Dr. N.L. Kachhara
We now study matter as described in modem science.
Elementary Particles
The matter is made up of molecules and these, in turn are made up of atoms. A typical atom consists of a nucleus composed of positively charged protons and neutral neutrons surrounded by a cloud of orbiting negatively charged electrons. Ernest Rutherford first postulated this model in 1913. At that time, it was thought that all matter consisted of these elementary particles. These particles are tabulated in Table 1.
Table 1 Elementary Particles
| Property | Electron | Proton | Neutron |
| Symbol | e- | p+ | n |
| Mass (kg) | 9.109x 10 -31 | 1.673x10-27 | 1.675x10-27 |
| Mass (MeV) | 0.51 | 938.2 | 939.6 |
| Electric Charge | -1 | +1 | 0 |
In 1928 Paul Dirac predicted that all particles should have opposites called anti-particles. The anti-particle of electron is positron. It is identical in every respect to the electron apart from its electric charge, which is (+1). When an electron and positron come in contact, they mutually annihilate each other producing a flood of energy in accordance with Einstein's equation.
Normally the total energy equivalent of the rest mass of particle is not released. About one thousand millionth fraction of this energy is released in chemical reactions. Even in a nuclear reaction about one percent of total energy is released. But interaction between a particle and anti particle releases full hundred percent energy equivalent to the rest mass of particle.
Both the proton and the neutron have anti particles. These also destroy each other if they meet with their particle. Ordinary matter is made up from particles. It appears that the universe is made up of ordinary matter. Matter composed of anti- particles is known as anti matter. Anti matter can be created in the laboratory but does not last long as it quickly comes in contact with ordinary matter and is destroyed.
It is now known that there are many more elementary particles than the six described above. These have been created using modem high-technology equipment. These have been divided into a number of groups depending on their properties. Most of these newly discovered particles have their anti-particles. The laws of physics are not quite the same for particles and antiparticles.
Leptons
The electron (e) is the simplest of the leptons. There are two heavier leptons called the muon (µ) and the tau (τ). Both are unstable and decay to simpler, more stable particles. Both have anti-particles. Muons are found in the air as cosmic rays enter the Earth's atmosphere and smash into atoms and molecules.
Another type of lepton is the enigmatic neutrino (ν). There are three types of neutrino, each one associated with one of the three leptons described above (e,µ,τ). They are called the electron neutrino (νe), muon neutrino (νµ) and tau neutrino (ντ).
Neutrinos hardly react with other types of matter. They can easily pass through the Earth. They have no electric charge. Each one has its anti-particle version so there are six types of neutrinos. Neutrinos have a very low mass and one type can change into one of the other two types. Leptons are never found in the nucleus of atoms. The six leptons are tabulated in Table 2
Table 2 Leptons
| Name of Lepton | Symbol | Mass (MeV) |
| Electron | e | 0.511 |
| Electron neutrino | νe | ~o |
| Muon | ν | 106 |
| Muon neutrino | νµ | ~0 |
| Tau | τ | 1777 |
| Tau neutrino | ντ | ~0 |
Baryons
The two most common baryons are the proton and neutron. They are both of similar mass but the proton has a single positive charge. They are collectively known as nucleons. Both are found in the nuclei of atoms, being kept there by the strong nuclear force that binds them together. In recent years it has been suggested that baryons are made up of even more elementary particles called quarks. Quarks are found in six types called flavours. In 1989 it was shown that only three pairs of quarks could exist. These correspond with the three leptons and the three neutrinos. Quarks are unusual in having fractional electric charges. Quarks are much smaller than wavelength of visible light and so do not have any colour in the normal sense. The quarks are tabulated in Table 3.
Table 3 Quarks
| Name of quark | Symbol | Charge | Mass (MeV) |
| up | u | + (2/3) | 2- 8 |
| Down | d | - (1/3) | 5-15 |
| Strangeness | s | - (1/3) | 100-300 |
| Charm | c | +(2/3) | 1000-1600 |
| Bottom (or Beauty) | b | - (1/3) | 4100-4500 |
| Top (or Truth) | t | + (2/3) | 180000 |
Baryons are made up of quark triplets. The proton is composed of two u quarks and a d quark
+ (2/3) + (2/3) - (1/3) = + 1
The neutron is made from two d quark and a u quark.
- (1/3) - (1/3) + (2/3) = 0
The proton and neutron are stable particles in most nuclei. Outside the nucleus or in certain unstable nuclei, neutrons decay. There exist other baryons, produced in high energy experiments that are less stable. These too are made up of quark triplets. Hundreds of these particles are known. All six quarks have their anti-quarks with charges opposite in value to their quark counterparts. The (u) anti- quark has a charge of - (2/3) while the (d) anti-quark has a charge of + (1/3). The anti-proton is made up of (u) (u) (d) and has a charge of -1.
