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A Brief History of Physics.


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Physics has brought not only fundamental changes in ideas about the material world, mathematics and philosophy, but also, through technology, a transformation of society. Physics is considered both a body of knowledge and the practice that makes and transmits it. The scientific revolution, beginning about year 1600, is a convenient boundary between ancient thought and classical physics. The year 1900 marks the beginnings of a more modern physics; today, the science shows no sign of completion, as more issues are raised, with questions rising from the age of the universe, to the nature of the vacuum, to the ultimate nature of the properties of subatomic particles. Partial theories are currently the best that physics has to offer, at the present time. The list of unsolved problems in physics is large.

Early physics.

laws of physics.
The laws of physics will eventually unlock the secrets of the universe.

Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. Also a mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Typically the behavior and nature of the world was explained by invoking the actions of gods. Eventually speculative natural explanations were proposed based on considering such questions; most of them were wrong, but this is part of the nature of the enterprise of systematic explanation, and even modern theories of quantum mechanics and relativity are merely considered "theories that haven't been broken yet". Physical theories in antiquity were largely couched in philosophical terms, and rarely verified by systematic experimental testing.

Indian contribution to physics.

In Lothal (c. 2400 BC), the ancient port city of the Harappan civilization, shell objects served as compasses to measure the angles of the 8-12 fold divisions of the horizon and sky in multiples of 40-360 degrees, and the positions of stars. In the late Vedic era (c. 9th-6th century BC), the Astronomer Yajnavalkya, in his Shatapatha Brahmana, referred to an early concept of Heliocentrism with the Earth being round and the Sun being the "centre of spheres". He measured the distances of the Moon and the Sun from the Earth as 108 times the diameters of these heavenly bodies, which were close to the modern values of 110.6 for the Moon and 107.6 for the Sun.

Indians in the Vedic era classified the material world into five basic elements: earth, fire, air, water and ether/space. From the 6th century BC, they formulated systemetic atomic theories, beginning with Kanada and Pakudha Katyayana. Indian atomists believed that an atom could be one of up to 9 elements, with each element having up to 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move and perform other actions, as well as elaborate theories of how atoms can form binary molecules that combine further to form larger molecules, and how particles first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter. This parallels with the structure of modern atomic theory, in which pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter. They had also suggested the possibility of splitting an atom, which as we know today, is the source of atomic energy.

The principle of relativity (not to be confused with Einstein's theory of relativity) was available in an embryonic form since the 6th century BC in the ancient Indian philosophical concept of "sapekshavad", literally "theory of relativity" in Sanskrit.

The Samkhya and Vaisheshika schools developed theories on light from the 6th-5th century BC. According to the Samkhya school, light is one of the five fundamental "subtle" elements out of which emerge the gross elements, which were taken to be continuous. The Vaisheshika school defined motion in terms of the non-instantaneous movement of the physical atoms. light rays were taken to be a stream of high velocity fire atoms, which can exhibit different characteristics depending on the speed and the arrangements of these particles. The Buddhists Dignaga (5th century) and Dharmakirti (7th century) developed a theory of light being composed of energy particles, similar to the modern concept of photons.

Veteran Australian indologist A. L. Basham concluded that "they were brilliant imaginative explanations of the physical structure of the world, and in a large measure, agreed with the discoveries of modern physics."

In 499, the mathematician-astronomer Aryabhata propounded a detailed model of the heliocentric Solar System of gravitation, where the Planets rotate on their axes causing day & night and follow elliptical orbits around the Sun causing year, and where the Planets and the Moon do not have their own light but reflect the light of the Sun. Aryabhata also correctly explained the causes of the solar and lunar eclipses and predicted their times, gave the radii of planetary orbits around the Sun, and accurately measured the lengths of the day, sidereal year, and the Earth's diameter and circumference. Brahmagupta, in his Brahma Sputa Siddhanta in 628, recognized gravity as a force of attraction and understood the law of gravitation.

