Relativity and the quantum

General relativity is one of the pillars of modern physics. It governs the large-scale phenomena of the cosmos, from planetary orbits in the solar system to stars, galaxies and the evolution of the universe as a whole.

Yet there is another pillar, at least as fundamental: quantum theory, which governs the properties of matter on microscopic scales. Developed in the early 20th century, it forms the basis of elementary particle physics, governs the behaviour of atoms, and lays the foundation of solid state physics. Wherever we go, we meet its applications, from laser pointers to CD players or the transistors in electronic devices.

Physicists have managed to formulate quantum theories describing electromagnetism and the elementary forces responsible for radioactive decay and the stability of atomic nuclei - theories that have been successfully tested at particle accelerators, and of which, it turns out special relativity is an integral part.

What's still missing is a quantum theory of gravity, though not for want of trying. There are a number of promising candidate theories that give a flavour of how quantum gravity might look like, but so far, none of them solves the problem completely.

Next page: Relativity in the micro-world