Physics, Classical

Physics, Classical

Classical physics is the science of physics as it was conceptualized and practiced in the three centuries prior to the advent of either quantum physics or relativity early in the twentieth century. The character of classical physics is well-represented by Isaac Newton's (1642–1727) formulation of the study of motion and James Clerk Maxwell's (1831–1879) approach to the study of electromagnetism.

Classical mechanics

Classical mechanics, the scientific study of motion in the style developed in the seventeenth century by Newton, is often taken as the foundational branch of classical physics. General physics courses commonly begin with the study of motion and use Newtonian mechanics as the setting in which numerous basic concepts, such as energy, force, and momentum, are first introduced.

Physics has long been concerned with understanding the nature and causes of motion. In the tradition of ancient Greek philosophy, the cosmos was thought to be divided into two distinctly differing realms—the terrestrial (near Earth) realm and the celestial realm (the region of the moon and beyond). As conceived in Greek thought, these two realms were not only spatially distinct, but they differed in character from one another in substantial ways. For one thing, the "natural" motions of things (motions that needed no further causation) in these two realms were presumed to be radically different.

According to Aristotle (384–322 b.c.e.), who was for nearly two millennia taken to be the authority on these matters, motion in the terrestrial realm required the continuous application of a cause. Remove the cause, and motion would cease. When a horse ceases to pull a cart, for instance, the cart comes to a halt. In Newton's formulation, however, what requires an active cause is not motion itself, but acceleration—any change in the speed or direction of motion. In effect, Newton's First Law of Motion asserts that the natural motion of things is uniform motion, straight-line motion at constant speed. Any deviation from this—any acceleration, that is—would require a cause. The name for this cause is force—specifically, the force exerted on one object by interaction with another. Expressed more traditionally, Newton's First Law states that unless acted upon by an applied force, an object will continue in a state of rest or uniform motion.

What happens when a force is applied to an object? The answer to that question is the subject of Newton's Second Law of Motion: When acted upon by an applied force, an object will accelerate; the resultant acceleration will be in the same direction as the applied force, and its magnitude will be directly proportional to the magnitude of the applied force and inversely proportional to the object's mass. Stated more succinctly, acceleration is proportional to force divided by mass. This statement, more than any other, functions as the core of Newtonian dynamics, Newton's formulation of the fundamental cause-effect relationship for motion. Force is the cause; acceleration is the effect. For a substantial class of motions, with exceptions to be noted later, this formulation continues to provide a fruitful way to predict or account for acceleration in response to applied forces.

Newton's Third Law of Motion is a statement about the character of the applied forces mentioned in the first two laws. All such forces occur in pairs and are the result of two bodies interacting with one another. When two bodies interact, says Newton, each exerts a force on the other. When bodies A and B interact, the force exerted on A by B is equal in magnitude and opposite in direction to the force exerted on B by A. This is sometimes abbreviated to read, "action equals reaction," but the meanings of action and reaction must be very carefully specified.

Among the various types of forces that contribute to the acceleration of terrestrial objects is the force of gravity—the force that causes apples, for example, to fall to the ground, or to "accelerate earthward." It was the genius of Newton that allowed him to consider the possibility that the orbital motion of the moon, which entails an acceleration toward the Earth, might also be a consequence of the Earth's gravitational attraction.

This suggestion required a remarkable break with Aristotelian tradition. According to Aristotle, the natural motion of the moon, of the planets, or of any other member of the celestial realm was entirely different from the terrestrial motions considered so far. The natural motion of celestial bodies was neither rest nor uniform straight-line motion. Rather, the motion of celestial bodies would necessarily be based on uniform circular motion, motion at constant speed on a circular path. In the spirit of this assumption, Claudius Ptolemy in the second century crafted a remarkably clever combination of uniform circular motions with which to describe the motions of the sun, moon, and planets relative to the central Earth.

However, building on the fruitful contributions of astronomers Nicolaus Copernicus (1473–1543), Galileo Galilei (1564–1642), and Johannes Kepler (1571–1630), Newton was able to demonstrate that Kepler's sun-centered model for planetary motions could be seen as but one more illustration of Newton's theory regarding the cause-effect relationship for motion. The moon was steered in its orbit around the Earth in response to a force exerted by the Earth on the moon. The Earth and the other planets orbited the sun in response to a force exerted on them by the sun. What was the force operating in these celestial motions? The same kind of force that caused apples to accelerate earthward—the universal gravitational force.

It was helpful to recognize gravity as a force exerted by one object on another. It was exceptionally insightful for Newton to propose that every pair of objects everywhere in the universe exerted gravitational forces on one another. Gone was the confusion of two kinds of natural motions. Gone was the even greater distinction between terrestrial and celestial realms—one characterized by imperfection and change, the other characterized by perfection and constancy. The cosmos is one system, not two. The world is a universe made of one set of substances and behaving according to one set of patterns. Classical mechanics provided the means to study all motions, both terrestrial and celestial, with one and the same methodology.

