Science

In 1900 the German physicist Max Planck proposed the then sensational idea that energy is not infinitely divisible but is always given off in set amounts, or quanta. Five years later, German-born American physicist Albert Einstein successfully used quanta to explain the photoelectric effect, which is the release of electrons when metals are bombarded by light. This, together with Einstein's special and general theories of relativity, challenged some of the most fundamental assumptions of the Newtonian era.

Unlike the laws of classical physics, quantum theory deals with events that occur on the smallest of scales. Quantum theory explains how subatomic particles form atoms, and how atoms interact when they combine to form chemical compounds. Quantum theory deals with a world where the attributes of any single particle can never be completely known—an idea known as the uncertainty principle, put forward by the German physicist Werner Heisenberg in 1927. But while there is uncertainty on the subatomic level, quantum physics successfully predicts the overall outcome of subatomic events, a fact that firmly relates it to the macroscopic world—that is, the one in which we live.

In 1934 Italian-born American physicist Enrico Fermi began a series of experiments in which he used neutrons (subatomic particles without an electric charge) to bombard atoms of various elements, including uranium. The neutrons combined with the nuclei of the uranium atoms to produce what he thought were elements heavier than uranium, known as transuranium elements. In 1939 other scientists demonstrated that in these experiments Fermi had not formed heavier elements, but instead had achieved the splitting, or fission, of the uranium atom's nucleus. These early experiments led to the development of fission as both an energy source (see Nuclear Energy) and a weapon (see Atomic Bomb).

These fission studies, coupled with the development of particle accelerators in the 1950s, initiated a long and remarkable journey into the nature of subatomic particles that continues today. Far from being indivisible, scientists now know that atoms are made up of 12 fundamental particles known as quarks and leptons, which combine in different ways to make all the kinds of matter currently known.

Advances in particle physics have been closely linked to progress in cosmology. From the 1920s onward, when the American astronomer Edwin Hubble showed that the universe is expanding, cosmologists have sought to rewind the clock and establish how the universe began. Today, most scientists believe that the universe started with a cosmic explosion some time between 10 and 20 billion years ago (see Big Bang Theory). However, the exact sequence of events surrounding its birth, and its ultimate fate, are still matters of ongoing debate.