Quantum electrodynamics

Quantum electrodynamics, often called QED, is a fundamental theory in particle physics. It is a relativistic quantum field theory that explains how light and matter interact. Specifically, QED describes all phenomena involving electrically charged particles. These particles interact by exchanging particles called photons, which are the carriers of light.

QED is a major achievement because it is the first theory to reach full agreement between quantum mechanics and special relativity. It serves as the quantum counterpart to classical electromagnetism. Because its predictions are so incredibly accurate, physicist Richard Feynman famously called it "the jewel of physics."

To understand how QED works, we can look at its three basic building blocks. First, a photon moves from one point in space and time to another. Second, an electron moves from one point in space and time to another. Third, an electron can emit or absorb a photon at a specific vertex, which is a junction where the particles meet.

Physicists use Feynman diagrams as a visual shorthand for these actions. In these drawings, a wavy line represents a photon, while a straight line represents an electron. The junction where they meet is the vertex. To calculate the probability of a complex interaction, scientists combine these simple actions using probability amplitudes.

Calculating these interactions requires following specific mathematical rules. One rule is the indistinguishability criterion. This means that if there is no way to observe which specific path a particle took, you cannot simply add the probabilities of the different paths. Instead, you must add the probability amplitudes first. The total probability is then found by squaring the absolute value of the sum of these amplitudes. Another rule is the independence criterion, which applies to processes that are not "entangled." For example, in a simple interaction like Compton scattering, an electron and a photon might move from point A to point C and B to point D.

However, the electron might also absorb the photon at a middle point before continuing. QED accounts for all these possible intermediate "virtual" processes to find a total result.

The history of QED is a long journey of solving mathematical puzzles. Paul Dirac began this work in the 1920s. He was the first to formulate a quantum theory describing the interaction of radiation and matter. Dirac also coined the term "quantum electrodynamics."

He described the electromagnetic field as an ensemble of harmonic oscillators. Over time, scientists like Wolfgang Pauli, Enrico Fermi, and Werner Heisenberg contributed to the field. However, by the late 1930s, researchers like Felix Bloch and Victor Weisskopf discovered a massive problem. When they tried to perform higher-order calculations, infinities emerged in the math. These infinities made the results meaningless and suggested that quantum mechanics and special relativity were incompatible.

A breakthrough occurred in 1947 thanks to Hans Bethe. Improved microwave technology allowed scientists to measure the Lamb shift and the magnetic moment of the electron very precisely. These experiments showed discrepancies that the old theory could not explain.

Bethe performed a non-relativistic computation of the hydrogen atom's energy shifts and found excellent agreement with experimental data. He used a process called renormalization. This involved attaching the mathematical infinities to the corrections of mass and charge. By fixing these to finite values found in experiments, the infinities were absorbed, yielding useful results. This method became a standard requirement for a theory to be considered acceptable.

Following Bethe's work, several physicists developed fully covariant formulations of QED. Shin'ichirō Tomonaga, Julian Schwinger, and Richard Feynman all produced work that allowed for finite computations at any order. Freeman Dyson later showed that Feynman's diagram-based approach was equivalent to the operator-based approaches of Schwinger and Tomonaga. For their massive contributions, Tomonaga, Schwinger, and Feynman were awarded the 1965 Nobel Prize in Physics.

While renormalization worked in practice, Feynman himself was somewhat skeptical of its mathematical validity. He famously referred to the process as a "shell game" or "hocus pocus." Despite his discomfort, the theory remains the most precisely tested theory in all of physics.

QED has served as a vital template for almost all modern particle physics. It provided the framework for quantum chromodynamics, which describes the strong force and was developed in the 1960s and 1970s. Furthermore, QED was merged with the weak nuclear force to create the electroweak force. This unification was achieved by scientists including Sheldon Glashow, Steven Weinberg, and Abdus Salam. By understanding how light and matter interact through QED, scientists have been able to build a complete picture of the fundamental forces that govern our universe.