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The air offensive against the RAF and British industry failed to have the desired effect. More might have been achieved had the OKL exploited their enemy's weak spot, the vulnerability of British sea communications. The Allies did so later when Bomber Command attacked rail communications and the United States Army Air Forces targeted oil, but that would have required an economic-industrial analysis of which the Luftwaffe was incapable. The OKL instead sought clusters of targets that suited the latest policy (which changed frequently), and disputes within the leadership were about tactics rather than strategy. Though militarily ineffective, the Blitz caused enormous damage to Britain's infrastructure and housing stock. It cost around 41,000 lives, and may have injured another 139,000.
The relieved British began to assess the impact of the Blitz in August 1941, and the RAF Air Staff used the German experience to improve Bomber Command's offensives. They concluded bombers should strike a single target each night and use more incendiaries because they had a greater impact on production than high explosives. They also noted regional production was severely disrupted when city centres were devastated through the loss of administrative offices, utilities and transport. They believed the Luftwaffe had failed in precision attack, and concluded the German example of area attack using incendiaries was the way forward for operations over Germany.
Some writers claim the Air Staff ignored a critical lesson, however: British morale did not break. Targeting German morale, as Bomber Command would do, was no more successful. Aviation strategists dispute that morale was ever a major consideration for Bomber Command. Throughout 1933–39 none of the 16 Western Air Plans drafted mentioned morale as a target. The first three directives in 1940 did not mention civilian populations or morale in any way. Morale was not mentioned until the ninth wartime directive on 21 September 1940. The 10th directive in October 1940 mentioned morale by name. However, industrial cities were only to be targeted if weather denied strikes on Bomber Command's main concern, oil.
AOC Bomber Command Arthur Harris did see German morale as a major objective. However, he did not believe that the morale-collapse could occur without the destruction of the German economy. The primary goal of Bomber Command's offensives was to destroy the German industrial base (economic warfare), and in doing so reduce morale. In late 1943, just before the Battle of Berlin, he declared the power of Bomber Command would enable it to achieve "a state of devastation in which surrender is inevitable." A summary of Harris' strategic intentions was clear:
A converse popular image arose of British people in the Second World War: a collection of people locked in national solidarity. This image entered the historiography of the Second World War in the 1980s and 1990s, especially after the publication of Angus Calder's book The Myth of the Blitz (1991). It was evoked by both the right and left political factions in Britain during the Falklands War when it was embedded in a nostalgic narrative in which the Second World War represented aggressive British patriotism successfully defending democracy. This imagery of people in the Blitz was and is powerfully portrayed in film, radio, newspapers and magazines. At the time it was a useful propaganda tool for home and foreign consumption. Historians' critical response to this construction focused on what were seen as over-emphasised claims of righteous nationalism and national unity. In the Myth of the Blitz, Calder exposed some of the counter-evidence of anti-social and divisive behaviours. What he saw as the myth—serene national unity—became "historical truth". In particular, class division was most evident.
In the wake of the Coventry Blitz, there was widespread agitation from the Communist Party over the need for bomb-proof shelters. Many Londoners, in particular, took to using the Underground railway system, without authority, for shelter and sleeping through the night there until the following morning. So worried were the Government over the sudden campaign of leaflets and posters distributed by the Communist Party in Coventry and London, that the Police were sent in to seize their production facilities. The Government, up until November 1940, was opposed to the centralised organisation of shelter. Home Secretary Sir John Anderson was replaced by Morrison soon afterwards, in the wake of a Cabinet reshuffle as the dying Neville Chamberlain resigned. Morrison warned that he could not counter the Communist unrest unless provision of shelters were made. He recognised the right of the public to seize tube stations and authorised plans to improve their condition and expand them by tunnelling. Still, many British citizens, who had been members of the Labour Party, itself inert over the issue, turned to the Communist Party. The Communists attempted to blame the damage and casualties of the Coventry raid on the rich factory owners, big business and landowning interests and called for a negotiated peace. Though they failed to make a large gain in influence, the membership of the Party had doubled by June 1941. The "Communist threat" was deemed important enough for Herbert Morrison to order, with the support of the Cabinet, the stoppage of the Daily Worker and The Week; the Communist newspaper and journal.
The brief success of the Communists also fed into the hands of the British Union of Fascists (BUF). Anti-Semitic attitudes became widespread, particularly in London. Rumours that Jewish support was underpinning the Communist surge were frequent. Rumours that Jews were inflating prices, were responsible for the Black Market, were the first to panic under attack (even the cause of the panic), and secured the best shelters via underhanded methods, were also widespread. Moreover, there was also racial antagonism between the small Black, Indian and Jewish communities. However, the feared race riots did not transpire despite the mixing of different peoples into confined areas.
In other cities, class conflict was more evident. Over a quarter of London's population had left the city by November 1940. Civilians left for more remote areas of the country. Upsurges in population south Wales and Gloucester intimated where these displaced people went. Other reasons, including industry dispersal may have been a factor. However, resentment of rich self-evacuees or hostile treatment of poor ones were signs of persistence of class resentments although these factors did not appear to threaten social order. The total number of evacuees numbered 1.4 million, including a high proportion from the poorest inner-city families. Reception committees were completely unprepared for the condition of some of the children. Far from displaying the nation's unity in time of war, the scheme backfired, often aggravating class antagonism and bolstering prejudice about the urban poor. Within four months, 88% of evacuated mothers, 86% of small children, and 43% of school children had been returned home. The lack of bombing in the Phoney War contributed significantly to the return of people to the cities, but class conflict was not eased a year later when evacuation operations had to be put into effect again.
In recent years a large number of wartime recordings relating to the Blitz have been made available on audiobooks such as The Blitz, The Home Front and British War Broadcasting. These collections include period interviews with civilians, servicemen, aircrew, politicians and Civil Defence personnel, as well as Blitz actuality recordings, news bulletins and public information broadcasts. Notable interviews include Thomas Alderson, the first recipient of the George Cross, John Cormack, who survived eight days trapped beneath rubble on Clydeside, and Herbert Morrison's famous "Britain shall not burn" appeal for more fireguards in December 1940.
The Endangered Species Act of 1973 (ESA; 16 U.S.C. § 1531 et seq.) is one of the few dozens of United States environmental laws passed in the 1970s, and serves as the enacting legislation to carry out the provisions outlined in The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). The ESA was signed into law by President Richard Nixon on December 28, 1973, it was designed to protect critically imperiled species from extinction as a "consequence of economic growth and development untempered by adequate concern and conservation." The U.S. Supreme Court found that "the plain intent of Congress in enacting" the ESA "was to halt and reverse the trend toward species extinction, whatever the cost." The Act is administered by two federal agencies, the United States Fish and Wildlife Service (FWS) and the National Oceanic and Atmospheric Administration (NOAA).