A particularly important Indian contribution was the Hindu-Arabic numerals. Modern physics can hardly be imagined without a system of arithmetic in which simple calculation is easy enough to make large calulations even possible. The modern positional numeral system (the Hindu-Arabic numeral system) and the number zero were first developed in India, along with the trigonometric functions of sine and cosine. These mathematical developments, along with the Indian developments in physics, were adopted by the Islamic Caliphate, from where they spread to Europe and other parts of the world.

Chinese contribution to physics.

In 1115 BC, the Chinese invented the first geared mechanism, the South Pointing Chariot, which was also the first to use a differential gear.

The Mo Ching (allegedly written by Mo Tzu) written around the 3rd century BC stated an early version of Newton's first law of motion:

"The cessation of motion is due to the opposing force ... If there is no opposing force ... the motion will never stop. This is as true as that an ox is not a horse."

Later contributions of the Chinese include the inventions of paper, printing, gunpowder, and the compass. They were also the first to discover negative numbers, which was important to mathematicians and physicists alike.

Greek and hellenistic contribution to physics

Western physics began with eminent Greek pre-Socratic philosophers such as Thales, Anaximander, possibly Pythagoras, Heraclitus, Anaxagoras, Empedocles and Philolaus, many of whom were involved in various schools. For example, Anaximander and Thales belonged to the Milesian school.

Plato, briefly and Aristotle at length, continued these studies of nature in their works, the earliest surviving complete treatises dealing with natural philosophy. Democritus, a contemporary, was of the school of Atomists who attempted to characterize the nature of matter.

Due to the absence of advanced experimental equipment such as Telescopes and accurate time-keeping devices, experimental testing of physical hypotheses was impossible or impractical. There were exceptions and there are anachronisms: for example, the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and also hydrostatics when, so the story goes, he noticed that his own body displaced a volume of water while he was getting into a bath one day. Another remarkable example was that of Eratosthenes, who deduced that the Earth was a sphere, and accurately calculated its circumference using the shadows of vertical sticks to measure the angle between two widely separated points on the Earth's surface. Greek mathematicians also proposed calculating the volume of objects like spheres and cones by dividing them into very thin disks and adding up the volume of each disk, using methods resembling integral calculus.

Modern knowledge of these early ideas in physics, and the extent to which they were experimentally tested, is sketchy. Almost all direct record of these ideas was lost when the Library of Alexandria was destroyed, around 400 AD. Perhaps the most remarkable idea we know of from this era was the deduction by Aristarchus of Samos that the Earth was a planet that traveled around the Sun once a year, and rotated on its axis once a day (accounting for the seasons and the cycle of day and night), and that the stars were other, very distant suns which also had their own accompanying Planets (and possibly, lifeforms upon those planets).

The discovery of the Antikythera mechanism points to a detailed understanding of movements of these astronomical objects, as well as a use of gear-trains that pre-dates any other known civilization's use of gears, except that of ancient China.

An early version of the steam engine, Hero's aeolipile was only a curiosity which did not solve the problem of transforming its rotational energy into a more usable form, not even by gears. The Archimedes screw is still in use today, to lift water from rivers onto irrigated farmland. The simple machines were unremarked, with the exception (at least) of Archimedes' elegant proof of the law of the lever. Ramps were in use several millennia before Archimedes, to build the Pyramids.

Regrettably, this period of inquiry into the nature of the world was eventually stifled by a tendency to accept the ideas of eminent philosophers, rather than to question and test those ideas. Pythagoras himself is said to have tried to suppress knowledge of the existence of irrational numbers, discovered by his own school, because they did not fit his number mysticism. For one thousand years following the destruction of the Library of Alexandria, Ptolemy's (not to be confused with the Egyptian Ptolemies) model of an Earth-centred universe in which the Planets are assumed to each move in a small circle, called an epicycle, which in turn moves along a larger circle called a deferent, was accepted as absolute truth.

