Introduction
All physical processes take place in space and in time. Every physical law in every physical domain contains, explicitly or otherwise, space-time relationships, viz., length(distance) and time interval (duration).
We know from experience that space and time possess certain symmetry properties, which impose restrictions on physical processes. Among these properties are space and time homogeneity.
Because of time homogeneity a physical phenomenon is always the same, if the conditions are the same, whenever it is observed. Two millennia have passed since Archimedes discovered the laws of buoyancy, yet today they can be readily developed, provided the conditions of observation are the same. The physical equivalence of different instants of time—the homogeneity of time-imposes certain restrictions on physical phenomena which find expression in the law of conservation of energy.
Because of space homogeneity a physical phenomenon is always the same, if the conditions are the same, wherever it is observed: the same physical experiment staged in Moscow or New York yields the same results. The physical equivalence of different points of space—the homogeneity of space—imposes certain restrictions on physical phenomena which find expression in the law ofconservation of (linear) momentum.
If these symmetry properties of space and time, which seem so obvious, did not exist, it would be useless to conduct scientific research and attempt to cognize the world. Imagine what would happen if there were no spatial homogeneity: one set of physical laws would hold in Moscow, another in London, yet another in New York! Non-homogeneity of time would make it impossible for people to advance their knowledge. The buoyancy principle discovered the other day would no longer be valid today, and a new set of experiments would have to be carried out, which would have to be repeated again tomorrow.
Insofar as the laws of conservation of energy and momentum follow from the very general symmetry properties of space and time the conservation laws are universal: they are equally valid in the physics of elementary particles and outer space, nuclear and solid-state physics. In fact, they are among the several most general laws which constitute the bedrock of contemporary physics.
It is impossible to investigate physical phenomena without introducing a frame of reference, reference bodies or landmarks relative to which observations are carried out. Imagine a highway crossing a very flat and monotonous plain. A car traveling along it in the distance at first seems to be standing still. But when one finds a landmark—a telegraph pole or a tree (usually this is done subconsciously)—and continues to observe the car for a period of time one detects a change in the mutual positions of the car and the landmark. The concept of frame of reference (coordinate system) is a fundamental one in physics.
A researcher embarking on physical investigations is free to choose any reference body he likes. It is natural to assume that the laws governing a given phenomenon do not depend on the choice of reference system. Experience, however, confirms this only with respect to a very special class of reference systems which move at a uniform velocity relative to each other. This is what is known as the relativity principle.
A space—time description of phenomena in terms of length and time interval is possible only when a reference system has been selected. The facts of life tell us that in studying physical phenomena lengths and time intervals do not change in passing over from one reference system to another. A navigator plotting a forthcoming flight on a map uses a ruler and a clock. He continues to use them quite confidently in flight without fearing that the length of the ruler or the rhythm of the clock might have changed, and he is quite justified in this.
One of the greatest contributions to twentieth-century physics was the postulate of the existence of a limiting velocity of propagation of anything whatsoever in nature, equal to the velocity of light in vacuum.
Experience indicates that as long as the velocities of investigated bodies are small compared with the limiting velocity, time intervals and lengths do not change in passing from one reference system to another. The science which treats of the motion of bodies in these conditions is called classical mechanics.
The velocities of bodies on earth and in the solar system (artificial earth satellites included) are incomparably smaller than the limiting velocity, and their motions are governed by the laws of classical mechanics to a high degree of accuracy.
When speeds are comparable with the limiting velocity, however, the properties of motion change: lengths and time intervals begin to depend on velocity. The mechanics based on these concepts is called relativistic mechanics.
Relativistic mechanics is, of course, more general, and in the special case of small velocities it turns into classical mechanics. Relativity theory does not reject classical mechanics: it brings in the necessary adjustments for high velocities.
Velocities comparable with the speed of light are found in the study of fast atomic particles. Experiments reveal that their motion is in fact subject to the laws of relativistic mechanics.
The physical entities of the real world include bodies and fields. A field can be described consistently only in terms of relativistic concepts. Unlike bodies, fields cannot generally be used as reference frames. In fact, for the first time since man began to investigate nature he is confronted with a physical entity which does not affect any of his senses directly.