One species in particular received widespread attention—the whooping crane. The species' historical range extended from central Canada South to Mexico, and from Utah to the Atlantic coast. Unregulated hunting and habitat loss contributed to a steady decline in the whooping crane population until, by 1890, it had disappeared from its primary breeding range in the north central United States. It would be another eight years before the first national law regulating wildlife commerce was signed, and another two years before the first version of the endangered species act was passed. The whooping crane population by 1941 was estimated at about only 16 birds still in the wild.
The Lacey Act of 1900 was the first federal law that regulated commercial animal markets. It prohibited interstate commerce of animals killed in violation of state game laws, and covered all fish and wildlife and their parts or products, as well as plants. Other legislation followed, including the Migratory Bird Conservation Act of 1929, a 1937 treaty prohibiting the hunting of right and gray whales, and the Bald Eagle Protection Act of 1940. These later laws had a low cost to society–the species were relatively rare–and little opposition was raised.
It authorized the Secretary of the Interior to list endangered domestic fish and wildlife and allowed the United States Fish and Wildlife Service to spend up to $15 million per year to buy habitats for listed species. It also directed federal land agencies to preserve habitat on their lands. The Act also consolidated and even expanded authority for the Secretary of the Interior to manage and administer the National Wildlife Refuge System. Other public agencies were encouraged, but not required, to protect species. The act did not address the commerce in endangered species and parts.
This first list is referred to as the "Class of '67" in The Endangered Species Act at Thirty, Volume 1, which concludes that habitat destruction, the biggest threat to those 78 species, is still the same threat to the currently listed species. It included only vertebrates because the Department of Interior's definition of "fish and wildlife" was limited to vertebrates. However, with time, researchers noticed that the animals on the endangered species list still were not getting enough protection, thus further threatening their extinction. The endangered species program was expanded by the Endangered Species Act of 1969.
The Endangered Species Conservation Act (P. L. 91-135), passed in December, 1969, amended the original law to provide additional protection to species in danger of "worldwide extinction" by prohibiting their importation and subsequent sale in the United States. It expanded the Lacey Act's ban on interstate commerce to include mammals, reptiles, amphibians, mollusks and crustaceans. Reptiles were added mainly to reduce the rampant poaching of alligators and crocodiles. This law was the first time that invertebrates were included for protection.
President Richard Nixon declared current species conservation efforts to be inadequate and called on the 93rd United States Congress to pass comprehensive endangered species legislation. Congress responded with a completely rewritten law, the Endangered Species Act of 1973 which was signed by Nixon on December 28, 1973 (Pub.L. 93–205). It was written by a team of lawyers and scientists, including the first appointed head of the Council on Environmental Quality (CEQ),an outgrowth of NEPA (The "National Environmental Policy Act of 1969") Dr. Russell E. Train. Dr. Train was assisted by a core group of staffers, including Dr. Earl Baysinger at EPA (currently Assistant Chief, Office of Endangered Species and International. Activities), Dick Gutting (U.S. Commerce Dept. lawyer, currently joined NOAA the previous year (1972), and Dr. Gerard A. "Jerry" Bertrand, a marine biologist (Ph.D, Oregon State University) by training, who had transferred from his post as the Scientific Adviser to the U.S Army Corps of Engineers, office of the Commandant of the Corp. to join the newly formed White House office. The staff, under Dr. Train's leadership, incorporated dozens of new principles and ideas into the landmark legislation; crafting a document that completely changed the direction of environmental conservation in the United States. Dr. Bertrand is credited with writing the most challenged section of the Act, the "takings" clause - Section 2.
A species can be listed in two ways. The United States Fish and Wildlife Service (FWS) or NOAA Fisheries (also called the National Marine Fisheries Service) can directly list a species through its candidate assessment program, or an individual or organizational petition may request that the FWS or NMFS list a species. A "species" under the act can be a true taxonomic species, a subspecies, or in the case of vertebrates, a "distinct population segment." The procedures are the same for both types except with the person/organization petition, there is a 90-day screening period.
During the listing process, economic factors cannot be considered, but must be " based solely on the best scientific and commercial data available." The 1982 amendment to the ESA added the word "solely" to prevent any consideration other than the biological status of the species. Congress rejected President Ronald Reagan's Executive Order 12291 which required economic analysis of all government agency actions. The House committee's statement was "that economic considerations have no relevance to determinations regarding the status of species."
Public notice is given through legal notices in newspapers, and communicated to state and county agencies within the species' area. Foreign nations may also receive notice of a listing. A public hearing is mandatory if any person has requested one within 45 days of the published notice. "The purpose of the notice and comment requirement is to provide for meaningful public participation in the rulemaking process." summarized the Ninth Circuit court in the case of Idaho Farm Bureau Federation v. Babbitt.
The provision of the law in Section 4 that establishes critical habitat is a regulatory link between habitat protection and recovery goals, requiring the identification and protection of all lands, water and air necessary to recover endangered species. To determine what exactly is critical habitat, the needs of open space for individual and population growth, food, water, light or other nutritional requirements, breeding sites, seed germination and dispersal needs, and lack of disturbances are considered.
All federal agencies are prohibited from authorizing, funding or carrying out actions that "destroy or adversely modify" critical habitats (Section 7(a) (2)). While the regulatory aspect of critical habitat does not apply directly to private and other non-federal landowners, large-scale development, logging and mining projects on private and state land typically require a federal permit and thus become subject to critical habitat regulations. Outside or in parallel with regulatory processes, critical habitats also focus and encourage voluntary actions such as land purchases, grant making, restoration, and establishment of reserves.
The ESA requires that critical habitat be designated at the time of or within one year of a species being placed on the endangered list. In practice, most designations occur several years after listing. Between 1978 and 1986 the FWS regularly designated critical habitat. In 1986 the Reagan Administration issued a regulation limiting the protective status of critical habitat. As a result, few critical habitats were designated between 1986 and the late 1990s. In the late 1990s and early 2000s, a series of court orders invalidated the Reagan regulations and forced the FWS and NMFS to designate several hundred critical habitats, especially in Hawaii, California and other western states. Midwest and Eastern states received less critical habitat, primarily on rivers and coastlines. As of December, 2006, the Reagan regulation has not yet been replaced though its use has been suspended. Nonetheless, the agencies have generally changed course and since about 2005 have tried to designate critical habitat at or near the time of listing.
Fish and Wildlife Service (FWS) and National Marine Fisheries Service (NMFS) are required to create an Endangered Species Recovery Plan outlining the goals, tasks required, likely costs, and estimated timeline to recover endangered species (i.e., increase their numbers and improve their management to the point where they can be removed from the endangered list). The ESA does not specify when a recovery plan must be completed. The FWS has a policy specifying completion within three years of the species being listed, but the average time to completion is approximately six years. The annual rate of recovery plan completion increased steadily from the Ford administration (4) through Carter (9), Reagan (30), Bush I (44), and Clinton (72), but declined under Bush II (16 per year as of 9/1/06).
