The RCA Corporation was a major American electronics company, founded as the Radio Corporation of America in 1919. It was a wholly owned subsidiary of General Electric. An innovative and progressive company, RCA was the dominant electronics and communications firm in the United States for over five decades. RCA was at the forefront of the mushrooming radio industry in the early 1920s, as a major manufacturer of radio receivers, the exclusive manufacturer of the first superheterodyne models. RCA created the first American radio network, the National Broadcasting Company; the company was a pioneer in the introduction and development of television, both black-and-white and color. During this period, RCA was identified with the leadership of David Sarnoff, he was general manager at the company's founding, became president in 1930, remained active, as chairman of the board, until the end of 1969. RCA's impregnable stature began to weaken in the mid-1970s, as it attempted to diversify and expand into a multifaceted conglomerate.
The company suffered enormous financial losses in the mainframe computer industry and other failed projects such as the CED videodisc. In 1986, RCA was reacquired by General Electric, which over the next few years liquidated most of the corporation's assets. Today, RCA exists as a brand name only. RCA originated as a reorganization of the Marconi Wireless Telegraph Company of America. In 1897, the Wireless Telegraph and Signal Company, was founded in London to promote the radio inventions of Guglielmo Marconi; as part of worldwide expansion, in 1899 American Marconi was organized as a subsidiary company, holding the rights to use the Marconi patents in the United States and Cuba. In 1912 it took over the assets of the bankrupt United Wireless Telegraph Company, from that point forward it had been the dominant radio communications company in the United States. With the entry of the United States into World War One in April 1917, the government took over most civilian radio stations, to use them for the war effort.
Although the overall U. S. government plan was to restore civilian ownership of the seized radio stations once the war ended, many Navy officials hoped to retain a monopoly on radio communication after the war. Defying instructions to the contrary, the Navy began purchasing large numbers of stations outright. With the conclusion of the conflict, Congress turned down the Navy's efforts to have peacetime control of the radio industry, instructed the Navy to make plans to return the commercial stations it controlled, including the ones it had improperly purchased, to the original owners. Due to national security considerations, the Navy was concerned about returning the high-powered international stations to American Marconi, since a majority of its stock was in foreign hands, the British largely controlled the international undersea cables; this concern was increased by the announcement in late 1918 of the formation of the Pan-American Wireless Telegraph and Telephone Company, a joint venture between American Marconi and the Federal Telegraph Company, with plans to set up service between the United States and South America.
The Navy had installed a high-powered Alexanderson alternator, built by General Electric, at the American Marconi transmitter site in New Brunswick, New Jersey. It proved to be superior for transatlantic transmissions to the spark transmitters, traditionally used by the Marconi companies. Marconi officials were so impressed by the capabilities of the Alexanderson alternators that they began making preparations to adopt them as their standard transmitters for international communication. A tentative plan made with General Electric proposed that over a two-year period the Marconi companies would purchase most of GE's alternator production. However, this proposal was met with disapproval, on national security grounds, by the U. S. Navy, concerned that this would guarantee British domination of international radio communication; the Navy, claiming it was acting with the support of President Wilson, looked for an alternative that would result in an "all-American" company taking over the American Marconi assets.
In April 1919 two naval officers, Admiral H. G. Bullard and Commander S. C. Hooper, met with GE's president, Owen D. Young, asking that he suspend the pending alternator sales to the Marconi companies; this move would leave General Electric without a buyer for its transmitters, so the officers proposed that GE purchase American Marconi, use the assets to form its own radio communications subsidiary. Young consented to this proposal, effective November 20, 1919, transformed American Marconi into the Radio Corporation of America; the new company was promoted as being a patriotic gesture. RCA's incorporation papers required that its officers needed to be U. S. citizens, with a majority of its stock held by Americans. RCA retained most of the American Marconi staff, although Owen Young became the new company's head as the chairman of the board. Former American Marconi vice president and general manager E. J. Nally become RCA's first president. Nally's term ended on December 31, 1922, he was succeeded the next day by Major General James G. Harbord.