Persian and Islamic contribution to physics.

With civilization dominated by the Roman Empire, many Greek doctors began to practice medicine for the Roman elite, but sadly the physical sciences were not so well supported. Following the collapse of the Roman Empire, Europe saw a decline in interest in classical culture which some have called the Dark Ages, though modern scholars do not use this phrase, and almost all scientific research ground to a halt.

In the Middle East however, many Greek and Hellenistic natural philosophers were able to find support for their work, and Islamic scholars built upon their previous work in astronomy and mathematics while developing such new fields as alchemy (chemistry). After the Arabs conquered Persia, many scientists arose among the Persians, who preserved Hellenistic physics, which faded away in Europe at the time, and studied Indian physics after conquering parts of India. The Persians, as well as the Arabs, went on to make many improvements on the Indian and Hellenistic concepts.

A Persian scientist Mohammad al-Fazari invented the astrolabe, an astronomical instrument and analog computer that was important in locating and predicting the positions of the Sun, Moon, Planets and stars. Mu?ammad ibn Musa al-?warizmi gave his name to what we now call an algorithm, and developed modern algebra, which was derived from the Arabic word al-jabr from the title of his treatise Hisab al-jabr w’al-muqabala.

The Persian scientist Alhazen Abu Ali al-Hasan ibn al-Haytham (c. 965-1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light, and went on to discover the laws of refraction.

He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Kitab-at-Manazir was translated into Latin in the Middle Ages, as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the Sun and the Moon when near the horizon. Through these extensive researches on optics, is considered as the father of modern optics.

Al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny particles travelling in straight lines, are reflected from objects into our eyes. He understood that light must travel at a large but finite velocity, and that refraction is caused by the velocity being different in different substances. He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.

Medieval European contributions to physics.

In the 12th century, the birth of medieval university and the rediscovery of the works of ancient philosophers through contact with the Arabs, during the process of Reconquista and the Crusades, started an intellectual revitalization of Europe.

By the 13th century, precursors of the modern scientific method can be seen already on Robert Grosseteste's emphasis on mathematics as a way to understand nature and on the empirical approach admired by Roger Bacon.

Bacon conducted experiments into optics, although much of it was similar to what had been done and was being done at the time by Arab scholars. He did make a major contribution to the development of science in medieval Europe by writing to the Pope to encourage the study of natural science in university courses and compiling several volumes recording the state of scientific knowledge in many fields at the time. He described the possible construction of a telescope, but there is no strong evidence of his having made one. He recorded the manner in which he conducted his experiments in precise detail so that others could reproduce and independently test his results - a cornerstone of the scientific method, and a continuation of the work of researchers like Al Battani.

In the 14th century, some scholars, such as Jean Buridan and Nicolas Oresme, started to question the received wisdom of Aristotle's mechanics. In particular, Buridan developed the theory of impetus which was the first step towards the modern concept of inertia.

In his turn, Oresme showed that the reasons proposed by the physics of Aristotle against the movement of the Earth were not valid and adduced the argument of simplicity for the theory that the Earth moves, and not the heavens. In the whole of his argument in favor of the earth's motion Oresme is both more explicit and much clearer than that given two centuries later by Copernicus. He was also the first to assume that color and light are of the same nature and the discoverer of the curvature of light through atmospheric refraction; even though, up to now, the credit for this latter achievement has been given to Hooke.

In the 14th century Europe was rocked by the Black Death which led to much social upheaval. In spite of this pause, the 15th century saw the artistic flourishing of the Renaissance. The rediscovery of ancient texts was improved when many Byzantine scholars had to seek refuge in the West after the fall of Constantinople in 1453. Meanwhile, the invention of printing was to democratize learning and allow a faster propagation of new ideas. All that paved the way to the Scientific Revolution, which may also be understood as a resumption of the process of scientific change halted around the middle of the 14th century.

Modern physics.