The question to be answered is whether a listed species will be harmed by the action and, if so, how the harm can be minimized. If harm cannot be avoided, the project agency can seek an exemption from the Endangered Species Committee, an ad hoc panel composed of members from the executive branch and at least one appointee from the state where the project is to occur. Five of the seven committee members must vote for the exemption to allow taking (to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or significant habitat modification, or to attempt to engage in any such conduct) of listed species.
Long before the exemption is considered by the Endangered Species Committee, the Forest Service, and either the FWS or the NMFS will have consulted on the biological implications of the timber harvest. The consultation can be informal, to determine if harm may occur; and then formal if the harm is believed to be likely. The questions to be answered in these consultations are whether the species will be harmed, whether the habitat will be harmed and if the action will aid or hinder the recovery of the listed species.
There have been six instances as of 2009 in which the exemption process was initiated. Of these six, one was granted, one was partially granted, one was denied and three were withdrawn. Donald Baur, in The Endangered Species Act: law, policy, and perspectives, concluded," ... the exemption provision is basically a nonfactor in the administration of the ESA. A major reason, of course, is that so few consultations result in jeopardy opinions, and those that do almost always result in the identification of reasonable and prudent alternatives to avoid jeopardy."
More than half of habitat for listed species is on non-federal property, owned by citizens, states, local governments, tribal governments and private organizations. Before the law was amended in 1982, a listed species could be taken only for scientific or research purposes. The amendment created a permit process to circumvent the take prohibition called a Habitat Conservation Plan or HCP to give incentives to non-federal land managers and private landowners to help protect listed and unlisted species, while allowing economic development that may harm ("take") the species.
The person or organization submits a HCP and if approved by the agency (FWS or NMFS), will be issued an Incidental Take Permit (ITP) which allows a certain number of "takes" of the listed species. The permit may be revoked at any time and can allow incidental takes for varying amounts of time. For instance, the San Bruno Habitat Conservation Plan/ Incidental Take Permit is good for 30 years and the Wal-Mart store (in Florida) permit expires after one year. Because the permit is issued by a federal agency to a private party, it is a federal action-which means other federal laws can apply, such as the National Environmental Policy Act or NEPA. A notice of the permit application action is published in the Federal Register and a public comment period of 30 to 90 days begins.
The US Congress was urged to create the exemption by proponents of a conservation plan on San Bruno Mountain, California that was drafted in the early 1980s and is the first HCP in the nation. In the conference report on the 1982 amendments, Congress specified that it intended the San Bruno plan to act "as a model" for future conservation plans developed under the incidental take exemption provision and that "the adequacy of similar conservation plans should be measured against the San Bruno plan". Congress further noted that the San Bruno plan was based on "an independent exhaustive biological study" and protected at least 87% of the habitat of the listed butterflies that led to the development of the HCP.
Growing scientific recognition of the role of private lands for endangered species recovery and the landmark 1981 court decision in Palila v. Hawaii Department of Land and Natural Resources both contributed to making Habitat Conservation Plans/ Incidental Take Permits "a major force for wildlife conservation and a major headache to the development community", wrote Robert D.Thornton in the 1991 Environmental Law article, Searching for Consensus and Predictability: Habitat Conservation Planning under the Endangered Species Act of 1973.
The "No Surprises" rule is meant to protect the landowner if "unforeseen circumstances" occur which make the landowner's efforts to prevent or mitigate harm to the species fall short. The "No Surprises" policy may be the most controversial of the recent reforms of the law, because once an Incidental Take Permit is granted, the Fish and Wildlife Service (FWS) loses much ability to further protect a species if the mitigation measures by the landowner prove insufficient. The landowner or permittee would not be required to set aside additional land or pay more in conservation money. The federal government would have to pay for additional protection measures.
The "Safe Harbor" agreement is a voluntary agreement between the private landowner and FWS. The landowner agrees to alter the property to benefit or even attract a listed or proposed species in exchange for assurances that the FWS will permit future "takes" above a pre-determined level. The policy relies on the "enhancement of survival" provision of Section §1539(a)(1)(A). A landowner can have either a "Safe Harbor" agreement or an Incidental Take Permit, or both. The policy was developed by the Clinton Administration in 1999.
The Candidate Conservation Agreement is closely related to the "Safe Harbor" agreement, the main difference is that the Candidate Conservation Agreements With Assurances(CCA) are meant to protect unlisted species by providing incentives to private landowners and land managing agencies to restore, enhance or maintain habitat of unlisted species which are declining and have the potential to become threatened or endangered if critical habitat is not protected. The FWS will then assure that if, in the future the unlisted species becomes listed, the landowner will not be required to do more than already agreed upon in the CCA.
Two examples of animal species recently delisted are: the Virginia northern flying squirrel (subspecies) on August, 2008, which had been listed since 1985, and the gray wolf (Northern Rocky Mountain DPS). On April 15, 2011, President Obama signed the Department of Defense and Full-Year Appropriations Act of 2011. A section of that Appropriations Act directed the Secretary of the Interior to reissue within 60 days of enactment the final rule published on April 2, 2009, that identified the Northern Rocky Mountain population of gray wolf (Canis lupus) as a distinct population segment (DPS) and to revise the List of Endangered and Threatened Wildlife by removing most of the gray wolves in the DPS.
As of September 2012, fifty-six species have been delisted; twenty-eight due to recovery, ten due to extinction (seven of which are believed to have been extinct prior to being listed), ten due to changes in taxonomic classification practices, six due to discovery of new populations, one due to an error in the listing rule, and one due to an amendment to the Endangered Species Act specifically requiring the species delisting. Twenty-five others have been down listed from "endangered" to "threatened" status.
Opponents of the Endangered Species Act argue that with over 2,000 endangered species listed, and only 28 delisted due to recovery, the success rate of 1% over nearly three decades proves that there needs to be serious reform in their methods to actually help the endangered animals and plants. Others argue that the ESA may encourage preemptive habitat destruction by landowners who fear losing the use of their land because of the presence of an endangered species; known colloquially as "Shoot, Shovel and Shut-Up." One example of such perverse incentives is the case of a forest owner who, in response to ESA listing of the red-cockaded woodpecker, increased harvesting and shortened the age at which he harvests his trees to ensure that they do not become old enough to become suitable habitat. While no studies have shown that the Act's negative effects, in total, exceed the positive effects, many economists believe that finding a way to reduce such perverse incentives would lead to more effective protection of endangered species.
According to research published in 1999 by Alan Green and the Center for Public Integrity (CPI), loopholes in the ESA are commonly exploited in the exotic pet trade. Although the legislation prohibits interstate and foreign transactions for list species, no provisions are made for in-state commerce, allowing these animals to be sold to roadside zoos and private collectors. Additionally, the ESA allows listed species to be shipped across state lines as long as they are not sold. According to Green and the CPI, this allows dealers to "donate" listed species through supposed "breeding loans" to anyone, and in return they can legally receive a reciprocal monetary "donation" from the receiving party. Furthermore, an interview with an endangered species specialist at the US Fish and Wildlife Service revealed that the agency does not have sufficient staff to perform undercover investigations, which would catch these false "donations" and other mislabeled transactions.
