Quantum field theory
In theoretical physics, quantum field theory is a theoretical framework that combines classical field theory, special relativity, quantum mechanics and is used to construct physical models of subatomic particles and quasiparticles. QFT treats particles as excited states of their underlying fields, which are—in a sense—more fundamental than the basic particles. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding fields; each interaction can be visually represented by Feynman diagrams, which are formal computational tools, in the process of relativistic perturbation theory. As a successful theoretical framework today, quantum field theory emerged from the work of generations of theoretical physicists spanning much of the 20th century, its development began in the 1920s with the description of interactions between light and electrons, culminating in the first quantum field theory — quantum electrodynamics. A major theoretical obstacle soon followed with the appearance and persistence of various infinities in perturbative calculations, a problem only resolved in the 1950s with the invention of the renormalization procedure.
A second major barrier came with QFT's apparent inability to describe the weak and strong interactions, to the point where some theorists called for the abandonment of the field theoretic approach. The development of gauge theory and the completion of the Standard Model in the 1970s led to a renaissance of quantum field theory. Quantum field theory is the result of the combination of classical field theory, quantum mechanics, special relativity. A brief overview of these theoretical precursors is in order; the earliest successful classical field theory is one that emerged from Newton's law of universal gravitation, despite the complete absence of the concept of fields from his 1687 treatise Philosophiæ Naturalis Principia Mathematica. The force of gravity as described by Newton is an "action at a distance" — its effects on faraway objects are instantaneous, no matter the distance. In an exchange of letters with Richard Bentley, Newton stated that "it is inconceivable that inanimate brute matter should, without the mediation of something else, not material, operate upon and affect other matter without mutual contact."
It was not until the 18th century that mathematical physicists discovered a convenient description of gravity based on fields — a numerical quantity assigned to every point in space indicating the action of gravity on any particle at that point. However, this was considered a mathematical trick. Fields began to take on an existence of their own with the development of electromagnetism in the 19th century. Michael Faraday coined the English term "field" in 1845, he introduced fields as properties of space having physical effects. He argued against "action at a distance", proposed that interactions between objects occur via space-filling "lines of force"; this description of fields remains to this day. The theory of classical electromagnetism was completed in 1862 with Maxwell's equations, which described the relationship between the electric field, the magnetic field, electric current, electric charge. Maxwell's equations implied the existence of electromagnetic waves, a phenomenon whereby electric and magnetic fields propagate from one spatial point to another at a finite speed, which turns out to be the speed of light.
Action-at-a-distance was thus conclusively refuted. Despite the enormous success of classical electromagnetism, it was unable to account for the discrete lines in atomic spectra, nor for the distribution of blackbody radiation in different wavelengths. Max Planck's study of blackbody radiation marked the beginning of quantum mechanics, he treated atoms, which absorb and emit electromagnetic radiation, as tiny oscillators with the crucial property that their energies can only take on a series of discrete, rather than continuous, values. These are known as quantum harmonic oscillators; this process of restricting energies to discrete values is called quantization. Building on this idea, Albert Einstein proposed in 1905 an explanation for the photoelectric effect, that light is composed of individual packets of energy called photons; this implied that the electromagnetic radiation, while being waves in the classical electromagnetic field exists in the form of particles. In 1913, Niels Bohr introduced the Bohr model of atomic structure, wherein electrons within atoms can only take on a series of discrete, rather than continuous, energies.
This is another example of quantization. The Bohr model explained the discrete nature of atomic spectral lines. In 1924, Louis de Broglie proposed the hypothesis of wave-particle duality, that microscopic particles exhibit both wave-like and particle-like properties under different circumstances. Uniting these scattered ideas, a coherent discipline, quantum mechanics, was formulated between 1925 and 1926, with important contributions from de Broglie, Werner Heisenberg, Max Born, Erwin Schrödinger, Paul Dirac, Wolfgang Pauli.:22-23In the same year as his paper on the photoelectric effect, Einstein published his theory of special relativity, built on Maxwell's electromagnetism. New rules, called Lorentz transformation, were given for the way time and space coordinates of an event change under changes in the observer's velocity, the distinction between time and space was blurred.:19 It was proposed that all physical laws must be the same for observers at different velocities, i.e. that physical laws be invariant under Lorentz transformations.
