The Schrödinger Equation: The Heart of Quantum Mechanics
By the early 1900s, physics had a problem. Classical physics, including Newton’s laws, worked well for planets, cars and falling objects, but it could not explain how atoms behave. Experiments showed that light can act both as a wave and as a particle, and that each type of atom gives off light only at certain fixed colours. Classical physics could not explain these facts.
In 1924, the French physicist Louis de Broglie proposed a bold idea: if light can behave like particles, then particles such as electrons might also behave like waves.
The Austrian physicist Erwin Schrödinger, then a professor at the University of Zurich, took this idea further. Around Christmas 1925, during a stay in the Swiss mountain town of Arosa, he worked out an equation describing how these “matter waves” behave. He published his theory in 1926 in a series of four papers.
What does the equation do?
It describes a quantity called the wavefunction, written as Ψ (psi). The wavefunction contains all the information that can be known about a quantum system, such as an electron in an atom. Schrödinger’s equation tells us how that wavefunction changes over time, in the same way Newton’s laws tell us how an object moves.
What does the wavefunction mean?
Schrödinger first thought it described the electron’s charge spread out in space, but that idea did not hold up. In 1926, the German physicist Max Born gave the explanation still used today: the wavefunction gives probabilities. It does not say exactly where an electron is, only how likely it is to be found in each place. At the smallest scales, nature is described in terms of chances, not certainties.
Its first great success
Schrödinger applied his equation to the hydrogen atom, the simplest atom. The equation produced the atom’s allowed energy levels, which match the colours of light hydrogen is observed to emit. Earlier, Niels Bohr’s model had obtained the same levels only by adding special rules. In Schrödinger’s equation, they followed naturally from the mathematics.
Two theories, one physics
A year earlier, in 1925, Werner Heisenberg, working with Max Born and Pascual Jordan, had developed a very different-looking version of quantum mechanics based on tables of numbers called matrices. In 1926, Schrödinger showed that the two approaches give the same results. They were two ways of describing the same physics.
Why it matters
Quantum mechanics explains why atoms are stable, how chemical bonds form, and why some materials conduct electricity while others do not. Schrödinger’s equation is its central working tool. The understanding behind transistors, computer chips, lasers and LEDs depends on quantum mechanics, and so does the operation of quantum computers.
The equation does have limits. It applies to particles moving much slower than light. In 1928, Paul Dirac found an equation for electrons moving close to the speed of light.
Recognition
In 1933, Schrödinger and Dirac shared the Nobel Prize in Physics. Two years later, Schrödinger described his famous thought experiment of a cat that, according to quantum theory, would have to be considered both alive and dead until observed. He meant it to show how strange quantum theory becomes when applied to everyday objects.
A hundred years later, his equation remains one of the foundations of modern physics.