title: Count Dooku

text: In the 2008 animated series "", set in between "Attack of the Clones" and "Revenge of the Sith", Dooku is the political leader of the Separatists, and the main antagonist. In addition to sending Grievous and Ventress on missions to antagonize the Republic, he works with the terrorist group Death Watch to give the Republic a reason to send a military presence to Mandalore, which would play in his favor. The plan falls through when Duchess Satine Kryze (Anna Graves) of Mandalore urges the Galactic Senate to hold off a military force.


title: Count Dooku

text: In the third season, Dooku is forced to eliminate his apprentice Ventress to prove his loyalty to Darth Sidious. Ventress survives, however, and works with Mother Talzin (Barbara Goodson) to kill Dooku by giving him Savage Opress (Clancy Brown) as a replacement apprentice. During a confrontation between Dooku and Ventress, Savage turns on both. In the fourth season, Dooku defeats Anakin in three separate lightsaber duels, and gets his revenge on Ventress by having General Grievous order the systematic genocide of the Nightsisters. In the fifth season, Dooku plays minor roles via hologram in guiding King Rash of Onderon and Grievous taking over Florrum.


title: Count Dooku

text: In the sixth season, Dooku finds out the clone trooper Tup executed Order 66 prematurely and works behind the scenes to stop the Republic's investigation. He then manipulates the Banking Clan and its representative Rush Clovis (Robin Atkin Downes) into putting all their resources in the hands of the Sith, bringing war to the planet Scipio. Later, the Jedi find a lightsaber belonging to deceased Jedi Master Sifo-Dyas—whom Dooku murdered years earlier—and start an investigation. Sidious forces Dooku to clean up their trail. Dooku confronts Anakin and Obi-Wan on Oba Diah, revealing his alter-ego Darth Tyranus to the Jedi, and they realize that it was he who created the clone army. Some further investigation by Yoda prompts Dooku and Sidious to perform a Sith ritual in an unsuccessful attempt to break the Jedi Master; in a vision experienced by Yoda, Dooku fights Anakin, who swiftly defeats and executes him, in a manner very similar to his eventual demise.


title: Count Dooku

text: Count Dooku does not directly appear in the seventh season. However, as the events of "Revenge of the Sith" are concurrent with the season's final arc, Ahsoka Tano is informed of his demise at Anakin's hands by Obi-Wan, who tells her that it is important to capture Maul, being the only way the Jedi can discover the true identity of Darth Sidious after Dooku's death.


title: Count Dooku

text: A younger Count Dooku serves as one of the two protagonists of animated miniseries "Tales of the Jedi", voiced once again by Corey Burton. He is the focus of the second, third, and fourth episodes, which depict his life as a Jedi and his eventual fall to the dark side. The miniseries also focuses on Dooku's friendship with his padawan Qui-Gon Jinn, with original actor Liam Neeson returning to voice the character.


title: Count Dooku

text: An unfinished sixth season arc of "The Clone Wars" was adapted into the 2015 novel "Dark Disciple" by Christie Golden. In the story, Dooku commands a genocidal attack on the planet Mahranee, leading the Jedi Council to send Master Quinlan Vos on a mission to assassinate Dooku. Vos partners with Dooku's former apprentice Asajj Ventress for the mission. The two confront Dooku on the planet Raxus Secundus, where Dooku was being awarded at a ceremony, but are no match for the Sith Lord, who captures Vos and forces Ventress to flee. Dooku takes Vos to Serenno, where he tortures him, eventually turning him to the dark side. Sometime later, the Jedi Council liberates Vos with the assistance of Ventress. However, upon deducing that he has turned to the dark side, the Council sends him to face Dooku again as a test of loyalty, with Obi-Wan Kenobi and Anakin Skywalker tailing him. Vos defeats Dooku, but spares his life and instead demands Dooku to lead him to Darth Sidious. Obi-Wan and Anakin arrest them and take them to Kenobi's "Venator"-class Star Destroyer. Vos and Dooku escape aboard Ventress' ship, killing two Jedi and many clone troopers in the process. They are shot down and crash-land on Christophsis, where they seek refuge in a Separatist tower, which is soon under attack by Jedi and Republic forces. Dooku attempts to kill Vos with Force lightning, but Ventress sacrifices her life and saves Vos. After killing Ventress, Dooku escapes the planet.


title: Count Dooku

text: In the audiobook "Dooku: Jedi Lost" by Cavan Scott, Dooku meets his sister Jenza, brother Ramil, and father Count Gora, when he visits his homeworld of Serenno as a Jedi Initiate. Dooku begins corresponding with Jenza, keeping their communications a secret for years. The audiobook also details several adventures Dooku has with his best friend, Sifo-Dyas, and Dyas's Jedi Master, Lene Kostana. During one mission, Dooku sees visions of many different futures through the Force, which shake him to the core. Eventually, Dooku becomes a Jedi Knight, trains two Padawans to knighthood, and joins the Jedi Council, hoping to bring about real change in the Republic. However, after he saves his homeworld of Serenno from invaders, he decides to remain to help his planet rebuild, leaving the Jedi Order and becoming Count Dooku of Serenno.


title: Count Dooku

text: In "", Dooku is mentioned by Duke Solha and Padmé Amidala while they fight at Mokivj. Solha establishes that the Count had dispatched him and his siblings to Mokivj to produce cortosis B2 super battle droids for the Clone Wars.


title: Count Dooku

text: The novel "Master & Apprentice" by Claudia Gray recounts several adventures from Qui-Gon Jinn's apprenticeship under Dooku, as well as Dooku's obsession with Jedi prophecies and seeming use of Force lightning during a mission. The novel also reveals that Dooku had one other apprentice before Qui-Gon, Rael Averross.


title: Count Dooku

text: Dooku appears extensively in the "Star Wars" Expanded Universe, including novels, comic books and the 2003 TV series "". In 2014, Lucasfilm labeled such material non-canon to the franchise, and rebranded them as "Star Wars Legends".


title: Count Dooku

text: In the "" series, set during the Clone Wars, Dooku trains multiple Dark Jedi apprentices, most of whom he uses as minions. His apprentices include Ventress, Tol Skorr and renegade Jedi Quinlan Vos. Vos initially intended to infiltrate the Separatists as a spy for the Jedi Council but instead nearly falls to the dark side.


title: Count Dooku

text: In Jude Watson's "Legacy of the Jedi", Dooku appears in Parts 1, 2, and 4, and is mentioned in Part 3. In Part 1, he is first tempted by the dark side of the Force as a child when his best friend and fellow Padawan Lorian Nod steals an ancient Sith Holocron from the Jedi Archives. When Nod is caught, Nod lies and says that the theft was Dooku's idea. However, Dooku manages to convince the Jedi Council of the truth, and Nod is then expelled from the Jedi Order. It is later implied that Dooku himself steals the Holocron because he is intrigued by the Sith's open embrace of power and realizes that he is just as ruthless as they are. Nod's treachery leaves Dooku with a great bitterness and intolerance of any form of betrayal, as well as an even more pronounced coldness to strangers. In Part 2, set 13 years later, he is a Jedi Knight, and takes Qui-Gon Jinn as his Padawan apprentice. By now, he is capable of masking emotions such as anger and hate that the Jedi Code forbids, stating that they in fact make him stronger. He encounters Nod while defending a Senator from space pirates, and defeats him in combat, almost executing him. However, Qui-Gon convinces him to spare Nod's life, and he and Dooku turn Nod over to the authorities. In Part 3, set 32 years later, Qui-Gon, now a Jedi Knight, remarks that he has not seen Dooku in years, noting that their relationship was never friendly. Finally in Part 4, set in the midst of the Clone Wars, Dooku, now a Sith, kills Nod after his former friend refuses to join the Separatists.


title: Count Dooku

text: In Sean Stewart's "", Dooku attempts to trap Yoda by offering to negotiate an end to the Clone Wars. Dooku attempts unsuccessfully to sway Yoda to his cause, while Yoda nearly convinces Dooku to return to the Jedi Order. When Anakin and Obi-Wan appear unexpectedly, Dooku believes that Yoda was trying to set him up to be captured, and renounces his former master once and for all. In the novel, it is also revealed that Dooku always resented his parents for "giving him away" to the Jedi Order.


title: Count Dooku

text: In James Luceno's "Labyrinth of Evil", Dooku engineers Grievous' transformation into a cyborg and trains him in lightsaber combat. He then schemes with Sidious to invade Coruscant in what he believes to be a plot to kill Obi-Wan and initiate Anakin into the Sith.


title: Count Dooku

text: Matthew Stover's expands upon Dooku's character: it portrays him as an evil man who has no concept of loyalty or friendship, and who despises the galaxy's non-human species. It also explains that Dooku believes that Palpatine's staged kidnapping is part of a plan to kill Obi-Wan and turn Anakin to the dark side. In this scenario, once the Republic becomes the Galactic Empire, Dooku and Palpatine will rule the galaxy together, while Anakin will command a “Sith Army” formed from the remains of the Jedi. The novelization depicts Dooku's death scene from his point of view. In his final moments, he realizes that Palpatine used him as a means to engineer the war and as a placeholder for Anakin, whom he intended to be his apprentice all along.


title: Count Dooku

text: During the 2003 animated micro-series "", Count Dooku leads the Separatists from behind the scenes, taking the dark assassin Asajj Ventress (voiced by Grey DeLisle) as his apprentice while training General Grievous in lightsaber combat. In the final episode, he plays a part in Grievous' attack on Coruscant and kidnapping Palpatine, setting the stage for "Revenge of the Sith".


