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## A brief history of quantum field theory

The world we live in is made up of fundamental particles interacting through the fundamental forces. One of the greatest aims of theoretical physics is to describe all of these forces and particles in one all-encompassing theory. Something called *quantum field theory* has been hugely successful in this context, but what exactly is it? And does it answer all the questions?

Based on interviews with some of the main players in theoretical physics, this series of accessible articles traces the history of quantum field theory, from its inception at the beginning of the twentieth century to the tantalising questions that are still open today. It's a story of pain and triumph, hardship and success. Happy reading!

Schrödinger's equation — what is it and what does it mean? — Let's start at the beginning. In the 1920s the Austrian physicist Erwin Schrödinger came up with what has become the central equation of quantum mechanics. It tells you all there is to know about a quantum physical system and it also predicts famous quantum weirdnesses such as superposition and quantum entanglement. In this three-part series, we introduce Schrödinger's equation, investigate its meaning, and put it in its historical context.

Let me take you down, cos we're going to ... quantum fields — You may have heard of quantum theory and you probably know what a field is. But what is quantum field theory? This article gives a first glimpse of the idea and how it might be applied to describe the interaction of light and matter.

The problem with infinity — After successfully applying quantum mechanics to the electromagnetic field, physicists faced a problem of boundless proportions: every calculation they made returned infinity as the answer.

Taming QED — We discover how an ingenious trick put QED back on track, taming those unwieldy infinities and causing one physicist to exclaim that god is great!

Quantum pictures — So the theory was tamed, but still so complicated as to be practically useless. Until the stick-like drawings known as Feynman diagrams, policed by a young Freeman Dyson, came to the rescue.

Strong but free — With QED now in the bag, the next on the list was a new one: the strong nuclear force. The early 1950s were an experimental gold mine with new particles produced in accelerators almost every week. Yet the strong nuclear force that acted between them defied theoretical description, sending physicists on a long and arduous journey that culminated in several Nobel prizes and the exotic concept of "asymptotic freedom".

Going with the flow — Asymptotic freedom allowed the strong force to be described by a quantum field theory. The problem was, however, the calculations only worked at high energies. Similarly, it seemed that quantum electrodynamics, the theory that described the interaction of light and matter, only worked at sufficiently low energies. If they did not work at all energy scales, how could these be thought of as valid theories? What is a valid theory, anyway? It was clear that quantum field theory needed some new ideas.