Mesons
In a nucleus, the protons and neutrons are not really separate entities, each with its own distinct identity. They change into each other by rapidly passing particles called pions (π) between themselves. Pions are the most common of the mesons. Mesons are composed of quark / anti-quark pair. The positive pion (π+) is made from a u quark and d anti-quarks. The negative pion (π-) is made from a d quark and u anti-quark. Kaons are short lived mesons that decay into simpler particles. Normally, particles and ant-particles decay in a similar way. All of the above particles are referred to as fermions. Particles have a property called spin. The spin of fermions has half integer values (1/2, 3/2, etc.). Because of this spin, fermions obey the Pauli Exclusion Principle. This means that two fermions cannot occupy the same energy states. With electrons this gives rise to atoms whose electrons are distributed in shells. These shells give atoms their differing chemical properties.
Forces
There is another type of particle called boson. Bosons are known as force carriers Bosons have integer spin (0, 1, and 2). Bosons do not obey the Pauli Exclusion Principle.
When two particles interact they exchange a boson. The bosons exchanged between matter particles are said to be virtual particles because, unlike 'real' particles, a particle detector cannot directly detect them. We know they exist, however, because they do have a measurable effect; they give rise to forces between matter particles.
Bosons can be grouped into four categories according to the strength of the force that they carry and the particles with which they interact. It should be emphasized that this division into four classes is man-made; it is convenient for the construction of partial theories, but it may not correspond to anything deeper. Ultimately, most physicists hope to find a unified theory that will explain all the four forces as different aspects of a single force.
The first category is the gravitational force. This force is universal, that is, every particle feels the force of gravity, according to its mass or energy. Gravity is the weakest of the four forces by a long way; it is so weak that we would not notice it at all were it not for two special properties that it has; it can act over large distances, and it is always attractive. This means that the very weak gravitational forces between individual particles in two large bodies, such as the earth and the sun, can all add up to produce a significant force. Some people suggest the existence of a graviton to carry the gravitational force.
The next category is the electromagnetic force, which interacts with electrically charged particles like electrons and quarks, but not with uncharged particles such as gravitons. It is much stronger than the gravitational force. The electromagnetic force between electrons is about 1042 times bigger than the gravitational force. The force between two positive (or negative) charges is repulsive, but the force is attractive between a positive and a negative charge. A large body, such as earth and the sun, contains nearly equal number of positive and negative charges. Thus the attractive and repulsive forces between the individual particles nearly cancel each other out, and there is very little net electromagnetic force. However on the small scales of atoms and molecules, electromagnetic forces dominate. The electromagnetic attraction between negatively charged electrons and positively charged protons in the nucleus causes the electrons to orbit the nucleus of the atom; just as gravitational attraction causes the earth to orbit the sun. The electromagnetic attraction is pictured as being caused by the exchange of large number of virtual mass less particles of spin 1, called photons.
The third category is called the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the protons and neutrons together in the nucleus of an atom. It is believed that this force is carried by spin-1 particle, called gluon, which interacts only with itself and with the quarks. The strong nuclear fore has a curious property called confinement. It always binds particles into combinations that have no clour.
The strong force is 100 times stronger than the electromagnetic force. In fact, it is the strongest force in nature. It has the shortest range of all forces, equal to about the diameter of the proton. Strong force interactions are very fast i.e. they take about 10-22 second.
The fourth category is called the weak nuclear force which is responsible for radioactivity and which acts on all matter particles of spin ½ but not on particles of spin 0, 1 or 2 such as photon and gravitons. Three bosons (w+, w- and z0) carry the weak nuclear force. This is the force responsible for beta decay in which a neutron spontaneously disintegrates into a proton by emitting an electron and mass less neutrino. The electrons, which are emitted in this process, become powerful radiations and are used in biology, medicine and industry. Weak interactions are also fast and take about 10-10 second.
All known particles fall into two classes, bosons or fermions. Many bosons can occupy the same state at the same time. This is not true for fermions only one fermion can occupy a given state at a given time. This is why solids can't pass through one another. Why we can't walk through walls, because of Pauli repulsion the inability of fermions (matter) to share the same space the way bosons (forces) can. Bosons do not have antiparticles. They are mass less. Photon, gluon and graviton do not have electric charge. The graviton has not yet been observed directly or indirectly.
The number of "basic forces" has changed over the years. The electric and magnetic forces once thought separate gradually become unified as electromagnetic forces. More recently, weak interactions have joined electromagnetic interactions to become electro weak interaction. In all likely hood, strong and gravitational interactions will eventually join electro weak to give us one grand system of interactions between objects in our universe.