title: Count Dooku

text: A number of toys based on Count Dooku have been produced, including the Lego set "Lego Star Wars: Duel on Geonosis" which recreates the duel between Yoda and Dooku portrayed in "Attack of the Clones", and a Hasbro Count Dooku lightsaber.


title: Earl Grey tea

text: Earl Grey tea is a tea blend which has been flavoured with oil of bergamot. The rind's fragrant oil is added to black tea to give Earl Grey its unique taste. Traditionally, Earl Grey was made from black teas such as Chinese keemun, and therefore intended to be served without milk. However, tea companies have since begun to offer Earl Grey made from stronger teas such as Ceylons, which are better suited to the addition of milk or cream. Some blend the tea with lapsang souchong tea which lends a smoky character. Other varieties have been introduced as well, such as green or oolong.


title: Earl Grey tea

text: The earliest reference to tea flavoured with bergamot dates to 1824; however the article in question makes no mention of Earl Grey. The use of bergamot seems to have been used to enhance the taste of low-quality teas. This practice seems to have been disreputable, as in 1837 charges were laid against a company accused of adding bergamot to misrepresent their tea as a superior product (at a higher price).


title: Earl Grey tea

text: It has been suggested that the Earl Grey blend, or "Earl Grey's Mixture", may have been named after Charles Grey, 2nd Earl Grey, British Prime Minister in the 1830s. However, the fact that adding bergamot to teas was viewed as disreputable near the time of his death suggests that, while it is possible that the second Earl Grey encountered tea flavoured with bergamot, it seems rather unlikely that he would have championed it. Nonetheless, there have been a number of theories attempting to link the tea to the earl.


title: Earl Grey tea

text: One legend claims that a grateful Chinese mandarin whose son was rescued from drowning by one of Lord Grey's men first presented the blend to the Earl in 1803. The tale appears to be apocryphal, as Lord Grey never set foot in China and the use of bergamot oil to scent tea was then unknown in China. However, this tale is subsequently told (and slightly corrected) on the Twinings website, as "having been presented by an envoy on his return from China".


title: Earl Grey tea

text: Another legend claims that he received it as a gift, probably a diplomatic perquisite, of tea flavoured with bergamot oil, perhaps as a result of his ending the monopoly held by the East India Company on trade between Britain and China.


title: Earl Grey tea

text: According to the Grey family, the tea was specially blended by a Chinese mandarin for Lord Grey, to suit the water at Howick Hall, the family seat in Northumberland, using bergamot in particular to offset the preponderance of lime in the local water. Lady Grey used it to entertain in London as a political hostess, and it proved so popular that she was asked if it could be sold to others, which is how Twinings came to market it as a brand.


title: Earl Grey tea

text: Jacksons of Piccadilly claim they originated Earl Grey's Tea, Lord Grey having given the recipe to George Charlton, partner at Robert Jackson & Co., in 1830. According to Jacksons, the original recipe has been in constant production and has never left their hands. Theirs has been based on Chinese black tea since the beginning.


title: Earl Grey tea

text: Other theories for the provenance of the tea suggest it may have been developed independently of the 2nd Earl of Grey, with the Earl title being added at a later date.


title: Earl Grey tea

text: References have been found in old advertisements dating to the 1850s and 1860s (after the death of the second Earl of Grey) to "Grey's Tea" or "Grey's mixture", with the earliest being attributed to a tea merchant known as William Grey in 1852. The first known published references to an "Earl Grey" tea are advertisements by Charlton & Co. of Jermyn Street in London in the 1880s. It has been suggested that the "Earl" title was added to make the tea seem more posh, or alternately that it became associated with Henry Grey, 3rd Earl Grey, who was alive at the time where the "Earl" title began to be attributed to the tea.


title: Earl Grey tea

text: A 2010 survey found that a significant number of people in the United Kingdom associate drinking Earl Grey tea with being "posh" or "middle class".


title: Earl Grey tea

text: "Earl Grey" as applied to tea is not a registered trademark, and numerous tea companies produce their own versions of Earl Grey tea, using a wide variety of tea leaves and additives.


title: Earl Grey tea

text: Bergamot orange ("Citrus bergamia") is a small citrus tree which blossoms during the winter and is grown commercially in Calabria, Italy. It is probably a hybrid of "Citrus limetta" (sweet lime) and "Citrus aurantium" (bitter orange).


title: Earl Grey tea

text: Like other black teas, Earl Grey tea leaves undergo an oxidation process. The oxidation of the leaves results in a change of the chemical composition of the leaves and a stronger flavour. Occasionally, green tea leaves or oolong leaves are used in place of the black tea leaves. When this is the case, the oxidation process is skipped and the tea becomes Earl Green tea. There are two methods of flavouring the black tea leaves to get Earl Grey tea. The first, which is said to result in a stronger citrus flavour, is the coating or spraying of the black tea leaves with bergamot essential oils. The second method is the addition of dried bergamot orange rinds to the Earl Grey tea blend. With this method, the citrus flavour infuses the black tea leaves during the brewing process.


title: Earl Grey tea

text: The brewing process varies significantly, depending on personal preference. Opinions vary on the preferred method of boiling the water, as well as the ideal steeping time. Steeping for too long results in a bitter flavour; two to three minutes is a common recommendation, although some recommend as long as five minutes in order to "enjoy the full benefit of the citrus aroma."


title: Earl Grey tea

text: Earl Grey tea is used as a flavouring for many types of cakes and confectionery, such as chocolates, as well as savoury sauces. For sauces, the flavour is normally created by adding tea bags to the basic stock, boiling for a few minutes and then discarding the bags. For sweet recipes, loose tea is often added to melted butter or hot cream and strained after the flavour is


title: Earl Grey tea

text: Like other black teas, the polyphenols and caffeine in Earl Grey tea are reputed to have cardiovascular, digestive, and cancer-preventitive benefits, but research in these areas remains inconclusive.


title: Earl Grey tea

text: The bergamot oil used to flavour Earl Grey tea may provide health benefits as well as risks to its consumers. Some studies have suggested that the oil may be able to lower cholesterol and have some analgesic properties.


title: Earl Grey tea

text: Consuming too much of bergamot teas can lead to issues with potassium intake. Lowered potassium in the body can lead to experiencing cramping and muscle twitching.


title: Earl Grey tea

text: In one case study, a patient who consumed four litres of Earl Grey tea per day reported muscle cramps, which were attributed to the function of the bergapten in bergamot oil as a potassium channel blocker. The symptoms subsided upon reducing his consumption of Earl Grey tea to one litre per day.


title: Earl Grey tea

text: Bergamot is a source of bergamottin which, along with the chemically related compound 6',7'-dihydroxybergamottin, is known to be responsible for grapefruit–drug interactions in which the consumption of the juice affects the metabolism of a variety of pharmaceutical drugs.


title: Earl Grey tea

text: In several studies, application of high concentrations of some brands of bergamot oil directly to the skin was shown to increase redness after exposure to ultraviolet light; however, this should not apply to ordinary oral consumption of Earl Grey tea.


title: Hawking radiation

text: Hawking radiation is theoretical black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical argument for its existence in 1974. Hawking radiation is a purely kinematic effect that is generic to Lorentzian geometries containing event horizons or local apparent horizons.


title: Hawking radiation

text: Hawking radiation reduces the mass and rotational energy of black holes and is therefore also theorized to cause black hole evaporation. Because of this, black holes that do not gain mass through other means are expected to shrink and ultimately vanish. For all except the smallest black holes, this would happen extremely slowly. The radiation temperature is inversely proportional to the black hole's mass, so micro black holes are predicted to be larger emitters of radiation than larger black holes and should dissipate faster.


title: Hawking radiation

text: Black holes are astrophysical objects of interest primarily because of their compact size and immense gravitational attraction. They were first predicted by Einstein's 1915 theory of general relativity, before astrophysical evidence began to mount half a century later.


title: Hawking radiation

text: A black hole can form when enough matter or energy is compressed into a volume small enough that the escape velocity is greater than the speed of light. Nothing can travel that fast, so nothing within a distance, proportional to the mass of the black hole, can escape beyond that distance. The region beyond which not even light can escape is the event horizon; an observer outside it cannot observe, become aware of, or be affected by events within the event horizon. The essence of a black hole is its event horizon, a theoretical demarcation between events and their causal relationships.


title: Hawking radiation

text: Alternatively, using a set of infalling coordinates in general relativity, one can conceptualize the event horizon as the region beyond which space is infalling faster than the speed of light. (Although nothing can travel "through" space faster than light, space itself can infall at any speed.) Once matter is inside the event horizon, all of the matter inside falls inexorably into a gravitational singularity, a place of infinite curvature and zero size, leaving behind a warped spacetime devoid of any matter. A classical black hole is pure empty spacetime, and the simplest (nonrotating and uncharged) is characterized just by its mass and event horizon.


title: Hawking radiation

text: Our current understandings of quantum physics can be used to investigate what may happen in the region around the event horizon. In 1974, British physicist Stephen Hawking used quantum field theory in curved spacetime to show that in theory, the force of gravity at the event horizon was strong enough to cause thermal radiation to be emitted and energy to "leak" into the wider universe from a tiny distance around and outside the event horizon. In effect this energy acted as if the black hole itself was slowly evaporating (although it actually came from outside it).