Talmud Torah schools were created in the Jewish world, both Ashkenazic and Sephardic, as a form of religious school for boys of modest backgrounds, where they were given an elementary education in Hebrew, the Scriptures, the Talmud. This was meant to prepare them for Yeshiva or in the movement's modern form, for Jewish education at a high school level; the Talmud Torah was modelled after the Cheder, a traditional form of schooling whose essential elements it incorporated, with changes appropriate to its public form rather than the heder's "private" financing through less formal or institutionalized mechanisms, including tuition fees and donations. The father was traditionally the sole teacher of his children in Jewish history; the institution known as the "bei rav" or "bet rabban", or as the "bei safra" or "bet sefer", is said to have been originated by Ezra' and his Great Assembly, who provided a public school in Jerusalem to secure the education of fatherless boys of the age of sixteen years and upward.
But the school system did not develop until Joshua ben Gamla the high priest caused public schools to be opened in every town and hamlet for all children above six or seven years of age. The expense was borne by the community, strict discipline was observed. Rav, ordered Samuel b. Shilat to deal tenderly with the pupils, to refrain from corporal punishment, or at most to use a shoe-strap in correcting pupils for inattention. A stupid pupil was made monitor until able to grasp the art of learning. Raba fixed the number of pupils at twenty-five for one teacher. Only married men were engaged as teachers, but there is a difference of opinion regarding the qualification of the "melammed". Raba preferred one who taught his pupils much though somewhat carelessly, while Rav Dimi of Nehardea preferred one who taught his pupils little, but that as an error in reading once adopted is hard to correct, it is, of course, assumed that both qualifications were to be found in one person. The teaching in the Talmud Torah consumed the whole day, in the winter months a few hours of the night besides.
Teaching was suspended in the afternoon of Friday, in the afternoon of the day preceding a holy day. On Sabbaths and holy days no new lessons were assigned. In times influenced by the Christian parochial schools of the thirteenth century, the reading of the prayers and benedictions and the teaching of the principles of the Jewish faith were included. In every community an organization called "Hevra Talmud Torah" was formed, whose duty was to create a fund and provide means for the support of public schools, to control all teachers and pupils. R. Asher ben Jehiel ruled to allow withdrawals from the funds of the Talmud Torah for the purpose of meeting the annual tax collected by the local governor, since otherwise great hardships would fall upon the poor, who were liable to be stripped of all their belongings if they failed in the prompt payment of their taxes. On the other hand, money from the general charity fund was at times employed to support the Talmud Torah, donations for a synagogue or cemetery were used.
Because Talmudic and Torah education was traditionally deemed obligatory for males and not females, Talmud Torahs were traditionally single-sex institutions. It is common in the present day for men to continue their full-time Torah studies well into their third decade of life while females pursue "secular" education such as computer programming and home economics; the Talmud Torah organization in Rome included eight societies in 1554, was reconstituted August 13, 1617. Certain synagogues assumed the name "Talmud Torah," as in the case of one at Fes, Morocco in 1603 and one at Cairo; this was because the school was held in or adjoined the synagogue. The income of the society was derived from several sources: one-sixth of the Monday and Thursday contributions in the synagogues and other places of worship. A legacy for the support of a yeshivah and Talmud Torah in a certain town, if accompanied by a provision that it may be managed "as the son of the testator may see fit," may be transferred, it was declared, to a yeshivah elsewhere.
The election of officers was made by ballot: three gabba'im, three vice-gabba'im, a treasurer. Only learned and honorable men over 36 years of age were eligible for election; the taḳḳanot regulating these sources of the Talmud Torah's income were in existence in the time of R. Moses Isserles. R. Yoel Sirkis, rabbi of Cracow in 1638, endorsed these regulations and added many others, all of which were confirmed at a general assembly of seventy representatives of the congre
Washington, D. C. formally the District of Columbia and referred to as Washington or D. C. is the capital of the United States. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, first President of the United States and Founding Father; as the seat of the United States federal government and several international organizations, Washington is an important world political capital. The city is one of the most visited cities in the world, with more than 20 million tourists annually; the signing of the Residence Act on July 16, 1790, approved the creation of a capital district located along the Potomac River on the country's East Coast. The U. S. Constitution provided for a federal district under the exclusive jurisdiction of the U. S. Congress, the District is therefore not a part of any state; the states of Maryland and Virginia each donated land to form the federal district, which included the pre-existing settlements of Georgetown and Alexandria.
