Only a few months after the discovery of X-Rays by Roentgen in 1896, Henri Bacquerel accidentally discovered the phenomenon of radioactivity in uranium sulphate. The invisible Bacquerel rays which possessed the ability to affect wrapped photographic paper were soon found to be emitted by a number of other heavy elements such as thorium, actinium etc. But the most famous was the discovery of radioactive elements Radium and Polonium by the Curies in 1998. The radiation emitted by radioactive substances was soon identified to consist of three different kinds of emissions named alpha, beta and gamma rays after the Greek letters. The alpha radiation was found to consist of positively charged particles having a positive electric charge of magnitude twice that of the electron and a mass four times that of the hydrogen atom. It was immediately concluded and is still believed that the alpha particle is the same as the bare nucleus of a Helium atom which has lost both its electrons. It is emitted at a velocity one fifteenth that of light and is stopped by a 0.1 millimeter thick foil of aluminum.
Beta rays were found to consist of negatively charged particles of very light mass emitted with a velocity that varied between one third to 99.8 percent of the velocity of light and penetrative power nearly one hundred times that of the alpha particles. The variations in their masses were later accounted for by the special theory of relativity and these were identified as electrons. Gamma rays have been found to be electromagnetic radiations similar to X-rays, but of extremely short wavelength of the order of one angstrom or 10^-10 meters and an ability to penetrate several centimeters of lead.
In 1911, Rutherford came forth with his own physical model for subatomic structure, as an interpretation for some unexpected experimental results. In it, the atom is made up of a central charge now called nucleus surrounded by a cloud of orbiting electrons. In subsequent years, rules were established for the numbers of atoms in various orbits and it was explained that electromagnetic radiation was released by electrons shifting between orbits and releasing energy when they moved from an orbit of higher energy to one of lower energy.
The discovery of the neutron by Chadwick in 1932 completed the well known picture of the nuclear structure consisting of protons and neutrons, the number of protons, carrying a positive charge equal and opposite that of electrons, being the same as electrons in the atom, and various isotopes being the result of differing numbers of electrically neutral or chargeless neutrons whose mass is very nearly the same as protons which is about 1836 times that of the electron.
The good thing about the electron is that anybody can see it in the electrical arc formed between two separated ends of a conductor if an electrical circuit is broken. It is this seeing and believing that lies at the heart of willing acceptance of later discoveries of subatomic particles none of which can be actually seen but whose existence is proven by inference. The good thing about atomic models is that they work in predicting properties of materials and have resulted in the development of science and technology of electronics which has revolutionized our lives.
Democritus's atoms were no longer the fundamental particles, but instead were composed of varying numbers of fundamental particles which were then established as being the electron, the proton and the neutron. The measurement of the rate of decay in radioactive materials, commonly denoted by half-life, made it possible to estimate the age of the earth which is now believed to be about 4.6x10^9 (four thousand six hundred million) years
In the year 1900, Wilson, Elster and Geital discovered that charged electroscopes exhibited a small residual leak in spite of the best insulation. The surrounding air was eliminated as a possible cause since its conductivity was found to be constant. In 1903, Rutherford and Cooke demonstrated by the use of absorbing screens of iron and lead that the radiation responsible for the discharge of the electroscope came from outside the instrument. Initially, it was thought that the radiation was the result of the contamination of Earth's surface and the surrounding air by radioactive materials which had also been recently discovered. However, when Gokel, Hess and Kolhorster during 1909-1914 sent sealed ionization chambers up in balloons up to 9,000 meters high, it was found that the intensity of radiation increased with height continuously, as much as five to ten times the value at ground level. Hence it was concluded that an extremely penetrative type of radiation, whose origin was entirely beyond our atmosphere, was falling upon the earth from above the atmosphere and it was called cosmic radiation. The methods developed and used in the study of cosmic radiation include ionization chambers, photographic emulsions and bubble chambers developed in 1952 by D.A. Glaser.
One of the greatest achievements of the cosmic rays studies was the discovery in 1932 by C.D. Anderson of the "Positron" or the antiparticle of the electron which is identical to the electron except that it carries a positive charge of the same magnitude. The positron had, however been predicted theoretically by P.A.M. Dirac in 1928. Similarly, two years after Yukawa's theoretical prediction, mesotrons or mesons were discovered in 1937 by Neddermeyer, Anderson, Street and Stevenson by a study of cloud chamber tracks of cosmic radiation.
The development of the cyclotron by Prof. E.O. Lawrence in 1932 provided a controllable means of producing beams of high energy accelerated particles that could be used to bombard other particles and nuclei and break them into pieces to render different elements and new fundamental particles. The technique is called transmutation, and has resulted in a proliferation of newly identified fundamental particles of diverse sizes and qualities. The second half of the twentieth century has seen the construction of a number of particle accelerators in various parts of the world; and could well be called the era of the elementary particles, as far as physics is concerned.
The first `atomic pile' -- the forerunner of nuclear reactors -- was activated by Enrico Fermi in December 1942 in a Squash court in Chicago University which among other things produced a significant quantity of the radioactive element Plutonium that had not existed naturally on earth before then and was hailed as the first synthesized or artificial element beyond the natural ninety-two. This great achievement, unfortunately, created the delusion among many that man had after all succeeded in overtaking nature and was now more powerful than God, if One existed. The conceit showed itself in the pathetic development and tragic use of atomic bombs much before the nuclear reactor could be used to produce useful energy and medicinal radioisotopes, apart from research in nuclear physics.
These studies have resulted in the identification of five basic types of forces that exist in nature as 1) gravity, 2) electric force, 3) strong nuclear forces which hold neutrons and protons together in the nucleus, 4) weak nuclear forces which control the change in charge states of nucleons, and 5) color forces which are postulated to operate on quark particles only about which we shall learn later. Of course, all other forces that we experience in our daily lives are supposed to be the complex consequences of these five. The method by which atomic, nuclear and subatomic particles interact is believed to be via the exchange of energy which can alternatively be considered as particle exchange. The interaction between electric charges is known to proceed by the exchange of photons which are massless particles which can carry force or energy for an infinite distance at the speed of light. Any particle that travels at the speed of light has to be massless (i.e have zero rest mass) in order for the theory of relativity to be held true, as a particle with any initial mass would assume infinite mass at the velocity of light. The hypothetical particle graviton carries the gravitational force effect between particles of matter. Neutrino and antinutrino are also chargeless and massless particles which can only be distinguished from photons by having different spin. Then, of course, there are muons, pions, kaons, lambda particles, sigma particles, omega particles, psi particles and a whole host of antiparticles, some stable and some unstable. The unstable only last between 10^-17 and 10^-21 seconds, some of which are called resonances. "Strange" particles have life times of the order of 10^-9 seconds. There are quark particles which were initially regarded as the fundamental building blocks of all particles and which possess the property of charm. Two varieties of the quarks have been named truth and beauty. Then of course, there are monopoles which as the very name suggests should be a sort of half-magnet that would be attracted by the north pole of a magnet and repelled by the south pole in any orientation or vice versa. In fact, it is curious that so far radions (particles of radio frequency radiation) and thermion (particles of heat radiation) do not seem to have made their mark. However, the growing diversity of fundamental particles is by no means a cause for discord. Some physicists are optimistic that it may be possible to identify a group or family of particles which could form the basis of the synthesis of all other particles; and it may even be possible to integrate these particles with the quantum theory and, in turn, with the theory of relativity, thus producing the grand unification and the final answers to all physical questions.