The properties of fields are studied from the behavior of bodies in them. Two fields are said to be physically identical if their action on the same body is the same. Thus, by studying the behavior of “sensible”, ponderable entities—bodies—man has been able to cognize the properties of intangible, imponderable entities, fields.
What better example of man’s amazing aptitude for cognition ?
In ordinary conditions we deal with two types of field, electromagnetic and gravitational. The properties of the electromagnetic field are more or less familiar to people who have taken a secondary-school course in physics. The properties of the gravitational field, however, are less familiar.
It should be noted, first of all, that gravitational fields are of importance only on the cosmic scale. The ratio of the intensity of gravitational interactions between elementary particles to their electromagnetic interactions is of the order of 
Gravitational fields occupy a unique position in nature. For one, every physical thing produces a gravitational field: particles, bodies and fields, even gravitational fields themselves. Secondly, gravitational fields shape and determine, as it were, the geometrical structure of space and time.
The properties of gravitational fields viewed on the vast scale of the observable universe seem to indicate that the spatial structure of the universe is changing with time in the direction of expansion. In fact, the spectacular effect of the expanding universe has been observed.
The field of physical research ranges from the boundless regions of the macrocosmos to the minute spaces of the microcosmos. When the observable phenomena involve very small masses (particles) and take place in very small regions of space, with dimensions of the order of  cm, these are atomic phenomena.
In investigating atomic phenomena man has once again displayed the might of his intellect. The atomic world is invisible, inaudible and intangible to our senses; atomic dimensions are so tiny as to defy the imagination, yet man has developed a picture of the atom and fathomed its laws. How was this achieved ?
Physical science deals only with things that are directly or indirectly observable. Any observation of a physical thing presumes interactions with its environment. A physical entity displays its properties only in interactions with something external with respect to it. In order to obtain a comprehensive description of the properties of such an entity it must be placed in different conditions. Not infrequently, especially where atomic phenomena are concerned, an object may display apparently contradictory, mutually exclusive properties. In fact, in atomic phenomena such seemingly paradoxical combinations of incompatible properties are rather the rule than the exception.
The foregoing can be illustrated by the following analogy. A stranger is always a sort of “thing in itself”, an unknown entity, until one comes into contact with him. In order to get to know the man, his disposition and other properties, one must observe him in different circumstances and situations. One often finds that the behavior (“properties”) of a person (“object”) in various circumstances is so different that one finds it hard to believe that it is the same person. This is just about what happened when the properties of the atom were investigated.
An observation of a microscopic object is an interaction which inevitably involves a disturbance, or perturbation of the investigated system. If the disturbance is small and can be neglected, the observed phenomenon is said to be of a macroscopic nature. If, on the other hand, it is impossible to neglect some limiting perturbation—associated with the existence of a quantum of action—the phenomenon is said to be belong to the microscopic sphere.
The discovery that there exists a universal minimum perturbation, which it is impossible to avoid no matter how refined the observations, was a tremendous triumph of the human intellect. A submicroscopic system (known also as a quantum object) cannot be observed without causing appreciable perturbations on it. A consequence of this is that quantum objects, such as atoms and molecules, are subject to uncertainty relationships.
In classical mechanics, when a certain quantity characterizing similar objects is measured in identical conditions, the results are, quite naturally, the same. When quantum objects of the same kind are measured in the same conditions (compatible with their submicroscopic physical nature) the results are specific, but different. The law of nature in such cases is that the various results are always obtained in the same proportion to the total number of measurements.
Quantum phenomena tax the imagination much more than classical processes. Depending on the conditions of observation, quantum systems may behave either as waves or as classical particles. In classical mechanics a moving body is characterized by a specific position in space and velocity at any given time. With quantum objects, however, it is either the position or the velocity that can be specified at a given instant, but not both at the same time. Try to visualize such a situation !
The very small regions of space that have been studied so far display all the basic symmetry properties of space and time, in particular, homogeneity. Hence, the laws of conservation of momentum and energy are valid for quantum systems.