The City of Washington was founded in 1791 to serve as the new national capital. In 1846, Congress returned the land ceded by Virginia. Washington had an estimated population of 702,455 as of July 2018, making it the 20th most populous city in the United States. Commuters from the surrounding Maryland and Virginia suburbs raise the city's daytime population to more than one million during the workweek. Washington's metropolitan area, the country's sixth largest, had a 2017 estimated population of 6.2 million residents. All three branches of the U. S. federal government are centered in the District: Congress and the U. S. Supreme Court. Washington is home to many national monuments, museums situated on or around the National Mall; the city hosts 177 foreign embassies as well as the headquarters of many international organizations, trade unions, non-profit, lobbying groups, professional associations, including the World Bank Group, the International Monetary Fund, the Organization of American States, AARP, the National Geographic Society, the Human Rights Campaign, the International Finance Corporation, the American Red Cross.
A locally elected mayor and a 13‑member council have governed the District since 1973. However, Congress may overturn local laws. D. C. residents elect a non-voting, at-large congressional delegate to the House of Representatives, but the District has no representation in the Senate. The District receives three electoral votes in presidential elections as permitted by the Twenty-third Amendment to the United States Constitution, ratified in 1961. Various tribes of the Algonquian-speaking Piscataway people inhabited the lands around the Potomac River when Europeans first visited the area in the early 17th century. One group known as the Nacotchtank maintained settlements around the Anacostia River within the present-day District of Columbia. Conflicts with European colonists and neighboring tribes forced the relocation of the Piscataway people, some of whom established a new settlement in 1699 near Point of Rocks, Maryland. In his Federalist No. 43, published January 23, 1788, James Madison argued that the new federal government would need authority over a national capital to provide for its own maintenance and safety.
Five years earlier, a band of unpaid soldiers besieged Congress while its members were meeting in Philadelphia. Known as the Pennsylvania Mutiny of 1783, the event emphasized the need for the national government not to rely on any state for its own security. Article One, Section Eight, of the Constitution permits the establishment of a "District as may, by cession of particular states, the acceptance of Congress, become the seat of the government of the United States". However, the Constitution does not specify a location for the capital. In what is now known as the Compromise of 1790, Alexander Hamilton, Thomas Jefferson came to an agreement that the federal government would pay each state's remaining Revolutionary War debts in exchange for establishing the new national capital in the southern United States. On July 9, 1790, Congress passed the Residence Act, which approved the creation of a national capital on the Potomac River; the exact location was to be selected by President George Washington, who signed the bill into law on July 16.
Formed from land donated by the states of Maryland and Virginia, the initial shape of the federal district was a square measuring 10 miles on each side, totaling 100 square miles. Two pre-existing settlements were included in the territory: the port of Georgetown, founded in 1751, the city of Alexandria, founded in 1749. During 1791–92, Andrew Ellicott and several assistants, including a free African American astronomer named Benjamin Banneker, surveyed the borders of the federal district and placed boundary stones at every mile point. Many of the stones are still standing. A new federal city was constructed on the north bank of the Potomac, to the east of Georgetown. On September 9, 1791, the three commissioners overseeing the capital's construction named the city in honor of President Washington; the federal district was named Columbia, a poetic name for the United States in use at that time. Congress held its first session in Washington on November 17, 1800. Congress passed the District of Columbia Organic Act of 1801 that organized the District and placed the entire territory under the exclusive control of the federal
Pearl Harbor is a lagoon harbor on the island of Oahu, west of Honolulu. It has been long visited by the Naval fleet of the United States, before it was acquired from the Hawaiian Kingdom by the U. S. with the signing of the Reciprocity Treaty of 1875. Much of the harbor and surrounding lands is now a United States Navy deep-water naval base, it is the headquarters of the United States Pacific Fleet. The U. S. government first obtained exclusive use of the inlet and the right to maintain a repair and coaling station for ships here in 1887. The attack on Pearl Harbor by the Empire of Japan on December 7, 1941, was the immediate cause of the United States' entry into World War II. Pearl Harbor was an extensive shallow embayment called Wai Momi or Puʻuloa by the Hawaiians. Puʻuloa was regarded as the home of the shark goddess, Kaʻahupahau, her brother, Kahiʻuka, in Hawaiian legends. According to tradition, the head of the powerful Ewa chiefs, is credited with cutting a navigable channel near the present Puʻuloa saltworks, by which he made the estuary, known as "Pearl River," accessible to navigation.