Two difficulties remained. Observationally, the Schrödinger equation underlying q
In mathematical physics, Minkowski space is a combination of three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two events is independent of the inertial frame of reference in which they are recorded. Although developed by mathematician Hermann Minkowski for Maxwell's equations of electromagnetism, the mathematical structure of Minkowski spacetime was shown to be an immediate consequence of the postulates of special relativity. Minkowski space is associated with Einstein's theory of special relativity and is the most common mathematical structure on which special relativity is formulated. While the individual components in Euclidean space and time may differ due to length contraction and time dilation, in Minkowski spacetime, all frames of reference will agree on the total distance in spacetime between events; because it treats time differently than it treats the 3 spatial dimensions, Minkowski space differs from four-dimensional Euclidean space.
In 3-dimensional Euclidean space, the isometry group is the Euclidean group. It is generated by rotations and translations; when time is amended as a fourth dimension, the further transformations of translations in time and Galilean boosts are added, the group of all these transformations is called the Galilean group. All Galilean transformations preserve the 3-dimensional Euclidean distance; this distance is purely spatial. Time differences are separately preserved as well; this changes in the spacetime of special relativity, where time are interwoven. Spacetime is equipped with an indefinite non-degenerate bilinear form, variously called the Minkowski metric, the Minkowski norm squared or Minkowski inner product depending on the context; the Minkowski inner product is defined as to yield the spacetime interval between two events when given their coordinate difference vector as argument. Equipped with this inner product, the mathematical model of spacetime is called Minkowski space; the analogue of the Galilean group for Minkowski space, preserving the spacetime interval is the Poincaré group.
In summary, Galilean spacetime and Minkowski spacetime are, when viewed as manifolds the same. They differ in; the former has the Euclidean distance function and time together with inertial frames whose coordinates are related by Galilean transformations, while the latter has the Minkowski metric together with inertial frames whose coordinates are related by Poincaré transformations. In 1905–06 Henri Poincaré showed that by taking time to be an imaginary fourth spacetime coordinate ict, where c is the speed of light and i is the imaginary unit, a Lorentz transformation can formally be regarded as a rotation of coordinates in a four-dimensional space with three real coordinates representing space, one imaginary coordinate representing time, as the fourth dimension. In physical spacetime special relativity stipulates that the quantity − t 2 + x 2 + y 2 + z 2 is invariant under coordinate changes from one inertial frame to another, i. e. under Lorentz transformations. Here the speed of light c is, following Poincaré, set to unity.
In the space suggested by him where physical spacetime is coordinatized by ↦, call it coordinate space, Lorentz transformations appear as ordinary rotations preserving the quadratic form x 2 + y 2 + z 2 + t 2 on coordinate space. The naming and ordering of coordinates, with the same labels for space coordinates, but with the imaginary time coordinate as the fourth coordinate, is conventional; the above expression, while making the former expression more familiar, may be confusing because it is not the same t that appears in the latter as in the former. Rotations in planes spanned by two space unit vectors appear in coordinate space as well as in physical spacetime appear as Euclidean rotations and are interpreted in the ordinary sense; the "rotation" in a plane spanned by a space unit vector and a time unit vector, while formally still a rotation in coordinate space, is a Lorentz boost in physical spacetime with real inertial coordinates. The analogy with Euclidean rotations is thus only partial.
This idea was elaborated by Hermann Minkowski, who used it to restate the Maxwell equations in four dimensions, showing directly their invariance under the Lorentz transformation. He further reformulated in four dimensions the then-recent theory of special relativity of Einstein. From this he concluded that time and space should be treated and so arose his concept of events taking place in a unified four-dimensional spacetime continuum. In a further development in his 1908 "Space and Time" lecture, Minkowski gave an alternative formulation of this idea that used a real time coordinate instead of an imaginary one, representing the four variables of space and time in coordinate form in a four dimensional real vector space. Points in this space correspond to events in spacetime. In this space, there is a defined light-cone associated with each point, events not on the light-cone are classified by their relation to the apex as spacelike or timel
Columbia University is a private Ivy League research university in Upper Manhattan, New York City. Established in 1754, Columbia is the oldest institution of higher education in New York and the fifth-oldest institution of higher learning in the United States, it is one of nine colonial colleges founded prior to the Declaration of Independence, seven of which belong to the Ivy League. It has been ranked by numerous major education publications as among the top ten universities in the world. Columbia was established as King's College by royal charter of George II of Great Britain in reaction to the founding of Princeton University in New Jersey, it was renamed Columbia College in 1784 following the Revolutionary War and in 1787 was placed under a private board of trustees headed by former students Alexander Hamilton and John Jay. In 1896, the campus was moved from Madison Avenue to its current location in Morningside Heights and renamed Columbia University. Columbia scientists and scholars have played an important role in the development of notable scientific fields and breakthroughs including: brain-computer interface.