Green and the CPI further noted another exploit of the ESA in their discussion of the critically endangered cotton-top tamarin (Saguinus oedipus). Not only had they found documentation that 151 of these primates had inadvertently made their way from the Harvard-affiliated New England Regional Primate Research Center into the exotic pet trade through the aforementioned loophole, but in October 1976, over 800 cotton-top tamarins were imported into the United States in order to beat the official listing of the species under the ESA.
Section 6 of the Endangered Species Act provided funding for development of programs for management of threatened and endangered species by state wildlife agencies. Subsequently, lists of endangered and threatened species within their boundaries have been prepared by each state. These state lists often include species which are considered endangered or threatened within a specific state but not within all states, and which therefore are not included on the national list of endangered and threatened species. Examples include Florida, Minnesota, Maine, and California.
A reward will be paid to any person who furnishes information which leads to an arrest, conviction, or revocation of a license, so long as they are not a local, state, or federal employee in the performance of official duties. The Secretary may also provide reasonable and necessary costs incurred for the care of fish, wildlife, and forest service or plant pending the violation caused by the criminal. If the balance ever exceeds $500,000 the Secretary of the Treasury is required to deposit an amount equal to the excess into the cooperative endangered species conservation fund.
Vacuum is space void of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum.
The quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (10−12) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm3. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. According to modern understanding, even if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics. In the electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether. In modern particle physics, the vacuum state is considered the ground state of matter.
Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism, which posited void and atom as the fundamental explanatory elements of physics. Following Plato, even the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, itself, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite literally nothing at all, which cannot rightly be said to exist. Aristotle believed that no void could occur naturally, because the denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void.
In his Physics, book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continue ad infinitum, there being no reason that something would come to rest anywhere in particular. Although Lucretius argued for the existence of vacuum in the first century BC and Hero of Alexandria tried unsuccessfully to create an artificial vacuum in the first century AD, it was European scholars such as Roger Bacon, Blasius of Parma and Walter Burley in the 13th and 14th century who focused considerable attention on these issues. Eventually following Stoic physics in this instance, scholars from the 14th century onward increasingly departed from the Aristotelian perspective in favor of a supernatural void beyond the confines of the cosmos itself, a conclusion widely acknowledged by the 17th century, which helped to segregate natural and theological concerns.
Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his or her breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs. Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia. Injuries caused by rapid decompression are called barotrauma. A pressure drop of 13 kPa (100 Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.
Almost two thousand years after Plato, René Descartes also proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of his namesake coordinate system and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct. With the acquiescence of Cartesian mechanical philosophy to the "brute fact" of action at a distance, and at length, its successful reification by force fields and ever more sophisticated geometric structure, the anachronism of empty space widened until "a seething ferment" of quantum activity in the 20th century filled the vacuum with a virtual pleroma.
In 1930, Paul Dirac proposed a model of the vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea. This theory helped refine the predictions of his earlier formulated Dirac equation, and successfully predicted the existence of the positron, confirmed two years later. Werner Heisenberg's uncertainty principle formulated in 1927, predict a fundamental limit within which instantaneous position and momentum, or energy and time can be measured. This has far reaching consequences on the "emptiness" of space between particles. In the late 20th century, so-called virtual particles that arise spontaneously from empty space were confirmed.
In general relativity, a vanishing stress-energy tensor implies, through Einstein field equations, the vanishing of all the components of the Ricci tensor. Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and gravitational waves (technically, these phenomena are the components of the Weyl tensor). The black hole (with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.
But although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites. Most artificial satellites operate in this region called low Earth orbit and must fire their engines every few days to maintain orbit.[citation needed] The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails, a proposed propulsion system for interplanetary travel.[citation needed] Planets are too massive for their trajectories to be significantly affected by these forces, although their atmospheres are eroded by the solar winds.
In the medieval Middle Eastern world, the physicist and Islamic scholar, Al-Farabi (Alpharabius, 872–950), conducted a small experiment concerning the existence of vacuum, in which he investigated handheld plungers in water.[unreliable source?] He concluded that air's volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent. However, according to Nader El-Bizri, the physicist Ibn al-Haytham (Alhazen, 965–1039) and the Mu'tazili theologians disagreed with Aristotle and Al-Farabi, and they supported the existence of a void. Using geometry, Ibn al-Haytham mathematically demonstrated that place (al-makan) is the imagined three-dimensional void between the inner surfaces of a containing body. According to Ahmad Dallal, Abū Rayhān al-Bīrūnī also states that "there is no observable evidence that rules out the possibility of vacuum". The suction pump later appeared in Europe from the 15th century.
Medieval thought experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, as Walter Burley postulated, whether a 'celestial agent' prevented the vacuum arising. The commonly held view that nature abhorred a vacuum was called horror vacui. Speculation that even God could not create a vacuum if he wanted to was shut down[clarification needed] by the 1277 Paris condemnations of Bishop Etienne Tempier, which required there to be no restrictions on the powers of God, which led to the conclusion that God could create a vacuum if he so wished. Jean Buridan reported in the 14th century that teams of ten horses could not pull open bellows when the port was sealed.
In 1654, Otto von Guericke invented the first vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that teams of horses could not separate two hemispheres from which the air had been partially evacuated. Robert Boyle improved Guericke's design and with the help of Robert Hooke further developed vacuum pump technology. Thereafter, research into the partial vacuum lapsed until 1850 when August Toepler invented the Toepler Pump and Heinrich Geissler invented the mercury displacement pump in 1855, achieving a partial vacuum of about 10 Pa (0.1 Torr). A number of electrical properties become observable at this vacuum level, which renewed interest in further research.
While outer space provides the most rarefied example of a naturally occurring partial vacuum, the heavens were originally thought to be seamlessly filled by a rigid indestructible material called aether. Borrowing somewhat from the pneuma of Stoic physics, aether came to be regarded as the rarefied air from which it took its name, (see Aether (mythology)). Early theories of light posited a ubiquitous terrestrial and celestial medium through which light propagated. Additionally, the concept informed Isaac Newton's explanations of both refraction and of radiant heat. 19th century experiments into this luminiferous aether attempted to detect a minute drag on the Earth's orbit. While the Earth does, in fact, move through a relatively dense medium in comparison to that of interstellar space, the drag is so minuscule that it could not be detected. In 1912, astronomer Henry Pickering commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".
The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~6997100000000000000♠1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.
The SI unit of pressure is the pascal (symbol Pa), but vacuum is often measured in torrs, named for Torricelli, an early Italian physicist (1608–1647). A torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often measured in millimeters of mercury (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure.
