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Thursday, November 6, 2025

Alternate history

From Wikipedia, the free encyclopedia
A painting by Jakub Różalski depicts an alternate history of the 1920s in Iron Harvest, in which rural peasants must contend with giant mechanical walking tanks.

Alternate history (also called alternative history, allohistoryalthist, or simply A.H.) is a subgenre of speculative fiction in which one or more historical events have occurred but are resolved differently than in real history. As conjecture based upon historical fact, alternate history stories propose "what if?" scenarios about pivotal events in human history, and present outcomes very different from the historical record. Some alternate histories are considered a subgenre of science fiction, or historical fiction.

Since the 1950s, as a subgenre of science fiction, some alternative history stories have featured the tropes of time travel between histories, the psychic awareness of the existence of an alternative universe by the inhabitants of a given universe, and time travel that divides history into various timestreams.

Definition

Often described as a subgenre of science fiction, alternative history is a genre of fiction wherein the author speculates upon how the course of history might have been altered if a particular historical event had an outcome different from the real life outcome. An alternate history requires three conditions: (i) A point of divergence from the historical record, before the time in which the author is writing; (ii) A change that would alter known history; and (iii) An examination of the ramifications of that alteration to history. Occasionally, some types of genre fiction are misidentified as alternative history, specifically science fiction stories set in what was then the future, but is now the past. Examples include the novels 2001: A Space Odyssey (1968) by Arthur C. Clarke and 1984 (1949) by George Orwell, as well as the movie 2012 (2009). None of these stories present an alternate history of events before the time of writing.

Similar to the genre of alternative history, there is also the genre of secret history - which can be either fictional or non-fictional - which documents events that might have occurred in history, but which had no effect upon the recorded historical outcome. Alternative history also is thematically related to, but distinct from, counterfactual history, which is a form of historiography that explores historical events in an extrapolated timeline in which key historical events either did not occur or had an outcome different from the historical record, in order to understand what did happen.

History of literature

Antiquity and medieval

Title page of the first Spanish-language translation of Joanot Martorell's Tirant lo Blanch (originally in Catalan)

The earliest example of alternate (or counterfactual) history is found in Livy's Ab Urbe Condita Libri (book IX, sections 17–19). Livy contemplated an alternative 4th century BC in which Alexander the Great had survived to attack Europe as he had planned; asking, "What would have been the results for Rome if she had been engaged in a war with Alexander?" Livy concluded that the Romans would likely have defeated Alexander. An even earlier possibility is Herodotus's Histories, which contains speculative material.

Another example of counterfactual history was posited by cardinal and Doctor of the Church Peter Damian in the 11th century. In his famous work De Divina Omnipotentia, a long letter in which he discusses God's omnipotence, he treats questions related to the limits of divine power, including the question of whether God can change the past, for example, bringing about that Rome was never founded:

I see I must respond finally to what many people, on the basis of your holiness's [own] judgment, raise as an objection on the topic of this dispute. For they say: If, as you assert, God is omnipotent in all things, can he manage this, that things that have been made were not made? He can certainly destroy all things that have been made, so that they do not exist now. But it cannot be seen how he can bring it about that things that have been made were not made. To be sure, it can come about that from now on and hereafter Rome does not exist; for it can be destroyed. But no opinion can grasp how it can come about that it was not founded long ago...

One early work of fiction detailing an alternate history is Joanot Martorell's 1490 epic romance Tirant lo Blanch, which was written when the fall of Constantinople to the Turks was still a recent and traumatic memory for Christian Europe. It tells the story of the knight Tirant the White from Brittany who travels to the embattled remnants of the Byzantine Empire. He becomes a Megaduke and commander of its armies and manages to fight off the invading Ottoman armies of Mehmet II. He saves the city from Islamic conquest, and even chases the Turks deeper into lands they had previously conquered.

19th century

One of the earliest works of alternate history published in large quantities for the reception of a large audience may be Louis Geoffroy's Histoire de la Monarchie universelle : Napoléon et la conquête du monde (1812–1832) (History of the Universal Monarchy: Napoleon and the Conquest of the World) (1836), which imagines Napoleon's First French Empire emerging victorious in the French invasion of Russia in 1812 and in an invasion of England in 1814, later unifying the world under Bonaparte's rule.

The Glorious Appearing of Jesus to the Nephites by William Armitage

The Book of Mormon (published 1830) is described as an "alternative history" by Richard Lyman Bushman, a biographer of Joseph Smith. Smith claimed to have translated the document from golden plates, which told the story of a Jewish group who migrated from Israel to the Americas and inhabited the region from about 600 B.C. to 400 A.D., becoming the ancestors of Native Americans. In the 2005 biography Joseph Smith: Rough Stone Rolling, Bushman wrote that the Book of Mormon "turned American history upside down [and] works on the premise that a history—a book—can reconstitute a nation. It assumes that by giving a nation an alternative history, alternative values can be made to grow."

