Материал: Андрианова English Thesaurus of Student-Physicist 2013

times of the year when the ether wind was expected to change direction, but the results were always the same: no fringe shift of the magnitude required was ever observed.

Fig. 26. Thesaurus for Unit 26

146

In later years, when more was known about the nature of light, the idea of an ether that permeates all of space was relegated to the theoretical graveyard. Light is now understood to be an electromagnetic wave, which requires no medium for its propagation. As a result, the idea of an ether in which these waves could travel became unnecessary.

Einstein's principle of relativity

In 1905 Albert Einstein proposed a theory that resolved this contradiction but at the same time completely altered our notions of space and time. He based his special theory of relativity on two postulates. The first postulate asserts that all the laws of physics are the same in all reference frames moving with constant velocity relative to each other. This postulate is a sweeping generalization of the principle of Galilean relativity, which refers only to the laws of mechanics. From an experimental point of view, Einstein's principle of relativity means that any kind of experiment - mechanical, thermal, optical, or electrical - performed in a laboratory at rest, must give the same result when performed in a laboratory moving at a constant speed past the first one. The postulate 2 asserts that the speed of light in a vacuum has the same value in all inertial reference frames, regardless of the velocity of the observer or the velocity of the source emitting the light.

If we accept Einstein's theory of relativity, we must conclude that uniform relative motion is unimportant when measuring the speed of light. At the same time, we have to adjust our commonsense notions of space and time and be prepared for some rather bizarre consequences. In relativistic mechanics, there is no such thing as absolute length or absolute time. Further, events at different locations that are observed to occur simultaneously in one frame are not observed to be simultaneous in another frame moving uniformly past the first. The principle of relativity states that there is no preferred inertial frame of reference.

Properly describing the motion of particles within the framework of special relativity requires generalizing Newton's laws of motion and the definitions of momentum and energy. These generalized definitions reduce to the classical (nonrelativistic) definitions when velocity (v) is much less than speed of light (c). The relativistic equation for momentum reduces to the classical expression when v is small compared with с.

The definition of momentum required generalization to make it compatible with the principle of relativity. Likewise, the definition of kinetic energy requires modification in relativistic mechanics. Einstein

147

found the correct expression for the kinetic energy of an object shows the amazing result that a stationary particle with zero kinetic energy has an energy proportional to its mass.

Pair production and annihilation

In general, converting mass into energy is a low-yield process. Burning wood or coal, or even the fission or fusion processes convert only a very small percentage of the available energy. An exception is the reaction of matter with antimatter. A common process in which a photon creates matter is called pair production. In this process, an electron and a positron are simultaneously produced, while the photon disappears. In order for pair production to occur, energy, momentum, and charge must all be conserved during the process. It's impossible for a photon to produce a single electron because the photon has zero charge and charge would not be conserved in the process. Pair annihilation is a process in which an electron-positron pair produces two photons-the inverse of pair production. Momentum can be conserved only if two photons moving in opposite directions, both with the same energy and magnitude of momentum, are produced.

General relativity

Special relativity relates observations of inertial observers. Einstein sought a more general theory that would address accelerating systems. His search was motivated in part by the following curious fact: mass determines the inertia of an object and also the strength of the gravitational field. Einstein's remarkable theory of gravitation is known as general relativity, in 1916.

One interesting effect predicted by general relativity is that time scales are altered by gravity. A clock in the presence of gravity runs more slowly than one in which gravity is negligible. As a consequence, light emitted from atoms in a strong gravity field, such as the Sun's, is observed to have a lower frequency than the same light emitted by atoms in the laboratory. This gravitational shift has been detected in spectral lines emitted by atoms in massive stars. It has also been verified on Earth by comparing the frequencies of gamma rays emitted from nuclei separated vertically by about 20 m.

General relativity also predicts extreme states of matter created by gravitational collapse. If the concentration of mass becomes very great, as is believed to occur when a large star exhausts its nuclear fuel and collapses to a very small volume, a black hole may form. Here the curvature

148

of space-time is so extreme that all matter and light within a certain radius becomes trapped. There is strong evidence for the existence of a black hole having a mass of millions of Suns at the center of our galaxy.

1.What’s the reason of special theory of relativity?

2.What is necessary to do to describe a physical event?

3.What laws of mechanics must be according to the principle of Galilean relativity?

4.Describe the Michelson-Morley experiment.

5.At what speeds Newton’s laws are right?

6.What is pair annihilation?

7.What does general theory of relativity predict?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3. Fill in the gaps with the missing words from the list:

positron, inertial frames, Special relativity, time interval, ether hypothesis

149

1._____ is all about relating two such measurements - and this rather innocuous relating of measurements leads to some of the most bizarre consequences in physics.

2._____ of reference are those reference frames in which Newton's laws are valid.

3.The negative results of the Michelson-Morley experiment not only contradicted the _____, but also showed that it was impossible to measure the absolute velocity of Earth with respect to the ether frame.

4.In relativistic mechanics, the distance between two points and the

_____ between two events depend on the frame of reference in which they are measured.

5.The _____ is often called the antiparticle of the electron.

UNIT 27

QUANTUM PHYSICS

Read the following texts. Study the thesaurus on fig. 27, answer the questions after the texts

Although many problems were resolved by the theory of relativity in the early part of the 20th century, many other problems remained unsolved. Attempts to explain the behavior of matter on the atomic level with the laws of classical physics were consistently unsuccessful. Various phenomena, such as the electromagnetic radiation emitted by a heated object (blackbody radiation), the emission of electrons by illuminated metals (the photoelectric effect), and the emission of sharp spectral lines by gas atoms in an electric discharge tube, couldn't be understood within the framework of classical physics. Between 1900 and 1930, however, a modern version of mechanics called quantum mechanics or wave mechanics was highly successful in explaining the behavior of atoms, molecules, and nuclei.

The earliest ideas of quantum theory were introduced by Planck, and most of the subsequent mathematical developments, interpretations, and improvements were made by a number of distinguished physicists, including Einstein, Bohr, Schrodinger, de Broglie, Heisenberg, I Born, and Dirac. In this Unit we introduce the underlying ideas of quantum theory and the wave-particle nature of matter, and discuss some simple applications of quantum theory, including the photoelectric effect, the Compton effect, and x-rays.

150