Hydrostatic gauges (such as the mercury column manometer) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 torr (100 Pa) to above atmospheric. An important variation is the McLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10−6 torr (0.1 mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge.
Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 torr to 10−3 torr, but they are sensitive to the chemical composition of the gases being measured.
Ion gauges are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In the hot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10−3 torr to 10−10 torr. The principle behind cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10−2 torr to 10−9 torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.
Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level atmosphere. The colder air temperatures found at altitudes of up to 3 km generally compensate for the lower pressures there. Above this altitude, oxygen enrichment is necessary to prevent altitude sickness in humans that did not undergo prior acclimatization, and spacesuits are necessary to prevent ebullism above 19 km. Most spacesuits use only 20 kPa (150 Torr) of pure oxygen. This pressure is high enough to prevent ebullism, but decompression sickness and gas embolisms can still occur if decompression rates are not managed.
Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of hypoxia within minutes, but the symptoms are not nearly as graphic as commonly depicted in media and popular culture. The reduction in pressure lowers the temperature at which blood and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains above the internal body temperature of 37 °C. Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known as ebullism, is still a concern. The gas may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Swelling and ebullism can be restrained by containment in a flight suit. Shuttle astronauts wore a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa (15 Torr). Rapid boiling will cool the skin and create frost, particularly in the mouth, but this is not a significant hazard.
In ultra high vacuum systems, some very "odd" leakage paths and outgassing sources must be considered. The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and greases will boil off in extreme vacuums. The permeability of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.
In quantum mechanics and quantum field theory, the vacuum is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (the ground state of the Hilbert space). In quantum electrodynamics this vacuum is referred to as 'QED vacuum' to distinguish it from the vacuum of quantum chromodynamics, denoted as QCD vacuum. QED vacuum is a state with no matter particles (hence the name), and also no photons. As described above, this state is impossible to achieve experimentally. (Even if every matter particle could somehow be removed from a volume, it would be impossible to eliminate all the blackbody photons.) Nonetheless, it provides a good model for realizable vacuum, and agrees with a number of experimental observations as described next.
QED vacuum has interesting and complex properties. In QED vacuum, the electric and magnetic fields have zero average values, but their variances are not zero. As a result, QED vacuum contains vacuum fluctuations (virtual particles that hop into and out of existence), and a finite energy called vacuum energy. Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some experimentally verified effects of vacuum fluctuations include spontaneous emission and the Lamb shift. Coulomb's law and the electric potential in vacuum near an electric charge are modified.
Stars, planets, and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 6998320000000000000♠3.2×10−2 Pa at 100 kilometres (62 mi) of altitude, the Kármán line, which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar winds, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre.
Vacuum is useful in a variety of processes and devices. Its first widespread use was in the incandescent light bulb to protect the filament from chemical degradation. The chemical inertness produced by a vacuum is also useful for electron beam welding, cold welding, vacuum packing and vacuum frying. Ultra-high vacuum is used in the study of atomically clean substrates, as only a very good vacuum preserves atomic-scale clean surfaces for a reasonably long time (on the order of minutes to days). High to ultra-high vacuum removes the obstruction of air, allowing particle beams to deposit or remove materials without contamination. This is the principle behind chemical vapor deposition, physical vapor deposition, and dry etching which are essential to the fabrication of semiconductors and optical coatings, and to surface science. The reduction of convection provides the thermal insulation of thermos bottles. Deep vacuum lowers the boiling point of liquids and promotes low temperature outgassing which is used in freeze drying, adhesive preparation, distillation, metallurgy, and process purging. The electrical properties of vacuum make electron microscopes and vacuum tubes possible, including cathode ray tubes. The elimination of air friction is useful for flywheel energy storage and ultracentrifuges.
Manifold vacuum can be used to drive accessories on automobiles. The best-known application is the vacuum servo, used to provide power assistance for the brakes. Obsolete applications include vacuum-driven windscreen wipers and Autovac fuel pumps. Some aircraft instruments (Attitude Indicator (AI) and the Heading Indicator (HI)) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments, since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft—the engine and an external venturi. Vacuum induction melting uses electromagnetic induction within a vacuum.
Evaporation and sublimation into a vacuum is called outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. In man-made systems, outgassing has the same effect as a leak and can limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they obscure optical instruments or react with other materials. This is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.
To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind positive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.
The above explanation is merely a simple introduction to vacuum pumping, and is not representative of the entire range of pumps in use. Many variations of the positive displacement pump have been developed, and many other pump designs rely on fundamentally different principles. Momentum transfer pumps, which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps. Entrapment pumps can capture gases in a solid or absorbed state, often with no moving parts, no seals and no vibration. None of these pumps are universal; each type has important performance limitations. They all share a difficulty in pumping low molecular weight gases, especially hydrogen, helium, and neon.
The lowest pressure that can be attained in a system is also dependent on many things other than the nature of the pumps. Multiple pumps may be connected in series, called stages, to achieve higher vacuums. The choice of seals, chamber geometry, materials, and pump-down procedures will all have an impact. Collectively, these are called vacuum technique. And sometimes, the final pressure is not the only relevant characteristic. Pumping systems differ in oil contamination, vibration, preferential pumping of certain gases, pump-down speeds, intermittent duty cycle, reliability, or tolerance to high leakage rates.
Fluids cannot generally be pulled, so a vacuum cannot be created by suction. Suction can spread and dilute a vacuum by letting a higher pressure push fluids into it, but the vacuum has to be created first before suction can occur. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.
The Han dynasty (Chinese: 漢朝; pinyin: Hàn cháo) was the second imperial dynasty of China, preceded by the Qin dynasty (221–207 BC) and succeeded by the Three Kingdoms period (220–280 AD). Spanning over four centuries, the Han period is considered a golden age in Chinese history. To this day, China's majority ethnic group refers to itself as the "Han people" and the Chinese script is referred to as "Han characters". It was founded by the rebel leader Liu Bang, known posthumously as Emperor Gaozu of Han, and briefly interrupted by the Xin dynasty (9–23 AD) of the former regent Wang Mang. This interregnum separates the Han dynasty into two periods: the Western Han or Former Han (206 BC – 9 AD) and the Eastern Han or Later Han (25–220 AD).
The emperor was at the pinnacle of Han society. He presided over the Han government but shared power with both the nobility and appointed ministers who came largely from the scholarly gentry class. The Han Empire was divided into areas directly controlled by the central government using an innovation inherited from the Qin known as commanderies, and a number of semi-autonomous kingdoms. These kingdoms gradually lost all vestiges of their independence, particularly following the Rebellion of the Seven States. From the reign of Emperor Wu onward, the Chinese court officially sponsored Confucianism in education and court politics, synthesized with the cosmology of later scholars such as Dong Zhongshu. This policy endured until the fall of the Qing dynasty in AD 1911.
