History of physics

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.Fact|date=September 2008 Prior to this time, a unified field of “physics” did not exist in the way that the term is currently understood. [Harvtxt|Dear|1995] Fact|date=September 2008

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 began in Antiquity with the Babylonians and with Hellenistic writers such as Archimedes and Ptolemy, then passed on to the Arabic-speaking world where they were critiqued and developed into a more physical and experimental tradition by scientists such as Ibn al-Haytham and Abū Rayhān Bīrūnī, [Harvtxt|Glick|Livesey|Wallis|2005|p=89-90] Mariam Rozhanskaya and I. S. Levinova (1996), "Statics", p. 642, in Harvtxt|Rashed|Morelon|1996|pp=614-642: quote|"Arabic statics was an essential link in the progress of world science. It played an important part in the prehistory of classical mechanics in medieval Europe. Without it classical mechanics proper could probably not have been created."] before eventually passing on to Western Europe where they were studied by scholars such as Roger Bacon and Witelo. They were thought of as technical in character and many philosophers generally did not perceive their descriptive content 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. The theory of impetus, the ancestor to the concepts of inertia and momentum, also belonged to this philosophical tradition, and was developed by medieval philosophers such as John Philoponus, Avicenna and Jean Buridan. The physical traditions in ancient China and India were also largely philosophical. In the philosophical tradition of "physics", 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, many philosophers did not view artificial experiments 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.

Emergence of experimental method and physical optics

The use of experiments in the sense of empirical procedures [Harvtxt|Smith|1996|p=x] in geometrical optics dates back to second century Roman Egypt, where Ptolemy carried out several early such experiments on reflection, refraction and binocular vision. [Harvtxt|Smith|1996|p=18] Due to his Platonic methodological paradigm of "saving the appearances", however, he discarded or rationalized any empirical data that did not support his theories, [Harvtxt|Smith|1996|p=19] as the idea of experiment did not hold any importance in Antiquity. [Harvtxt|Tybjerg|2002|p=350] The incorrect emission theory of vision thus continued to dominate optics through to the 10th century.

The turn of the second millennium saw the emergence of experimental physics with the development of an experimental method emphasizing the role of experimentation as a form of proof in scientific inquiry, and the development of physical optics where the mathematical discipline of geometrical optics was successfully unified with the philosophical field of physics. The Iraqi physicist, Ibn al-Haytham (Alhazen), is considered a central figure in this shift in physics from a philosophical activity to an experimental and mathematical one, and the shift in optics from a mathematical discipline to a physical and experimental one.Harvtxt|Toomer|1964] [Harvtxt|Rashed|Armstrong|1994|pp=345-6] Harvtxt|Smith|1996|p=57] Due to his positivist approach,Harvtxt|Rashed|2007|p=19: quote|"In reforming optics he as it were adopted ‘‘positivism’’ (before the term was invented): we do not go beyond experience, and we cannot be content to use pure concepts in investigating natural phenomena. Understanding of these cannot be acquired without mathematics. Thus, once he has assumed light is a material substance, Ibn al-Haytham does not discuss its nature further, but confines himself to considering its propagation and diffusion. In his optics ‘‘the smallest parts of light’’, as he calls them, retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."] his "Doubts Concerning Ptolemy" insisted on scientific demonstration and criticized Ptolemy's confirmation bias and conjectural undemonstrated theories. [Harvtxt|Sabra|1998|p=300] His "Book of Optics" (1021) was the earliest successful attempt at unifying a mathematical discipline (geometrical optics) with the philosophical field of physics, to create the modern science of physical optics. An important part of this was the intromission theory of vision, which in order to prove, he developed an experimental method to test his hypothesis.Harvtxt|Thiele|2005a: quote|“Through a closer examination of Ibn al-Haytham's conceptions of mathematical models and of the role they play in his theory of sense perception, it becomes evident that he was the true founder of physics in the modern sense of the word; in fact he anticipated by six centuries the fertile ideas that were to mark the beginning of this new branch of science.”] Harvtxt|Thiele|2005b: quote|"Schramm showed that already some centuries before Galileo, experimental physics had its roots in Ibn al-Haytham."] Harvtxt|Sabra|2003|pp=91-2] Harvtxt|Gorini|2003: quote|"According to the majority of the historians al-Haytham was the pioneer of the modern scientific method. With his book he changed the meaning of the term optics and established experiments as the norm of proof in the field. His investigations are based not on abstract theories, but on experimental evidences and his experiments were systematic and repeatable."] He conducted various experiments to prove his intromission theory [G. A. Russell, "Emergence of Physiological Optics", pp. 686-7, in Harvtxt|Rashed|Morelon|1996] and other hypotheses on light and vision. [Harvtxt|Sabra|1989] The "Book of Optics" established experimentation as the norm of proof in optics, and gave optics a physico-mathematical conception at a much earlier date than the other mathematical disciplines. [Harv|Dijksterhuis|2004|pp=113-5: quote|"Through the influential work of Alhacen the onset of a physico-mathematical conception of optics was established at a much earlier time than would be the case in the other mathematical sciences."] His "On the Light of the Moon" also attempted to combine mathematical astronomy with physics, a field now known as astrophysics, to formulate several astronomical hypotheses which he proved through experimentation.