Making due allowance for legendary amplification, the estuary had an outlet for its waters where the present gap is. During the early 19th century, Pearl Harbor was not used for large ships due to its shallow entrance; the interest of United States in the Hawaiian Islands grew as a result of its whaling and trading activity in the Pacific. As early as 1820, an "Agent of the United States for Commerce and Seamen" was appointed to look after American business in the Port of Honolulu; these commercial ties to the American continent were accompanied by the work of the American Board of Commissioners for Foreign Missions. American missionaries and their families became an integral part of the Hawaiian political body. Throughout the 1820s and 1830s, many American warships visited Honolulu. In most cases, the commanding officers carried letters from the U. S. Government giving advice on governmental affairs and of the relations of the island nation with foreign powers. In 1841, the newspaper Polynesian, printed in Honolulu, advocated that the U.
S. establish a naval base in Hawaii for protection of American citizens engaged in the whaling industry. The British Hawaiian Minister of Foreign Affairs Robert Crichton Wyllie, remarked in 1840 that "... my opinion is that the tide of events rushes on to annexation to the United States." From the conclusion of the Civil War, to the purchase of Alaska, to the increased importance of the Pacific states, the projected trade with countries in Asia and the desire for a duty-free market for Hawaiian staples, Hawaiian trade expanded. In 1865, the North Pacific Squadron was formed to embrace Hawaii. Lackawanna in the following year was assigned to cruise among the islands, "a locality of great and increasing interest and importance." This vessel surveyed the Northwestern Hawaiian Islands toward Japan. As a result, the United States claimed Midway Island; the Secretary of the Navy was able to write in his annual report of 1868, that in November 1867, 42 American flags flew over whaleships and merchant vessels in Honolulu to only six of other nations.
This increased activity caused the permanent assignment of at least one warship to Hawaiian waters. It praised Midway Island as possessing a harbor surpassing Honolulu's. In the following year, Congress approved an appropriation of $50,000 on March 1, 1869, to deepen the approaches to this harbor. After 1868, when the Commander of the Pacific Fleet visited the islands to look after American interests, naval officers played an important role in internal affairs, they served as arbitrators in business disputes, negotiators of trade agreements and defenders of law and order. Periodic voyages among the islands and to the mainland aboard U. S. warships were arranged for members of the Hawaiian royal family and important island government officials. When King Lunalilo died in 1873, negotiations were underway for the cession of Pearl Harbor as a port for the duty-free export of sugar to the U. S. With the election of King Kalākaua in March 1874, riots prompted landing of sailors from USS Tuscarora and Portsmouth.
The British warship, HMS Tenedos landed a token force. During the reign of King Kalākaua the United States was granted exclusive rights to enter Pearl Harbor and to establish "a coaling and repair station." Although this treaty continued in force until August 1898, the U. S. did not fortify Pearl Harbor as a naval base. As it had for 60 years, the shallow entrance constituted a formidable barrier against the use of the deep protected waters of the inner harbor; the United States and the Hawaiian Kingdom signed the Reciprocity Treaty of 1875 as supplemented by Convention on December 6, 1884, the Reciprocity Treaty was made by James Carter and ratified it in 1887. On January 20, 1887, the United States Senate allowed the Navy to exclusive right to maintain a coaling and repair station at Pearl Harbor.. The Spanish–American War of 1898 and the desire for the United States to have a permanent presence in the Pacific both contributed to the decision. Following the overthrow of the Hawaiian Kingdom, the United States Navy established a base on the island in 1899.