The Han dynasty was an age of economic prosperity and saw a significant growth of the money economy first established during the Zhou dynasty (c. 1050–256 BC). The coinage issued by the central government mint in 119 BC remained the standard coinage of China until the Tang dynasty (618–907 AD). The period saw a number of limited institutional innovations. To pay for its military campaigns and the settlement of newly conquered frontier territories, the government nationalized the private salt and iron industries in 117 BC, but these government monopolies were repealed during the Eastern Han period. Science and technology during the Han period saw significant advances, including papermaking, the nautical steering rudder, the use of negative numbers in mathematics, the raised-relief map, the hydraulic-powered armillary sphere for astronomy, and a seismometer employing an inverted pendulum.
The Xiongnu, a nomadic steppe confederation, defeated the Han in 200 BC and forced the Han to submit as a de facto inferior partner, but continued their raids on the Han borders. Emperor Wu of Han (r. 141–87 BC) launched several military campaigns against them. The ultimate Han victory in these wars eventually forced the Xiongnu to accept vassal status as Han tributaries. These campaigns expanded Han sovereignty into the Tarim Basin of Central Asia, divided the Xiongnu into two separate confederations, and helped establish the vast trade network known as the Silk Road, which reached as far as the Mediterranean world. The territories north of Han's borders were quickly overrun by the nomadic Xianbei confederation. Emperor Wu also launched successful military expeditions in the south, annexing Nanyue in 111 BC and Dian in 109 BC, and in the Korean Peninsula where the Xuantu and Lelang Commanderies were established in 108 BC.
After 92 AD, the palace eunuchs increasingly involved themselves in court politics, engaging in violent power struggles between the various consort clans of the empresses and empress dowagers, causing the Han's ultimate downfall. Imperial authority was also seriously challenged by large Daoist religious societies which instigated the Yellow Turban Rebellion and the Five Pecks of Rice Rebellion. Following the death of Emperor Ling (r. 168–189 AD), the palace eunuchs suffered wholesale massacre by military officers, allowing members of the aristocracy and military governors to become warlords and divide the empire. When Cao Pi, King of Wei, usurped the throne from Emperor Xian, the Han dynasty ceased to exist.
China's first imperial dynasty was the Qin dynasty (221–206 BC). The Qin unified the Chinese Warring States by conquest, but their empire became unstable after the death of the first emperor Qin Shi Huangdi. Within four years, the dynasty's authority had collapsed in the face of rebellion. Two former rebel leaders, Xiang Yu (d. 202 BC) of Chu and Liu Bang (d. 195 BC) of Han, engaged in a war to decide who would become hegemon of China, which had fissured into 18 kingdoms, each claiming allegiance to either Xiang Yu or Liu Bang. Although Xiang Yu proved to be a capable commander, Liu Bang defeated him at Battle of Gaixia (202 BC), in modern-day Anhui. Liu Bang assumed the title "emperor" (huangdi) at the urging of his followers and is known posthumously as Emperor Gaozu (r. 202–195 BC). Chang'an was chosen as the new capital of the reunified empire under Han.
At the beginning of the Western Han dynasty, thirteen centrally controlled commanderies—including the capital region—existed in the western third of the empire, while the eastern two-thirds were divided into ten semi-autonomous kingdoms. To placate his prominent commanders from the war with Chu, Emperor Gaozu enfeoffed some of them as kings. By 157 BC, the Han court had replaced all of these kings with royal Liu family members, since the loyalty of non-relatives to the throne was questioned. After several insurrections by Han kings—the largest being the Rebellion of the Seven States in 154 BC—the imperial court enacted a series of reforms beginning in 145 BC limiting the size and power of these kingdoms and dividing their former territories into new centrally controlled commanderies. Kings were no longer able to appoint their own staff; this duty was assumed by the imperial court. Kings became nominal heads of their fiefs and collected a portion of tax revenues as their personal incomes. The kingdoms were never entirely abolished and existed throughout the remainder of Western and Eastern Han.
To the north of China proper, the nomadic Xiongnu chieftain Modu Chanyu (r. 209–174 BC) conquered various tribes inhabiting the eastern portion of the Eurasian Steppe. By the end of his reign, he controlled Manchuria, Mongolia, and the Tarim Basin, subjugating over twenty states east of Samarkand. Emperor Gaozu was troubled about the abundant Han-manufactured iron weapons traded to the Xiongnu along the northern borders, and he established a trade embargo against the group. Although the embargo was in place, the Xiongnu found traders willing to supply their needs. Chinese forces also mounted surprise attacks against Xiongnu who traded at the border markets. In retaliation, the Xiongnu invaded what is now Shanxi province, where they defeated the Han forces at Baideng in 200 BC. After negotiations, the heqin agreement in 198 BC nominally held the leaders of the Xiongnu and the Han as equal partners in a royal marriage alliance, but the Han were forced to send large amounts of tribute items such as silk clothes, food, and wine to the Xiongnu.
Despite the tribute and a negotiation between Laoshang Chanyu (r. 174–160 BC) and Emperor Wen (r. 180–157 BC) to reopen border markets, many of the Chanyu's Xiongnu subordinates chose not to obey the treaty and periodically raided Han territories south of the Great Wall for additional goods. In a court conference assembled by Emperor Wu (r. 141–87 BC) in 135 BC, the majority consensus of the ministers was to retain the heqin agreement. Emperor Wu accepted this, despite continuing Xiongnu raids. However, a court conference the following year convinced the majority that a limited engagement at Mayi involving the assassination of the Chanyu would throw the Xiongnu realm into chaos and benefit the Han. When this plot failed in 133 BC, Emperor Wu launched a series of massive military invasions into Xiongnu territory. Chinese armies captured one stronghold after another and established agricultural colonies to strengthen their hold. The assault culminated in 119 BC at the Battle of Mobei, where the Han commanders Huo Qubing (d. 117 BC) and Wei Qing (d. 106 BC) forced the Xiongnu court to flee north of the Gobi Desert.
In 121 BC, Han forces expelled the Xiongnu from a vast territory spanning the Hexi Corridor to Lop Nur. They repelled a joint Xiongnu-Qiang invasion of this northwestern territory in 111 BC. In that year, the Han court established four new frontier commanderies in this region: Jiuquan, Zhangyi, Dunhuang, and Wuwei. The majority of people on the frontier were soldiers. On occasion, the court forcibly moved peasant farmers to new frontier settlements, along with government-owned slaves and convicts who performed hard labor. The court also encouraged commoners, such as farmers, merchants, landowners, and hired laborers, to voluntarily migrate to the frontier.
Even before Han's expansion into Central Asia, diplomat Zhang Qian's travels from 139 to 125 BC had established Chinese contacts with many surrounding civilizations. Zhang encountered Dayuan (Fergana), Kangju (Sogdiana), and Daxia (Bactria, formerly the Greco-Bactrian Kingdom); he also gathered information on Shendu (Indus River valley of North India) and Anxi (the Parthian Empire). All of these countries eventually received Han embassies. These connections marked the beginning of the Silk Road trade network that extended to the Roman Empire, bringing Han items like silk to Rome and Roman goods such as glasswares to China.
