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Action at a distance is the concept in physics that an object's motion can be affected by another object without the two being in physical contact; that is, it is the concept of the non-local interaction of objects that are separated in space. Coulomb's law and Newton's law of universal gravitation are based on action at a distance.
Forces and Fields has eleven chapters. The first ten chapters consist of 5 or more sections. The eleventh, 2 sections. These chapters are titled The Logical Status of Theories, The Primitive Analogies, Mechanism in Greek Science, The Greek Inheritance, The Corpuscular Philosophy, The Theory of Gravitation, Action at a Distance, The Field Theories, The theory of Relativity, Modern Physics, and ...
Proper object-oriented programming involves design principles that avoid action at a distance. The Law of Demeter states that an object should only interact with other objects near itself. Should action in a distant part of the system be required then it should be implemented by propagating a message.
The action is defined by an integral, and the classical equations of motion of a system can be derived by minimizing the value of that integral. The action principle provides deep insights into physics, and is an important concept in modern theoretical physics. Various action principles and related concepts are summarized below.
A theory that includes the principle of locality is said to be a "local theory". This is an alternative to the concept of instantaneous, or "non-local" action at a distance . Locality evolved out of the field theories of classical physics .
Unlike instantaneous action at a distance theories of the early 1800s these "direct interaction" theories are based on interaction propagation at the speed of light. They differ from the classical field theory in three ways 1) no independent field is postulated; 2) the point charges do not act upon themselves; 3) the equations are time symmetric.
The Newtonian and action-principle forms are equivalent, and either one can solve the same problems, but selecting the appropriate form will make solutions much easier. The energy function in the action principles is not the total energy ( conserved in an isolated system ), but the Lagrangian , the difference between kinetic and potential energy.
Einstein also developed general relativity, in which spacetime was treated as a field and its curvature was the origin of the gravitational interactions, putting an end to action at a distance. In quantum field theory, fields become the fundamental objects of study, and particles are excitations of these fields. To differentiate from quantum ...