On December 7, 1941, the base was attacked by the Imperial Japanese Navy airplanes and midget submarines, causing the American entry into World War II. One of the main reasons that Pearl Harbor happened was because the United States had major communication breakdowns among several branches of the U. S. armed services and departments of the U. S. government. This led to the surprise Japanese attack at the Hawai
Paterson, New Jersey
Paterson is the largest city in and the county seat of Passaic County, New Jersey, United States. As of the 2010 United States Census, its population was 146,199, making it New Jersey's third-most-populous city. Paterson has the second-highest density of any U. S. city with over 100,000 people, behind only New York City. For 2017, the Census Bureau's Population Estimates Program calculated a population of 148,678, an increase of 1.7% from the 2010 enumeration, making the city the 174th-most-populous in the nation. Paterson is known as the "Silk City" for its dominant role in silk production during the latter half of the 19th century, it has since evolved into a major destination for Hispanic immigrants as well as for immigrants from India, South Asia, the Arab and Muslim world. Paterson has the second-largest Muslim population in the United States by percentage; the area of Paterson was inhabited by the Algonquian-speaking Native American Acquackanonk tribe of the Lenape known as the Delaware Indians.
The land was known as the Lenapehoking. The Dutch claimed the land as New Netherlands the British as the Province of New Jersey. In 1791 Alexander Hamilton, first United States Secretary of the Treasury, helped found the Society for the Establishment of Useful Manufactures, which helped encourage the harnessing of energy from the Great Falls of the Passaic River to secure economic independence from British manufacturers; the society founded Paterson. Paterson was named for William Paterson, signer of the Constitution and Governor of New Jersey, who signed the 1792 charter that established the Town of Paterson. Architect and city planner Pierre Charles L'Enfant, who had earlier developed the initial plans for Washington, D. C. was the first planner for the S. U. M. Project, his plan proposed to harness the power of the Great Falls through a channel in the rock and an aqueduct. The society's directors felt he was taking too long and was over budget, he was replaced by Peter Colt, who used a less complicated reservoir system to get the water flowing to factories in 1794.
Colt's system developed some problems and a scheme resembling L'Enfant's original plan was used after 1846. Paterson was formed as a township from portions of Acquackanonk Township on April 11, 1831, while the area was still part of Essex County, it became part of newly created Passaic County on February 7, 1837, was incorporated as a city on April 14, 1851, based on the results of a referendum held that day. The city was reincorporated on March 14, 1861; the industries developed in Paterson were powered by the 77-foot-high Great Falls and a system of water raceways that harnessed the falls' power, providing power for the mills in the area until 1914 and fostering the growth of the city around them. The district included dozens of mill buildings and other manufacturing structures associated with the textile industry and the firearms and railroad locomotive manufacturing industries. In the latter half of the 19th century silk production became the dominant industry and formed the basis of Paterson's most prosperous period, earning it the nickname "Silk City."In 1835 Samuel Colt began producing firearms in Paterson, but within a few years he moved his business to Hartford, Connecticut.
In the 19th century Paterson was the site of early experiments with submarines by Irish-American inventor John Philip Holland. Two of Holland's early models—one found at the bottom of the Passaic River—are on display in the Paterson Museum, housed in the former Rogers Locomotive and Machine Works near the Passaic Falls. Behind Newark and New York, the brewing industry was booming in Paterson in the late 1800s. Braun Brewery, Sprattler & Mennell, Graham Brewery, The Katz Brothers, Burton Brewery merged in 1890 to form Paterson Consolidated Brewing Company. Hinchliffe Brewing and Malting Company, founded in 1861, produced 75,000 barrels a year from its state-of-the-art facility at 63 Governor Street. All the breweries closed after Prohibition; the city was a mecca for immigrant laborers, who worked in its factories Italian weavers from the Naples region. Paterson was the site of historic labor unrest that focused on anti-child labor legislation, the six-month-long Paterson silk strike of 1913 that demanded the eight-hour day and better working conditions.