title: Hawking radiation

text: An important difference between the black hole radiation as computed by Hawking and thermal radiation emitted from a black body is that the latter is statistical in nature, and only its average satisfies what is known as Planck's law of black-body radiation, while the former fits the data better. Thus, thermal radiation contains information about the body that emitted it, while Hawking radiation seems to contain no such information, and depends only on the mass, angular momentum, and charge of the black hole (the no-hair theorem). This leads to the black hole information paradox.


title: Hawking radiation

text: However, according to the conjectured gauge-gravity duality (also known as the AdS/CFT correspondence), black holes in certain cases (and perhaps in general) are equivalent to solutions of quantum field theory at a non-zero temperature. This means that no information loss is expected in black holes (since the theory permits no such loss) and the radiation emitted by a black hole is probably the usual thermal radiation. If this is correct, then Hawking's original calculation should be corrected, though it is not known how (see below).


title: Hawking radiation

text: A black hole of one solar mass () has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of (about the mass of the Moon, or about across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits.


title: Hawking radiation

text: Hawking's discovery followed a visit to Moscow in 1973, where the Soviet scientists Yakov Zel'dovich and Alexei Starobinsky convinced him that rotating black holes ought to create and emit particles, while Russian physicist Vladimir Gribov believed that even a non-rotating black hole should emit radiation. When Hawking did the calculation, he found to his surprise that it was true. In 1972, Jacob Bekenstein conjectured that the black holes should have an entropy, where by the same year, he proposed no-hair theorems. Bekenstein's discovery and results are commended by Stephen Hawking, which also led him to think about radiation due to this formalism.


title: Hawking radiation

text: According to the physicist Dmitri Diakonov, there was an argument between Zeldovich and Vladimir Gribov at the Zeldovich Moscow 1972–1973 seminar. Zeldovich believed that only a rotating black hole could emit radiation, while Gribov believed that even a non-rotating black hole emits radiation due to the laws of quantum mechanics. This account is confirmed by Gribov's obituary in the Physics-Uspekhi by Vitaly Ginzburg and others.


title: Hawking radiation

text: Hawking radiation is required by the Unruh effect and the equivalence principle applied to black-hole horizons. Close to the event horizon of a black hole, a local observer must accelerate to keep from falling in. An accelerating observer sees a thermal bath of particles that pop out of the local acceleration horizon, turn around, and free-fall back in. The condition of local thermal equilibrium implies that the consistent extension of this local thermal bath has a finite temperature at infinity, which implies that some of these particles emitted by the horizon are not reabsorbed and become outgoing Hawking radiation.


title: Hawking radiation

text: The field theory is defined by a local path integral, so if the boundary conditions at the horizon are determined, the state of the field outside will be specified. To find the appropriate boundary conditions, consider a stationary observer just outside the horizon at position


title: Hawking radiation

text: which is Rindler in terms of . The metric describes a frame that is accelerating to keep from falling into the black hole. The local acceleration, , diverges as .


title: Hawking radiation

text: The horizon is not a special boundary, and objects can fall in. So the local observer should feel accelerated in ordinary Minkowski space by the principle of equivalence. The near-horizon observer must see the field excited at a local temperature


title: Hawking radiation

text: The gravitational redshift is given by the square root of the time component of the metric. So for the field theory state to consistently extend, there must be a thermal background everywhere with the local temperature redshift-matched to the near horizon temperature:


title: Hawking radiation

text: Thus a field theory defined on a black-hole background is in a thermal state whose temperature at infinity is


title: Hawking radiation

text: From the black-hole temperature, it is straightforward to calculate the black-hole entropy . The change in entropy when a quantity of heat is added is


title: Hawking radiation

text: The radius of a black hole is twice its mass in Planck units, so the entropy of a black hole is proportional to its surface area:


title: Hawking radiation

text: Assuming that a small black hole has zero entropy, the integration constant is zero. Forming a black hole is the most efficient way to compress mass into a region, and this entropy is also a bound on the information content of any sphere in space time. The form of the result strongly suggests that the physical description of a gravitating theory can be somehow encoded onto a bounding surface.


title: Hawking radiation

text: When particles escape, the black hole loses a small amount of its energy and therefore some of its mass (mass and energy are related by Einstein's equation ). Consequently, an evaporating black hole will have a finite lifespan. By dimensional analysis, the life span of a black hole can be shown to scale as the cube of its initial mass, and Hawking estimated that any black hole formed in the early universe with a mass of less than approximately 10 g would have evaporated completely by the present day.


title: Hawking radiation

text: In 1976, Don Page refined this estimate by calculating the power produced, and the time to evaporation, for a non-rotating, non-charged Schwarzschild black hole of mass . The time for the event horizon or entropy of a black hole to halve is known as the Page time. The calculations are complicated by the fact that a black hole, being of finite size, is not a perfect black body; the absorption cross section goes down in a complicated, spin-dependent manner as frequency decreases, especially when the wavelength becomes comparable to the size of the event horizon. Page concluded that primordial black holes could only survive to the present day if their initial mass were roughly or larger. Writing in 1976, Page using the understanding of neutrinos at the time erroneously worked on the assumption that neutrinos have no mass and that only two neutrino flavors exist, and therefore his results of black hole lifetimes do not match the modern results which take into account 3 flavors of neutrinos with nonzero masses. A 2008 calculation using the particle content of the Standard Model and the WMAP figure for the age of the universe yielded a mass bound of .


title: Hawking radiation

text: If black holes evaporate under Hawking radiation, a solar mass black hole will evaporate over 10 years which is vastly longer than the age of the universe. A supermassive black hole with a mass of 10 (100 billion) will evaporate in around . Some monster black holes in the universe are predicted to continue to grow up to perhaps 10 during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of up to 10 years.


title: Hawking radiation

text: The power emitted by a black hole in the form of Hawking radiation can be estimated for the simplest case of a nonrotating, non-charged Schwarzschild black hole of mass . Combining the formulas for the Schwarzschild radius of the black hole, the Stefan–Boltzmann law of blackbody radiation, the above formula for the temperature of the radiation, and the formula for the surface area of a sphere (the black hole's event horizon), several equations can be derived.


title: Hawking radiation

text: The Bekenstein–Hawking luminosity of a black hole, under the assumption of pure photon emission (i.e. that no other particles are emitted) and under the assumption that the horizon is the radiating surface is:


title: Hawking radiation

text: where is the luminosity, i.e., the radiated power, is the reduced Planck constant, is the speed of light, is the gravitational constant and is the mass of the black hole. It is worth mentioning that the above formula has not yet been derived in the framework of semiclassical gravity.


title: Hawking radiation

text: where and are the mass and (Schwarzschild) volume of the black hole. A black hole of one solar mass ( = ) takes more than to evaporate—much longer than the current age of the universe at . But for a black hole of , the evaporation time is . This is why some astronomers are searching for signs of exploding primordial black holes.


title: Hawking radiation

text: However, since the universe contains the cosmic microwave background radiation, in order for the black hole to dissipate, the black hole must have a temperature greater than that of the present-day blackbody radiation of the universe of 2.7 K. A study suggests that must be less than 0.8% of the mass of the Earth – approximately the mass of the Moon.


title: Hawking radiation

text: The trans-Planckian problem is the issue that Hawking's original calculation includes quantum particles where the wavelength becomes shorter than the Planck length near the black hole's horizon. This is due to the peculiar behavior there, where time stops as measured from far away. A particle emitted from a black hole with a frequency, if traced back to the horizon, must have had an infinite frequency, and therefore a trans-Planckian wavelength.


title: Hawking radiation

text: The Unruh effect and the Hawking effect both talk about field modes in the superficially stationary spacetime that change frequency relative to other coordinates that are regular across the horizon. This is necessarily so, since to stay outside a horizon requires acceleration that constantly Doppler shifts the modes.


title: Hawking radiation

text: An outgoing photon of Hawking radiation, if the mode is traced back in time, has a frequency that diverges from that which it has at great distance, as it gets closer to the horizon, which requires the wavelength of the photon to "scrunch up" infinitely at the horizon of the black hole. In a maximally extended external Schwarzschild solution, that photon's frequency stays regular only if the mode is extended back into the past region where no observer can go. That region seems to be unobservable and is physically suspect, so Hawking used a black hole solution without a past region that forms at a finite time in the past. In that case, the source of all the outgoing photons can be identified: a microscopic point right at the moment that the black hole first formed.


title: Hawking radiation

text: The quantum fluctuations at that tiny point, in Hawking's original calculation, contain all the outgoing radiation. The modes that eventually contain the outgoing radiation at long times are redshifted by such a huge amount by their long sojourn next to the event horizon that they start off as modes with a wavelength much shorter than the Planck length. Since the laws of physics at such short distances are unknown, some find Hawking's original calculation unconvincing.


title: Hawking radiation

text: The trans-Planckian problem is nowadays mostly considered a mathematical artifact of horizon calculations. The same effect occurs for regular matter falling onto a white hole solution. Matter that falls on the white hole accumulates on it, but has no future region into which it can go. Tracing the future of this matter, it is compressed onto the final singular endpoint of the white hole evolution, into a trans-Planckian region. The reason for these types of divergences is that modes that end at the horizon from the point of view of outside coordinates are singular in frequency there. The only way to determine what happens classically is to extend in some other coordinates that cross the horizon.


title: Hawking radiation

text: There exist alternative physical pictures that give the Hawking radiation in which the trans-Planckian problem is addressed. The key point is that similar trans-Planckian problems occur when the modes occupied with Unruh radiation are traced back in time. In the Unruh effect, the magnitude of the temperature can be calculated from ordinary Minkowski field theory, and is not controversial.


