- Newton was the first to describe the laws of the gravitational force; it was in the late XVII century.
- In XIX century, physicists pinned down the laws of the electromagnetic forces.
- Atoms were long thought of as elementary indivisible blocks of matter (Democritus, ~400 BC), in 1869 Mendeleev arranged them in the periodic table of elements.
- At the turn of the XX century, atoms were found to have a structure. It was realised that an atom consisted of a bunch of electrons (e) (Thomson, 1897) circling around a tiny nucleus (Ratcliffe and Rutherford, 1911).
- In 1930 based on the circumstantial evidence, Pauli argued that there must be one more particle present in radioactive decays. The particle was named a neutrino (n) and was found only in 1956. The force responsible for radioactive decays is called weak force.
- In 1932, with the discovery of neutrons by Chadwick, the structure of nuclei was finally pinned down: they were also composite and consisted of protons and neutrons closely packed together. A term of strong force emerged to name the force that held all of them together.
- In 1960s, it became clear that protons and neutrons themselves are made of sub-particles, quarks. A proton would have to have two so-called up-quarks (u) and one down-quark (d), while a neutron---one up-quark and two down-quarks.
- Electron, its neutrino, up- and down-quarks form the first generation of elementary particles. We call them elementary as they still look to us point-like (if they do have sizes, they must be smaller than 10-18 m) and show no signs of inner structure. However, there turned out to be two more generations of identical sets of particles, the only difference being their masses. Nobody knows why there are three duplicating generations. It all started from a muon (m), a cousin of an electron, discovered in 1937 (the legend goes that when it became clear that it was nothing else, but another "heavy electron", a famous theorist Rabi exclaimed Who ordered that?!!). The third generation started out from the discovery of another, even heavier, cousin of an electron, named t-lepton, in 1975, followed by the discovery of the b-quark in 1977. It is very tempting to assume that this new "periodic table of elementary particles" is a give-away sign of some deeper structure.
- Quanta of the electromagnetic force are called photons (g), of the strong force---gluons (g), of the weak force---Z and W Bosons (not bison!). The quantum of the gravitational force would be a graviton, but the quantum theory of gravitational force still remains elusive.
- Now all "elements" in the table shown above have been experimentally observed.