At the time of ancient Greeks, it was believed that all matter in the Universe was made up of four basic elements - earth, air, water and fire. Furthermore, it was also believed that matter was continuous , that is, one could divide a piece of matter into smaller and smaller bits without any limit: one never came up against a grain of matter that could not be divided further. However, some Greeks, such as Democritus, believed that all the matter was made up of small particles, which could not be further divided, and these tiny particles were called atoms. (The word atom means “indivisible” in Greek.) However, our understanding of nature have been changed by years of research and now we have a better explanation of how our Universe works. There are four fundamental forces which dominate the Universe and on the basis of these forces the Universe works. Here is a brief explanation of these forces.
Gravitational Force
The gravity is the weakest of all the forces. It is a universal force, that is, every particle feels the force of gravity according to its mass and energy. This is the force due to which planets orbit around the sun. The gravitational force has two special properties that it can act over long distances and it is always attractive. This means that the very weak gravitational forces between the individual particles in two large bodies, such as the earth and the sun, can all add up to produce a significant force. The other three forces are either short range, or are sometimes attractive and some-times repulsive, so they tend to cancel out. Before Einstein, gravity was only thought as a force and no one knew what caused this force. However, Einstein changed our view of gravity. According to Einstein, gravity is caused by the curvature of space-time. In Einstein's view, space-time is like a fabric, which is curved by mass. Every body which has some significant amount mass curves space-time. The larger the mass of the object the more it curves space-time fabric. The more the curvature of space-time around an object the more amount of gravity it has. The planets move around sun due to this curvature of space-time. Below is the demonstration of gravity in 2D.
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| Visualization Of Gravity In 2D |
In the quantum mechanical way of looking at the gravitational field, the force between two matter particles is pictured as being carried by a particle of spin 2 called the graviton. This has no mass of its own, so the force that it carries is long range. The gravitational force between the sun and the earth is ascribed to the exchange of gravitons between the particles that make up these two bodies. Although the exchanged particles are virtual, they certainly do produce a measurable effect. They make the earth orbit the sun. Real gravitons make up what classical physicists would call gravitational waves, which are very weak.
Electromagnetic Force
The next 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
two electrons is about a million million million million million million
million (1 with forty-two zeros after it) times bigger than the gravitational
force. However, there are two kinds of electric charge, positive and negative.
The force between two positive charges is repulsive, as is the force between
two negative charges, but the force is attractive between a positive and a
negative charge. A large body, such as the earth or the sun, contains nearly
equal numbers 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
numbers of virtual massless particles of spin 1, called photons. Again, the
photons that are exchanged are virtual particles. However, when an electron
changes from one allowed orbit to another one nearer to the nucleus, energy is
released and a real photon is emitted – which can be observed as visible light
by the human eye, if it has the right wave-length, or by a photon detector such
as photographic film. Equally, if a real photon collides with an atom, it may
move an electron from an orbit nearer the nucleus to one farther away. This
uses up the energy of the photon, so it is absorbed.
Weak Nuclear Force
The third 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 photons and
gravitons. The weak nuclear force was not well understood until 1967, when
Abdus Salam at Imperial College, London, and Steven Weinberg at Harvard both
proposed theories that unified this interaction with the electromagnetic force,
just as Maxwell had unified electricity and magnetism about a hundred years
earlier. They suggested that in addition to the photon, there were three other
spin-1 particles, known collectively as massive vector bosons, that carried the
weak force. These were called W+ (pronounced W plus), W- (pronounced W minus),
and Zº (pronounced Z naught), and each had a mass of around 100 GeV (GeV stands
for gigaelectron-volt, or one thousand million electron volts). The Weinberg-Salam
theory exhibits a property known as spontaneous symmetry breaking. This means
that what appear to be a number of completely different particles at low
energies are in fact found to be all the same type of particle, only in
different states. At high energies all these particles behave similarly. The
effect is rather like the behavior of a roulette ball on a roulette wheel. At
high energies (when the wheel is spun quickly) the ball behaves in essentially
only one way – it rolls round and round. But as the wheel slows, the energy of
the ball decreases, and eventually the ball drops into one of the thirty-seven
slots in the wheel. In other words, at low energies there are thirty-seven
different states in which the ball can exist. If, for some reason, we could
only observe the ball at low energies, we would then think that there were
thirty-seven different types of ball. In the Weinberg-Salam theory, at energies
much greater than 100 GeV, the three new particles and the photon would all
behave in a similar manner. But at the lower particle energies that occur in
most normal situations, this symmetry between the particles would be broken.
WE, W, and Zº would acquire large masses, making the forces they carry have a
very short range. At the time that Salam and Weinberg proposed their theory,
few people believed them, and particle accelerators were not powerful enough to
reach the energies of 100 GeV required to produce real W+, W-, or Zº particles.
However, over the next ten years or so, the other predictions of the theory at
lower energies agreed so well with experiment that, in 1979, Salam and Weinberg
were awarded the Nobel Prize for physics, together with Sheldon Glashow, also
at Harvard, who had suggested similar unified theories of the electromagnetic
and weak nuclear forces. The Nobel committee was spared the embarrassment of
having made a mistake by the discovery in 1983 at CERN (European Centre for
Nuclear Research) of the three massive partners of the photon, with the correct
predicted masses and other properties. Carlo Rubbia, who led the team of
several hundred physicists that made the discovery, received the Nobel Prize in
1984, along with Simon van der Meer, the CERN engineer who developed the
antimatter storage system employed. (It is very difficult to make a mark in
experimental physics these days unless you are already at the top.)
Strong Nuclear Force
The fourth is 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 another spin-1 particle, called the gluon, which interacts only with itself and with the quarks. The strong nuclear force has a curious property called confinement: it always binds particles together into combinations that have no color. One cannot have a single quark on its own because it would have a color (red, green, or blue). Instead, a red quark has to be joined to a green and a blue quark by a “string” of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron. Another possibility is a pair consisting of a quark and an antiquark (red + anti-red, or green + anti-green, or blue + antiblue = white). Such combinations make up the particles known as mesons, which are unstable because the quark and antiquark can annihilate each other, producing electrons and other particles. Similarly, confinement prevents one having a single gluon on its own, because gluons also have color. Instead, one has to have a collection of gluons whose colors add up to white. Such a collection forms an unstable particle called a glueball. The fact that confinement prevents one from observing an isolated quark or gluon might seem to make the whole notion of quarks and gluons as particles somewhat metaphysical. However, there is another property of the strong nuclear force, called asymptotic freedom, that makes the concept of quarks and gluons well defined. At normal energies, the strong nuclear force is indeed strong, and it binds the quarks tightly together. However, experiments with large particle accelerators indicate that at high energies the strong force becomes much weaker, and the quarks and gluons behave almost like free particles.
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