What a quantum is
Lesson 1 of 23 in the course “Quantum”. Free, with animations and a quiz.
The world under a magnifier
A ball, a car, you: everything big follows the physics you know. Throw a ball and you can say exactly where it’ll be a second later. But shrink further and further, down to atoms, and the rules change. An atom is hundreds of thousands of times smaller than the width of a hair. Down there particles behave in ways that break common sense. The rules of that tiny world are quantum physics. No formulas needed: you’ll get pictures, experiments and examples.
Energy comes in portions
1900. German physicist Max Planck is trying to explain what colour red-hot metal glows. The maths won’t work until he makes a bold assumption: light gives off energy not smoothly but in tiny portions. That smallest portion was named a quantum, from the Latin quantum, “how much”. Think of a ramp versus stairs: on a ramp you can stand at any height, on stairs only on a step. The micro-world is built like stairs.
Light is a stream of particles
1905, Albert Einstein. The puzzle: shine light on a metal and electrons fly out. But red light doesn’t knock them out, even very bright red. Yet dim ultraviolet does it instantly. Einstein explained it: light travels in portions, particles called photons, and the energy of one photon depends on its colour. A red photon is too weak, no matter how many you add. Einstein’s 1921 Nobel Prize was for exactly this. Solar panels and your phone’s camera run on this effect.
An electron lives on steps
1913, the Dane Niels Bohr. An electron in an atom can’t circle just anywhere: it has allowed energy levels, like steps. When the electron jumps down a step, it releases a photon. The height of the jump sets the photon’s energy, and so its colour. Every chemical element has its own steps. That’s why in fireworks strontium always burns red, sodium yellow and copper blue. That’s not paint, that’s quantum physics.
Why we don’t notice it
If the micro-world is so strange, why does your cup of coffee behave normally? Quantum effects show when an object is small, cold and well isolated. A cup holds trillions of trillions of atoms, all constantly bumping into air and light. Those bumps wipe out the quantum weirdness and leave the familiar world. Physicists call this decoherence. We’ll unpack it in the lesson on measurement; for now remember: big and warm hides the quantum.
The 2025 Nobel: a quantum in your hand
For a long time quantum tricks were thought to belong only to single atoms. In 1984–1985 John Clarke, Michel Devoret and John Martinis showed otherwise. An electrical circuit you can hold in your hand also jumps between energy steps. And it passes through a barrier that ordinary physics says it can’t cross: that’s called tunnelling. In 2025 this won the Nobel Prize in Physics. Google’s and IBM’s quantum chips are built on this idea.
Check yourself
- Why did ultraviolet knock out an electron while bright red light didn’t?
Correct answer: One photon’s energy depends on its colour. One photon knocks out one electron. An ultraviolet photon carries enough energy, a red one doesn’t. That’s what Einstein explained in 1905. - Why does strontium in fireworks always give red?
Correct answer: Its electrons jump between its own steps. Strontium has its own energy levels. A jump down of the same height always gives a photon of the same energy, so the same colour. That’s how chemists identify elements by their light. - Which fact answers your friend best?
Correct answer: 2025 Nobel: a palm-sized circuit behaves quantumly. Clarke, Devoret and Martinis showed that a large electrical circuit jumps between energy levels and tunnels like a single particle. Superconducting qubits grew out of this.