Galileo Galilei and the rise of physico-mathematics

In the 17th century, natural philosophers began to mount a sustained attack on the Scholastic philosophical program, and supposed that mathematical descriptive schemes adopted from such fields as mechanics and astronomy could actually yield universally valid characterizations of motion. The Tuscan mathematician Galileo Galilei was the central figure in the shift to this perspective. As a mathematician, Galileo’s role in the university culture of his era was subordinated to the three major topics of study: law, medicine, and theology (which was closely allied to philosophy). Galileo, however, felt that the descriptive content of the technical disciplines warranted philosophical interest, particularly because mathematical analysis of astronomical observations—notably the radical analysis offered by astronomer Nicolaus Copernicus concerning the relative motions of the sun, earth, moon, and planets—indicated that philosophers’ statements about the nature of the universe could be shown to be in error. Galileo also performed mechanical experiments, and insisted that motion itself—regardless of whether that motion was natural or artificial—had universally consistent characteristics that could be described mathematically.

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. [Harvtxt|Drake|1978] [Harvtxt|Biagioli|1993]

Galileo’s interest in the mechanical experimentation and mathematical description in motion established a new natural philosophical tradition focused on experimentation. This tradition, combining with the non-mathematical emphasis on the collection of "experimental histories" by philosophical reformists such as William Gilbert and Francis Bacon, 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.

The Cartesian philosophy of motion

The French philosopher René Descartes was well-connected to, and influential within, the experimental philosophy networks. Descartes had a more ambitious agenda, however, which was geared toward replacing the Scholastic philosophical tradition altogether. Questioning the reality interpreted through the senses, Descartes sought to reestablish philosophical explanatory schemes by reducing all perceived phenomena to being attributable to the motion of an invisible sea of “corpuscles”. (Notably, he reserved human thought and God from his scheme, holding these to be separate from the physical universe). In proposing this philosophical framework, Descartes supposed that different kinds of motion, such as that of planets versus that of terrestrial objects, were not fundamentally different, but were merely different manifestations of an endless chain of corpuscular motions obeying universal principles. Particularly influential were his explanation for circular astronomical motions in terms of the vortex motion of corpuscles in space (Descartes argued, in accord with the beliefs, if not the methods, of the Scholastics, that a vacuum could not exist), and his explanation of gravity in terms of corpuscles pushing objects downward. [Harvtxt|Shea|1991] [Harvtxt|Garber|1992] [Harvtxt|Gaukroger|2002]

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

In the late 17th and early 18th centuries, the Cartesian mechanical tradition was challenged by another philosophical tradition established by the Cambridge University mathematician Isaac Newton. Where Descartes held that all motions should be explained with respect to the immediate force exerted by corpuscles, Newton chose to describe universal motion with reference to a set of fundamental mathematical principles: his three laws of motion and the law of gravitation, which he introduced in his 1687 work "Mathematical Principles of Natural Philosophy". Using these principles, Newton removed the idea that objects followed paths determined by natural shapes (such as Kepler’s idea that planets moved naturally in ellipses), and instead demonstrated that not only regularly observed paths, but all the future motions of any body could be deduced mathematically based on knowledge of their existing motion, their mass, and the forces acting upon them. However, observed celestial motions did not precisely conform to a Newtonian treatment, and Newton, who was also deeply interested in theology, imagined that God intervened to ensure the continued stability of the solar system.