title: Hawking radiation

text: The formulas from the previous section are applicable only if the laws of gravity are approximately valid all the way down to the Planck scale. In particular, for black holes with masses below the Planck mass (~), they result in impossible lifetimes below the Planck time (~). This is normally seen as an indication that the Planck mass is the lower limit on the mass of a black hole.


title: Hawking radiation

text: In a model with large extra dimensions (10 or 11), the values of Planck constants can be radically different, and the formulas for Hawking radiation have to be modified as well. In particular, the lifetime of a micro black hole with a radius below the scale of the extra dimensions is given by equation 9 in Cheung (2002) and equations 25 and 26 in Carr (2005).


title: Hawking radiation

text: where is the low-energy scale, which could be as low as a few TeV, and is the number of large extra dimensions. This formula is now consistent with black holes as light as a few TeV, with lifetimes on the order of the "new Planck time" ~.


title: Hawking radiation

text: A detailed study of the quantum geometry of a black hole event horizon has been made using loop quantum gravity. Loop-quantization does not reproduce the result for black hole entropy originally discovered by Bekenstein and Hawking, unless the value of a free parameter is set to cancel out various constants such that the Bekenstein–Hawking entropy formula is reproduced. However, quantum gravitational corrections to the entropy and radiation of black holes have been computed based on the theory.


title: Hawking radiation

text: Based on the fluctuations of the horizon area, a quantum black hole exhibits deviations from the Hawking spectrum that would be observable were X-rays from Hawking radiation of evaporating primordial black holes to be observed. The quantum effects are centered at a set of discrete and unblended frequencies highly pronounced on top of Hawking radiation spectrum.


title: Hawking radiation

text: In June 2008, NASA launched the Fermi space telescope, which is searching for the terminal gamma-ray flashes expected from evaporating primordial black holes. As of 2018, none had been detected.


title: Hawking radiation

text: If speculative large extra dimension theories are correct, then CERN's Large Hadron Collider may be able to create micro black holes and observe their evaporation. No such micro black hole has been observed at CERN.


title: Hawking radiation

text: Under experimentally achievable conditions for gravitational systems, this effect is too small to be observed directly. It was predicted that Hawking radiation could be studied by analogy using sonic black holes, in which sound perturbations are analogous to light in a gravitational black hole and the flow of an approximately perfect fluid is analogous to gravity (see "Analog models of gravity"). Observations of Hawking radiation were reported, in sonic black holes employing Bose–Einstein condensates.


title: Hawking radiation

text: In September 2010 an experimental set-up created a laboratory "white hole event horizon" that the experimenters claimed was shown to radiate an optical analog to Hawking radiation. However, the results remain unverified and debatable, and its status as a genuine confirmation remains in doubt.


title: Modem

text: A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more carrier wave signals to encode digital information, while the receiver demodulates the signal to recreate the original digital information. The goal is to produce a signal that can be transmitted easily and decoded reliably. Modems can be used with almost any means of transmitting analog signals, from light-emitting diodes to radio.


title: Modem

text: Early modems were devices that used audible sounds suitable for transmission over traditional telephone systems and leased lines. These generally operated at 110 or 300 bits per second (bit/s), and the connection between devices was normally manual, using an attached telephone handset. By the 1970s, higher speeds of 1,200 and 2,400 bit/s for asynchronous dial connections, 4,800 bit/s for synchronous leased line connections and 35 kbit/s for synchronous conditioned leased lines were available. By the 1980s, less expensive 1,200 and 2,400 bit/s dialup modems were being released, and modems working on radio and other systems were available. As device sophistication grew rapidly in the late 1990s, telephone-based modems quickly exhausted the available bandwidth, reaching 56 kbit/s.


title: Modem

text: The rise of public use of the internet during the late 1990s led to demands for much higher performance, leading to the move away from audio-based systems to entirely new encodings on cable television lines and short-range signals in subcarriers on telephone lines. The move to cellular telephones, especially in the late 1990s and the emergence of smartphones in the 2000s led to the development of ever-faster radio-based systems. Today, modems are ubiquitous and largely invisible, included in almost every mobile computing device in one form or another, and generally capable of speeds on the order of tens or hundreds of megabytes per second.


title: Modem

text: Modems are frequently classified by the maximum amount of data they can send in a given unit of time, usually expressed in bits per second (symbol bit/s, sometimes abbreviated "bps") or rarely in bytes per second (symbol B/s). Modern broadband modem speeds are typically expressed in megabits per second (Mbit/s).


title: Modem

text: Historically, modems were often classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency-shift keying with two possible frequencies, corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using phase-shift keying.


title: Modem

text: Many modems are variable-rate, permitting them to be used over a medium with less than ideal characteristics, such as a telephone line that is of poor quality or is too long. This capability is often adaptive so that a modem can discover the maximum practical transmission rate during the connect phase, or during operation.


title: Modem

text: Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. The earliest devices that satisfy the definition of a modem may be the multiplexers used by news wire services in the 1920s.


title: Modem

text: In 1941, the Allies developed a voice encryption system called SIGSALY which used a vocoder to digitize speech, then encrypted the speech with one-time pad and encoded the digital data as tones using frequency shift keying. This was also a digital modulation technique, making this an early modem.


title: Modem

text: Commercial modems largely did not become available until the late 1950s, when the rapid development of computer technology created demand for a method of connecting computers together over long distances, resulting in the Bell Company and then other businesses producing an increasing number of computer modems for use over both switched and leased telephone lines.


title: Modem

text: Later developments would produce modems that operated over cable television lines, power lines, and various radio technologies, as well as modems that achieved much higher speeds over telephone lines.


title: Modem

text: A dial-up modem transmits computer data over an ordinary switched telephone line that has not been designed for data use. This contrasts with leased line modems, which also operate over lines provided by a telephone company, but ones which are intended for data use and do not impose the same signaling constraints.


title: Modem

text: The modulated data must fit the frequency constraints of a normal voice audio signal. Early modems, including acoustic coupled modems, relied on the communicating parties or an automatic calling unit to dial and establish a voice connection before switching their modems to line; more modern devices are able to perform the actions needed to connect a call through a telephone exchange, e.g., picking up the line, dialing, understanding signals sent back by phone company equipment (dialtone, ringing, busy signal) recognizing incoming ring signals and answering calls.


title: Modem

text: Dial-up modems have been made in a wide variety of speeds and capabilities, with many capable of testing the line they are calling over and selecting the most advanced signaling mode that the line can support. Generally speaking, the fastest dialup modems ever available to consumers never exceeded 56 kbit/s and never achieved that speed in both directions.


title: Modem

text: The dial-up modem was once a widely known technology, since it was mass-marketed to consumers in many countries for dial-up internet access. In the 1990s, tens of millions of people in the United States used dial-up modems for internet access.


title: Modem

text: Dial-up service has since been largely supplanted by broadband internet, such as DSL, which typically still uses a modem, but of a very different type which may still operate over a normal phone line, but with substantially relaxed constraints.


title: Modem

text: Mass production of telephone line modems in the United States began as part of the SAGE air-defense system in 1958, connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the United States and Canada.


title: Modem

text: Shortly afterwards in 1959, the technology in the SAGE modems was made available commercially as the Bell 101, which provided 110 bit/s speeds. Bell called this and several other early modems "datasets".


title: Modem

text: The Bell 103A standard was introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz.


title: Modem

text: The 103 modem would eventually become a de facto standard once third-party (non-AT&T modems) reached the market, and throughout the 1970s, independently made modems compatible with the Bell 103 de facto standard were commonplace. Example models included the Novation CAT and the Anderson-Jacobson. A lower-cost option was the Pennywhistle modem, designed to be built using readily available parts.


title: Modem

text: Teletype machines were granted access to remote networks such as the Teletypewriter Exchange using the Bell 103 modem. AT&T also produced reduced-cost units, the originate-only 113D and the answer-only 113B/C modems.


title: Modem

text: The "201A" "Data-Phone" was a synchronous modem using two-bit-per-symbol phase-shift keying (PSK) encoding, achieving 2,000 bit/s half-duplex over normal phone lines. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase.


title: Modem

text: In early 1973, Vadic introduced the "VA3400" which performed full-duplex at 1,200 bit/s over a normal phone line.


title: Modem

text: In November 1976, AT&T introduced the 212A modem, similar in design, but using the lower frequency set for transmission. It was not compatible with the VA3400, but it would operate with 103A modem at 300 bit/s.


title: Modem

text: In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.


title: Modem

text: A significant advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it introduced a command language which allowed the computer to make control requests, such as commands to dial or answer calls, over the same RS-232 interface used for the data connection. The command set used by this device became a de facto standard, the Hayes command set, which was integrated into devices from many other manufacturers.


title: Modem

text: Automatic dialing was not a new capability – it had been available via separate Automatic Calling Units, and via modems using the X.21 interface – but the Smartmodem made it available in a single device that could be used with even the most minimal implementations of the ubiquitous RS-232 interface, making this capability accessible from virtually any system or language.


title: Modem

text: The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features. This eventually led to legal action over use of the patented Hayes command language.


title: Modem

text: Dial modems generally remained at 300 and 1,200 bit/s (eventually becoming standards such as V.21 and V.22) into the mid-1980s.


