The modern discipline of physics emerged in the 17th century following in traditions of inquiry established by Galileo Galilei, René Descartes, Isaac Newton, and other natural philosophers. Prior to this time, ?physics? did not exist in the way that the term is currently understood.

Elements of what became physics were drawn primarily from the fields of astronomy, optics, and mechanics, which were methodologically united through the study of geometry. These disciplines were passed from individuals in Classical Antiquity, such as Archimedes and Ptolemy, to the Arabic-speaking world, where they were critiqued and developed, before eventually passing back to Western Europe. They were thought of as technical in character, and their descriptive content was not generally perceived as representing a philosophically significant knowledge of the natural world.

Meanwhile, philosophy, including what was called ?physics?, focused on explanatory (rather than descriptive) schemes developed around the Aristotelian idea of the four types of ?causes?. According to Aristotelian and, later, Scholastic physics, things moved in the way that they did because it was part of their essential nature to do so. Celestial objects were thought to move in circles, because perfect circular motion was considered an innate property of objects that existed in the uncorrupted realm of the celestial spheres.

Motions below the lunar sphere were seen as imperfect, and thus could not be expected to exhibit consistent motion. More idealized motion in the ?sublunary? realm could only be achieved through artifice, and prior to the 17th century, artificial experiments were not widely thought of as a valid means of learning about the natural world. Instead, physical explanations in the sublunary realm revolved around tendencies. Stones contained the element earth, and earthy objects tended to move in a straight line toward the center of the universe (which the earth was supposed to be situated around) unless otherwise prevented from doing so. Other physical explanations, which would not later be considered within the bounds of physics, followed similar reasoning. For instance, people tended to think, because people were, by their essential nature, thinking animals.^[1]

Galileo Galilei

Galileo Galilei (1564-1642)

Galileo used his 1609 telescopic discovery of the moons of Jupiter, as published in his Sidereus Nuncius in 1610, to procure a position in the Medici court with the dual title of mathematician and philosopher. As a court philosopher, he was expected to engage in debates with philosophers in the Aristotelian tradition, and received a large audience for his own publications, such as The Assayer and Discourses and Mathematical Demonstrations Concerning Two New Sciences, which was published abroad after he was placed under house arrest for his publication of Dialogue Concerning the Two Chief World Systems in 1632. ^[2][3]

Galileo?s interest in the mechanical experimentation and mathematical description in motion established a new natural philosophical tradition focused on experimentation. This tradition drew a significant following in the years leading up to and following Galileo?s death, including Evangelista Torricelli and the participants in the Accademia del Cimento in Italy; Marin Mersenne and Blaise Pascal in France; Christiaan Huygens in the Netherlands; and Robert Hooke and Robert Boyle in England.

René Descartes and the Cartesian Philosophy of Motion

René Descartes (1596-1650)

Descartes, like Galileo, was convinced of the importance of mathematical explanation, and he and his followers were key figures in the development of mathematics and geometry in the 17th century. Cartesian mathematical descriptions of motion held that all mathematical formulations had to be justifiable in terms of direct physical action, a position held by Huygens and the German philosopher Gottfried Leibniz, who, while following in the Cartesian tradition, developed his own philosophical alternative to Scholasticism, which he outlined in his 1714 work, The Monadology.

Newtonian Motion versus Cartesian Motion

Sir Isaac Newton, (1643-1727)

Gottfried Leibniz, (1646-1716)

Newton?s principles (but not his mathematical treatments) proved controversial with Continental philosophers, who found his lack of metaphysical explanation for movement and gravitation philosophically unacceptable. Beginning around 1700, a bitter rift opened between the Continental and British philosophical traditions, which were stoked by heated, ongoing, and viciously personal disputes between the followers of Newton and Leibniz concerning priority over the analytical techniques of calculus, which each had developed independently. Initially, the Cartesian and Leibnizian traditions prevailed on the Continent (leading to the dominance of the Leibnizian calculus notation everywhere except Britain). Newton himself remained privately disturbed at the lack of a philosophical understanding of gravitation, while insisting in his writings that none was necessary to infer its reality. As the 18th century progressed, Continental natural philosophers increasingly accepted the Newtonians? willingness to forgo ontological metaphysical explanations for mathematically described motions. ^[7][8][9]

Rational Mechanics in the 18th Century

Leonhard Euler, (1707-1783)

The mathematical analytical traditions established by Newton and Leibniz flourished during the 18th century as more mathematicians learned calculus and elaborated upon its initial formulation. The application of mathematical analysis to problems of motion was known as rational mechanics, or mixed mathematics (and was later termed classical mechanics). This work primarily revolved around celestial mechanics, although other applications were also developed, such as the Swiss mathematician Daniel Bernoulli?s treatment of fluid dynamics, which he introduced in his 1738 work Hydrodynamica.^[10]