Homogeneity is one of the most important symmetry properties of space but not the only one. Thus, space possesses symmetry between right and left. The mirror image of space is wholly equivalent to the original. As we have seen, the symmetry properties of space and time are associated with certain conservation laws. In nonquantum physics mirror symmetry introduces no new conservation laws. In quantum physics, however, the law of conservation of parity comes into effect. Parity is a concept which characterizes the state of quantum objects.
Our daily experience tells us that time flows in one direction only. It is impossible to bring back one’s youth, yet such things were permissible in the framework of non-quantum physics. Only quantum theory offers a sound explanation of the anisotropy of time by virtue of which the direct and reverse passages of time are not equivalent.
In very small regions of space, fields, like particles, are subject to the laws of quantum. The quantized field, like all other fields, is a real relativistic entity. The particle and the field constitute the two fundamental concepts of physics. The synthesis of relativistic and quantum ideas has resulted in a merger of the two and contributed to the remarkable discovery of so-called antiparticles, which were first predicted theoretically and whose existence was later confirmed by laboratory experiment.
One of the most wonderful properties of particle-anti-particle pairs is their ability for production and annihilation. Such a pair, which possesses mass, is capable of transforming into particles possessing no mass and , conversely, particles with no mass may produce particle-antiparticle pairs having mass.
Quantum phenomena taking place in regions of space with dimensions of the order of  to  cm are beautifully explained by contemporary physics. In smaller regions of space, however, physicists feel less confident of their definitions. Investigation of the phenomena characteristic of such regions has practically only just begun. It constitutes the subject of the physics of elementary particles.
In spite of the difficulties of staging laboratory experiments and their prohibitive cost, quite a lot of empirical data have been accumulated in this sphere. A very nice and promising classification of the known elementary particles has been developed which has made possible the theoretical prediction of particles. Many of these predicted particles have now been discovered, and today some thirty or so particles are known.
A classification of interactions of elementary particles has been elaborated. Remarkable internal symmetry properties and new types of conservation laws associated with them have been discovered. Certain types of particle interactions have been found to violate some of the known conservative laws. Thus, in so-called weak interactions parity is not conserved.
The tremendous faculties of the human brain for speculative reasoning have been demonstrated time and again. Equally staggering is the craftsmanship of contemporary experimentalists. The neutrino, one of the most remarkable of the elementary particles, whose existence was predicted thirty years ago, has no mass, travels with the speed of light and practically does not interact with matter. A neutrino could travel through an iron sphere millions of times larger than the earth’s orbit around the sun and experience only one interaction. Neutrinos travel freely through the earth, the solar system and whole galaxies. It would seem that there was no way whatsoever of intercepting neutrinos, and yet it has been done. The success of the physical experiment to detect the neutrino represents another triumph of the human mind. And still, many more mysteries lie in store. Already now some wonderful links are being suggested between the physics of elementary particles and the physics of outer space.
Some elementary particles, called nucleons, are capable of forming stable systems: atomic nuclei. There are not many different types of nuclei in nature. The spatial domain of the atomic nucleus is of dimension  cm. Nucleons form into nuclei in conditions existing in the interior of stars, where the “stellar state” of matter prevails.
In ordinary terrestrial conditions matter is made up of atoms and molecules. Macroscopic bodies are assemblies countless numbers of atoms and molecules. All the properties of macroscopic bodies—elasticity and flowability, thermal and electric properties, magnetic, chemical and optical properties, the properties of solid, liquid, and gaseous states—are explained by the properties of atoms and the bonds between them. There can be no doubt that the phenomena of life, too, can be explained within the framework of known physical concepts.
It should be clear from what has been said that physical concepts and ideas are the simplest, the most general and the most profound, with them it is possible to study the whole of nature according to a unified method of research.
The purpose of physics is to establish physical laws, that is, to determine the relationships that exist between the physical quantities charaterizing a phenomenon.
Physicists use the rules and methods of mathematics to formulate their conclusions. Nevertheless, physics differs radically from mathematics in that it cannot be divorced from experiment. Accordingly, it is essentially made up of two sciences, experimental physics and theoretical physics.
In the course of its evolution physics has developed some more or less general principles of research, e.g., (1) science deals only with observable things; (2) every object displays its properties only in interactions with something external with respect to it (the so-called external classical conditions, which will be discussed later on).
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