title: Modem

text: In 1984, V.22bis was created, a 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212. This bit rate increases was achieved by defining four or eight distinct symbols, which allowed the encoding of two or three bits per symbol instead of only one. By the late 1980s, many modems could support improved standards like this, and 2,400-bit/s operation was becoming common.


title: Modem

text: Increasing modem speed greatly improved the responsiveness of online systems and made file transfer practical. This led to rapid growth of online services with large file libraries, which in turn gave more reason to own a modem. The rapid update of modems led to a similar rapid increase in BBS use.


title: Modem

text: The introduction of microcomputer systems with internal expansion slots made small internal modems practical. This led to a series of popular modems for the S-100 bus and Apple II computers that could directly dial out, answer incoming calls, and hang up entirely from software, the basic requirements of a bulletin board system (BBS). The seminal CBBS for instance was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed.


title: Modem

text: Echo cancellation became a feature of modems in this period, which improved the bandwidth available to both modems by allowing them to ignore their own reflected signals.


title: Modem

text: Additional improvements were introduced by quadrature amplitude modulation (QAM) encoding, which increased the number of bits per symbol to four through a combination of phase shift and amplitude.


title: Modem

text: Transmitting at 1,200 baud produced the 4,800 bit/s V.27ter standard, and at 2,400 baud the 9,600 bit/s V.32. The carrier frequency was 1,650 Hz in both systems.


title: Modem

text: The introduction of these higher-speed systems also led to the development of the digital fax machine during the 1980s. While early fax technology also used modulated signals on a phone line, digital fax used the now-standard digital encoding used by computer modems. This eventually allowed computers to send and receive fax images.


title: Modem

text: In the early 1990s, V.32 modems operating at 9,600 bit/s were introduced, but were expensive and were only starting to enter the market when V.32bis was standardized, which operated at 14,400 bit/s.


title: Modem

text: Rockwell International's chip division developed a new driver chip set incorporating the V.32bis standard and aggressively priced it. Supra, Inc. arranged a short-term exclusivity arrangement with Rockwell, and developed the SupraFAXModem 14400 based on it. Introduced in January 1992 at (or less), it was half the price of the slower V.32 modems already on the market. This led to a price war, and by the end of the year V.32 was dead, never having been really established, and V.32bis modems were widely available for .


title: Modem

text: V.32bis was so successful that the older high-speed standards had little advantages. USRobotics (USR) fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as "V.32ter", but neither non-standard modem sold well.


title: Modem

text: Consumer interest in these proprietary improvements waned during the lengthy introduction of the V.34 standard. While waiting, several companies decided to release hardware and introduced modems they referred to as "V.FAST".


title: Modem

text: In order to guarantee compatibility with V.34 modems once a standard was ratified (1994), manufacturers used more flexible components, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips. This would allow later firmware updates to conform with the standards once ratified.


title: Modem

text: The ITU standard V.34 represents the culmination of these joint efforts. It employed the most powerful coding techniques available at the time, including channel encoding and shape encoding. From the mere four bits per symbol (), the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and modems. This rate is near the theoretical Shannon limit of a phone line.


title: Modem

text: While speeds had been available for leased-line modems for some time, they did not become available for dial up modems until the late 1990s.


title: Modem

text: In the late 1990s, technologies to achieve speeds above began to be introduced. Several approaches were used, but all of them began as solutions to a single fundamental problem with phone lines.


title: Modem

text: By the time technology companies began to investigate speeds above , telephone companies had switched almost entirely to all-digital networks. As soon as a phone line reached a local central office, a "line card" converted the analog signal from the subscriber to a digital one and conversely. While digitally encoded telephone lines notionally provide the same bandwidth as the analog systems they replaced, the digitization itself placed constraints on the "types" of waveforms that could be reliably encoded.


title: Modem

text: The first problem was that the process of analog-to-digital conversion is intrinsically lossy, but second, and more importantly, the digital signals used by the telcos were not "linear": they did not encode all frequencies the same way, instead utilizing a nonlinear encoding (μ-law and a-law) meant to favor the nonlinear response of the human ear to voice signals. This made it very difficult to find a encoding that could survive the digitizing process.


title: Modem

text: Modem manufacturers discovered that, while the analog to digital conversion could not preserve higher speeds, "digital-to-analog" conversions could. Because it was possible for an ISP to obtain a direct digital connection to a telco, a "digital modem" one that connects directly to a digital telephone network interface, such as T1 or PRI could send a signal that utilized every bit of bandwidth available in the system. While that signal still had to be converted back to analog at the subscriber end, that conversion would not distort the signal in the same way that the opposite direction did.


title: Modem

text: The first 56k dial-up option was a proprietary design from USRobotics, which they called "X2" because 56k was twice the speed (×2) of 28k modems.


title: Modem

text: At that time, USRobotics held a 40% share of the retail modem market, while Rockwell International held an 80% share of the modem chipset market. Concerned with being shut out, Rockwell began work on a rival 56k technology. They joined with Lucent and Motorola to develop what they called "K56Flex" or just "Flex".


title: Modem

text: Both technologies reached the market around February 1997; although problems with K56Flex modems were noted in product reviews through July, within six months the two technologies worked equally well, with variations dependent largely on local connection characteristics.


title: Modem

text: The retail price of these early 56K modems was about , compared to for standard 33k modems. Compatible equipment was also required at the Internet service providers (ISPs) end, with costs varying depending on whether their current equipment could be upgraded. About half of all ISPs offered 56k support by October 1997. Consumer sales were relatively low, which USRobotics and Rockwell attributed to conflicting standards.


title: Modem

text: In February 1998, The International Telecommunication Union (ITU) announced the draft of a new standard V.90 with strong industry support. Incompatible with either existing standard, it was an amalgam of both, but was designed to allow both types of modem by a firmware upgrade. The V.90 standard was approved in September 1998 and widely adopted by ISPs and consumers.


title: Modem

text: The V.92 standard was approved by ITU in November 2000 and utilized digital PCM technology to increase the upload speed to a maximum of .


title: Modem

text: The high upload speed was a tradeoff. upstream rate would reduce the downstream as low as due to echo effects on the line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a plain 33. analog connection in order to maintain a high digital downstream of or higher.


title: Modem

text: V.92 also added two other features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods of time while they answer a call. The second feature is the ability to quickly connect to one's ISP, achieved by remembering the analog and digital characteristics of the telephone line and using this saved information when reconnecting.


title: Modem

text: These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines). For a complete list see the companion article "list of device bandwidths". A baud is one symbol per second; each symbol may encode one or more data bits.


title: Modem

text: Many dial-up modems implement standards for data compression to achieve higher effective throughput for the same bitrate. V.44 is an example used in conjunction with V.92 to achieve speeds greater than 56k over ordinary phone lines.


title: Modem

text: As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase apparent throughput. This server-side compression can operate much more efficiently than the on-the-fly compression performed within modems, because the compression techniques are content-specific (JPEG, text, EXE, etc.). Website text, images, and Flash media are typically compacted to approximately 4%, 12%, and 30%, respectively. The drawback is a loss in quality, as they use lossy compression which causes images to become pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up".


title: Modem

text: These accelerated downloads are integrated into the Opera and Amazon Silk web browsers, using their own server-side text and image compression.


title: Modem

text: Dial-up modems can attach in two different ways: with an acoustic coupler, or with a direct electrical connection.


title: Modem

text: The case "Hush-A-Phone Corp. v. United States", which legalized acoustic couplers, applied only to mechanical connections to a telephone set, not electrical connections to the telephone line. The "Carterfone" decision of 1968, however, permitted customers to attach devices directly to a telephone line as long as they followed stringent Bell-defined standards for non-interference with the phone network. This opened the door to independent (non-AT&T) manufacture of direct-connect modems, that plugged directly into the phone line rather than via an acoustic coupler.


title: Modem

text: While "Carterfone" required AT&T to permit connection of devices, AT&T successfully argued that they should be allowed to require the use of a special device to protect their network, placed in between the third-party modem and the line, called a Data Access Arrangement or DAA. The use of DAAs was mandatory from 1969 to 1975 when the new FCC Part 68 rules allowed the use of devices without a Bell-provided DAA, subject to equivalent circuitry being included in the third-party device.


title: Modem

text: While Bell (AT&T) provided modems that attached via direct wire connection to the phone network as early as 1958, their regulations at the time did not permit the direct electrical connection of any non-Bell device to a telephone line. However, the Hush-a-Phone ruling allowed customers to attach any device "to a telephone set" as long as it did not interfere with its functionality. This allowed third-party (non-Bell) manufacturers to sell modems utilizing an "acoustic coupler".


title: Modem

text: With an acoustic coupler, an ordinary telephone handset was placed in a cradle containing a speaker and microphone positioned to match up with those on the handset. The tones used by the modem were transmitted and received into the handset, which then relayed them to the phone line.


title: Modem

text: Because the modem was not electrically connected, it was incapable of picking up, hanging up or dialing, all of which required direct control of the line. Touch-tone dialing would have been possible, but touch-tone was not universally available at this time. Consequently, the dialing process was executed by the user lifting the handset, dialing, then placing the handset on the coupler. To accelerate this process, a user could purchase a "dialer" or "Automatic Calling Unit".


title: Modem

text: Early modems – could not place or receive calls on their own, but required human intervention for these steps.


title: Modem

text: As early as 1964, Bell provided "Automatic Calling Units" that connected separately to a second serial port on a host machine and could be commanded to open the line, dial a number, and even ensure the far end had successfully connected before transferring control to the modem. Later on, third-party models would become available, sometimes known simply as "dialers", and features such as the ability to automatically sign in to time-sharing systems.