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. [Harvtxt|Hall|1980] [Harvtxt|Bertolini Meli|1993] [Harvtxt|Guicciardini|1999]

Rational mechanics in the 18th century

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". [Harvtxt|Darrigol|2005]

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. [Harvtxt|Bos|1980] 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. [Harvtxt|Greenberg|1986] [Harvtxt|Guicciardini|1989] [Harvtxt| Guicciardini|1999] [Harvtxt|Garber|1999]

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. [Harvtxt|Ben-Chaim|2004]

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).

While it was recognized early in the 18th century that finding absolute theories of electrostatic and magnetic force akin to Newton’s principles of motion would be an important achievement, none were forthcoming. This impossibility only slowly disappeared as experimental practice became more widespread and more refined in the early years of the 19th century in places such as the newly-established Royal Institution in London, where John Dalton argued for an atomistic interpretation of chemistry, Thomas Young argued for the interpretation of light as a wave, and Michael Faraday established the phenomenon of electromagnetic induction. Meanwhile, the analytical methods of rational mechanics began to be applied to experimental phenomena, most influentially with the French mathematician Joseph Fourier’s analytical treatment of the flow of heat, as published in 1822. [Harvtxt|Heilbron|1979] [Harvtxt|Buchwald|1989] [Harvtxt|Golinski|1999]

Thermodynamics, statistical mechanics, and electromagnetic theory

The establishment of a mathematical physics of energy between the 1850s and the 1870s expanded substantially on the physics of prior eras and challenged traditional ideas about how the physical world worked. While Pierre-Simon Laplace’s work on celestial mechanics solidified a deterministically mechanistic view of objects obeying fundamental and totally reversible laws, the study of energy and particularly the flow of heat, threw this view of the universe into question. Drawing upon the engineering theory of Lazare and Sadi Carnot, and Émile Clapeyron; the experimentation of James Prescott Joule on the interchangeability of mechanical, chemical, thermal, and electrical forms of work; and his own Cambridge mathematical tripos training in mathematical analysis; the Glasgow physicist William Thomson and his circle of associates established a new mathematical physics relating to the exchange of different forms of energy and energy’s overall conservation (what is still accepted as the “first law of thermodynamics”). Their work was soon allied with the theories of similar but less-known work by the German physician Julius Robert von Mayer and physicist and physiologist Hermann von Helmholtz on the conservation of forces.

Taking his mathematical cues from the heat flow work of Joseph Fourier (and his own religious and geological convictions), Thomson believed that the dissipation of energy with time (what is accepted as the “second law of thermodynamics”) represented a fundamental principle of physics, which was expounded in Thomson and Peter Guthrie Tait’s influential work "Treatise on Natural Philosophy". However, other interpretations of what Thomson called thermodynamics were established through the work of the German physicist Rudolf Clausius. His statistical mechanics, which was elaborated upon by Ludwig Boltzmann and the British physicist James Clerk Maxwell, held that energy (including heat) was a measure of the speed of particles. Interrelating the statistical likelihood of certain states of organization of these particles with the energy of those states, Clausius reinterpreted the dissipation of energy to be the statistical tendency of molecular configurations to pass toward increasingly likely, increasingly disorganized states (coining the term “entropy” to describe the disorganization of a state). The statistical versus absolute interpretations of the second law of thermodynamics set up a dispute that would last for several decades (producing arguments such as “Maxwell's demon”), and that would not be held to be definitively resolved until the behavior of atoms was firmly established in the early 20th century. [Harvtxt|Smith|Wise|1989] [Harvtxt|Smith|1998]

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. [Harvtxt|Buchwald|1985] [Harvtxt|Jungnickel and McCormmanch|1986] [Harvtxt|Hunt|1991] [Harvtxt|Buchwald|1994] Research on the transmission of electromagnetic waves began soon after, with the experiments conducted by physicists such as Nikola Tesla, Jagadish Chandra Bose and Guglielmo Marconi during the 1890s leading to the invention of radio.

The emergence of a new physics circa 1900

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 and chemical 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.

Radical new physical theories also began to emerge in this same period. In 1905 Albert Einstein, then a Bern patent clerk, argued that the speed of light was a constant in all inertial reference frames and that electromagnetic laws should remain valid independent of reference frame—assertions which rendered the ether “superfluous” to physical theory, and that held that observations of time and length varied relative to how the observer was moving with respect to the object being measured (what came to be called the “special theory of relativity”). It also followed that mass and energy were interchangeable quantities according to the equation E=mc². In another paper published the same year, Einstein asserted that electromagnetic radiation was transmitted in discrete quantities (“quanta”), according to a constant that the theoretical physicist Max Planck had posited in 1900 to arrive at an accurate theory for the distribution of blackbody radiation—an assumption that explained the strange properties of the photoelectric effect. The Danish physicist Niels Bohr used this same constant in 1913 to explain the stability of Rutherford’s atom as well as the frequencies of light emitted by hydrogen gas.