It was defeated with workers forced to return under pre-strike conditions. Factory workers labored long hours for low wages under dangerous conditions and lived in crowded tenement buildings around the mills; the factories moved to the South, where there were no labor unions, still moved overseas. In 1919 Paterson was one of eight locations bombed by self-identified anarchists. In 1932 Paterson opened Hinchliffe Stadium, a 10,000-seat stadium named in honor of John V. Hinchliffe, the city's mayor at the time. Hinchliffe Stadium served as the site for high school and professional athletic events. From 1933 to 1937 and 1939 to 1945, it was the home of the New York Black Yankees, from 1935 to 1936 the home of the New York Cubans of the Negro National League; the ballpark was a venue for professional football games and field events, boxing matches, auto and motorcycle racing. The comedy team of Bud Abbott and Lou Costello performed at Hinchliffe. Hinchliffe is one of only three Negro League stadiums left standing in the United States and is on the National Register of Historic Places.
Paterson Public Schools acquired the stadium in 1963 and used it for public school events until 1997, but it is now in di
Physics is the natural science that studies matter, its motion, behavior through space and time, that studies the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, its main goal is to understand how the universe behaves. Physics is one of the oldest academic disciplines and, through its inclusion of astronomy the oldest. Over much of the past two millennia, chemistry and certain branches of mathematics, were a part of natural philosophy, but during the scientific revolution in the 17th century these natural sciences emerged as unique research endeavors in their own right. Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, the boundaries of physics which are not rigidly defined. New ideas in physics explain the fundamental mechanisms studied by other sciences and suggest new avenues of research in academic disciplines such as mathematics and philosophy. Advances in physics enable advances in new technologies.
For example, advances in the understanding of electromagnetism and nuclear physics led directly to the development of new products that have transformed modern-day society, such as television, domestic appliances, nuclear weapons. Astronomy is one of the oldest natural sciences. Early civilizations dating back to beyond 3000 BCE, such as the Sumerians, ancient Egyptians, the Indus Valley Civilization, had a predictive knowledge and a basic understanding of the motions of the Sun and stars; the stars and planets were worshipped, believed to represent gods. While the explanations for the observed positions of the stars were unscientific and lacking in evidence, these early observations laid the foundation for astronomy, as the stars were found to traverse great circles across the sky, which however did not explain the positions of the planets. According to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, all Western efforts in the exact sciences are descended from late Babylonian astronomy.
Egyptian astronomers left monuments showing knowledge of the constellations and the motions of the celestial bodies, while Greek poet Homer wrote of various celestial objects in his Iliad and Odyssey. Natural philosophy has its origins in Greece during the Archaic period, when pre-Socratic philosophers like Thales rejected non-naturalistic explanations for natural phenomena and proclaimed that every event had a natural cause, they proposed ideas verified by reason and observation, many of their hypotheses proved successful in experiment. The Western Roman Empire fell in the fifth century, this resulted in a decline in intellectual pursuits in the western part of Europe. By contrast, the Eastern Roman Empire resisted the attacks from the barbarians, continued to advance various fields of learning, including physics. In the sixth century Isidore of Miletus created an important compilation of Archimedes' works that are copied in the Archimedes Palimpsest. In sixth century Europe John Philoponus, a Byzantine scholar, questioned Aristotle's teaching of physics and noting its flaws.
He introduced the theory of impetus. Aristotle's physics was not scrutinized until John Philoponus appeared, unlike Aristotle who based his physics on verbal argument, Philoponus relied on observation. On Aristotle's physics John Philoponus wrote: “But this is erroneous, our view may be corroborated by actual observation more than by any sort of verbal argument. For if you let fall from the same height two weights of which one is many times as heavy as the other, you will see that the ratio of the times required for the motion does not depend on the ratio of the weights, but that the difference in time is a small one, and so, if the difference in the weights is not considerable, that is, of one is, let us say, double the other, there will be no difference, or else an imperceptible difference, in time, though the difference in weight is by no means negligible, with one body weighing twice as much as the other”John Philoponus' criticism of Aristotelian principles of physics served as an inspiration for Galileo Galilei ten centuries during the Scientific Revolution.