From roughly 115 to 60 BC, Han forces fought the Xiongnu over control of the oasis city-states in the Tarim Basin. Han was eventually victorious and established the Protectorate of the Western Regions in 60 BC, which dealt with the region's defense and foreign affairs. The Han also expanded southward. The naval conquest of Nanyue in 111 BC expanded the Han realm into what are now modern Guangdong, Guangxi, and northern Vietnam. Yunnan was brought into the Han realm with the conquest of the Dian Kingdom in 109 BC, followed by parts of the Korean Peninsula with the colonial establishments of Xuantu Commandery and Lelang Commandery in 108 BC. In China's first known nationwide census taken in 2 AD, the population was registered as having 57,671,400 individuals in 12,366,470 households.
To pay for his military campaigns and colonial expansion, Emperor Wu nationalized several private industries. He created central government monopolies administered largely by former merchants. These monopolies included salt, iron, and liquor production, as well as bronze-coin currency. The liquor monopoly lasted only from 98 to 81 BC, and the salt and iron monopolies were eventually abolished in early Eastern Han. The issuing of coinage remained a central government monopoly throughout the rest of the Han dynasty. The government monopolies were eventually repealed when a political faction known as the Reformists gained greater influence in the court. The Reformists opposed the Modernist faction that had dominated court politics in Emperor Wu's reign and during the subsequent regency of Huo Guang (d. 68 BC). The Modernists argued for an aggressive and expansionary foreign policy supported by revenues from heavy government intervention in the private economy. The Reformists, however, overturned these policies, favoring a cautious, non-expansionary approach to foreign policy, frugal budget reform, and lower tax-rates imposed on private entrepreneurs.
Wang Mang initiated a series of major reforms that were ultimately unsuccessful. These reforms included outlawing slavery, nationalizing land to equally distribute between households, and introducing new currencies, a change which debased the value of coinage. Although these reforms provoked considerable opposition, Wang's regime met its ultimate downfall with the massive floods of c. 3 AD and 11 AD. Gradual silt buildup in the Yellow River had raised its water level and overwhelmed the flood control works. The Yellow River split into two new branches: one emptying to the north and the other to the south of the Shandong Peninsula, though Han engineers managed to dam the southern branch by 70 AD.
The period between the foundation of the Han dynasty and Wang Mang's reign is known as the Western Han dynasty (simplified Chinese: 西汉; traditional Chinese: 西漢; pinyin: Xī Hàn) or Former Han dynasty (simplified Chinese: 前汉; traditional Chinese: 前漢; pinyin: Qiánhàn) (206 BC – 9 AD). During this period the capital was at Chang'an (modern Xi'an). From the reign of Guangwu the capital was moved eastward to Luoyang. The era from his reign until the fall of Han is known as the Eastern Han dynasty (simplified Chinese: 东汉; traditional Chinese: 東漢; pinyin: Dōng Hàn) or the Later Han dynasty (simplified Chinese: 后汉; traditional Chinese: 後漢; pinyin: Hòu Hàn) (25–220 AD).
The Eastern Han, also known as the Later Han, formally began on 5 August 25, when Liu Xiu became Emperor Guangwu of Han. During the widespread rebellion against Wang Mang, the state of Goguryeo was free to raid Han's Korean commanderies; Han did not reaffirm its control over the region until AD 30. The Trưng Sisters of Vietnam rebelled against Han in AD 40. Their rebellion was crushed by Han general Ma Yuan (d. AD 49) in a campaign from AD 42–43. Wang Mang renewed hostilities against the Xiongnu, who were estranged from Han until their leader Bi (比), a rival claimant to the throne against his cousin Punu (蒲奴), submitted to Han as a tributary vassal in AD 50. This created two rival Xiongnu states: the Southern Xiongnu led by Bi, an ally of Han, and the Northern Xiongnu led by Punu, an enemy of Han.
During the turbulent reign of Wang Mang, Han lost control over the Tarim Basin, which was conquered by the Northern Xiongnu in AD 63 and used as a base to invade Han's Hexi Corridor in Gansu. Dou Gu (d. 88 AD) defeated the Northern Xiongnu at the Battle of Yiwulu in AD 73, evicting them from Turpan and chasing them as far as Lake Barkol before establishing a garrison at Hami. After the new Protector General of the Western Regions Chen Mu (d. AD 75) was killed by allies of the Xiongnu in Karasahr and Kucha, the garrison at Hami was withdrawn. At the Battle of Ikh Bayan in AD 89, Dou Xian (d. AD 92) defeated the Northern Xiongnu chanyu who then retreated into the Altai Mountains. After the Northern Xiongnu fled into the Ili River valley in AD 91, the nomadic Xianbei occupied the area from the borders of the Buyeo Kingdom in Manchuria to the Ili River of the Wusun people. The Xianbei reached their apogee under Tanshihuai (檀石槐) (d. AD 180), who consistently defeated Chinese armies. However, Tanshihuai's confederation disintegrated after his death.
Ban Chao (d. AD 102) enlisted the aid of the Kushan Empire, occupying the area of modern India, Pakistan, Afghanistan, and Tajikistan, to subdue Kashgar and its ally Sogdiana. When a request by Kushan ruler Vima Kadphises (r. c. 90–c. 100 AD) for a marriage alliance with the Han was rejected in AD 90, he sent his forces to Wakhan (Afghanistan) to attack Ban Chao. The conflict ended with the Kushans withdrawing because of lack of supplies. In AD 91, the office of Protector General of the Western Regions was reinstated when it was bestowed on Ban Chao.
In addition to tributary relations with the Kushans, the Han Empire received gifts from the Parthian Empire, from a king in modern Burma, from a ruler in Japan, and initiated an unsuccessful mission to Daqin (Rome) in AD 97 with Gan Ying as emissary. A Roman embassy of Emperor Marcus Aurelius (r. 161–180 AD) is recorded in the Hou Hanshu to have reached the court of Emperor Huan of Han (r. AD 146–168) in AD 166, yet Rafe de Crespigny asserts that this was most likely a group of Roman merchants. Other travelers to Eastern-Han China included Buddhist monks who translated works into Chinese, such as An Shigao of Parthia, and Lokaksema from Kushan-era Gandhara, India.
Emperor Zhang's (r. 75–88 AD) reign came to be viewed by later Eastern Han scholars as the high point of the dynastic house. Subsequent reigns were increasingly marked by eunuch intervention in court politics and their involvement in the violent power struggles of the imperial consort clans. With the aid of the eunuch Zheng Zhong (d. 107 AD), Emperor He (r. 88–105 AD) had Empress Dowager Dou (d. 97 AD) put under house arrest and her clan stripped of power. This was in revenge for Dou's purging of the clan of his natural mother—Consort Liang—and then concealing her identity from him. After Emperor He's death, his wife Empress Deng Sui (d. 121 AD) managed state affairs as the regent empress dowager during a turbulent financial crisis and widespread Qiang rebellion that lasted from 107 to 118 AD.