In the English language, the first known complete alternate history may be Nathaniel Hawthorne's short story "P.'s Correspondence", published in 1845. It recounts the tale of a man who is considered "a madman" due to his perceptions of a different 1845, a reality in which long-dead famous people, such as the poets Robert Burns, Lord Byron, Percy Bysshe Shelley, and John Keats, the actor Edmund Kean, the British politician George Canning, and Napoleon Bonaparte, are still alive.

The first novel-length alternate history in English would seem to be Castello Holford's Aristopia (1895). While not as nationalistic as Geoffroy's Napoléon et la conquête du monde, 1812–1823, Aristopia is another attempt to portray a Utopian society. In Aristopia, the earliest settlers in Virginia discover a reef made of solid gold and are able to build a Utopian society in North America.

Early 20th century and the era of the pulps

In 1905, H. G. Wells published A Modern Utopia. As explicitly noted in the book itself, Wells's main aim in writing it was to set out his social and political ideas, the plot serving mainly as a vehicle to expound them. This book introduced the idea of a person being transported from a point in our familiar world to the precise geographical equivalent point in an alternate world in which history had gone differently. The protagonists undergo various adventures in the alternate world, and then are finally transported back to our world, again to the precise geographical equivalent point. Since then, that has become a staple of the alternate history genre.

A number of alternate history stories and novels appeared in the late 19th and early 20th centuries (see, for example, Joseph Edgar Chamberlin's The Ifs of History [1907] and Charles Petrie's If: A Jacobite Fantasy [1926]). In 1931, British historian Sir John Squire collected a series of essays from some of the leading historians of the period for his anthology If It Had Happened Otherwise. In that work, scholars from major universities, as well as important non-academic authors, turned their attention to such questions as "If the Moors in Spain Had Won" and "If Louis XVI Had Had an Atom of Firmness". The essays range from serious scholarly efforts to Hendrik Willem van Loon's fanciful and satiric portrayal of an independent 20th-century New Amsterdam, a Dutch city-state on the island of Manhattan. Among the authors included were Hilaire Belloc, André Maurois, and Winston Churchill.

One of the entries in Squire's volume was Churchill's "If Lee Had Not Won the Battle of Gettysburg", written from the viewpoint of a historian in a world in which the Confederacy had won the American Civil War. The entry considers what would have happened if the North had been victorious (in other words, a character from an alternate world imagines a world more like the real one we live in, although it is not identical in every detail). Speculative work that narrates from the point of view of an alternate history is variously known as "recursive alternate history", a "double-blind what-if", or an "alternate-alternate history". Churchill's essay was one of the influences behind Ward Moore's alternate history novel Bring the Jubilee in which General Robert E. Lee won the Battle of Gettysburg and paved the way for the eventual victory of the Confederacy in the American Civil War (named the "War of Southron Independence" in this timeline). The protagonist, the autodidact Hodgins Backmaker, travels back to the aforementioned battle and inadvertently changes history, which results in the emergence of our own timeline and the consequent victory of the Union instead.

The American humorist author James Thurber parodied alternate history stories about the American Civil War in his 1930 story "If Grant Had Been Drinking at Appomattox", which he accompanied with this very brief introduction: "Scribner's magazine is publishing a series of three articles: 'If Booth Had Missed Lincoln', 'If Lee Had Won the Battle of Gettysburg', and 'If Napoleon Had Escaped to America'. This is the fourth".

Another example of alternate history from this period (and arguably the first that explicitly posited cross-time travel from one universe to another as anything more than a visionary experience) is H.G. Wells' Men Like Gods (1923) in which the London-based journalist Mr. Barnstable, along with two cars and their passengers, is mysteriously teleported into "another world", which the "Earthlings" call Utopia. Being far more advanced than Earth, Utopia is some 3000 years ahead of humanity in its development. Wells describes a multiverse of alternative worlds, complete with the paratime travel machines that would later become popular with American pulp writers. However, since his hero experiences only a single alternate world, the story is not very different from conventional alternate history.

In the 1930s, alternate history moved into a new arena. The December 1933 issue of Astounding published Nat Schachner's "Ancestral Voices", which was quickly followed by Murray Leinster's "Sidewise in Time" (1934). While earlier alternate histories examined reasonably-straightforward divergences, Leinster attempted something completely different. In his "World gone mad", pieces of Earth traded places with their analogs from different timelines. The story follows Professor Minott and his students from a fictitious Robinson College as they wander through analogues of worlds that followed a different history.[citation needed] "Sidewise in Time" has been described as "the point at which the alternate history narrative first enters science fiction as a plot device" and is the story for which the Sidewise Award for Alternate History is named.

The world in 1964 in the novel Fatherland in which the Nazis won World War II

A somewhat similar approach was taken by Robert A. Heinlein in his 1941 novelette Elsewhen in which a professor trains his mind to move his body across timelines. He then hypnotizes his students so that they can explore more of them. Eventually, each settles into the reality that is most suitable for him or her. Some of the worlds they visit are mundane, some are very odd, and others follow science fiction or fantasy conventions.

World War II produced alternate history for propaganda: both British and American authors wrote works depicting Nazi invasions of their respective countries as cautionary tales.

