History of nuclear science

Five centuries before the birth of Christ, Greek philosopher Democritus proposed that all matter was made up of tiny, indivisible particles. He called them atomos, or uncuttable.

Ernest Rutherford

It was more than two thousand years before humankind began to really delve into the atom and its properties.

In a series of theories put forward over more than a century, the structure of the atom was defined.

Englishman John Dalton's theory, published in 1803, proposed that: each element was composed of the same kind of atoms unique to that element; compounds were composed of atoms in specific ratios; and chemical reactions were rearrangements of atoms.

Another Englishman, Joseph John Thompson, was awarded the Nobel Prize in Physics in 1906 for discovering the electron. He later theorised in his 'plum pudding' model that the atom was a sphere of diffuse positive electricity in which negative particles were embedded.

Two years later, New Zealander Ernest Rutherford collected the Nobel Prize in Chemistry for his work on the structure of the atom. He proposed that the atom was mostly empty space, but with a dense, positively-charged nucleus orbited by negative electrons.

In 1922, Denmark's Niels Bohr took out the Nobel Prize in Physics, for his services in the investigation of the structure of atoms and of the radiation emanating from them. In 1913, he had proposed that only certain electron orbits were allowed, and that transitions between them involved the absorption or release of energy.

After further research, by the 1920s scientists had decided upon the overall structure of the atom: that it consists of a dense nucleus of protons and neutrons, surrounded by electrons existing in different 'clouds' at various energy levels.

The book was certainly not ruled off at neutrons, protons and electrons. By the 1930s, scientists were beginning to find other elementary particles and the search continues.

The elementary particles making up an atom have been placed in two categories: fermions and bosons, depending on their spin.

Fermions comprise leptons and quarks. The leptons are the electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. Just to show scientists do have a sense of humour, the different types, or flavours, of quarks are up, down, charm, strange, top (or truth), and bottom (or beauty).

The bosons include the photon, graviton and Higgs boson.

The fermions are regarded as the basic constituents of nuclear and atomic structure, or matter, while bosons transmit the fundamental forces of nature between them.

Perhaps the biggest experiment in international nuclear physics is the search for the almost fabled Higgs boson, which may be about to reach a climax at the European Organisation for Nuclear Research (CERN) in Switzerland.

Scientists believe there is a unifying force for light, electricity, magnetism and some forms of radioactivity. Finding the proof has been elusive.

According to CERN, in order for this unification to work mathematically, it requires that particles carrying this force have no mass.

We know from experiments that this is not true, so physicists Peter Higgs, Robert Brout and Francsois Englert came up with a solution to solve this conundrum, CERN said.

They suggested that all particles had no mass just after the Big Bang. As the Universe cooled and the temperature fell below a critical value, an invisible force field called the 'Higgs field' was formed together with the associated 'Higgs boson'.

The field prevails throughout the cosmos: any particles that interact with it are given a mass via the Higgs boson. The more they interact, the heavier they become, whereas particles that never interact are left with no mass at all.

The difficulty has been that no-one has been able to detect the Higgs boson, or 'god particle'.

The 27km circumference Large Hadron Collider at CERN, which cost almost $US10 billion to build, began its search for the Higgs boson in 2008.