title: Modem

text: Prior to the 1990s, modems contained all the electronics and intelligence to convert data in discrete form to an analog (modulated) signal and back again, and to handle the dialing process, as a mix of discrete logic and special-purpose chips. This type of modem is sometimes referred to as "controller-based".


title: Modem

text: In 1993, Digicom introduced the "Connection 96 Plus", a modem which replaced the discrete and custom components with a general purpose digital signal processor, which could be reprogrammed to upgrade to newer standards.


title: Modem

text: As this design trend spread, both terms – "soft modem" and "Winmodem" – obtained a negative connotation in non-Windows-based computing circles because the drivers were either unavailable for non-Windows platforms, or were only available as unmaintainable closed-source binaries, a particular problem for Linux users.


title: Modem

text: Later in the 1990s, software-based modems became available. These are essentially sound cards, and in fact a common design uses the AC'97 audio codec, which provides multichannel audio to a PC and includes three audio channels for modem signals.


title: Modem

text: The audio sent and received on the line by a modem of this type is generated and processed entirely in software, often in a device driver. There is little functional difference from the user's perspective, but this design reduces the cost of a modem by moving most of the processing power into inexpensive software instead of expensive hardware DSPs or discrete components.


title: Modem

text: Soft modems of both types either are internal cards or connect over external buses such as USB. They never utilize RS-232 because they require high bandwidth channels to the host computers to carry the raw audio signals generated (sent) or analyzed (received) by software.


title: Modem

text: Since the interface is not RS-232, there is no standard for communication with the device directly. Instead, soft modems come with drivers which create an emulated RS-232 port, which standard modem software (such as an operating system dialer application) can communicate with.


title: Modem

text: "Voice" and "fax" are terms added to describe any dial modem that is capable of recording/playing audio or transmitting/receiving faxes. Some modems are capable of all three functions.


title: Modem

text: Voice modems are used for computer telephony integration applications as simple as placing/receiving calls directly through a computer with a headset, and as complex as fully automated robocalling systems.


title: Modem

text: Fax modems can be used for computer-based faxing, in which faxes are sent and received without inbound or outbound faxes ever needing to ever be printed on paper. This differs from efax, in which faxing occurs over the internet, in some cases involving no phone lines whatsoever.


title: Modem

text: The ITU-T V.150.1 Recommendation defines procedures for the inter-operation of PSTN to IP gateways. In a classic example of this setup, each dial-up modem would connect to a modem relay gateway. The gateways are then connected to an IP network (such as the Internet). The analog connection from the modem is terminated at the gateway and the signal is demodulated. The demodulated control signals are transported over the IP network in an RTP packet type defined as State Signaling Events (SSEs). The data from the demodulated signal is sent over the IP network via a transport protocol (also defined as an RTP payload) called Simple Packet Relay Transport (SPRT). Both the SSE and SPRT packet formats are defined in the V.150.1 Recommendation (Annex C and Annex B respectively). The gateway at the remote end that receives the packets uses the information to re-modulate the signal for the modem connected at that end.


title: Modem

text: While the V.150.1 Recommendation is not widely deployed, a pared down version of the recommendation called "Minimum Essential Requirements (MER) for V.150.1 Gateways" (SCIP-216) is used in Secure Telephony applications.


title: Modem

text: While traditionally a hardware device, fully software-based modems with the ability to be deployed in a cloud environment (such as Microsoft Azure or AWS) do exist. Leveraging a Voice-over-IP (VoIP) connection through a SIP Trunk, the modulated audio samples are generated and sent over an IP network via RTP and an uncompressed audio codec (such as G.711 μ-law or a-law).


title: Modem

text: A 1994 Software Publishers Association found that although 60% of computers in US households had a modem, only 7% of households went online. A CEA study in 2006 found that dial-up Internet access was declining in the US. In 2000, dial-up Internet connections accounted for 74% of all US residential Internet connections. The United States demographic pattern for dial-up modem users per capita has been more or less mirrored in Canada and Australia for the past 20 years.


title: Modem

text: Dial-up modem use in the US had dropped to 60% by 2003, and stood at 36% in 2006. Voiceband modems were once the most popular means of Internet access in the US, but with the advent of new ways of accessing the Internet, the traditional 56K modem was losing popularity. The dial-up modem is still widely used by customers in rural areas where DSL, cable, wireless broadband, satellite, or fiber optic service are either not available or they are unwilling to pay what the available broadband companies charge. In its 2012 annual report, AOL showed it still collected around $700 million in fees from about three million dial-up users.


title: Modem

text: TDD devices are a subset of the teleprinter intended for use by the deaf or hard of hearing, essentially a small teletype with a built-in dial-up modem and acoustic coupler. The first models produced in 1964 utilized FSK modulation much like early computer modems.


title: Modem

text: A leased line modem also uses ordinary phone wiring, like dial-up and DSL, but does not use the same network topology. While dial-up uses a normal phone line and connects through the telephone switching system, and DSL uses a normal phone line but connects to equipment at the telco central office, leased lines do not terminate at the telco.


title: Modem

text: Leased lines are pairs of telephone wire that have been connected together at one or more telco central offices so that they form a continuous circuit between two subscriber locations, such as a business' headquarters and a satellite office. They provide no power or dialtone - they are simply a pair of wires connected at two distant locations.


title: Modem

text: A dialup modem will not function across this type of line, because it does not provide the power, dialtone and switching that those modems require. However, a modem with leased-line capability can operate over such a line, and in fact can have greater performance because the line is not passing through the telco switching equipment, the signal is not filtered, and therefore greater bandwidth is available.


title: Modem

text: Leased-line modems can operate in 2-wire or 4-wire mode. The former uses a single pair of wires and can only transmit in one direction at a time, while the latter uses two pairs of wires and can transmit in both directions simultaneously. When two pairs are available, bandwidth can be as high as 1.5 Mbit/s, a full data T1 circuit.


title: Modem

text: The term "broadband" was previously used to describe communications faster than what was available on voice grade channels.


title: Modem

text: The term "broadband" gained widespread adoption in the late 1990s to describe internet access technology exceeding the 56 kilobit/s maximum of dialup. There are many broadband technologies, such as various DSL (digital subscriber line) technologies and cable broadband.


title: Modem

text: DSL technologies such as ADSL, HDSL, and VDSL use telephone lines (wires that were installed by a telephone company and originally intended for use by a telephone subscriber) but do not utilize most of the rest of the telephone system. Their signals are not sent through ordinary phone exchanges, but are instead received by special equipment (a DSLAM) at the telephone company central office.


title: Modem

text: Because the signal does not pass through the telephone exchange, no "dialing" is required, and the bandwidth constraints of an ordinary voice call are not imposed. This allows much higher frequencies, and therefore much faster speeds. ADSL in particular is designed to permit voice calls and data usage over the same line simultaneously.


title: Modem

text: Similarly, cable modems use infrastructure originally intended to carry television signals, and like DSL, typically permit receiving television signals at the same time as broadband internet service.


title: Modem

text: Different terms are used for broadband modems, because they frequently contain more than just a modulation/demodulation component.


title: Modem

text: Because high-speed connections are frequently used by multiple computers at once, many broadband modems do not have direct (e.g. USB) PC connections, but connect over a network such as Ethernet or Wi-Fi. Early broadband modems offered Ethernet handoff allowing the use of one or more public IP addresses, but no other services such as NAT and DHCP that would allow multiple computers to share one connection. This led to many consumers purchasing separate "broadband routers," placed between the modem and their network, to perform these functions.


title: Modem

text: Eventually, ISPs began providing residential gateways which combined the modem and broadband router into a single package that provided routing, NAT, security features, and even Wi-Fi access in addition to modem functionality, so that subscribers could connect their entire household without purchasing any extra equipment. Even later, these devices were extended to provide "triple play" features such as telephony and television service. Nonetheless, these devices are still often referred to simply as "modems" by service providers and manufacturers.


title: Modem

text: Consequently, the terms "modem", "router", and "gateway" are now used interchangeably in casual speech, but in a technical context "modem" may carry a specific connotation of basic functionality with no routing or other features, while the others describe a device with features such as NAT.


title: Modem

text: Broadband modems may also handle authentication such as PPPoE. While it is often possible to authenticate a broadband connection from a users PC, as was the case with dial-up internet service, moving this task to the broadband modem allows it to establish and maintain the connection itself, which makes sharing access between PCs easier since each one does not have to authenticate separately. Broadband modems typically remain authenticated to the ISP as long as they are powered on.


title: Modem

text: Any communication technology sending digital data wirelessly involves a modem. This includes direct broadcast satellite, WiFi, WiMax, mobile phones, GPS, Bluetooth and NFC.


title: Modem

text: Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fiber optic is not economical.


title: Modem

text: Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many wireless communication links to work simultaneously on different frequencies.


title: Modem

text: Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time. Typically, transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure. Transparent modems are most commonly used by utility companies for data collection.


title: Modem

text: Smart modems come with media access controllers inside, which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world.


title: Modem

text: Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, 5G etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a laptop, mobile phone or other device, or be connected externally. External wireless modems include connect cards, USB modems, and cellular routers.


title: Modem

text: Most GSM wireless modems come with an integrated SIM cardholder (i.e. Huawei E220, Sierra 881.) Some models are also provided with a microSD memory slot and/or jack for additional external antenna, (Huawei E1762, Sierra Compass 885.)


