Perbezaan antara semakan "Sejarah fizik"

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tiada ringkasan suntingan
Pada zaman purba, tindakan-tindakan dan sifat-sifat dunia biasanya diterangkan melalui tindakan [[Dewa|dewa-dewa]]. Akhirnya, penjelasan yang mengagak-agak telah dicadangkan; bagaimanapun kebanyakannya kini didapati salah, tetapi ini hanya merupakan sebahagian daripada sifat-sifat usaha penjelasan sistematik, dan teori-teori [[mekanik kuantum]] dan [[teori kerelatifan|kerelatifan]] yang moden juga dianggap hanya sebagai "teori-teori yang masih belum dibuktikan salah". Teori-teori fizik pada zaman purba biasanya dikemukakan dari segi [[falsafah]], dan jarang ditentusahkan melalui ujian [[uji kaji]] yang sistematik.
{{BI|History of physics}}
===Indian contributions===
{{See|Science and technology in ancient India}}
In [[Lothal]] (c. 2400 BC), the ancient [[port city]] of the [[Indus Valley Civilization|Harappan civilization]], shell objects served as [[compass|compasses]] to measure the angles of the 8&ndash;12 fold divisions of the horizon and sky in multiples of 40&ndash;360 degrees, and the positions of stars. <ref name="time1"> {{cite web|url= |title=Astronomsko društvo JAVORNIK |accessdate=2007-01-12 |date=2006-06-27 }}</ref> In the late [[Vedic civilization|Vedic era]] (c. [[9th century BC|9th]]&ndash;[[6th century BC]]), the [[Astronomer|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.<ref name='time2'> {{cite web|url= |accessdate=2007-01-12 |last=Vepa |first=Kosla |title=Indic Studies Foundation }}</ref> <!-- Note a better source could be found, as this infomation is pretty far down in this timeline. -->
Indians in the Vedic era classified the material world into five basic elements: earth, fire, air, water and [[Aether (classical element)|ether]]/space. <ref name='vedic1'> {{cite web|url= |title=Appendix A. Ayurveda's History, Beliefs and Practices |accessdate=2007-01-12 }}</ref> From the 6th century BC, they formulated systematic [[Atomic theory|atomic theories]], beginning with [[Kanada]] and [[Pakudha Katyayana]]. Indian [[atomism|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.<ref name='vedic1'/> This parallels with the structure of [[Elementary particle|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]].{{cn}}
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&ndash;[[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 [[Buddhist]]s [[Dignāga]] ([[5th century]]) and [[Dharmakirti]] ([[7th century]]) developed a theory of light being composed of energy particles, similar to the modern concept of [[photon]]s.
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 [[Indian mathematics|mathematician]]-[[:Category:Indian astronomers|astronomer]] [[Aryabhata]] propounded a detailed model of the heliocentric [[solar system]] of [[gravitation]], where the planets rotate on their [[Axis of rotation|axes]] causing day & night and follow [[ellipse|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 eclipse|solar]] and [[lunar eclipse|lunar]] [[eclipse]]s and predicted their times, gave the [[radius|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 ''[[Brahmasphutasiddhanta|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 calculations even possible. The modern [[Positional notation|positional]] [[numeral system]] (the [[Hindu-Arabic numeral system]]) and the number [[0 (number)|zero]] were first developed in India, along with the [[trigonometric function]]s of sine and cosine. These mathematical developments, along with the Indian developments in physics, were adopted by the [[Islam]]ic [[Caliph]]ate, from where they spread to Europe and other parts of the world.
===Chinese contributions===
{{See|Science and technology in China}}
In 1115 BC, the [[Zhonghua minzu|Chinese]] invented the first geared mechanism, the [[South Pointing Chariot]], which was also the first to use a [[differential gear]].<ref name="china1"> {{cite web|url= |title=Ruts in Written History |accessdate=2007-01-13 }}</ref> <!-- There has to be a better source then this, but this was the best that I could find. Their should be a source somewhere that describes more fully what this is. -->
The ''Mo Ching'' (allegedly written by [[Mo Tzu]]) written around the [[3rd century BC]] stated an early version of [[Newton's laws of motion|Newton's first law of motion]]:
"''The cessation of motion is due to the opposing force&nbsp;... If there is no opposing force&nbsp;... the motion will never stop. This is as true as that an ox is not a horse.''"<ref name='china2'> {{cite web|url= |title=No. 2080: THE SURVIVAL OF INVENTION |accessdate=2007-01-13 |last=Lienhard |first=John H. }}</ref>
===Greek and Hellenistic contributions===
Western physics began with eminent [[Ancient Greece|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 [[Atomism|Atomists]] who attempted to characterize the nature of matter.
