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(fluid mechanics)
With respect to wave components, the nonlinear action by which properties of fluid flow (such as momentum, energy, vorticity), are transferred from one portion of the wave spectrum to another, or viewed in another manner, between eddies of different size-scales.
A process in which two or more bodies exert mutual forces on each other.
The phenomenon which causes the response to applying two treatments not to be the simple sum of the responses to each treatment.


(STATISTICS) the compounded effect that two or more INDEPENDENT VARIABLES may have on the DEPENDENT VARIABLE when they act together. In examining the effect of the variables in an experiment, the individual effect of each may not explain the total variation. In such instances it is therefore appropriate to use a statistical test such as ANALYSIS OF VARIANCE which is designed to assess the effect of the interaction between the variables as well as the specific effect of each.



one of the basic philosophical categories, reflecting the processes by which different objects act upon one another, the mutual conditioning and change of state or reciprocal transition of different objects, and the generating of one object by another. Interaction represents a type of immediate or mediated, external or internal, relations, or connections. The properties of an object are manifested and can be known only in interaction with other objects. “Interaction is the first thing that we encounter when we consider matter in motion” (F. Engels; see K. Marx and F. Engels, Soch., 2nd ed., vol. 20, p. 546).

The concept of “interaction” is closely related to the concept of “structure.” Interaction serves as an integrating factor by means of which parts are joined together to form a particular type of whole. For example, electromagnetic interaction between the nucleus and the electrons creates the structure of the atom. The interaction of people with each other and with the world—in other words, social practice— determines the structure of society, as well as human behavior and consciousness.

Interaction has an objective and universal character. It embraces all forms of being and all forms of their reflection. The interrelation of all structural levels of being and the material unity of the world are achieved as a result of the universality of interaction. The absolute nature of interaction does not appear immediately, but in limited, finite forms, and in this sense interaction is relative. The relative character of interaction also consists in the fact that it occurs at a finite velocity. There exists a spatiotemporal limit beyond which the immediate interaction of a given object with others does not take place. However, mediated interaction can take place among objects at any distance from one another. The chain of interaction is continuous; it has neither a beginning nor an end. Every phenomenon is only a link in the universal chain of interaction.

The principle of interaction assumes concrete form in the doctrine of causality. It is precisely interaction that defines the relationship of cause and effect: an object acted upon by a cause is not passive—it reacts, and thus causality is transformed into interaction. Each of the interacting elements acts simultaneously both as a cause of the other element and as its effect. “In the most immediate way interaction represents the reciprocal causality of substances that are presupposed and determined by each other; each substance is simultaneously active and passive relative to the other” (Hegel, Soch., vol. 5, Moscow, 1937, p. 691).

Interaction conditions the development of objects. It is in fact the interaction of opposites, or contradiction, that is the deepest source and the basic and ultimate cause of the origin, self-movement, and development of objects and of their result or their origin. The self-contained interaction of natural forces and processes as the source of the self-movement and development of things excludes the intervention of super-natural “absolute” sources of the motion and organization of the material world. Each form of motion of matter has at its basis specific types of interaction of structural elements. Moreover, the interaction of the parts of a developing system are at the same time the regulating and guiding factor that determines the direction of its development. Each qualitatively distinct system has its own special type of interaction. Contemporary natural science has shown that all interaction is linked with material fields and is accompanied by the transfer of matter, motion, and information. Interaction can be achieved only by means of a specific material vehicle. The contemporary classification of interaction is based on a distinction between interaction of forces and interaction of information.

In physics there are four basic types of interaction of forces, which provide a key to the understanding of the infinite variety of physical processes: gravitational interaction, electromagnetic interaction, strong (nuclear fusion) interaction, and weak (fission) interaction. Each type of interaction in physics is characterized by a special measure. Contemporary biology is concerned with the investigation of interaction on various levels, including that of the molecule, cell, organism, population, and species and the biocenotic level. Still more complex forms of interaction characterize the life of society. According to Marx’ definition, society is “the product of the interaction of people” (see K. Marx and F. Engels, Soch., 2nd ed., vol. 27, p. 402). The classic examples of investigation of the manifold interactions within society as an integral, internally differentiated, self-developing system are K. Marx’ Das Kapital and V. I. Lenin’s The Development of Capitalism in Russia.