The Columbia University Physics Department has been affiliated with 33 Nobel Prize winners as alumni, faculty or research staff, the third most of any American institution behind MIT and Harvard. In addition, 22 Nobel Prize winners in Physiology and Medicine have been affiliated with Columbia, the third most of any American institution; the university's research efforts include the Lamont-Doherty Earth Observatory, Goddard Institute for Space Studies and accelerator laboratories with major technology firms such as IBM. Columbia is one of the fourteen founding members of the Association of American Universities and was the first school in the United States to grant the M. D. degree. The university administers the Pulitzer Prize annually. Columbia is organized into twenty schools, including three undergraduate schools and numerous graduate schools, it maintains research centers outside of the United States known as Columbia Global Centers. In 2018, Columbia's undergraduate acceptance rate was 5.1%, making it one of the most selective colleges in the United States, the second most selective in the Ivy League after Harvard.
Columbia is ranked as the 3rd best university in the United States by U. S. News & World Report behind Princeton and Harvard. In athletics, the Lions field varsity teams in 29 sports as a member of the NCAA Division I Ivy League conference; the university's endowment stood at $10.9 billion in 2018, among the largest of any academic institution. As of 2018, Columbia's alumni and affiliates include: five Founding Fathers of the United States — among them an author of the United States Constitution and co-author of the Declaration of Independence. S. presidents. Discussions regarding the founding of a college in the Province of New York began as early as 1704, at which time Colonel Lewis Morris wrote to the Society for the Propagation of the Gospel in Foreign Parts, the missionary arm of the Church of England, persuading the society that New York City was an ideal community in which to establish a college. However, it was not until the founding of the College of New Jersey across the Hudson River in New Jersey that the City of New York considered founding a college.
In 1746, an act was passed by the general assembly of New York to raise funds for the foundation of a new college. In 1751, the assembly appointed a commission of ten New York residents, seven of whom were members of the Church of England, to direct the funds accrued by the state lottery towards the foundation of a college. Classes were held in July 1754 and were presided over by the college's first president, Dr. Samuel Johnson. Dr. Johnson was the only instructor of the college's first class, which consisted of a mere eight students. Instruction was held in a new schoolhouse adjoining Trinity Church, located on what is now lower Broadway in Manhattan; the college was founded on October 31, 1754, as King's College by royal charter of King George II, making it the oldest institution of higher learning in the state of New York and the fifth oldest in the United States. In 1763, Dr. Johnson was succeeded in the presidency by Myles Cooper, a graduate of The Queen's College, an ardent Tory. In the charged political climate of the American Revolution, his chief opponent in discussions at the college was an undergraduate of the class of 1777, Alexander Hamilton.
The American Revolutionary War broke out in 1776, was catastrophic for the operation of King's College, which suspended instruction for eight years beginning in 1776 with the arrival of the Continental Army. The suspension continued through the military occupation of New York City by British troops until their departure in 1783; the college's library was looted and its sole building requisitioned for use as a military hospital first by American and British forces. Loyalists were forced to abandon their King's College in New York, seized by the rebels and renamed Columbia College; the Loyalists, led by Bishop Charles Inglis fled to Windsor, Nova Scotia, where the
The European Organization for Nuclear Research, known as CERN, is a European research organization that operates the largest particle physics laboratory in the world. Established in 1954, the organization is based in a northwest suburb of Geneva on the Franco–Swiss border and has 23 member states. Israel is the only non-European country granted full membership. CERN is an official United Nations Observer; the acronym CERN is used to refer to the laboratory, which in 2016 had 2,500 scientific and administrative staff members, hosted about 12,000 users. In the same year, CERN generated 49 petabytes of data. CERN's main function is to provide the particle accelerators and other infrastructure needed for high-energy physics research – as a result, numerous experiments have been constructed at CERN through international collaborations; the main site at Meyrin hosts a large computing facility, used to store and analyse data from experiments, as well as simulate events. Researchers need remote access to these facilities, so the lab has been a major wide area network hub.
