What is matter?

All matter consists of basic substances, or elements, with well definedphysical and chemical properties, ranging from hydrogen, the lightest, touranium and beyond.
Each element consists of building blocks - atoms - uniqueto the element, but the different atoms can combine to form anenormous variety of compounds from simple water to complexproteins. Yet, as scientists first discovered towards the end of the19th century atoms are not the simplest building bricks of matter, so what isthe most basic of our building blocks?

What is the atom made up of?

To find out what the atom was made up of theories had to be worked out,explained and proved. The structure of the atom was discovered in anexperiment where protons were fired at a piece of gold foil. Approximately onein eightthousend was bounced back, and when it was bounced back it returnedwith greater speed. This suggested a small, positively charged nucleus, withnegatively charged particles orbiting the nucleus, since like forces repel, andso the positive particle colliding with the nucleus would be powerfullyrepelled.

Splitting the atom

We now know that most of the mass of an atom is concentratedin a small, dense, positively-charged nucleus. A cloud of tinynegatively-charged electrons envelopes the nucleus, but at arelatively large distance, so that much of the volume of an atomis empty space.
In most atoms the nucleus contains two types of particle ofalmost equal mass: positively-charged protons and electrically-neutral neutrons. To make the atom neutral overall the number ofprotons exactly balances the number of electrons.
This picture of the atom stems largely from pioneering work atCambridge and Manchester Universities. At Cambridge in the1890's, two physicists began unwittingly to probe the worldwithin the atom. One, Joseph ('J.J.') Thomson, discovered thefirst known subatomic particle, the electron, while one of hisstudents, Ernest Rutherford, started to explore the newphenomenon of radioactivity, in which atoms change from onekind to another. This was to lead Rutherford eventually to thediscovery of the nucleus, in work with Hans Geiger (of Geigercounter fame) and Ernest Marsden at Manchester University in1909-10.
Later back at Cambridge, Rutherford found that atoms containpositively-charged particles, identical to the nucleus of hydrogen.He called the particles protons. And at Cambridge in 1932, JamesChadwick showed that the nucleus must also contain neutrons.By this time Rutherford and his colleagues had established much ofthe modern picture of the atom.

A Pageant of Particles

This was only the beginning. The electron, proton and neutronproved to be the first members of a rich pageant of subatomicparticles. During the 1930's and 40's, many physicists studiedcosmic rays, the steady rain of high energy subatomic particlesthat originate in outer space.
The collisions of high-energy cosmic rays with atoms in theatmosphere prised open the nucleus to reveal new kinds ofshortlived particles that could be seen only through the tracksleft behind in sensitive detectors. There were particles such asthe muon, which behaves like an electron, but is 210 timesheavier; thepion, which is just a little heavier than the muon; thekaon at little more than half the protons mass; and the lambda,which is about 20 per cent heavier than the proton.

Enter the Positron

One particularly important particle, discovered in 1932 by CarlAnderson at the California Institute of Technology is the positron- as light as an electron, but with positive charge. Its existence, atfirst a puzzle, was soon explained by Paul Dirac, a theoreticalphysicist at Cambridge University.
According to Dirac's theory the positron is a particle withexactly opposite properties to an electron - an anti-electron.The theory showed how an electron and a positron can emergetogether from pure energy provided the energy is sufficient tosupply the total mass of the two particles in accordance withEinstein's equation, E=mc^2.
If they collide, the particle and antiparticle disappear to leaveonly energy - an act of mutual destruction called annihilation.Experiments have since demonstrated that most other particles-protons, neutrons, muons and so on-have antiparticles too.


The result of matter and antimatter colliding: theyannihilate each other, creating conditions like those that might have existedin the first fractions of a second after the Big Bang.

Cosmic Mimics

By the early 1950s, the study of these particles had become abranch of physics in its own right- particle physics had come ofage. To aid them, the physicists had machines that couldaccelerate protons and electrons to high energies, mimicking thecosmic rays but in more controlled conditions.
The 3.8 metre wide LEP tunnel, 100 metres below theFrench and Swiss countryside, has taken eight years to complete. The tunnelcarries bunches of accelerated particles in a 27km long aluminium 'beampipe'.
Work in the early 1930s by John Cockcroft and Ernest Walton atCambridge, and by Ernest Lawrence and Stanley Livingstone atBerkley in California, had provided the first artificially acceleratedprotons. Their pioneering ideas gave birth in the 195O's and 60's tolarge machines capable of producing millions of protons,electrons, pions or kaons each second. With the invention ofmore sophisticated detectors to complement the accelerators,physicists now had the tools to study the many varieties ofparticle in detail.

Into inner space

The results of this onslaught on the realm of 'innerspace' have beenspectacular. We know that matter has a deeper layer revealed only as we probe it more energetically, inaccelerators and in studies of cosmic rays.
The proton, neutron, pion, kaon, lambda and many other subatomic particles arethemselves complex structures, based on only a few more basic particles - the quarksand their corresponding antiquarks.There are probably at least six types quark in three pairs, which have beennamed: up, down; charm, strange; top andbottom - although the top quark has yet tobe positively identified. These quarks combine in groups of threeto form the proton, neutron, lambda and related particles calledbaryons. The quarks can also bind with antiquarks to makeparticles such as pions and kaons, which are collectively known asmesons.


The fundamental particles of Nature appear to fallinto two categories: leptons and quarks.
The electron and muon, on the other hand, are not made fromquarks but appear as far as we can tell, to be indivisible. Theybelong to a separate family of particles called leptons, which alsoinclude a third still heavier charged particle, thetau, as well asneutrinos - particles that are almost massless, neutral and difficult todetect.