Galileo cited Philoponus in his works when arguing that Aristotelian physics was flawed. In the 1300s Jean Buridan, a teacher in the faculty of arts at the University of Paris, developed the concept of impetus, it was a step toward the modern ideas of momentum. Islamic scholarship inherited Aristotelian physics from the Greeks and during the Islamic Golden Age developed it further placing emphasis on observation and a priori reasoning, developing early forms of the scientific method; the most notable innovations were in the field of optics and vision, which came from the works of many scientists like Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi and Avicenna. The most notable work was The Book of Optics, written by Ibn al-Haytham, in which he conclusively disproved the ancient Greek idea about vision, but came up with a new theory. In the book, he presented a study of the phenomenon of the camera obscura (his thousand-year-old
General relativity is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present; the relation is specified by the Einstein field equations, a system of partial differential equations. Some predictions of general relativity differ from those of classical physics concerning the passage of time, the geometry of space, the motion of bodies in free fall, the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the gravitational time delay; the predictions of general relativity in relation to classical physics have been confirmed in all observations and experiments to date.
Although general relativity is not the only relativistic theory of gravity, it is the simplest theory, consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity. Einstein's theory has important astrophysical implications. For example, it implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not light, can escape—as an end-state for massive stars. There is ample evidence that the intense radiation emitted by certain kinds of astronomical objects is due to black holes; the bending of light by gravity can lead to the phenomenon of gravitational lensing, in which multiple images of the same distant astronomical object are visible in the sky. General relativity predicts the existence of gravitational waves, which have since been observed directly by the physics collaboration LIGO.
In addition, general relativity is the basis of current cosmological models of a expanding universe. Acknowledged as a theory of extraordinary beauty, general relativity has been described as the most beautiful of all existing physical theories. Soon after publishing the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new relativistic framework. In 1907, beginning with a simple thought experiment involving an observer in free fall, he embarked on what would be an eight-year search for a relativistic theory of gravity. After numerous detours and false starts, his work culminated in the presentation to the Prussian Academy of Science in November 1915 of what are now known as the Einstein field equations; these equations specify how the geometry of space and time is influenced by whatever matter and radiation are present, form the core of Einstein's general theory of relativity. The Einstein field equations are nonlinear and difficult to solve.
Einstein used approximation methods in working out initial predictions of the theory. But as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the Schwarzschild metric; this solution laid the groundwork for the description of the final stages of gravitational collapse, the objects known today as black holes. In the same year, the first steps towards generalizing Schwarzschild's solution to electrically charged objects were taken, which resulted in the Reissner–Nordström solution, now associated with electrically charged black holes. In 1917, Einstein applied his theory to the universe as a whole, initiating the field of relativistic cosmology. In line with contemporary thinking, he assumed a static universe, adding a new parameter to his original field equations—the cosmological constant—to match that observational presumption. By 1929, the work of Hubble and others had shown that our universe is expanding; this is described by the expanding cosmological solutions found by Friedmann in 1922, which do not require a cosmological constant.
Lemaître used these solutions to formulate the earliest version of the Big Bang models, in which our universe has evolved from an hot and dense earlier state. Einstein declared the cosmological constant the biggest blunder of his life. During that period, general relativity remained something of a curiosity among physical theories, it was superior to Newtonian gravity, being consistent with special relativity and accounting for several effects unexplained by the Newtonian theory. Einstein himself had shown in 1915 how his theory explained the anomalous perihelion advance of the planet Mercury without any arbitrary parameters. A 1919 expedition led by Eddington confirmed general relativity's prediction for the deflection of starlight by the Sun during the total solar eclipse of May 29, 1919, making Einstein famous, yet the theory entered the mainstream of theoretical physics and astrophysics only with the developments between 1960 and 1975, now known as the golden age of general relativity. Physicists began to understand the concept of a black hole, to identify quasars as one of these objects' astrophysical manifestations.
More precise solar system tests confirmed the theory's predictive power, relativistic cosmology, became amenable to direct observational tests. Over the years, general relativity has acqui