CERN is the birthplace of the World Wide Web. The convention establishing CERN was ratified on 29 September 1954 by 12 countries in Western Europe; the acronym CERN represented the French words for Conseil Européen pour la Recherche Nucléaire, a provisional council for building the laboratory, established by 12 European governments in 1952. The acronym was retained for the new laboratory after the provisional council was dissolved though the name changed to the current Organisation Européenne pour la Recherche Nucléaire in 1954. According to Lew Kowarski, a former director of CERN, when the name was changed, the abbreviation could have become the awkward OERN, Werner Heisenberg said that this could "still be CERN if the name is ". CERN's first president was Sir Benjamin Lockspeiser. Edoardo Amaldi was the general secretary of CERN at its early stages when operations were still provisional, while the first Director-General was Felix Bloch; the laboratory was devoted to the study of atomic nuclei, but was soon applied to higher-energy physics, concerned with the study of interactions between subatomic particles.
Therefore, the laboratory operated by CERN is referred to as the European laboratory for particle physics, which better describes the research being performed there. At the sixth session of the CERN Council, which took place in Paris from 29 June - 1 July 1953, the convention establishing the organization was signed, subject to ratification, by 12 states; the convention was ratified by the 12 founding Member States: Belgium, France, the Federal Republic of Germany, Italy, the Netherlands, Sweden, the United Kingdom, Yugoslavia. Several important achievements in particle physics have been made through experiments at CERN, they include: 1973: The discovery of neutral currents in the Gargamelle bubble chamber. In September 2011, CERN attracted media attention when the OPERA Collaboration reported the detection of faster-than-light neutrinos. Further tests showed that the results were flawed due to an incorrectly connected GPS synchronization cable; the 1984 Nobel Prize for Physics was awarded to Carlo Rubbia and Simon van der Meer for the developments that resulted in the discoveries of the W and Z bosons.
The 1992 Nobel Prize for Physics was awarded to CERN staff researcher Georges Charpak "for his invention and development of particle detectors, in particular the multiwire proportional chamber". The 2013 Nobel Prize for Physics was awarded to François Englert and Peter Higgs for the theoretical description of the Higgs mechanism in the year after the Higgs boson was found by CERN experiments; the World Wide Web began as a CERN project named ENQUIRE, initiated by Tim Berners-Lee in 1989 and Robert Cailliau in 1990. Berners-Lee and Cailliau were jointly honoured by the Association for Computing Machinery in 1995 for their contributions to the development of the World Wide Web. Based on the concept of hypertext, the project was intended to facilitate the sharing of information between researchers; the first website was activated in 1991. On 30 April 1993, CERN announced. A copy of the original first webpage, created by Berners-Lee, is still published on the World Wide Web Consortium's website as a historical document.
Prior to the Web's development, CERN had pioneered the introduction of Internet technology, beginning in the early 1980s. More CERN has become a facility for the development of grid computing, hosting projects including the Enabling Grids for E-sciencE and LHC Computing Grid, it hosts the CERN Internet Exchange Point, one of the two main internet exchange points in Switzerland. CERN operates a network of a decelerator; each machine in the chain increases the energy of particle beams before delivering them
Enrico Fermi was an Italian and naturalized-American physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and the "architect of the atomic bomb", he was one of few physicists to excel in both theoretical physics and experimental physics. Fermi held several patents related to the use of nuclear power, was awarded the 1938 Nobel Prize in Physics for his work on induced radioactivity by neutron bombardment and for the discovery of transuranium elements, he made significant contributions to the development of statistical mechanics, quantum theory, nuclear and particle physics. Fermi's first major contribution involved the field of statistical mechanics. After Wolfgang Pauli formulated his exclusion principle in 1925, Fermi followed with a paper in which he applied the principle to an ideal gas, employing a statistical formulation now known as Fermi–Dirac statistics. Today, particles that obey the exclusion principle are called "fermions".