Classical electromagnetism

Classical electromagnetism provided a systematic account of numerous phenomena involving the interaction of electric charges and currents. Electric charges at rest were considered to be the source of electric fields—modifications in the nature of space that cause other charges to experience a force. Electric charges in motion, giving rise to an electric current, were considered to be the source of magnetic fields, modifications in the nature of space that could be detected by a magnetic compass and caused other electric currents to experience a force. Given any static distribution of electric charge, the configuration of the resultant electric field could be computed. Given any distribution of electric currents, the configuration of the resultant magnetic field could be computed. Given these electric and magnetic field configurations, the forces on all electric charges and currents could be predicted.

In addition to phenomena involving static charge distributions and steady electric currents, another important category of phenomena arises from dynamically changing configurations of charge or current. When charge or current configurations change, the resultant electric and magnetic fields will also change. However, changes in these field configurations must propagate at a finite speed—now called the speed of light, approximately 300,000 kilometers per second. Electromagnetic radiation is the phenomenon of traveling variations, or waves, in electric and magnetic field strength caused by accelerated electric charges. The electromagnetic spectrum spans the full range of wavelength values from very short to very long—from gamma rays, X-rays, and ultraviolet to visible light, infrared, microwaves and radio waves. Maxwell's equations—four mathematical statements that systematically integrated the work of predecessors like Charles-Augustin de Coulomb (1736–1806), Hans Christian Oersted (1777–1851), Michael Faraday (1791–1867), and André-Marie Ampère (1775–1836)—were taken to be the complete specification of all electromagnetic phenomena, including electromagnetic radiation.

Limitations of classical physics

Until the early twentieth century, classical physics appeared to be adequate to account for all observed phenomena. But new discoveries soon demonstrated that, although classical physics would continue to provide a convenient and powerful means of dealing with many phenomena, it needed to be supplemented with other theoretical strategies based on differing sets of assumptions regarding the fundamental character of the physical universe. In the arena of electromagnetism, for instance, classical physics assumed that electromagnetic energy could be continuously varied in value and that its transmission could be fully described in terms of traveling electromagnetic waves. However, in order to account for such phenomena as blackbody radiation (electromagnetic energy radiated by any warm object) and the photoelectric effect (electrons ejected from the surface of a metal illuminated by light), physicists had to propose and accept the idea that electromagnetic energy was transmitted in particle-like quanta of energy, now called photons. Phenomena in which the photon character of electromagnetic radiation plays a central role requires the employment of quantum physics in place of classical physics.

Quantum physics is also needed to account for the behavior of extremely small systems like atoms and molecules. The motion of electrons relative to atomic nuclei cannot be adequately described in the language of classical mechanics. Contrary to Newtonian expectations, the energy of atoms and molecules is not continuously variable, but is quantized—restricted to certain specific values. And, contrary to the expectations of classical electromagnetism, electrons in motion relative to atomic nuclei do not radiate energy continuously, but only when making a transition from one stable energy state to another of lower energy value. Consistent with the Principle of Conservation of Energy, the amount of energy lost by the atom is exactly equal to the energy carried away by the emitted photon.

A second shortcoming of classical physics becomes evident when Newtonian mechanics attempts to deal with things that are moving at very high speed relative to an observer. When this speed becomes a substantial fraction of the speed of light, several Newtonian expectations require modification. Many of these modifications are accounted for by the Special Theory of Relativity proposed by Albert Einstein (1879–1955) in 1905. The relationship between kinetic energy (energy associated with motion) and speed must be modified. Distance and time intervals once thought to be invariant become dependent on relative motion. Even the mass of an object is measured differently by different observers. Other modifications are accounted for by Einstein's General Theory of Relativity, published in 1916, which deals with the interaction of mass and the geometry of space. The General Theory describes the force of gravity in a manner very different from Newton's and is able to account for several discrepancies between observation and Newtonian predictions.

Religious concerns and classical physics

Classical physics gave support to the idea that the world was fundamentally deterministic. Given full information about the configuration and motion of some system today, its entire future could, in principle, be computed. Its future was considered to be fully determined by its present. But is there room in such a universe for contingency or choice? The apparent absence of choice presents difficulties for religious concepts like human responsibility and human accountability to God for obedience to revealed standards for moral action.

Another religious concern arises when one inquires about the character and role of divine action in the universe. When Newton considered the future motions of the planets in the solar system, for instance, he judged that this set of orbital motions was inherently unstable and would, from time to time, need to be adjusted by God to restore the desired array of orbits. This introduction of occasional supernatural interventions may be considered a form of the God of the gaps approach to divine action: the universe is presumed to lack some quality or capability that must be compensated for by direct divine action. In the case of planetary motions, for example, Newton considered the universe to lack the capability of maintaining a stable set of orbits. This "capability gap" could, however, be bridged with occasional acts of supernatural intervention. Eventually, however, it was demonstrated that the system of planetary orbits was, in fact, stable, thereby removing the need for occasional gap-bridging interventions. When a "gap" of this sort becomes filled, the God of the gaps becomes superfluous. For this reason, many contemporary theologians are inclined to see divine action, not as a supernatural compensation for capability gaps in the universe, but as an essential aspect of an enriched concept of what takes place naturally.

See also Aristotle; Determinism; Divine Action; God of the Gaps; Gravitation; Newton, Isaac; Physics, Quantum; Relativity, General Theory of; Relativity, Special Theory of; Wave-particle Duality

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howard j. van till