Classical physics

In the history of physics with the name of classical physics are grouped all the areas and models of physics that do not consider the phenomena described in the macrocosm by general relativity and in the microcosm by quantum mechanics, theories that define instead the so-called modern physics.

For this reason it is possible to classify as classical physics all the theories formulated before the twentieth century, at the beginning of which appeared the first works of Max Planck based on the quantum hypothesis. Some later theories, such as special relativity, can be considered classical or modern. Therefore are included in classical physics the theories on mechanics, including acoustics, thermodynamics, electromagnetism, including optics, and Newtonian gravity.

In the seventeenth century the scientific method was developed and opened a phase of in-depth investigation of nature conducted by famous scientists such as Galileo Galilei, Isaac Newton and Gottfried Wilhelm von Leibniz. The studies focused on the motion of bodies and its causes, with particular regard to celestial mechanics, marked by the comparison between the geocentric theory and the heliocentric one. The attention of physics in the next two centuries was extended to electrostatics and magnetism, thermodynamics and finally to electrodynamics.

Classical electrodynamics represented the first unification of theories that describe different phenomena, such as electricity, magnetism and light, in a single mathematical synthesis formulated by James Clerk Maxwell. However, it was thanks to the study of Maxwell’s equations that classical physics entered in crisis. Some physical phenomena occurring at microscopic and macroscopic scale such as the study of the shape of black body spectrum, the failure of the luminiferous ether theory and the discovery of phenomena such as the photoelectric effect, Brownian motion, the model of hydrogen atom, Bragg diffraction, the non invariance in form of Maxwell equations with respect to Galileo transformations, the precession of the perihelion of Mercury orbit etc., generated a series of contradictions that in a short time put in crisis the complex apparatus of classical physics, opening the way to special relativity and quantum mechanics and all the modern physics of the twentieth century.

All classical physics had been built on the basis of Newtonian mechanistic model of the universe. The scenario of Newtonian universe in which all physical phenomena took place was the three-dimensional space of classical Euclidean geometry, an absolute space, always still and immutable. All changes occurring in the physical world were described as a function of a separate dimension, called time, also absolute, which had no connection with the material world and flowed uniformly from the past to the future, through the present. The elements of the Newtonian world that moved in this absolute space and time were the material particles. In the mathematical equations these were treated as “material points” and Newton considered them small, solid, indestructible objects of which all matter was composed. This model was quite similar to that of the Greek atomists. Both were based on the distinction between full and empty, between matter and space, and in both models particles were always identical to themselves in mass and shape so matter was always conserved and essentially inert.

The important difference between Democritus atomism and Newton atomism is in the force acting between material particles: defined as gravity force that depends only on masses and mutual distance between particles. It was believed that the particles and the forces acting between them were created by God, and therefore could not be subjected to further analysis. In Newton’s mechanics, all physical events are reduced to the motion of material points in space, motion caused by their mutual attraction, that is, by the force of gravity. To express in a precise mathematical form the effect of this force on a material point Newton had to invent the new concepts and mathematical techniques of differential calculus. Newton’s equations related to the motion of bodies are the basis of classical mechanics.

According to Newton, in the beginning God created material particles, the forces acting between them, and the fundamental laws of motion. In this way, the entire universe was set in motion and has continued to function, like a machine, governed by immutable laws ever since. The philosophical basis of this way of thinking was the logical consequence of the fundamental division introduced by Descartes between the Ego and the world. It was therefore believed that the world could be described without taking into account the human observer. In the eighteenth and nineteenth century there is a huge success of Newtonian mechanics. Newton himself applied his theory to the motion of the planets and was able to explain the fundamental characteristics of the solar system. However his planetary model was extremely simplified – there was neglected, for example, the gravitational influence between planets – so that resulted some irregularities that Newton could not explain. He solved this problem by supposing that God was always present in the universe to correct these irregularities.

The great mathematician Laplace refined Newton’s calculations, showing that his laws of motion ensured the stability of the solar system and treated the universe as a perfectly self-regulating machine. Physicists also applied Newton’s laws of motion to fluid motion and vibrations of elastic bodies. Even heat theory could be reduced to mechanics when it was understood that heat is energy associated to a complicated motion of “agitation” of molecules. The extraordinary success of the mechanistic model gave birth to the physicists of the early nineteenth century the belief that the universe was actually a huge mechanical system that worked according to Newton’s laws of motion. These laws were seen as the fundamental laws of nature, and Newton’s mechanics came to be regarded as the definitive theory of natural phenomena.

However, less than one hundred years later, the Newtonian model showed its limits, showing that none of its aspects had absolute validity. The study of electric and magnetic phenomena, could not be adequately described, but it involved the introduction of a new type of force. Michael Faraday and Clerk Maxwell discovered the theory of electromagnetism. They replaced the concept of force with the one of force field, being the first to go beyond the boundaries of Newtonian physics, they established that every positive and negative charge instead of attracting each other (as it was believed until then), creates in the surrounding space a “perturbation”, so that another charge, if present, feels a force. This condition of space that has the ability to produce a force is called “field”. It is generated by a single charge and exists independently from the fact that another charge is present or not and feels the effect. From this theory, called electrodynamics, it was understood, that light is nothing but a rapidly alternating electromagnetic field that moves in space as a wave.

Today we know that radio waves, light waves or X-rays, are all electromagnetic waves, that is oscillating electric and magnetic fields that differ only in the frequency of oscillation, and that visible light is only a small fraction of the electromagnetic spectrum.

At the beginning of the twentieth century, therefore, there were two valid theories with which physicists explained phenomena: Newton’s mechanics and Maxwell’s electrodynamics; consequently, the Newtonian model was no longer the basis of all physics.

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