Table of Mechanicks.
Table of Mechanicks, 1728 Cyclopaedia.

The scientific revolution which begun from the late 16th century can be viewed as a flowering of the Renaissance and the portal to modern civilization. This was in part brought about by the rediscovery of those elements of ancient Greek, Indian, Chinese and Islamic culture preserved and further developed by the Islamic world from the 8th to the 15th centuries, and translated by Christian monks into Latin, such as the Almagest.

It started with only a few researchers, evolving into an enterprise which continues to the present day. Starting with astronomy, the principles of natural philosophy crystallized into fundamental laws of physics which were enunciated and improved in the succeeding centuries. By the 19th century, the sciences had segmented into multiple fields with specialized researchers and the field of physics, although logically pre-eminent, no longer could claim sole ownership of the entire field of scientific research

16th century physics.

In the 16th century Nicolaus Copernicus revived Aristarchus' heliocentric model of the Solar System in Europe (which survived primarily in a passing mention in The Sand Reckoner of Archimedes). When this model was published at the end of his life, it was with a preface by Andreas Osiander that piously represented it as only a mathematical convenience for calculating the positions of planets, and not an account of the true nature of the planetary orbits.

In England William Gilbert (1544-1603) studied magnetism and published a seminal work, De Magnete (1600), in which he thoroughly presented his numerous experimental results.

17th century physics.

In the early 17th century Johannes Kepler formulated a model of the Solar System based upon the five Platonic solids, in an attempt to explain why the orbits of the Planets had the relative sizes they did. His access to extremely accurate astronomical observations by Tycho Brahe enabled him to determine that his model was inconsistent with the observed orbits. After a heroic seven-year effort to more accurately model the motion of the planet Mars (during which he laid the foundations of modern integral calculus) he concluded that the Planets follow not circular orbits, but elliptical orbits with the Sun at one focus of the ellipse. This breakthrough overturned a millennium of dogma based on Ptolemy's idea of "perfect" circular orbits for the "perfect" heavenly bodies. Kepler then went on to formulate his three laws of planetary motion. He also proposed the first known model of planetary motion in which a force emanating from the Sun deflects the Planets from their "natural" motion, causing them to follow curved orbits.

An important device, the vernier, which allows the accurate mechanical measurement of angles and distances was invented by a Frenchman, Pierre Vernier in 1631. It is in widespread use in scientific laboratories and machine shops to this day.

Otto von Guericke constructed the first air pump in 1650 and demonstrated the physics of the vacuum and atmospheric pressure using the Magdeburg hemispheres. Later, he turned his interests to static electricity, and he invented a mechanical device consisting of a sphere of sulfur that could be turned on a crank and repeatedly charged and discharged to produce electric sparks.

In 1656 the Dutch physicist and astronomer, Christian Huygens invented a mechanical clock using a pendulum that swung through an elliptical arc, powered by a falling counterweight, to usher in the era of accurate timekeeping.

The first quantitative estimate of the speed of light was made in 1676 by Ole Rřmer, by timing the motions of Jupiter's satellite Io with a telescope.

During the early 17th century, Galileo Galilei pioneered the use of experiment to validate physical theories, which is the key idea in the scientific method. Galileo's use of experiment, and the insistence of Galileo and Kepler that observational results must always take precedence over theoretical results (in which they followed the precepts of Aristotle if not his practice), brushed away the acceptance of dogma, and gave birth to an era where scientific ideas were openly discussed and rigorously tested. Galileo formulated and successfully tested several results in dynamics, including the correct law of accelerated motion, the parabolic trajectory, the relativity of unaccelerated motion, and an early form of the Law of Inertia.

In 1687, Isaac Newton published the Principia Mathematica, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Law of Gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

18th century physics.

From the 18th century onwards, thermodynamic concepts were developed by Robert Boyle, Thomas Young, and many others, concurrently with the development of the steam engine, onward into the next century. In 1733, Daniel Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. Benjamin Thompson demonstrated the conversion of unlimited mechanical work into heat.