title: Modem

text: The CDMA (EVDO) versions do not typically use R-UIM cards, but use Electronic Serial Number (ESN) instead.


title: Modem

text: Until the end of April 2011, worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3:1 because USB modems can be easily discarded. Embedded modems may overtake separate modems as tablet sales grow and the incremental cost of the modems shrinks, so by 2016, the ratio may change to 1:1.


title: Modem

text: Like mobile phones, mobile broadband modems can be SIM locked to a particular network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using an 'unlock code'.


title: Modem

text: A modem that connects to a fiber optic network is known as an optical network terminal (ONT) or optical network unit (ONU). These are commonly used in fiber to the home installations, installed inside or outside a house to convert the optical medium to a copper Ethernet interface, after which a router or gateway is often installed to perform authentication, routing, NAT, and other typical consumer internet functions, in addition to "triple play" features such as telephony and television service.


title: Modem

text: Fiber optic systems can use quadrature amplitude modulation to maximize throughput. 16QAM uses a 16-point constellation to send four bits per symbol, with speeds on the order of 200 or 400 gigabits per second. 64QAM uses a 64-point constellation to send six bits per symbol, with speeds up to 65 terabits per second. Although this technology has been announced, it may not yet be commonly used.


title: Modem

text: Although the name "modem" is seldom used, some high-speed home networking applications do use modems, such as powerline ethernet. The G.hn standard for instance, developed by ITU-T, provides a high-speed (up to 1 Gbit/s) local area network using existing home wiring (power lines, phone lines, and coaxial cables). G.hn devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal for transmission over the wire.


title: Modem

text: As described above, technologies like Wi-Fi and Bluetooth also use modems to communicate over radio at short distances.


title: Modem

text: A null modem cable is a specially wired cable connected between the serial ports of two devices, with the transmit and receive lines reversed. It is used to connect two devices directly without a modem. The same software or hardware typically used with modems (such as Procomm or Minicom) could be used with this type of connection.


title: Modem

text: A null modem adapter is a small device with plugs on both end which is placed on the end of a normal "straight-through" serial cable to convert it into a null-modem cable.


title: Modem

text: A "short haul modem" is a device that bridges the gap between leased-line and dial-up modems. Like a leased-line modem, they transmit over "bare" lines with no power or telco switching equipment, but are not intended for the same distances that leased lines can achieve. Ranges up to several miles are possible, but significantly, short-haul modems can be used for "medium" distances, greater than the maximum length of a basic serial cable but still relatively short, such as within a single building or campus. This allows a serial connection to be extended for perhaps only several hundred to several thousand feet, a case where obtaining an entire telephone or leased line would be overkill.


title: Modem

text: While some short-haul modems do in fact use modulation, low-end devices (for reasons of cost or power consumption) are simple "line drivers" that increase the level of the digital signal but do not modulate it. These are not technically modems, but the same terminology is used for them.


title: Train

text: In rail transport, a train (from Old French , from Latin , "to pull, to draw") is a series of connected vehicles that run along a railway track and transport people or freight. Trains are typically pulled or pushed by locomotives (often known simply as "engines"), though some are self-propelled, such as multiple units. Passengers and cargo are carried in railroad cars, also known as wagons. Trains are designed to a certain gauge, or distance between rails. Most trains operate on steel tracks with steel wheels, the low friction of which makes them more efficient than other forms of transport.


title: Train

text: Trains have their roots in wagonways, which used railway tracks and were powered by horses or pulled by cables. Following the invention of the steam locomotive in the United Kingdom in 1804, trains rapidly spread around the world, allowing freight and passengers to move over land faster and cheaper than ever possible before. Rapid transit and trams were first built in the late 1800s to transport large numbers of people in and around cities. Beginning in the 1920s, and accelerating following World War II, diesel and electric locomotives replaced steam as the means of motive power. Following the development of cars, trucks, and extensive networks of highways which offered greater mobility, as well as faster airplanes, trains declined in importance and market share, and many rail lines were abandoned. The spread of buses led to the closure of many rapid transit and tram systems during this time as well.


title: Train

text: Since the 1970s, governments, environmentalists, and train advocates have promoted increased use of trains due to their greater fuel efficiency and lower greenhouse gas emissions compared to other modes of land transport. High-speed rail, first built in the 1960s, has proven competitive with cars and planes over short to medium distances. Commuter rail has grown in importance since the 1970s as an alternative to congested highways and a means to promote development, as has light rail in the 21st century. Freight trains remain important for the transport of bulk commodities such as coal and grain, as well as being a means of reducing road traffic congestion by freight trucks.


title: Train

text: While conventional trains operate on relatively flat tracks with two rails, a number of specialized trains exist which are significantly different in their mode of operation. Monorails operate on a single rail, while funiculars and rack railways are uniquely designed to traverse steep slopes. Experimental trains such as high speed maglevs, which use magnetic levitation to float above a guideway, are under development in the 2020s and offer higher speeds than even the fastest conventional trains. Development of trains which use alternative fuels such as natural gas and hydrogen is another 21st century development.


title: Train

text: Trains are an evolution of wheeled wagons running on stone wagonways, the earliest of which were built by Babylon circa 2,200 BCE. Starting in the 1500s, wagonways were introduced to haul material from mines; from the 1790s, stronger iron rails were introduced. Following early developments in the second half of the 1700s, in 1804 a steam locomotive built by British inventor Richard Trevithick powered the first ever steam train. Outside of coal mines, where fuel was readily available, steam locomotives remained untried until the opening of the Stockton and Darlington Railway in 1825. British engineer George Stephenson ran a steam locomotive named Locomotion No. 1 on this long line, hauling over 400 passengers at up to . The success of this locomotive, and Stephenson's Rocket in 1829, convinced many of the value in steam locomotives, and within a decade the stock market bubble known as "Railway Mania" started across the United Kingdom.


title: Train

text: News of the success of steam locomotives quickly reached the United States, where the first steam railroad opened in 1829. American railroad pioneers soon started manufacturing their own locomotives, designed to handle the sharper curves and rougher track typical of the country's railroads.The other nations of Europe also took note of British railroad developments, and most countries on the continent constructed and opened their first railroads in the 1830s and 1840s, following the first run of a steam train in France in late 1829. In the 1850s, trains continued to expand across Europe, with many influenced by or purchases of American locomotive designs. Other European countries pursued their own distinct designs. Around the world, steam locomotives grew larger and more powerful throughout the rest of the century as technology advanced.


title: Train

text: Trains first entered service in South America, Africa, and Asia through construction by imperial powers, which starting in the 1840s built railroads to solidify control of their colonies and transport cargo for export. In Japan, which was never colonized, railroads first arrived in the early 1870s. By 1900, railroads were operating on every continent besides uninhabited Antarctica.


title: Train

text: Even as steam locomotive technology continued to improve, inventors in Germany started work on alternative methods for powering trains. Werner von Siemens built the first train powered by electricity in 1879, and went on to pioneer electric trams. Another German inventor, Rudolf Diesel, constructed the first diesel engine in the 1890s, though the potential of his invention to power trains was not realized until decades later. Between 1897 and 1903, tests of experimental electric locomotives on the Royal Prussian Military Railway in Germany demonstrated they were viable, setting speed records in excess of .Early gas powered "doodlebug" self-propelled railcars entered service on railroads in the first decade of the 1900s. Experimentation with diesel and gas power continued, culminating in the German "Flying Hamburger" in 1933, and the influential American EMD FT in 1939. These successful diesel locomotives showed that diesel power was superior to steam, due to lower costs, ease of maintenance, and better reliability. Meanwhile, Italy developed an extensive network of electric trains during the first decades of the 20th century, driven by that country's lack of significant coal reserves.


title: Train

text: World War II brought great destruction to existing railroads across Europe, Asia, and Africa. Following the war's conclusion in 1945, nations which had suffered extensive damage to their railroad networks took the opportunity provided by Marshall Plan funds (or economic assistance from the USSR and Comecon, for nations behind the Iron Curtain) and advances in technology to convert their trains to diesel or electric power. France, Russia, Switzerland, and Japan were leaders in adopting widespread electrified railroads, while other nations focused primarily on dieselization. By 1980, the majority of the world's steam locomotives had been retired, though they continued to be used in parts of Africa and Asia, along with a few holdouts in Europe and South America. China was the last country to fully dieselize, due to its abundant coal reserves; steam locomotives were used to haul mainline trains as late as 2005 in Inner Mongolia.


title: Train

text: Trains began to face strong competition from automobiles and freight trucks in the 1930s, which greatly intensified following World War II. After the war, air transport also became a significant competitor for passenger trains. Large amounts of traffic shifted to these new forms of transportation, resulting in a widespread decline in train service, both freight and passenger. A new development in the 1960s was high-speed rail, which runs on dedicated rights of way and travels at speeds of or greater. The first high-speed rail service was the Japanese Shinkansen, which entered service in 1964. In the following decades, high speed rail networks were developed across much of Europe and Eastern Asia, providing fast and reliable service competitive with automobiles and airplanes. The first high-speed train in the Americas was Amtrak's Acela in the United States, which entered service in 2000.