Pauli postulated the existence of an uncharged invisible particle emitted along with an electron during beta decay, to satisfy the law of conservation of energy. Fermi took up this idea, developing a model that incorporated the postulated particle, which he named the "neutrino", his theory referred to as Fermi's interaction and now called weak interaction, described one of the four fundamental interactions in nature. Through experiments inducing radioactivity with the discovered neutron, Fermi discovered that slow neutrons were more captured by atomic nuclei than fast ones, he developed the Fermi age equation to describe this. After bombarding thorium and uranium with slow neutrons, he concluded that he had created new elements. Although he was awarded the Nobel Prize for this discovery, the new elements were revealed to be nuclear fission products. Fermi left Italy in 1938 to escape new Italian racial laws that affected his Jewish wife, Laura Capon, he emigrated to the United States, where he worked on the Manhattan Project during World War II.
Fermi led the team that designed and built Chicago Pile-1, which went critical on 2 December 1942, demonstrating the first human-created, self-sustaining nuclear chain reaction. He was on hand when the X-10 Graphite Reactor at Oak Ridge, went critical in 1943, when the B Reactor at the Hanford Site did so the next year. At Los Alamos, he headed F Division, part of which worked on Edward Teller's thermonuclear "Super" bomb, he was present at the Trinity test on 16 July 1945, where he used his Fermi method to estimate the bomb's yield. After the war, Fermi served under J. Robert Oppenheimer on the General Advisory Committee, which advised the Atomic Energy Commission on nuclear matters. After the detonation of the first Soviet fission bomb in August 1949, he opposed the development of a hydrogen bomb on both moral and technical grounds, he was among the scientists who testified on Oppenheimer's behalf at the 1954 hearing that resulted in the denial of Oppenheimer's security clearance. Fermi did important work in particle physics related to pions and muons, he speculated that cosmic rays arose when material was accelerated by magnetic fields in interstellar space.
Many awards and institutions are named after Fermi, including the Enrico Fermi Award, the Enrico Fermi Institute, the Fermi National Accelerator Laboratory, the Fermi Gamma-ray Space Telescope, the Enrico Fermi Nuclear Generating Station, the synthetic element fermium, making him one of 16 scientists who have elements named after them. Enrico Fermi was born in Rome, Italy, on 29 September 1901, he was the third child of Alberto Fermi, a division head in the Ministry of Railways, Ida de Gattis, an elementary school teacher. His sister, was two years older than he, his brother Giulio a year older. After the two boys were sent to a rural community to be wet nursed, Enrico rejoined his family in Rome when he was two and a half. Although he was baptised a Roman Catholic in accordance with his grandparents' wishes, his family was not religious; as a young boy he shared the same interests as his brother Giulio, building electric motors and playing with electrical and mechanical toys. Giulio died during an operation on a throat abscess in 1915 and Maria died in an airplane crash near Milan in 1959.
At a local market Fermi found a physics book, the 900-page Elementorum physicae mathematicae. Written in Latin by Jesuit Father Andrea Caraffa, a professor at the Collegio Romano, it presented mathematics, classical mechanics, astronomy and acoustics as they were understood at the time of its 1840 publication. With scientifically inclined friend, Enrico Persico, Fermi pursued projects such as building gyroscopes and measuring the acceleration of Earth's gravity. A colleague of Fermi's father gave him books on physics and mathematics which he assimilated quickly. Fermi graduated from high school in July 1918, at Amidei's urging applied to the Scuola Normale Superiore in Pisa. Having lost one son, his parents only reluctantly allowed him to live in the school's lodgings for four years. Fermi took first place in the difficult entrance exam, which included an essay on the theme of "Specific characteristics of Sounds". At the Scuola Normale Superiore Fermi played pranks with fellow student Franco Rasetti.
Fermi was advised by Luigi Puccianti, director of the physics laborat
Padua is a city and comune in Veneto, northern Italy. It is the capital of the economic and communications hub of the area. Padua's population is 214,000; the city is sometimes included, with Venice and Treviso, in the Padua-Treviso-Venice Metropolitan Area which has a population of c. 2,600,000. Padua stands on 29 km southeast of Vicenza; the Brenta River, which once ran through the city, still touches the northern districts. Its agricultural setting is the Venetian Plain. To the city's south west lies the Euganaean Hills, praised by Lucan and Martial, Ugo Foscolo, Shelley, it hosts the University of Padua, founded in 1222, where Galileo Galilei was a lecturer between 1592 and 1610. The city is picturesque, with a dense network of arcaded streets opening into large communal piazze, many bridges crossing the various branches of the Bacchiglione, which once surrounded the ancient walls like a moat. Padua is the setting for most of the action in Shakespeare's The Taming of the Shrew. There is a play by the Irish writer Oscar Wilde entitled The Duchess of Padua.