Rational mechanics dealt primarily with the development of elaborate mathematical treatments of observed motions, using Newtonian principles as a basis, and emphasized improving the tractability of complex calculations and developing of legitimate means of analytical approximation. By the end of the century analytical treatments were rigorous enough to verify the stability of the solar system solely on the basis of Newton?s laws without reference to divine intervention?even as deterministic treatments of systems as simple as the three body problem in gravitation remained intractable.^[11]

British work, carried on by mathematicians such as Brook Taylor and Colin Maclaurin, fell behind Continental developments as the century progressed. Meanwhile, work flourished at scientific academies on the Continent, led by such mathematicians as Daniel Bernoulli, Leonhard Euler, Joseph-Louis Lagrange, Pierre-Simon Laplace, and Adrien-Marie Legendre. At the end of the century, the members of the French Academy of Sciences had attained clear dominance in the field.^{[12][13][14][15]}

Physical Experimentation in the 18th and Early-19th Centuries

At the same time, the experimental tradition established by Galileo and his followers persisted. The Royal Society and the French Academy of Sciences were major centers for the performance and reporting of experimental work, and Newton was himself an influential experimenter, particularly in the field of optics, where he was recognized for his prism experiments dividing white light into its constituent spectrum of colors, as published in his 1704 book Opticks (which also advocated a particulate interpretation of light). Experiments in mechanics, optics, magnetism, static electricity, chemistry, and physiology were not clearly distinguished from each other during the 18th century, but significant differences in explanatory schemes and, thus, experiment design were emerging. Chemical experimenters, for instance, defied attempts to enforce a scheme of abstract Newtonian forces onto chemical affiliations, and instead focused on the isolation and classification of chemical substances and reactions.^[16]

Nevertheless, the separate fields remained tied together, most clearly through the theories of weightless ?imponderable fluids", such as heat (?caloric?), electricity, and phlogiston (which was rapidly overthrown as a concept following Lavoisier?s identification of oxygen gas late in the century). Assuming that these concepts were real fluids, their flow could be traced through a mechanical apparatus or chemical reactions. This tradition of experimentation led to the development of new kinds of experimental apparatus, such as the Leyden Jar and the Voltaic Pile; and new kinds of measuring instruments, such as the calorimeter, and improved versions of old ones, such as the thermometer. Experiments also produced new concepts, such as the University of Glasgow experimenter Joseph Black?s notion of latent heat and Philadelphia intellectual Benjamin Franklin?s characterization of electrical fluid as flowing between places of excess and deficit (a concept later reinterpreted in terms of positive and negative charges).

Michael Faraday (1791-1867) delivering the 1856 Christmas Lecture at the Royal Institution.

Thermodynamics, Statistical Mechanics, and Electromagnetic Theory

William Thomson (1824-1907), later Lord Kelvin

Ludwig Boltzmann (1844-1906)

Meanwhile, the new physics of energy transformed the analysis of electromagnetic phenomena, particularly through the introduction of the concept of the field and the publication of Maxwell?s 1873 Treatise on Electricity and Magnetism, which also drew upon theoretical work by German theoreticians such as Carl Friedrich Gauss and Wilhelm Weber. The encapsulation of heat in particulate motion, and the addition of electromagnetic forces to Newtonian dynamics established an enormously robust theoretical underpinning to physical observations. The prediction that light represented a transmission of energy in wave form through a ?luminiferous ether?, and the seeming confirmation of that prediction with Helmholtz student Heinrich Hertz?s 1888 detection of electromagnetic radiation, was a major triumph for physical theory and raised the possibility that even more fundamental theories based on the field could soon be developed.^{[22][23][24][25]}

The Emergence of a New Physics circa 1900

Marie Sk?odowska Curie (1867-1934)

The triumph of Maxwell?s theories was undermined by inadequacies that had already begun to appear. The Michelson-Morley experiment failed to detect a shift in the speed of light, which would have been expected as the earth moved at different angles with respect to the ether. The possibility explored by Hendrik Lorentz, that the ether could compress matter, thereby rendering it undetectable, presented problems of its own as a compressed electron (detected in 1897 by British experimentalist J. J. Thomson) would prove unstable. Meanwhile, other experimenters began to detect unexpected forms of radiation: Wilhelm Röntgen caused a sensation with his discovery of x-rays in 1895; in 1896 Henri Becquerel discovered that certain kinds of matter emit radiation on their own accord. Marie and Pierre Curie coined the term ?radioactivity? to describe this property of matter, and isolated the radioactive elements radium and polonium. Ernest Rutherford and Frederick Soddy identified two of Becquerel?s forms of radiation with electrons and the element helium. In 1911 Rutherford established that the bulk of mass in atoms are concentrated in positively-charged nuclei with orbiting electrons, which was a theoretically unstable configuration. Studies of radiation and radioactive decay continued to be a preeminent focus for physical research through the 1930s, when the discovery of nuclear fission opened the way to the practical exploitation of what came to be called ?atomic? energy.