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.

The quantized theory of the atom gave way to a full-scale quantum mechanics in the 1920s. The quantum theory (which previously relied in the “correspondence” at large scales between the quantized world of the atom and the continuities of the “classical” world) was accepted when the Compton Effect established that light carries momentum and can scatter off particles, and when Louis de Broglie asserted that matter can be seen as behaving as a wave in much the same way as electromagnetic waves behave like particles (wave-particle duality). New principles of a “quantum” rather than a “classical” mechanics, formulated in matrix-form by Werner Heisenberg, Max Born, and Pascual Jordan in 1925, were based on the probabilistic relationship between discrete “states” and denied the possibility of causality. Erwin Schrödinger established an equivalent theory based on waves in 1926; but Heisenberg’s 1927 “uncertainty principle” (indicating the impossibility of precisely and simultaneously measuring position and momentum) and the “Copenhagen interpretation” of quantum mechanics (named after Bohr’s home city) continued to deny the possibility of fundamental causality, though opponents such as Einstein would assert that “God does not play dice with the universe”. [Harvtxt|Kragh|1999] Also in the 1920s, Satyendra Nath Bose's work on photons and quantum mechanics provided the foundation for Bose-Einstein statistics, the theory of the Bose-Einstein condensate, and the discovery of the boson.

Constructing a new fundamental physics

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.). [Harvtxt|Schweber|1994]

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. [Harvtxt|Galison|1997]

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 SubatomicParticle|link=yes|Omega-, 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. [Harvtxt|Kragh|1999]

The “standard model” groups the electroweak interaction theory and quantum chromodynamics into a structure denoted by the gauge group "SU(3)×SU(2)×U(1)". The formulation of the unification of the electromagnetic and weak interactions in the standard model is due to Abdus Salam, Steven Weinberg and, subsequently, Sheldon Glashow. After the discovery, made at CERN, of the existence of neutral weak currents, [F. J. Hasert "et al." "Phys. Lett." 46B 121 (1973).] [F. J. Hasert "et al." "Phys. Lett." 46B 138 (1973).] [F. J. Hasert "et al." "Nucl. Phys." B73 1(1974).] [cite web|url=http://cerncourier.com/cws/article/cern/29168|title=The discovery of the weak neutral currents|date=2004-10-04|publisher=CERN courier|accessdate=2008-05-08] mediated by the SubatomicParticle|Z boson boson foreseen in the standard model, the physicists Salam, Glashow and Weinberg received the 1979 Nobel Prize in Physics for their electroweak theory. [cite web|title=The Nobel Prize in Physics 1979|url=http://www.nobel.se/physics/laureates/1979|publisher=Nobel Foundation|accessdate=2008-09-10]

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.

ee also

* Famous physicists
* Nobel Prize in physics

Further reading

* [http://www.newtonproject.sussex.ac.uk/prism.php?id=90 “Selected Works about Isaac Newton and His Thought”] from [http://www.newtonproject.sussex.ac.uk/ "The Newton Project"] .
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*Nina Byers and Gary Williams, ed., [http://www.cambridge.org/us/catalogue/catalogue.asp?isbn=9780521821971 "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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*Citation | last1=Rashed | first1=R. | last2=Armstrong | first2=Angela | year=1994 | title=The Development of Arabic Mathematics | publisher=Springer | isbn=0792325656 | oclc=29181926.
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*Citation |last=Rashed |first=R. |year=2007 |title=The Celestial Kinematics of Ibn al-Haytham |journal=Arabic Sciences and Philosophy |volume=17 |pages=7–55 |publisher=Cambridge University Press |doi=10.1017/S0957423907000355.
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*Citation |last=Smith |first=A. Mark |year=1996 |title=Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary |publisher=Diane Publishing |isbn=0871698625 |oclc=185537531 34724889.
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*Citation |last=Tybjerg |first=Karin |year=2002 |title=Book Review: Andrew Barker, "Scientiic Method in Ptolemy's Harmonics" |journal=The British Journal for the History of Science |volume=35 |publisher=Cambridge University Press |page=347-379 |doi=10.1017/S0007087402224784.