Due to the absence of advanced experimental equipment such as [[telescope]]s and accurate time-keeping devices, experimental testing of physical hypotheses was impossible or impractical. There were exceptions and there are [[anachronism]]s: 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 [[sphere]]s and [[cone (geometry)|cones]] by dividing them into very thin disks and adding up the volume of each disk, using methods resembling [[integral calculus]].
Modern knowledge of many 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 of Alexandria|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 [[planet]]s 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.
===Sumbangan Parsi dan Islam===
Al-Haytham juga memperdebatkan dengan tepat bahawa manusia boleh nampak objek-objek hanya kerana sinar-sinar cahaya matahari telah dimantulkan oleh objek-objek itu ke dalam mata. Beliau juga mempercayai bahawa arus-arus zarah halus bergerak pada garis yang lurus, dan memahami bahawa cahaya harus bergerak dengan kelajuan yang amat tinggi tetapi terhingga. Al-Haytham juga memahami bahawa pembiasan adalah disebabkan oleh halaju cahaya yang berbeza di dalam bahan-bahan yang berbeza. Beliau juga mengkaji cermin-cermin [[sfera]] dan [[parabola]], dan memahami bagaimana pembiasan [[kanta]] membenarkan imej-imej difokuskan serta dibesarkan. Al-Haytham juga berupaya mempergunakan matematik untuk menerangkan mengapa sebuah cermin sfera akan mewujudkan [[aberasi]].
{{BI|History of physics}}
===Medieval European contributions===
In the [[12th century]], the birth of [[medieval university]] and the rediscovery of the works of ancient philosophers through contact with the [[Arab]]s, 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 on [[Robert Grosseteste]]'s emphasis on [[mathematics]] as a way to understand nature and on the [[empiricism|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 Empire|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 ==
[[Image:Table_of_Mechanicks,_Cyclopaedia,_Volume_2.jpg|thumb|right|300px|''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 century|8th]] to the [[15th century|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 [[law of physics|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 ===
In the [[16th century]] [[Nicolaus Copernicus]] revived [[Aristarchus of Samos|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 ===
In the early [[17th century]] [[Johannes Kepler]] formulated a model of the solar system based upon the five [[Platonic solid]]s, 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 (planet)|Mars]] (during which he laid the foundations of modern [[integral calculus]]) he concluded that the planets follow not circular orbits, but [[ellipse|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 [[Laws of Kepler|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 Rømer]], 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 (mechanics)|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]].
[[René Descartes]], French mathematician, philosopher, and natural scientist, invented analytic geometry, and discovered the law of conservation of momentum. He outlined his views on the universe in his [[Principles of Philosophy]]. It was only after Newton published his ''Principia'' that Descartes was compelled to rethink his understanding of the Laws of Motion.
In [[1687]], [[Isaac Newton]] published the ''[[Philosophiae Naturalis Principia Mathematica|Principia Mathematica]],'' detailing two comprehensive and successful physical theories: [[Newton's laws of motion]], from which arise [[classical mechanics]]; and [[gravity|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 [[astronomy|astronomical]] phenomena using physical theories.