In 1746 an important step in the development of electricity was taken in the invention of the Leyden jar, a capacitor, that could store and discharge electrical charge in a controlled way. Benjamin Franklin effectively used them (together with von Guericke's generator) in his researches into the nature of electricity in 1752.

In about 1788, Joseph Louis Lagrange elaborated an important new formulation of mechanics using the calculus of variations, the principle of least action and the Euler-Lagrange equations.

19th century physics.

In a letter to the Royal Society in 1800, Alessandro Volta described his invention of the electric battery, thus providing for the first time the means to generate a constant electric current, and opening up a new field of physics for investigation.

The behavior of electricity and magnetism was studied by Michael Faraday, Georg Ohm, Hans Christian Řrsted, and others. Faraday, who began his career in Chemistry working under Humphry Davy at the Royal Institution, demonstrated that electrostatic phenomena, the action of the newly discovered electric pile or battery, electrochemical phenomena, and lightning were all different manifestations of electrical phenomena. Faraday further discovered in 1821 that electricity can cause rotational mechanical motion, and in 1831 discovered the principle of electromagnetic induction, by which means mechanical motion is converted into electricity. Thus it was Faraday who laid the foundations for both the electric motor and the electric generator.

In 1855, James Clerk Maxwell unified the two phenomena into a single theory of electromagnetism, described by Maxwell's equations. A prediction of this theory was that light is an electromagnetic wave. The discovery of the Hall effect in 1879 gave the first direct evidence that the carrier of electricity was negatively charged.

In 1847 James Prescott Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. However, the principle of conservation of energy had been suggested in various forms by perhaps a dozen German, French, British and other scientists during the first half of the 19th century. About the same time, entropy and the second law of thermodynamics were first clearly described in the work of Rudolf Clausius. In 1875 Ludwig Boltzmann made the important connection between the number of possible states that a system could occupy and its entropy. With two installments in 1876 and 1878, Josiah Willard Gibbs developed much of the theoretical formalism for thermodynamics, and a decade later firmly laid the foundation for statistical mechanics - much of which Ludwig Boltzmann had independently invented. In 1881 Gibbs also was very influential in moving much of the notation of physics from Hamilton's quaternions to vectors.

Classical mechanics was given a new formulation by William Rowan Hamilton, in 1833 with the introduction of what is now called the Hamiltonian, which a century later gave an entry to wave mechanical formulation of quantum mechanics.

Dimensional analysis was used for the first time in 1878 by Lord Rayleigh who was trying to understand why the sky is blue.

In 1887 the Michelson-Morley experiment was conducted and it was interpreted as counter to the generally held theory of the day, that the Earth was moving through a "luminiferous aether". Albert Abraham Michelson and Edward Morley were not fully convinced of the non-existence of the aether. Morley conducted further experiments with Dayton Miller with improved interferometers, again giving null results.

In 1887, Nikola Tesla investigated X-rays using his own devices as well as Crookes tubes. In 1895, Wilhelm Conrad Röntgen observed and analysed X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Pierre and Marie Curie and others. This initiated the field of nuclear physics.

In 1897, J.J. Thomson and Philipp Lenard studied cathode rays. Thomson deduced that were negatively charged particles, which he called "corpuscles", which came to be called electrons. Lenard showed that the particles ejected in the photoelectric effect were the same as those in cathode rays, and that their energy was independent of the intensity of the light, but was greater for short wavelengths of the incident light.

20th century physics.

The beginning of the 20th century brought the start of a revolution in physics. In 1904, Thomson proposed the first model of the atom, known as the plum pudding model. The existence of atoms of different weights had been proposed in 1808 by John Dalton to explain the law of multiple proportions. The convergence of various estimates of Avogadro's number lent decisive evidence for atomic theory. In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. The first quantum mechanical model of the atom, the Bohr model, was published in 1913 by Niels Bohr. Sir W. H. Bragg and his son Sir William Lawrence Bragg, also in 1913, began to unravel the arrangement of atoms in crystalline matter by the use of x-ray diffraction. Neutrons, the neutral nuclear constituents, were discovered in 1932 by James Chadwick.