Albert Einstein (1879-1955)

The Radical Years: General Relativity and Quantum Mechanics

The gradual acceptance of Einstein?s theories of relativity and the quantized nature of light transmission, and of Niels Bohr?s model of the atom created as many problems as they solved, leading to a full-scale effort to reestablish physics on new fundamental principles. Expanding relativity to cases of accelerating reference frames (the ?general theory of relativity?) in the 1910s, Einstein posited an equivalence between the inertial force of acceleration and the force of gravity, leading to the conclusion that space is curved and finite in size, and the prediction of such phenomena as gravitational lensing and the distortion of time in gravitational fields.

Niels Bohr (1885-1962)

Constructing a New Fundamental Physics

A ?Feynman diagram? of a renormalized vertex in quantum electrodynamics.

As the philosophically inclined continued to debate the fundamental nature of the universe, quantum theories continued to be produced, beginning with Paul Dirac?s formulation of a relativistic quantum theory in 1927. However, attempts to quantize electromagnetic theory entirely were stymied throughout the 1930s by theoretical formulations yielding infinite energies. This situation was not considered adequately resolved until after World War II ended, when Julian Schwinger, Richard Feynman, and Sin-Itiro Tomonaga independently posited the technique of ?renormalization?, which allowed for an establishment of a robust quantum electrodynamics (Q.E.D.).^[27]

Meanwhile, new theories of fundamental particles proliferated with the rise of the idea of the quantization of fields through ?exchange forces? regulated by an exchange of short-lived ?virtual? particles, which were allowed to exist according to the laws governing the uncertainties inherent in the quantum world. Notably, Hideki Yukawa proposed that the positive charges of the nucleus were kept together courtesy of a powerful but short-range force mediated by a particle intermediate in mass between the size of an electron and a proton. This particle, called the ?pion?, was identified in 1947, but it was part of a slew of particle discoveries beginning with the neutron, the ?positron? (a positively-charged ?antimatter? version of the electron), and the ?muon? (a heavier relative to the electron) in the 1930s, and continuing after the war with a wide variety of other particles detected in various kinds of apparatus: cloud chambers, nuclear emulsions, bubble chambers, and coincidence counters. At first these particles were found primarily by the ionized trails left by cosmic rays, but were increasingly produced in newer and more powerful particle accelerators.^[28]

Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider; an experiment done in order to investigate the properties of a quark gluon plasma such as the one thought to exist in the ultrahot first few microseconds after the big bang

The interaction of these particles by ?scattering? and ?decay? provided a key to new fundamental quantum theories. Murray Gell-Mann and Yuval Ne'eman brought some order to these new particles by classifying them according to certain qualities, beginning with what Gell-Mann referred to as the ?Eightfold Way?, but proceeding into several different ?octets? and ?decuplets? which could predict new particles, most famously the ?^-, which was detected at Brookhaven National Laboratory in 1964, and which gave rise to the ?quark? model of hadron composition. While the quark model at first seemed inadequate to describe strong nuclear forces, allowing the temporary rise of competing theories such as the S-Matrix, the establishment of quantum chromodynamics in the 1970s finalized a set of fundamental and exchange particles, which allowed for the establishment of a ?standard model? based on the mathematics of gauge invariance, which successfully described all forces except for gravity, and which remains generally accepted within the domain to which it is designed to be applied. ^[29]

While accelerators have confirmed most aspects of the standard model by detecting expected particle interactions at various collision energies, no theory reconciling the general theory of relativity with the standard model has yet been found, although ?string theory? has provided one promising avenue forward. Since the 1970s, fundamental particle physics has provided insights into early universe cosmology, particularly the ?big bang? theory proposed as a consequence of Einstein?s general theory. However, starting from the 1990s, astronomical observations have also provided new challenges, such as the need for new explanations of galactic stability (the problem of dark matter), and accelerating expansion of the universe (the problem of dark energy).

The Physical Sciences

With increased accessibility to and elaboration upon advanced analytical techniques in the 19th century, physics was defined as much, if not more, by those techniques than by the search for universal principles of motion and energy, and the fundamental nature of matter. Fields such as acoustics, geophysics, astrophysics, aerodynamics, plasma physics, low-temperature physics, and solid-state physics joined optics, fluid dynamics, electromagnetism, and mechanics as areas of physical research. In the 20th century, physics also became closely allied with such fields as electrical, aerospace, and materials engineering, and physicists began to work in government and industrial laboratories as much as in academic settings. Following World War II, the population of physicists increased dramatically, and came to be centered on the United States, while, in more recent decades, physics has become a more international pursuit than at any time in its previous history.

Further reading

• ?Selected Works about Isaac Newton and His Thought? from ''The Newton Project''.
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• Nina Byers and Gary Williams, ed., ''OUT OF THE SHADOWS:Contributions of 20th Century Women to Physics'' Cambridge University Press, 2006 ISBN 0-5218-2197-1

Notes

References

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See also

• Famous physicists
• Nobel Prize in physics

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