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 and galaxies — as well as the evolution of the universe as a whole.
Yet another pillar, quantum theory (which governs the properties of matter on microscopic scales), is at least as fundamental. Developed in the early 20th century, it forms the basis of elementary particle physics, governs the behavior of atoms, and lays the foundation of solid state physics. Wherever we go, we meet its applications — from laser pointers and CD players to 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 which include special relativity as an integral part.
However, a quantum theory of gravity is still missing — though not for want of trying. There are a number of promising candidate theories that hint at how quantum gravity might look, but so far, none of them solves the problem completely.