The city is known for being the city where Saint Anthony, a Portuguese Franciscan, spent part of his life and died in 1231. The original significance of the Roman name Patavium is uncertain, it may be connected with the ancient name of the River Po. Additionally, the root pat-, in the Indo-European language may refer to a wide open plain as opposed to nearby hills; the suffix -av (also found in the name of the rivers such as the Timavus and Tiliaventum is of Venetic origin indicating the presence of a river, which in the case of Padua is the Brenta. The ending - ium, signifies the presence of villages. Padua claims to be the oldest city in northern Italy. According to a tradition dated at least to the time of Virgil's Aeneid and to Livy's Ab Urbe Condita, Padua was founded in around 1183 BC by the Trojan prince Antenor. After the Fall of Troy, Antenor led a group of Trojans and their Paphlagonian allies, the Eneti or Veneti, who lost their king Pylaemenes to settle the Euganean plain in Italy.
Thus, when a large ancient stone sarcophagus was exhumed in the year 1274, officials of the medieval commune declared the remains within to be those of Antenor. An inscription by the native Humanist scholar Lovato dei Lovati placed near the tomb reads: This sepulchre excavated from marble contains the body of the noble Antenor who left his country, guided the Eneti and Trojans, banished the Euganeans and founded Padua However, more recent tests suggest the sepulchre dates to the between the 4th and 3rd centuries BC. Archeological remains confirm an early date for the foundation of the center of the town to between the 11th and 10th centuries BC. By the 5th century BC, rose on the banks of the river Brenta, which in the Roman era was called Medoacus Maior and until AD 589 followed the path of the present day Bacchiglione. Padua was one of the principal centers of the Veneti; the Roman historian Livy records an attempted invasion by the Spartan king Cleonimos around 302 BC. The Spartans came up the river but were defeated by the Veneti in a naval battle and gave up the idea of conquest.
Still the Veneti of Padua repulsed invasions by the Etruscans and Gauls. According to Livy and Silius Italicus, the Veneti, including those of Padua, formed an alliance with the Romans by 226 BC against their common enemies, first the Gauls and the Carthaginians. Men from Padua died beside the Romans at Cannae. With Rome's northwards expansion, Padua was assimilated into the Roman Republic. In 175 BC, Padua requested the aid of Rome in putting down a local civil war. In 91 BC, along with other cities of the Veneti, fought with Rome against the rebels in the Social War. Around 49 BC, Padua was made a Roman municipium under the Lex Julia Municipalis and its citizens ascribed to the Roman tribe, Fabia. At that time the population of the city was 40,000; the city was reputed for the wool of its sheep. In fact, the poet Martial remarks on the thickness of the tunics made there. By the end of the first century BC, Padua seems to have been the wealthiest city in Italy outside of Rome; the city became so powerful that it was able to raise two hundred thousand fighting men.
However, despite its wealth, the city was renowned for its simple manners and strict morality. This concern with morality is reflected in Livy's Roman History wherein he portrays Rome's rise to dominance as being founded upon her moral rectitude and discipline. Still Pliny, referring to one of his Paduan protégés' Paduan grandmother, Sarrana Procula, lauds her as more upright and disciplined than any of her strict fellow citizens. Padua provided the Empire with notable intellectuals. Nearby Abano was the birthplace, after many years spent in Rome, the deathplace of Livy, whose Latin was said by the critic Asinius Pollio to betray his Patavinitas. Padua was the birthplace of Thrasea Paetus, Asconius Pedianus, Valerius Flaccus. Christianity was introduced to much of the Veneto by Saint Prosdocimus, he is venerated as the first bishop of the city. His deacon, the Jewish convert Daniel, is a
Palermo is a city of Southern Italy, the capital of both the autonomous region of Sicily and the Metropolitan City of Palermo. The city is noted for its history, culture and gastronomy, playing an important role throughout much of its existence. Palermo is located in the northwest of the island of Sicily, right by the Gulf of Palermo in the Tyrrhenian Sea; the city was founded in 734 BC by the Phoenicians as Ziz. Palermo became a possession of Carthage. Two Greek colonies were established, known collectively as Panormos or "All-Port"; as Panormus, the town became part of Empire for over a thousand years. From 831 to 1072 the city was under Arab rule during the Emirate of Sicily when the city first became a capital; the Arabs shifted the Greek name into Bal ` the root for Palermo's present-day name. Following the Norman reconquest, Palermo became the capital of a new kingdom, the Kingdom of Sicily and the capital of the Holy Roman Empire under Emperor Frederick II and King Conrad IV; the population of Palermo urban area is estimated by Eurostat to be 855,285, while its metropolitan area is the fifth most populated in Italy with around 1.2 million people.