The Lorentz transformations, the fundamental equations of special relativity, were published in 1897 and 1900 and also by Joseph Larmor and by Hendrik Lorentz in 1899 and 1904. They both showed that Maxwell's equations were invariant under the transformations. In 1905, Einstein formulated the theory of special relativity. In 1915, Einstein extended special relativity to describe gravity with the general theory of relativity. One principal result of General relativity is the gravitational collapse into black holes, which was anticipated two centuries earlier, but elucidated by Robert Oppenheimer. Important exact solutions of Einstein's field equation were found by Karl Schwarzschild in 1915 and Roy Kerr only in 1963.

According to Cornelius Lanczos, any physical law which can be expressed as a variational principle describes an expression which is self-adjoint or Hermitian. Thus such an expression describes an invariant under a Hermitian transformation. Felix Klein's Erlangen program attempted to identify such invariants under a group of transformations. Noether's theorem identified the conditions under which the Poincaré group of transformations (what is now called a gauge group) for General relativity define conservation laws. The relationship of these invariants (the symmetries under a group of transformations) and what are now called conserved currents, depends on a variational principle, or action principle. Noether's papers made the requirements for the conservation laws precise. Noether's theorem remains right in line with current developments in physics to this day.

Beginning in 1900, Max Planck, Albert Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results,e.g. the photoelectric effect and the black body spectrum, by introducing discrete energy levels and in 1925 Wolfgang Pauli elucidated the Pauli exclusion principle and introduced the notion of quantized spin and fermions. In that year Erwin Schrödinger formulated wave mechanics, which provided a consistent mathematical method for describing a wide variety of physical situations such as the particle in a box and the quantum harmonic oscillator which he solved for the first time. Werner Heisenberg described, also in 1925, an alternative mathematical method, called matrix mechanics, which proved to be equivalent to wave mechanics. In 1928 Paul Dirac produced a relativistic formulation built on Heinsberg's matrix mechanics, and predicted the existence of the positron and founded quantum electrodynamics.

In quantum mechanics, the outcomes of physical measurements are inherently probabilistic. The theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales.

Quantum mechanics also provided the theoretical tools for understanding condensed matter physics, which studies the physical behavior of solids and liquids, including phenomena such as electrical conductivity in crystal structures. The pioneers of condensed matter physics include Felix Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. Much of the behavior of solids was elucidated within a few years with the discovery of the Fermi surface which was based on the idea of the Pauli exclusion principle applied to systems having many electrons. The understanding of the transport properties in semiconductors as described in William Shockley's Electrons and holes in semiconductors, with applications to transistor electronics enabled the electronic revolution of the twentieth century through the development of the ubiquitous, ultracheap transistor.

In 1929, Edwin Hubble published his discovery that the speed at which Galaxies recede positively correlates with their distance. This is the basis for understanding that the universe is expanding. Thus, the universe must have been smaller and therefore hotter in the past. In 1933 Karl Jansky at Bell Labs discovered the radio emission from the Milky Way, and thereby starting the science of radio astronomy.By the 1940s, researchers like George Gamow proposed the Big Bang theory, evidence for which was discovered in 1964; Enrico Fermi and Fred Hoyle were among the doubters in the 1940s and 1950s. Hoyle had dubbed Gamow's theory the Big Bang in order to debunk it. Today, it is one of the principal results of cosmology.