title: Train

text: Towards the end of the 20th century, increased awareness of the benefits of trains for transport led to a revival in their use and importance. Freight trains are significantly more efficient than trucks, while also emitting far fewer greenhouse gas emissions per ton-mile; passenger trains are also far more energy efficient than other modes of transport. According to the International Energy Agency, "On average, rail requires 12 times less energy and emits 7-11 times less GHGs per passenger-km travelled than private vehicles and airplanes, making it the most efficient mode of motorised passenger transport. Aside from shipping, freight rail is the most energy-efficient and least carbon-intensive way to transport goods." As such, rail transport is considered an important part of achieving sustainable energy. Intermodal freight trains, carrying double-stack shipping containers, have since the 1970s generated significant business for railroads and gained market share from trucks. Increased use of commuter rail has also been promoted as a means of fighting traffic congestion on highways in urban areas.


title: Train

text: Trains can be sorted into types based on whether they haul passengers or freight (though mixed trains which haul both exist), by their weight (heavy rail for regular trains, light rail for lighter rapid transit systems), by their speed, and by what form of track they use. Conventional trains operate on two rails, but several other types of track systems are also in use around the world.


title: Train

text: The railway terminology that is used to describe a train varies between countries. The two primary systems of terminology are International Union of Railways terms in much of the world, and Association of American Railroads terms in North America.


title: Train

text: Trains are typically defined as one or more locomotives coupled together, with or without cars. A collection of passenger or freight carriages connected together (not necessarily with a locomotive) is (especially in British and Indian English) typically referred to as a rake. A collection of rail vehicles may also be called a consist. A set of vehicles that are permanently or semi-permanently coupled together (such as the Pioneer Zephyr) is called a trainset. The term rolling stock is used to describe any kind of train vehicle.


title: Train

text: Bogies, also known in North America as trucks, support the wheels and axles of trains. Trucks range from just one axle to as many as four or more. Two-axle trucks are in the widest use worldwide, as they are better able to handle curves and support heavy loads than single axle trucks.


title: Train

text: Train vehicles are linked to one another by various systems of coupling. In much of Europe, India, and South America, trains primarily use buffers and chain couplers, while in the rest of the world knuckle couplers are used.


title: Train

text: Because trains are heavy, powerful brakes are needed to slow or stop trains, and because steel wheels on steel rails have relatively low friction, brakes must be distributed among as many wheels as possible. Early trains could only be stopped by manually applied hand brakes, requiring workers to ride on top of the cars and apply the brakes when the train went downhill. Hand brakes are still used to park cars and locomotives, but the predominant braking system for trains globally is air brakes, invented in 1869 by George Westinghouse. Air brakes are applied at once to the entire train using air hoses.


title: Train

text: For safety and communication, trains are equipped with bells, horns, and lights. Steam locomotives typically use steam whistles rather than horns. Other types of lights may be installed on locomotives and cars, such as classification lights, Mars Lights, and ditch lights.


title: Train

text: Locomotives are in most cases equipped with cabs, also known as driving compartments, where a train driver controls the train's operation. They may also be installed on unpowered train cars known as cab or control cars, to allow for a train to operate with the locomotive at the rear.


title: Train

text: To prevent collisions or other accidents, trains are often scheduled, and almost always are under the control of train dispatchers. Historically, trains operated based on timetables; most passenger trains continue to operate based on fixed schedules, though freight trains may instead run on an as-needed basis, or when enough freight cars are available to justify running a train.


title: Train

text: Simple repairs may be done while a train is parked on the tracks, but more extensive repairs will be done at a motive power depot. Similar facilities exist for repairing damaged or defective train cars. Maintenance of way trains are used to build and repair railroad tracks and other equipment.


title: Train

text: Train drivers, also known as engineers, are responsible for operating trains. Conductors are in charge of trains and their cargo, and help passengers on passenger trains. Brakeman, also known as trainmen, were historically responsible for manually applying brakes, though the term is used today to refer to crew members who perform tasks such as operating switches, coupling and uncoupling train cars, and setting handbrakes on equipment. Steam locomotives require a fireman who is responsible for fueling and regulating the locomotive's fire and boiler. On passenger trains, other crew members assist passengers, such as chefs to prepare food, and service attendants to provide food and drinks to passengers. Other passenger train specific duties include passenger car attendants, who assist passengers with boarding and alighting from trains, answer questions, and keep train cars clean, and sleeping car attendants, who perform similar duties in sleeping cars.


title: Train

text: Around the world, various track gauges are in use for trains. In most cases, trains can only operate on tracks that are of the same gauge; where different gauge trains meet, it is known as a break of gauge. Standard gauge, defined as between the rails, is the most common gauge worldwide, though both broad-gauge and narrow-gauge trains are also in use. Trains also need to fit within the loading gauge profile to avoid fouling bridges and lineside infrastructure with this being a potential limiting factor on loads such as intermodal container types that may be carried.


title: Train

text: Train accidents sometimes occur, including derailments (when a train leaves the tracks) and train wrecks (collisions between trains). Accidents were more common in the early days of trains, when railway signal systems, centralized traffic control, and failsafe systems to prevent collisions were primitive or did not yet exist. To prevent accidents, systems such as automatic train stop are used; these are failsafe systems that apply the brakes on a train if it passes a red signal and enters an occupied block, or if any of the train's equipment malfunctions. More advanced safety systems, such as positive train control, can also automatically regulate train speed, preventing derailments from entering curves or switches too fast.


title: Train

text: Modern trains have a very good safety record overall, comparable with air travel. In the United States between 2000 and 2009, train travel averaged 0.43 deaths per billion passenger miles traveled. While this was higher than that of air travel at 0.07 deaths per billion passenger miles, it was also far below the 7.28 deaths per billion passenger miles of car travel. In the 21st century, several derailments of oil trains caused fatalities, most notably the Canadian Lac-Mégantic rail disaster in 2013 which killed 47 people and leveled much of the town of Lac-Mégantic.


title: Train

text: The vast majority of train-related fatalities, over 90 percent, are due to trespassing on railroad tracks, or collisions with road vehicles at level crossings. Organizations such as Operation Lifesaver have been formed to improve safety awareness at railroad crossings, and governments have also launched ad campaigns. Trains cannot stop quickly when at speed; even an emergency brake application may still require more than a mile of stopping distance. As such, emphasis is on educating motorists to yield to trains at crossings and avoid trespassing.


title: Train

text: Steam locomotives work by using a boiler to heat water into steam, which powers the locomotive's pistons which are in turn connected to the wheels. In the mid 20th century, most steam locomotives were replaced by diesel or electric locomotives, which were cheaper, cleaner, and more reliable. Steam locomotives are still used in heritage railways operated in many countries for the leisure and enthusiast market.


title: Train

text: Diesel locomotives are powered with a diesel engine, which generates electricity to drive traction motors. This is known as a diesel–electric transmission, and is used on almost all diesels. Diesel power replaced steam for a variety of reasons: diesel locomotives were less complex, far more reliable, cheaper, cleaner, easier to maintain, and more fuel efficient.


title: Train

text: Electric trains receive their current via overhead lines or through a third rail electric system, which is then used to power traction motors that drive the wheels. Electric traction offers a lower cost per mile of train operation but at a higher initial cost, which can only be justified on high traffic lines. Even though the cost per mile of construction is much higher, electric traction is cheaper to operate thanks to lower maintenance and purchase costs for locomotives and equipment. Compared to diesel locomotives, electric locomotives produce no direct emissions and accelerate much faster, making them better suited to passenger service, especially underground.


title: Train

text: In the mid 1900s, gas turbine locomotives were developed and successfully used, though most were retired due to high fuel costs and poor reliability.


title: Train

text: In the 21st century, alternative fuels for locomotives are under development, due to increasing costs for diesel and a desire to reduce greenhouse gas emissions from trains. Examples include hydrail (trains powered by hydrogen fuel cells) and the use of compressed or liquefied natural gas.


title: Train

text: Train cars, also known as wagons, are unpowered rail vehicles which are typically pulled by locomotives. Many different types exist, specialized to handle various types of cargo. Some common types include boxcars (also known as covered goods wagons) that carry a wide variety of cargo, flatcars (also known as flat wagons) which have flat tops to hold cargo, hopper cars which carry bulk commodities, and tank cars which carry liquids and gases. Examples of more specialized types of train cars include bottle cars which hold molten steel, Schnabel cars which handle very heavy loads, and refrigerator cars which carry perishable goods.


title: Train

text: Early train cars were small and light, much like early locomotives, but over time they have become larger as locomotives have become more powerful.


title: Train

text: A passenger train is used to transport people along a railroad line. These trains may consist of unpowered passenger railroad cars (also known as coaches or carriages) hauled by one or more locomotives, or may be self-propelled; self propelled passenger trains are known as multiple units or railcars. Passenger trains travel between stations or depots, where passengers may board and disembark. In most cases, passenger trains operate on a fixed schedule and have priority over freight trains.


title: Train

text: Long distance passenger trains travel over hundreds or even thousands of miles between cities. The longest passenger train service in the world is Russia's Trans-Siberian Railway between Moscow and Vladivostok, a distance of . In general, long distance trains may take days to complete their journeys, and stop at dozens of stations along their routes. For many rural communities, they are the only form of public transportation available.


title: Train

text: Short distance or regional passenger trains have travel times measured in hours or even minutes, as opposed to days. They run more frequently than long distance trains, and are often used by commuters. Short distance passenger trains specifically designed for commuters are known as commuter rail.


title: Train

text: High speed trains are designed to be much faster than conventional trains, and typically run on their own separate tracks than other, slower trains. The first high speed train was the Japanese Shinkansen, which opened in 1964. In the 21st century, services such as the French TGV and German Intercity Express are competitive with airplanes in travel time over short to medium distances.