In the central area, the city has a population of around 676,000 people. The inhabitants are known as Palermitani or, panormiti; the languages spoken by its inhabitants are the Italian language and the Palermitano dialect of the Sicilian language. Palermo is Sicily's cultural and tourism capital, it is a city rich in history, art and food. Numerous tourists are attracted to the city for its good Mediterranean weather, its renowned gastronomy and restaurants, its Romanesque and Baroque churches and buildings, its nightlife and music. Palermo is the main Sicilian industrial and commercial center: the main industrial sectors include tourism, services and agriculture. Palermo has an international airport, a significant underground economy. In fact, for cultural and economic reasons, Palermo was one of the largest cities in the Mediterranean and is now among the top tourist destinations in both Italy and Europe, it is the main seat of the UNESCO World Heritage Site Arab-Norman Palermo and the Cathedral Churches of Cefalù and Monreale.
The city is going through careful redevelopment, preparing to become one of the major cities of the Euro-Mediterranean area. Roman Catholicism is important in Palermitano culture; the Patron Saint of Palermo is Santa Rosalia. The area attracts significant numbers of tourists each year and is known for its colourful fruit and fish markets at the heart of Palermo, known as Vucciria, Ballarò and Capo. Palermo lies in a basin, formed by the Papireto and Oreto rivers; the basin was named the Conca d'Oro by the Arabs in the 9th century. The city is surrounded by a mountain range, named after the city itself; these mountains face the Tyrrhenian Sea. Palermo is home to a natural port and offers excellent views to the sea from Monte Pellegrino. Palermo experiences a hot-summer subtropical Mediterranean climate, mild with moderate seasonality. Summers are long and dry due to the domination of subtropical high pressure system, while winters experience moderate temperatures and changeable, rainy weather due to the polar front.
Temperatures in autumn and spring are mild. Palermo is one of the warmest cities in Europe, with an average annual air temperature of 18.3 °C, it's one of the warmest cities in Italy. It receives 2,530 hours of sunshine per year. Snow is a rare occurrence having snowed about a dozen times since 1945. Since the 1940s to nowadays there have been at least five times when considerable snowfall has occurred. In 1949 and in 1956, when the minimum temperature went down to 0 °C, the city was blanketed by some centimetres of snow. Snowfalls occurred in 1981, 1986, 1999 and 2014; the average annual temperature of the sea is above 19 °C. In the period from November to May, the average sea temperature exceeds 18 °C and in the period from June to October, the average sea temperature exceeds 21 °C. Palermo is surrounded by mountains; some districts of the city are divided by the mountains themselves. It was difficult to reach the inner part of Sicily from the city because of the mounts; the tallest peak of the range is La Pizzuta, about 1,333 metres high.
However the most important mount is Monte Pellegrino, geographically separated from the rest of the range by a plain. The mount lies right in front of the Tyrrhenian Sea. Monte Pellegrino's cliff was described in the 19th century by Johann Wolfgang von Goethe, as "the most beautiful promontory in the world", in his essay "Italian Journey". Today both the Kemonia are covered up by buildings. However, the shape of the former watercourses can still be recognised today, because the streets that were built on them follow their shapes. Today the only waterway not drained yet is the Oreto river that divides the downtown of the city from the western uptown and the industrial districts. In the basins there were, many seasonal torrents that helped formed swampy plains, reclaimed during history.