In 1934, the Italian physicist Enrico Fermi had discovered strange results when bombarding uranium with neutrons, which he believed at first to have created transuranic elements. In 1939, it was discovered by the chemist Otto Hahn and the physicist Lise Meitner that what was actually happening was the process of nuclear fission, whereby the nucleus of uranium was actually being split into two pieces, releasing a considerable amount of energy in the process. At this point it became clear to a number of scientists independently that this process could potentially be harnessed to produce massive amount of energy, either as a civilian power source or as a weapon. Leó Szilárd actually filed a patent on the idea of a nuclear chain reaction in 1934. In America, a team led by Fermi and Szilárd achieved the first man-made nuclear chain reaction in 1942 in the world's first nuclear reactor, and in 1945 the world's first nuclear explosive was detonated at Trinity Site, north of Alamogordo, New Mexico. After the war, central governments would become the primary sponsors of physics. The scientific leader of the Allied project, theoretical physicist Robert Oppenheimer, noted the change of the imagined role of the physicist when he noted in a speech that:

"In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin, and this is a knowledge which they cannot lose." Though the process had begun with the invention of the cyclotron by Ernest O. Lawrence in the 1930s, nuclear physics in the postwar period entered into a phase of what historians have called "Big Science", requiring costly huge accelerators and particle detectors, and large collaborative laboratories to test open new frontiers. The primary patron of physics became central governments, who recognized that the support of "basic" research could sometimes lead to technologies useful to both military and industrial applications. Toward the end of the twentieth century, a European collaboration of 20 nations, CERN, became the largest Particle physics laboratory in the world.

Another "big science" was the science of ionized gases, plasma, which had begun with Crookes tubes late in the 19th century. Large international collaborations over the last half of the twentieth century embarked on a long range effort to produce commercial amounts of electricity through fusion power, which remains a distant goal.

Further understanding of the physics of metals, semiconductors and insulators led a team of three men at Bell labs, William Shockley, Walter Brattain and John Bardeen in 1947 to the first transistor and then to many important variations, especially the bipolar junction transistor. Further developments in the fabrication and miniaturization of integrated circuits in the years to come produced fast, compact computers that came to revolutionize the way physics was done--simulations and complex mathematical calculations became possible that were undreamed of even a few decades previous.

The discovery of nuclear magnetic resonance in 1946 led to many new methods for examining the structures of molecules and became a very widely used tool in analytical chemistry, and it gave rise to an important medical imaging technique, magnetic resonance imaging.

Starting in 1960 the military establishment of the United States began using atomic clocks to construct the global positioning system which in 1984 achieved its full configuration of 24 satellites in low Earth orbits. This came to have many important civilian and scientific uses as well.

Superconductivity, discovered in 1911 by Kamerlingh Onnes, was shown to be a quantum effect and was satisfactorily explained in 1957 by Bardeen, Cooper, and Schrieffer. A family of high temperature superconductors, based on cuprate perovskite, were discovered in 1986, and their understanding remains one of the major outstanding challenges for condensed matter theorists.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It achieved its modern form in the late 1940s with work by Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga, and Freeman Dyson. This provided the framework for modern particle physics, which studies fundamental forces and elementary particles. In 1954, Yang Chen Ning and Robert Mills developed a class of gauge theories, which provided the framework for the Standard Model. This was largely completed in the 1970s and successfully describes almost all elementary particles observed to date.

In 1974 Stephen Hawking discovered the spectrum of radiation emanating during the collapse of matter into black holes. These mysterious objects became objects of intense interest to astrophysicists and even the general public in the latter part of the twentieth century.

Attempts to unify quantum mechanics and General relativity made significant progress during the 1990s. At the close of the century, a theory of everything was still not in hand, but some of its characteristics were taking shape. String theory, loop Quantum gravity and black hole thermodynamics all predicted quantized Spacetime on the Planck scale.

A number of new efforts to understand the physical world arose in the last half of the twentiety century that generated widespread interest: fractals and scaling, self-organized criticality, complexity and chaos, power laws and noise, networks, non-equilibrium thermodynamics, sandpiles, nanotechnology, cellular automata and the anthropic principle were only a few of these important topics.




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