The category of interaction is an essential methodological principle for the cognition of natural and social phenomena. In order to really disclose the essence of an object it is necessary to discover its law-like interactions. Without the study of interaction in its general and concrete manifestations, it is impossible to understand the properties, structures, or laws of reality. “Not one phenomenon can be explained in and of itself (J. W. von Goethe, Izbr.filos. proizv., Moscow, 1964, p. 334). Any object can be understood and defined only within the system of its relations and interaction with other surrounding phenomena, their parts, aspects, and properties. Knowledge of things means knowledge of their interaction and is itself the result of the interaction of subject and object. Interaction is not only the initial but the terminal point of knowledge as well. “We cannot go beyond the knowledge of this interaction for the very reason that there is nothing behind it to know” (F. Engels; see K. Marx and F. Engels, Soch., 2nd ed., vol. 20, p. 546). The category of interaction occupies a fundamental place in the conceptual apparatus of contemporary theoretical thought.


Engels, F. Dialektika prirody. Moscow, 1955. Pages 129, 184, 312.
Grigor’ev, V. I., and G. la. Miakishev. Sily vprirode, 3rd ed. Moscow, 1969.
Uemov, A. I. Veshchi, svoistva i otnosheniia. Moscow, 1963.
Kedrov, B. M. Engel’s i dialektika estestvoznaniia. Moscow, 1970. Chapter 4.




in physics, the action of bodies or particles on each other, leading to a change in the state of their motion. In Newtonian mechanics the mutual action of bodies upon each other is quantitatively characterized by force. Potential energy is a more general characteristic of interaction.

The notion was originally maintained in physics that the interaction between bodies could be brought about directly across empty space, which does not take any part in the transmission of the interaction; in this case the transmission of interaction occurs instantaneously. Thus, it was thought that the movement of the earth must lead directly to changes in the gravitational force affecting the moon. This amounted to the concept of the so-called long-range order. However, these notions were abandoned, since they did not conform to the facts after the discovery and analysis of the electromagnetic field. It was proved that the interaction of electrically charged bodies does not occur instantaneously and that the movement of one charged particle leads to a change in the forces affecting other particles only after a finite period of time rather than at the same moment. A certain process, which propagates with a finite velocity, takes place in the space between the particles. Correspondingly, there is an “intermediary” that carries out the interaction between the charged particles. This intermediary was called the electromagnetic field. Each electrically charged particle creates an electromagnetic field, which affects the other particles. The velocity of propagation of an electromagnetic field is equal to the speed of light in a vacuum, ~300,000 km/sec. A new concept came into being—the short-range order, which was later extended to all other interactions. According to this concept, interactions between bodies occur by means of one or another of the fields that are continuously distributed in space. Thus, universal gravitation is brought about by a gravitational field.

The concept of interaction was changed substantially after the appearance of quantum field theory. According to this theory, every field consists of quantum particles—the quanta of these fields. Each field has its own corresponding particles. For example, the quanta of an electromagnetic field are photons. Charged particles are constantly emitting and absorbing photons, which also form the electromagnetic field surrounding the particles. The electromagnetic interaction in quantum field theory results from the exchange of particles by photons—that is, photons are the carriers of this interaction. Similarly, other types of interaction occur as a result of the exchange of particles by the quanta of corresponding fields.

In spite of the diversity of the actions of bodies on one another (depending on the interaction of their constituent elementary particles), only four types of fundamental interactions are presently known to exist in nature (according to modern data). These are, in order of increasing interaction intensity, gravitational interactions, weak interactions (responsible, for example, for the decay of elementary particles), electromagnetic interactions, and strong interactions (which provide, in particular, particle bonds in atomic nuclei: nuclear forces develop because of the fact that protons and neutrons are exchanged with the particles of a nuclear field—pi-mesons). The intensities of interactions are determined by the so-called coupling constants (in particular, the electric charge is the coupling constant for electromagnetic interactions).

Present-day quantum theory of electromagnetic interactions excellently describes all known electromagnetic phenomena. A quantitative theory of strong interactions, as well as weak interactions at high energy, has not yet been constructed. Quantum effects are held to be unimportant in the ordinary gravitational interactions of bodies.

In addition to force interactions, specific nonforce interactions, which do not depend on coupling constants, appear in systems consisting of identical particles (which are indistinguishable according to the identity principle, one of the principles of quantum mechanics). Thus, particles with half-integral spin undergo effective repulsion (in accordance with Pauli’s principle), whereas particles with integral spin undergo effective attraction. These nonforce interactions may also lead to a change in force interactions between particles.


Grigor’ev, V. I., and G. la. Miakishev. Sily v prirode, 3rd ed. Moscow, 1969.


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