Modern Physics
Relativity
Until the beginning of the twentieth century, many scientists had believed that Newton’s law of motion and the law of gravity offered a satisfactory basis for explaining the physical world. In 1905, Albert Einstein published his theory of special relativity, which showed that Newton’s laws were really versions of more general laws. Einstein’s theory is based on the following assumptions:
1. The measured value of the speed of light in a vacuum is always the same no matter how fast the observer or light source is moving.
2. The maximum velocity possible in the universe its that of light.
3. Absolute speed cannot be measured; only speed relative to some other object can be.
The theory states that to an observer at rest, a moving object will appear shorter and more massive than the same object at rest, and that a moving clock will appear to be going more slowly than the same one at rest.
 QUANTUM PHYSICS 12 pins buttons Mechanics Theory badges  US $9.99
|
 The Meaning of Quantum Theory A Guide for Students of US $6.45
|
 Quantum Kinetic Theory and Applications Electrons Pho US $9.99
|
 Einsteins Struggles with Quantum Theory by Andrew W US $156.78
|
 Many Body Problems and Quantum Field Theory An Intro US $73.03
|
 Constructive Quantum Field Theory Selected Papers US $73.80
|
Quantum Theory
Newton believed that visible light consists of particles, while a Dutch physicist, Christian Huygens, and the others felt light was a wave. Today both views are believed to be correct. Radiation has properties of both waves and particles. Max Planch, a German physicist, while studying radiation from so-called black bodies, concluded that his results could only be explained if energy were produced in individual bundles or packets, which he called quanta, rather than in waves. A quantum of energy emitted as visible light is called a photon. The amount of energy of each quantum is not the same. The amount of energy of each is proportional to the frequency of the radiation. Einstein soon showed that the photoelectric effect, which is the release of electrons by certain metals when irradiated by light, can only be explained in terms of the quantum theory. The quantum theory has since become the foundation of your understanding of matter.
The Principle of Uncertainty
Newton believed that if we knew the exact position and momentum ( mass multiplied by velocity) of a particle at some instant, it would be possible to calculate where the particle would be at any time in the future. In 1927, Wener Heisenberg, a German physicist, showed that this is not so. He proved that it is impossible to determine both the exact position and speed of any particle at the same time, since the act of measuring disturbs the particle and introduces an error into the measurement. This principle of uncertainty has led to quantum, or wave, mechanics, a new way of describing atomic particles.
The uncertainty principle tells us that any statement about any individual even that occurs within the confines of this basic uncertainty has no meaning and is not admissible in physics. The discovery of this principle has given physicist deeper insight into the laws of nature, but it has not altered the general pattern of physical concepts. Most observations are carried out with materials having large numbers of particles and individual events, and are statistical in nature. The laws of probability see to it that the sum total of the events is determined, although each individual event is uncertain.
Particle physics
At one time around the 1930 the only known particles smaller than atoms were electrons, protons, and photons. The electron is the carrier of negative electric charge( and some mass), the proton is the carrier of positive electric charge and mass, and the photon is the carrier of electromagnetic radiation. But there are many subatomic particles that have been found since 1930. Certain of these particles are considered basic in various theories. There are 22 of these.
More basic even than most of the 22 observed particles are quarks. While quarks have never been observed experimentally, 15 of the 22 particles are thought to be made from quarks. The 15 subatomic particles formed from quarks ( and other very short-lived particles also from quarks) are called baryons, meaning “heavy,” or mesons, meaning “middle”. Subatomic particles that are not formed from quarks are called leptons, meaning “light,” except for the photon, which is a class by itself. The mass of these particles is measured in millions of electron volts.
Four different forces cause the interactions between particles, including the change of an isolated particle to other particles of less mass. These forces are gravity, the electric force, the weak force, and the strong force. ( In some recent theories, the electric and the weak forces have been combined as the electroweak force.) Gravity is actually the weakest of the four forces.
Nearly all particles exist in at least two forms, one of which is called the antiparticle of the other. The anti particle of the proton is called the antiproton; the antiparticle of the neutron is the antineutron; and so forth for more particles. However, the antiparticle of the electron, which was the first antiparticle to be discovered, is usually called the positron. If a particle and its antiparticle meet, say an electron and a positron, they will destroy each other completely, producing photons that carry off the masses as energy. Similarly, a photons with sufficient energy can suddenly become a pair consisting of one electron and one positron, converting part of the energy of the photons into mass. Atoms can be made from antiprotons, antineutrons, and positron, forming antimatter, but they immediately interact with ordinary matter to turn into energy.
Nuclear Physics
Nuclear physics is concerned with changes in the nuclei of atoms.
Radioactivity
Antoine Henri Becquerel, a French scientist, discovered in 1896 that uranium gives off radiation that fogs photographic film. Soon after, Marie and Pierre Curie isolated two new radioactive elements, polonium and radium. Radioactivity is the spontaneous disintegration of the nuclei of certain atoms. As these atoms disintegrate , energy is released. The radioactivity of a substance decreases continually with time, and the rate of decrease is different for each element. The rate of radioactive decay is usually expressed as the half-life of that element, that is, the time for half the atoms to decompose. The half-life ranges from a fraction of a second for some isotopes to billions of years for others.
Radioactivity originated in the unstable nucleus of the atom. There are three general types of radioactive decay. These nucleus can emit a gamma ray, which is an electromagnetic wave of very high frequency. It can also emit an alpha particle, which is the equivalent of the nucleus of a helium atom containing two protons and two neutrons. Since the los of protons means a decrease in the atomic number( the number of protons in the nucleus), and since the atomic number determines the chemistry of the atom, the result is the formation of a new element. When radium loses an alpha particle, the products are helium and radon.
The third type of radioactive decay comes from the beta particle, which is an electron. We can imagine a neutron losing an electron and becoming a proton. Since a proton has been gained, the atomic number changes and a new element is produced.
Nuclear Fission
In 1939, Otto Han and Fritz Strassman, German physicists, discovered that if uranium is bombarded with neutrons, uranium is formed. This isotope is unstable and splits into several fragments in a process called nuclear fission. The fission process releases enormous amounts of energy because some mass is converted to energy. The theory of relativity states that when a mass is converted to energy, the energy is E = mc(square), where c is the speed of light. Since c(Square) is a very large number, E is great even for small values of m.
The fission process requires a neutron to start it, but the resulting fissions produce many neutrons. If these bombard other Uranium atons, a chain reaction can occur. This requires a ceratin critical mass of uranium, for otherwise the neutrons will escape. The fission bomb is an uncontroleed chain reaction.
To use the fission reaction to produce nuclear power, it is necessary to have a controleed reaction. The first controoled reaction was contained in a uranium-graphite pile into which cadmium rods were inserted. Cadmium abosrobs neutrons so the reaction can be slowed down by simply pushing the rods further into the pile. This is the basic principle of today’s nuclear reactors. The huge amount of heat that is produced is converted to stream, which in turn drives electric generators.
Nuclear Fusion
Another way to release very large amounts of energy is by fusion, or combining, the nuclei of certain light atoms to form heavier elements. In this process some mass is converted to energy. This fusion reaction is the source of energy of the stars, including our sun. In the sun, for hydrogen atoms fuse to make an isotope of helium. The helium nucleus formed has less mass than the sum of the four hydrogen nuclei. This mass is converted into energy. To start this reaction requires temperatures that are difficult to attain in a controlled way. We have succeeded in making hydrogen, or fusion, bombs by using fission bombs to attain the necessary temperatures. If we could control the fusion process, our energy problems would be over.