Modern Physics is based on the theory of relativity of Albert Einstein and the quantum theory of Max Planck and others. Matter and energy are not separate concepts, but are alternate forms of each other (Asinov, 2004).
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The theory of relativity is a description of space and time as determined by physical measurements. The origins of the theory date back to the principles of relative motion formulated by scientists in the 17th century. In its present form, however, the theory is largely the work of Albert Einstein. According to Einstein’s theory, space and time are relative concepts, and measurements of space and time depend on the state of motion of the observer (Lilley, 2001). Einstein’s theory consists of two parts: (1) the special, or restricted, theory, which concerns measurements made by observers moving at constant velocity with respect to each other; and (2) the general theory, which expands the special theory to include measurements by observers whose relative velocity is changing. The general theory applies the principles of relativity to gravitation. The special theory of relativity was published in 1905; the general theory, in 1916. Einstein’s theory of relativity has been of great importance in modern physics.
For example, the special theory showed scientists that it is possible to unleash the energy contained in the nucleus of the atom. The theory has influenced all branches of physics dealing with electromagnetic radiation and high-speed particles. It has had a profound effect on astronomy and the related science of cosmology, which attempts to explain the origin and structure of the universe. The body of scientific principles developed before Einstein’s time is referred to as classical physics.
When applied to everyday situations, these principles are still valid (Crease, 2006). The theory of relativity differs significantly from classical physics only when dealing with objects moving at extremely high speed, with objects having very strong gravitational fields, or with the universe on a broad scale. In addition, Einstein formulated his special theory of relativity on two basic assumptions: 1. The laws of physics are the same for all observers moving with a uniform motion relative to each other. 2. The speed of light in a vacuum is a universal constant, the same for all observers regardless of their relative motion or the motion of the light source. The first statement incorporates the relativity principle of classical mechanics, but it is more comprehensive.
Einstein was thinking not only of mechanical laws but also of the laws governing light and other electromagnetic phenomena. The second statement means that it is futile for an experimenter to try to determine his velocity through space by using a beam of light as a gauge. It is futile because regardless of the speed of the observer, his measurement of the speed of light will always give the same value.
This statement implies that nature offers no absolute reference system for the comparison of time or distance. Quantum theory is the theory that radiant energy is emitted (given off) and absorbed in units, or quanta, rather than in a steady stream. The quantum theory revolutionized physics in the early 1900’s; its introduction is often considered the beginning of modern physics (Apfel, 2005). The quantum theory has inspired some of the most brilliant scientific work of the 2oth century, and a number of Nobel prizes have been awarded for achievements related to the theory. Max Planck, a German physicist, originated the quantum theory in 1900. Certain studies that had been made of radiant energy led Planck to the conclusion that radiant energy is not emitted in a steady stream, like water from a hose, but in pieces, or units (which he called quanta), like bullets from a machine gun. His theory startled the scientific world, which had accepted the concept of classical physics that the emission and absorption of energy, like all other physical processes, takes place in a continuous manner (Davies, 2001).
In 1905, Albert Einstein helped establish the quantum theory with his explanation of the photoelectric effect—the ejection of electrons from a metal surface exposed to light. Einstein showed that the energy of light is concentrated into particle-like quanta. (According to classical physics, the energy of a light wave, like the energy of a water wave, should be evenly spread over the wave).
Today scientists usually treat light as a wave in processes that involve its transmission, and as quanta, called photons, in processes that involve its emission or absorption. The quantum theory has led to important discoveries about photoelectricity, photochemistry, the specific heats of solids, and the structure of atoms and certain activities of atoms, and to many other findings about matter and energy. The science that applies the quantum theory to matter is called quantum, or wave, mechanics (Davies, 2001). Modern physics is broken up into various fields of study. Atomic physics is the study of the structure of atoms and the behavior of electrons, one of the kinds of particles that make up the atom. The study of the structure and behavior of the atom by chemists and physicists has led to the military and commercial development of atomic energy, and has contributed greatly to man’s understanding of the universe. Atoms are so small that a hundred million of them, placed side by side, would make a row only about one inch (2. 5 cm) long.
Individual atoms can be viewed only with certain types of nonoptical microscopes that have extremely high magnifications. An atom is the smallest unit of matter, however. Protons, neutrons, and electrons—the particles that make up the atom—are smaller, as are muons, mesons, and a number of other particles (Crease, 2006). Each chemical element has its own chemical identity and behaves differently from every other element. Atoms of the same element are identical in chemical behavior’ they differ from the atoms of other elements in chemical behavior, in structure, and (usually) in weight.
In the fifth century, B. C., Empedocles, a Greek philosopher, developed the theory that all matter in the universe was composed of various combinations and proportions of four elementary substances—earth, air, fire, and water. Metals, for example, were considered to be composed of earth and fire, since they could be produced by placing ores (earth) in a flame. The shinier a metal, the more fire it was believed to contain (Crease, 2006). Nuclear physics is the branch of physics that deals with the atomic nucleus, or inner core of the atom.
The British physicist Lord Rutherford is regarded as the founder of nuclear physics. In 1911, he developed the following laboratory: An atom matter consists of a heavy inner core (the nucleus) that is surrounded by lightweight particles called electrons. The electrons each have a negative electric charge, while the nucleus has a positive charge (Cottingham, & Greenwood, 2006). The electrons can be pictured as orbiting the nucleus, just as the earth and other planets orbit the sun. According to currently accepted theories, the atomic nucleus is made up of positively charged particles called protons and uncharged (neutral) particles called neutrons. A proton has nearly the same weight as a neutron, about 1, 840 times that of an electron. The protons and neutrons are held together in the nucleus by a type of force called the strong force. The arrangement of protons and neutrons in the nucleus is termed nuclear structure.
Whenever the nuclear protons and neutrons are torn apart or otherwise rearranged, nuclear energy is released. The amount of energy released by such nuclear reactions is much greater than that released by chemical reactions, such as those that take place when coal burns or TNT explodes. Nuclear weapons are designed to release a large amount of nuclear energy suddenly, producing a powerful explosion (Asinov, 2004). Released in a slow, controlled manner in nuclear energy is used to generate electricity in many countries. The development of nuclear energy has been the supreme technical achievement of nuclear physics, and the various aspects of nuclear energy require the services of thousands of nuclear physicists and nuclear engineers.
Nuclear physicists are employed by government, private industry, and educational institutions. Many nuclear physicists hold teaching positions at universities, but devote most of their time to research (Asinov, 2004). In the United States, research projects in nuclear science are financed both by the government and by private foundations. Reference: Apfel, N. H. (2005).
It’s All Elementary: from Atoms to the Quantum World of quarks, Leptons, and Gluons (Lothrop, Lee & Shepard). Asinov, Isaac (2004). The History of Physics (Walker). Cottingham, W. M. & Greenwood, A. D.
(2006). An Introduction to Nuclear Physics (Cambridge University). Crease, Robert (2006). The Age of Discovery: Makers of the Revolution in 20th Century Physics (Macmillan). Davies, Paul (2001).
Other worlds: a Portrait of Nature in Rebellion—Space, Superspace, and the Quantum Universe (Simon & Schuster). Lilley, Sam (2001). Discovering Relativity for Yourself (Cambridge University).