Classical and Modern Physics

Classical and Modern Physics
The earliest recorded e fforts to systematically assemble knowledge
concerning motion came from ancient Greece. In the system of natural
philosophy set forth by Aristotle (384--322 b.c.), expla nations of physical
phenomena were deduced from assumptions about the world, rather than
derived from experimentation. For ex ample, it was a fundamental
assumption that every substance had a "natural place" in the universe.
Motion was ruled to be the result of a substance trying to reach its natural
plac e. Because of the agreement between the deductions of Aristotelian
physics and the motions observed throughout the physical universe, and
because there was no tradition of experimentation that could overturn the
ancient physics, th e Greek view was accepted for nearly 2000 years. It
was the Italian scientist Galileo Galilei (1564--1642) whose brilliant
experiments on motion established for all time the absolute necessity of
experimentation in physics and initiated the disintegration of Aristotelian
physics. Within 100 years, Isaac Newton had generalized the results of
Galileo's experiments into his three spectacularly successful laws of
motion, and the natural philosophy of Aristotle was gone.
Ex perimenta tion during the next 200 years brought a flood of
discoveries, inspiring the development of physical theories to e xplain
them. By the end of the nineteenth century, Newton's laws for the
motions of mechanical systems had been joined by equally impressive
laws from Maxwell, Joule, Carnot, and others to describe
ele ctromagnetism and thermodynamics. The subjects that occupied
physical scientists through the end of the nineteenth century---mechanic s,
light, heat, sound, electricity, and magnetism---are usually referre d to as
classical physics.
Th e remarkable success of classical physics led many scientists to
believe that the description of the physical universe was complete.
However, the discoveries of X rays by Roentgen in 1895 and of nuclear
radioactivity by Becq uerel in 1896 seemed to be outside the framework
of classica l physics. The theory of special relativity proposed by Albert
Einstein in 1905 contradicted the ideas of space and time of Galileo and
Newton. In the same year, Einstein suggested that light energy is
quantized; that is, that light comes in discrete packets rather than being
wavelike and continuous as had been assumed in classic al physics. The
generalization of this insight to the quantization of all types of energy is a
central idea of quantum mechanics, one that has many amazing and
important consequences. The application of special relativity and,
particularly, quantum theory to such microscopic systems as atoms,
molecule s, and nuclei has led to a detailed understanding of solids,
liquids, and g ases and is often referred to as modern physics.
Except for the interiors of atoms and for motions at speeds near the
speed of light, classical physics correctly and precisely describes the
behavior of the physical world.