When Empress Dowager Deng died, Emperor An (r. 106–125 AD) was convinced by the accusations of the eunuchs Li Run (李閏) and Jiang Jing (江京) that Deng and her family had planned to depose him. An dismissed Deng's clan members from office, exiled them and forced many to commit suicide. After An's death, his wife, Empress Dowager Yan (d. 126 AD) placed the child Marquess of Beixiang on the throne in an attempt to retain power within her family. However, palace eunuch Sun Cheng (d. 132 AD) masterminded a successful overthrow of her regime to enthrone Emperor Shun of Han (r. 125–144 AD). Yan was placed under house arrest, her relatives were either killed or exiled, and her eunuch allies were slaughtered. The regent Liang Ji (d. 159 AD), brother of Empress Liang Na (d. 150 AD), had the brother-in-law of Consort Deng Mengnü (later empress) (d. 165 AD) killed after Deng Mengnü resisted Liang Ji's attempts to control her. Afterward, Emperor Huan employed eunuchs to depose Liang Ji, who was then forced to commit suicide.
Students from the Imperial University organized a widespread student protest against the eunuchs of Emperor Huan's court. Huan further alienated the bureaucracy when he initiated grandiose construction projects and hosted thousands of concubines in his harem at a time of economic crisis. Palace eunuchs imprisoned the official Li Ying (李膺) and his associates from the Imperial University on a dubious charge of treason. In 167 AD, the Grand Commandant Dou Wu (d. 168 AD) convinced his son-in-law, Emperor Huan, to release them. However the emperor permanently barred Li Ying and his associates from serving in office, marking the beginning of the Partisan Prohibitions.
Following Huan's death, Dou Wu and the Grand Tutor Chen Fan (陳蕃) (d. 168 AD) attempted a coup d'état against the eunuchs Hou Lan (d. 172 AD), Cao Jie (d. 181 AD), and Wang Fu (王甫). When the plot was uncovered, the eunuchs arrested Empress Dowager Dou (d. 172 AD) and Chen Fan. General Zhang Huan (張奐) favored the eunuchs. He and his troops confronted Dou Wu and his retainers at the palace gate where each side shouted accusations of treason against the other. When the retainers gradually deserted Dou Wu, he was forced to commit suicide.
The Partisan Prohibitions were repealed during the Yellow Turban Rebellion and Five Pecks of Rice Rebellion in 184 AD, largely because the court did not want to continue to alienate a significant portion of the gentry class who might otherwise join the rebellions. The Yellow Turbans and Five-Pecks-of-Rice adherents belonged to two different hierarchical Daoist religious societies led by faith healers Zhang Jue (d. 184 AD) and Zhang Lu (d. 216 AD), respectively. Zhang Lu's rebellion, in modern northern Sichuan and southern Shaanxi, was not quelled until 215 AD. Zhang Jue's massive rebellion across eight provinces was annihilated by Han forces within a year, however the following decades saw much smaller recurrent uprisings. Although the Yellow Turbans were defeated, many generals appointed during the crisis never disbanded their assembled militia forces and used these troops to amass power outside of the collapsing imperial authority.
General-in-Chief He Jin (d. 189 AD), half-brother to Empress He (d. 189 AD), plotted with Yuan Shao (d. 202 AD) to overthrow the eunuchs by having several generals march to the outskirts of the capital. There, in a written petition to Empress He, they demanded the eunuchs' execution. After a period of hesitation, Empress He consented. When the eunuchs discovered this, however, they had her brother He Miao (何苗) rescind the order. The eunuchs assassinated He Jin on September 22, 189 AD. Yuan Shao then besieged Luoyang's Northern Palace while his brother Yuan Shu (d. 199 AD) besieged the Southern Palace. On September 25 both palaces were breached and approximately two thousand eunuchs were killed. Zhang Rang had previously fled with Emperor Shao (r. 189 AD) and his brother Liu Xie—the future Emperor Xian of Han (r. 189–220 AD). While being pursued by the Yuan brothers, Zhang committed suicide by jumping into the Yellow River.
General Dong Zhuo (d. 192 AD) found the young emperor and his brother wandering in the countryside. He escorted them safely back to the capital and was made Minister of Works, taking control of Luoyang and forcing Yuan Shao to flee. After Dong Zhuo demoted Emperor Shao and promoted his brother Liu Xie as Emperor Xian, Yuan Shao led a coalition of former officials and officers against Dong, who burned Luoyang to the ground and resettled the court at Chang'an in May 191 AD. Dong Zhuo later poisoned Emperor Shao.
After Cao's defeat at the naval Battle of Red Cliffs in 208 AD, China was divided into three spheres of influence, with Cao Cao dominating the north, Sun Quan (182–252 AD) dominating the south, and Liu Bei (161–223 AD) dominating the west. Cao Cao died in March 220 AD. By December his son Cao Pi (187–226 AD) had Emperor Xian relinquish the throne to him and is known posthumously as Emperor Wen of Wei. This formally ended the Han dynasty and initiated an age of conflict between three states: Cao Wei, Eastern Wu, and Shu Han.
Each successive rank gave its holder greater pensions and legal privileges. The highest rank, of full marquess, came with a state pension and a territorial fiefdom. Holders of the rank immediately below, that of ordinary marquess, received a pension, but had no territorial rule. Officials who served in government belonged to the wider commoner social class and were ranked just below nobles in social prestige. The highest government officials could be enfeoffed as marquesses. By the Eastern Han period, local elites of unattached scholars, teachers, students, and government officials began to identify themselves as members of a larger, nationwide gentry class with shared values and a commitment to mainstream scholarship. When the government became noticeably corrupt in mid-to-late Eastern Han, many gentrymen even considered the cultivation of morally grounded personal relationships more important than serving in public office.
The farmer, or specifically the small landowner-cultivator, was ranked just below scholars and officials in the social hierarchy. Other agricultural cultivators were of a lower status, such as tenants, wage laborers, and in rare cases slaves. Artisans and craftsmen had a legal and socioeconomic status between that of owner-cultivator farmers and common merchants. State-registered merchants, who were forced by law to wear white-colored clothes and pay high commercial taxes, were considered by the gentry as social parasites with a contemptible status. These were often petty shopkeepers of urban marketplaces; merchants such as industrialists and itinerant traders working between a network of cities could avoid registering as merchants and were often wealthier and more powerful than the vast majority of government officials. Wealthy landowners, such as nobles and officials, often provided lodging for retainers who provided valuable work or duties, sometimes including fighting bandits or riding into battle. Unlike slaves, retainers could come and go from their master's home as they pleased. Medical physicians, pig breeders, and butchers had a fairly high social status, while occultist diviners, runners, and messengers had low status.

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