=== 18th century ===
From the [[18th century]] onwards, [[thermodynamics|thermodynamic]] concepts were developed by [[Robert Boyle]], [[Thomas Young (scientist)|Thomas Young]], and many others, concurrently with the development of the steam engine, onward into the next century.{{cn}} 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.{{cn}}
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]].{{cn}}
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.{{cn}}
{{section stub}}
=== 19th century ===
In a letter to the [[Royal Society]] in [[1800]], [[Alessandro Volta]] described his invention of the [[Battery (electricity)|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.{{cn}}
The behavior of [[electricity]] and [[magnetism]] was studied by [[Michael Faraday]], [[Georg Ohm]], [[Hans Christian Ørsted]], 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 (electricity)|battery]], electrochemical phenomena, and [[lightning]] were all different manifestations of electrical phenomena.{{cn}} 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]].{{cn}}
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 radiation|electromagnetic wave]]. The discovery of the [[Hall effect]] in [[1879]] gave the first direct evidence that the carrier of electricity was negatively charged.{{cn}}
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.{{cn}} 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, [[Willard Gibbs|Josiah Willard Gibbs]] developed much of the theoretical formalism for [[thermodynamics]], and a decade later firmly laid the foundation for [[statistical mechanics]] &mdash; 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 [[vector (spatial)|vectors]].{{cn}}
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. During its first [[1868]] meeting, notable [[oceanographer]] [[Matthew F. Maury]] helped launch the [[American Association for the Advancement of Science]] (AAAS).{{cn}}
[[Dimensional analysis]] was used for the first time in 1878 by [[Lord Rayleigh]] who was trying to understand why the [[Diffuse sky radiation|sky is blue]].{{cn}}
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]]".{{cn}} [[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.{{cn}}
In [[1887]], [[Nikola Tesla]] investigated [[X-ray]]s 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 Curie|Pierre]] and [[Maria Sklodowska-Curie|Marie Curie]] and others. This initiated the field of [[nuclear physics]].{{cn}}
In [[1897]], [[J.J. Thomson]] and [[Philipp Lenard]] studied [[cathode ray tube|cathode rays]]. Thomson deduced that they were composed of negatively charged particles, which he called "''corpuscles''", later realized to be [[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.{{cn}}
=== 20th century - The Dawn of Modern 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 [[atom/plum pudding|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 [[Rutherford scattering|scattering experiments]] the existence of a compact [[atomic nucleus]], with positively charged constituents dubbed [[proton]]s. 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]]. [[Neutron]]s, 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 relativity|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]]<ref>{{cite book | last = Lanczos | first = Cornelius | title = The Variational Principles of Mechanics | year = 1986 | publisher = Dover Publication | location = New York | id = ISBN 0-486-65067-7 }}</ref> 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 law]]s. 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 (physics)|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 (physics)|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 [[probability|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, ultra-cheap [[transistor]].
In [[1929]], [[Edwin Hubble]] published his discovery that the speed at which galaxies appear to 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 initiated the science of [[radio astronomy]]. By the [[1940]]s, researchers like [[George Gamow]] proposed the ''[[Big Bang]]'' theory,<ref>Alpher, Herman, and Gamow. ''Nature'' '''162''', 774 (1948).</ref> evidence for which was discovered in [[1964]];<ref>{{cite web|last=Wilson |first=Robert W. |authorlink=Robert Woodrow Wilson|date=1978 |url= |title=The cosmic microwave background radiation |format=PDF |accessdate=2006-06-07 }} Wilson's Nobel Lecture.</ref> [[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 [[physical cosmology|cosmology]].
In [[1934]], the Italian physicist [[Enrico Fermi]] had discovered strange results when bombarding [[uranium]] with [[neutron]]s, 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 test|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 (physics)|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&mdash;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 [[John Bardeen|Bardeen]], [[Leon Neil Cooper|Cooper]], and [[John Robert Schrieffer|Schrieffer]]. A family of [[High-temperature superconductivity|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 force]]s and [[elementary particles]]. In [[1954]], [[Yang Chen Ning]] and [[Robert Mills (physicist)|Robert Mills]] developed a class of [[gauge theory|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 [[Hawking radiation|spectrum of radiation]] emanating during the collapse of matter into [[black hole]]s. These mysterious objects became 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]], [[Telecommunications network|networks]], [[non-equilibrium thermodynamics]], [[Bak-Tang-Wiesenfeld sandpile|sandpiles]], [[nanotechnology]], [[cellular automata]] and the [[anthropic principle]] were only a few of these important topics.
==Lihat juga==
* [ Fizik India]
{{BI|History of physics}}
[[Kategori:Sejarah fizik| ]]