More than 2000 years ago the Greek philosopher Democritus (460-371 B.C.) formulated the atomic hypothesis: All matter consists of smallest (indivisible) entities (atoms), separated by empty space. Combinations of the different kinds of atoms form all the things in nature.	The Greek philosopher Democritus (460-371 B.C.), founder of the atomic hypothesis.
Richard Feynman from the Californian Institute of Technology,Nobel prize 1965 for his work on Quantum Electrodynamics.	Richard Feynman, the well known American physicist (Nobel prize 1965), remarked that this is the most important and far-reaching hypothesis ever formulated about nature.

It was, however, only in the nineteenth century that the first concrete experimental evidence for the existence of atoms emerged. From about the year 1860 onwards the approximate size of atoms became known - 10-8cm - among the pioneers in this area the names of Clausius, Maxwell and Boltzmann stand out. The picture shows Ludwig Boltzmann (1844-1906), one of the great early pioneers of atomic physics. In his time his views on atoms met with great scepticism and even hostility, something which appears totally incomprehensible today, when we can now make single atoms visible and even shove them around. To visualize the size of atoms, the figure shows a sphere with a diameter of 1/100000 mm, consisting of 17000 atoms of Copper, prepared at HASYLAB at DESY. Mini-spheres of this kind, called clusters, are nowadays an interesting object of research.

Ludwig Boltzmann (1844-1906), founder of statistical thermodynamics.

A sphere with a diameter of 1/100.000mm, consisting out of 17000 Copper atoms
(HASYLAB, DESY)

Ernest Rutherford, who discovered the atomic nucleus

Rutherford and his collaborators discovered the atomic nucleus in 1910: more than 99.9% of the atomic mass is concentrated in a tiny nucleus.

Schematic view of an atom with its nucleus, which consists of protons and neutrons

Its size is only about 10-12 cm, i.e. 1000 times smaller than an atom. From this it can be seen, that the atoms and therefore ordinary matter are mainly empty space. Matter can therefore be compressed enormously, e.g. in a neutron star, where gravitation exerts a crushing force, and matter is compressed by a factor 1000000 million - the pyramid of Cheops would fit into a nutshell at that density. This discovery of the atomic nucleus marked the beginning of nuclear physics. It was soon realized that the atomic nucleus consists of two building blocks, protons and neutrons. The proton, the nucleus of the Hydrogen atom, is the lightest atomic nucleus.

In 1956 Hofstadter and his collaborators measured the size of the proton for the first time, by using the world's biggest (at that time) linear accelerator to shoot high energy electrons at hydrogen (Nobel prize 1961). They found a size of about 10-13cm, which is about 1/10 the size of a nucleus. This measurement indicated that there might be something inside the proton, and raised doubts, whether it was truly elementary.

The size of the proton is measured by the deflection of an electron as it passes close by.

SLAC, the two mile long linear electron accelerator in Stanford, California.

In 1962 the gigantic, two mile long electron linear accelerator (SLAC) in Stanford, California started operations. It is still the largest accelerator of this type in the world, and at that time it delivered electrons with the highest energy in the world, about 30 billion electron volts.

When these electrons were directed on protons, a structure inside the proton, in the form of even smaller point-like particles, became visible: the quarks (Nobel prize 1990 for J.I.Friedman,H.W.Kendall and R.E.Taylor).

An electron from SLAC is deflected sharply as it hits a quark inside the proton

In the sixties two large proton-synchrotrons, each providing protons with energies of about 300 billion electron volts, were built at CERN in Geneva and at Fermilab (FNAL) in the USA. They produced muon- and neutrino beams at much higher energy than the beams at SLAC. These beams were even better suited to explore the structure of the proton, and the combination of the two species of particles yielded additional information.
One could e.g. directly count the number of quarks in the proton; three were found, two up-quarks and one down-quark, as expected from the quark model of the proton.
 

The strong QCD force, mediated by gluons, keeps the quarks imprisoned inside the proton.

Why are the quarks imprisoned inside the proton and don't escape? An explanation was found by the introduction of a new kind of force, described by a theory called Quantum Chromodynamics (QCD). It is mediated by quanta known as gluons. The first indirect evidence for these gluons was seen at SLAC, and then more clearly at CERN and FNAL.

The year 1979 marks the actual discovery of gluons at the PETRA Electron-Positron Storage Ring at DESY. This important measurement did much to establish the new theory of forces between quarks. (High Energy Physics Prize of the European Physical Society for Günter Wolf, Paul Söding, Sau Lan Wu and Björn Wiik).

High Energy Physics Prize of the European Physical Society for G.Wolf, B.Wiik, P.Söding and S.L.Wu (v.l.n.r.) for the discovery of the gluons.

The new force mediated by gluons is very strong, more than 100 times stronger than the conventional nuclear force that binds protons and neutrons in the nucleus together. The conventional nuclear force, which e.g. is responsible for nuclear energy, is now seen as an indirect consequence of the basic gluonic force of QCD, analogous to the force that binds neutral atoms in molecules, which stems from residual electrical charges.

As a consequence of these gigantic forces, quarks are unable to really separate from the proton completely and be observed as isolated particles. Nobody has ever seen a quark directly!

The proton according to the new realistic quark model: Besides the three quarks of the naive model, there are the gluon strings, which can break and form numerous quark-antiquark pairs of the 'sea'.

What happens if one tries to separate a quark from the proton by brute force, for example if one hits a quark inside the proton with the high energy provided by the HERA storage ring?
In this case the gluon string, which connects the quarks inside the proton, ruptures and on the break points a new quark and antiquark are formed. These quarks and antiquarks immediately combine with other quarks to form bound states called mesons and baryons. It is therefore impossible to create a free quark, in the same way that is it not possible to create an isolated magnetic north or south pole by breaking a bar magnet: the result of such an operation is to produce two smaller bar magnets, each with a north and a south pole.

The gluon strings connecting the quarks inside the proton can also break spontaneously, giving birth to new quark-antiquark pairs. Therefore it is necessary to give up the old picture of the proton containing just three quarks; this old picture, the 'naive quark model', is now replaced by a more realistic model of the proton, containing quarks, antiquarks and gluons.