we can only know some things about it, but never everything at once!

The Heisenberg's uncertainty principle says that you can’t know exactly where a particle is and how fast it's moving at the same time. It was formulated by the German physicist and Nobel laureate Werner Heisenberg in 1927.

In Book 1, Millie comes to understand Heisenberg's uncertainty principle when she inquires about the position of the electron that has become a wave. The response she receives is: "Probably not far away, but it could be anywhere."

At the same time, Millie can know the exact position of the other electron, the stationary particle in front of her, but she cannot determine its speed because it had to stop to interact with her.

This principle also applies in physics laboratories; when attempting to confine a particle within a limited space, you inadvertently interfere with its momentum, making precise measurement impossible. Conversely, while you can accurately measure the speed of the wave associated with the particles, you cannot pinpoint their exact location.