Quantum theory grew from the ideas of Max Planck. He proposed that atoms could only emit (give out) energy in fixed units called quanta. The theory has been developed to explain the behavior of particles and the energy they emit.

A quantum is the smallest possible amount
of energy. A quantum of light (or other electromagnetic radiation) is called a photon.
A photon may be emitted by an electron as it makes a QUANTUM LEAP.

## WHY IS QUANTUM THEORY SO STRANGE?

Quantum theory shows that energy behaves as both waves and particles at the same time. Quanta are packets of energy that exist in lumps or units like separate particles—but when they travel, they spread out like waves on the surface of a pond.

## HOW DO SCIENTISTS USE QUANTUM THEORY?

Although quantum theory is hard to understand, it is one of the most accurate scientific theories ever developed. With quantum theory, scientists can calculate precisely the properties of atoms, molecules, and materials. Quantum theory is used to design electronic components, new materials, and drugs. Without it there would be no computers, cellular phones, or many other recent inventions.

## BIOGRAPHY: MAX PLANCK German, 1858–1947

Max Planck was born in
Kiel, Germany. He was outstanding at all subjects at school and was a talented musician, but he decided to devote his life to physics. Planck’s constant (fixed number) is used to calculate the energy of quanta. He was awarded the Nobel Prize for Physics in 1918.

Electrons in atoms occupy shells. In each shell, an electron has a certain energy. If an electron moves to a higher or lower energy shell, it is said to make a quantum leap.

### WHAT IS THE UNCERTAINTY PRINCIPLE?

Quantum theory shows that the position and the speed of a quantum, such as a photon or an electron, cannot both be known exactly. The more accurately we know the position, the more uncertain we are of the speed, and vice versa. The uncertainty principle shows that we can only calculate probabilities, not certainties.