Time travel to create historical divergences

The period around World War II also saw the publication of the time travel novel Lest Darkness Fall by L. Sprague de Camp in which an American academic travels to Italy at the time of the Byzantine invasion of the Ostrogoths. De Camp's time traveler, Martin Padway, is depicted as making permanent historical changes and implicitly forming a new time branch, thereby making the work an alternate history.

In William Tenn's short story Brooklyn Project (1948), a tyrannical US Government brushes aside the warnings of scientists about the dangers of time travel and goes on with a planned experiment - with the result that minor changes to the prehistoric past cause Humanity to never have existed, its place taken by tentacled underwater intelligent creatures - who also have a tyrannical government which also insists on experimenting with time-travel.

In Ray Bradbury's classic short story "A Sound of Thunder" (1952) a group of hunters travel to the Late Cretaceous to hunt dinosaurs whose death would not be considered consequential as they are about to die a natural death within two minutes of the encounter. To minimize risking changes history they are told to stay on a levitating antigravity path that touches nothing. However one of the hunters stumbles off the path, inadvertently crushing a butterfly. When the group returns they find that history became significantly harsher and a fascist is now President.

Time travel as the cause of a point of divergence (POD), which can denote either the bifurcation of a historical timeline or a simple replacement of the future that existed before the time-travelling event, has continued to be a popular theme. In Ward Moore's Bring the Jubilee (1953), the protagonist lives in an alternate history in which the Confederacy has won the American Civil War. He travels backward through time and brings about a Union victory at the Battle of Gettysburg.

When a story's assumptions about the nature of time travel lead to the complete replacement of the visited time's future, rather than just the creation of an additional time line, the device of a "time patrol" is often used where guardians move through time to preserve the "correct" history.

A more recent example is Making History by Stephen Fry in which a time machine is used to alter history so that Adolf Hitler was never born. That ironically results in a more competent leader of Nazi Germany and results in the country's ascendancy and longevity in the altered timeline.

Quantum theory of many worlds

While many justifications for alternate histories involve a multiverse, the "many world" theory would naturally involve many worlds, in fact a continually exploding array of universes. In quantum theory, new worlds would proliferate with every quantum event, and even if the writer uses human decisions, every decision that could be made differently would result in a different timeline. A writer's fictional multiverse may, in fact, preclude some decisions as humanly impossible, as when, in Night Watch, Terry Pratchett depicts a character informing Vimes that while anything that can happen, has happened, nevertheless there is no history whatsoever in which Vimes has ever murdered his wife. When the writer explicitly maintains that all possible decisions are made in all possible ways, one possible conclusion is that the characters were neither brave, nor clever, nor skilled, but simply lucky enough to happen on the universe in which they did not choose the cowardly route, take the stupid action, fumble the crucial activity, etc.; few writers focus on this idea, although it has been explored in stories such as Larry Niven's story All the Myriad Ways, where the reality of all possible universes leads to an epidemic of suicide and crime because people conclude their choices have no moral import.

In any case, even if it is true that every possible outcome occurs in some world, it can still be argued that traits such as bravery and intelligence might still affect the relative frequency of worlds in which better or worse outcomes occurred (even if the total number of worlds with each type of outcome is infinite, it is still possible to assign a different measure to different infinite sets). The physicist David Deutsch, a strong advocate of the many-worlds interpretation of quantum mechanics, has argued along these lines, saying that "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives. When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen." This view is perhaps somewhat too abstract to be explored directly in science fiction stories, but a few writers have tried, such as Greg Egan in his short story The Infinite Assassin, where an agent is trying to contain reality-scrambling "whirlpools" that form around users of a certain drug, and the agent is constantly trying to maximize the consistency of behavior among his alternate selves, attempting to compensate for events and thoughts he experiences, he guesses are of low measure relative to those experienced by most of his other selves.

Many writers—perhaps the majority—avoid the discussion entirely. In one novel of this type, H. Beam Piper's Lord Kalvan of Otherwhen, a Pennsylvania State Police officer, who knows how to make gunpowder, is transported from our world to an alternate universe where the recipe for gunpowder is a tightly held secret and saves a country that is about to be conquered by its neighbors. The paratime patrol members are warned against going into the timelines immediately surrounding it, where the country will be overrun, but the book never depicts the slaughter of the innocent thus entailed, remaining solely in the timeline where the country is saved.

The cross-time theme was further developed in the 1960s by Keith Laumer in the first three volumes of his Imperium sequence, which would be completed in Zone Yellow (1990). Piper's politically more sophisticated variant was adopted and adapted by Michael Kurland and Jack Chalker in the 1980s; Chalker's G.O.D. Inc trilogy (1987–89), featuring paratime detectives Sam and Brandy Horowitz, marks the first attempt at merging the paratime thriller with the police procedural. Kurland's Perchance (1988), the first volume of the never-completed "Chronicles of Elsewhen", presents a multiverse of secretive cross-time societies that utilize a variety of means for cross-time travel, ranging from high-tech capsules to mutant powers. Crosstime Traffic is a 6-book series written by Harry Turtledove aimed at teenagers featuring a variant of H. Beam Piper's paratime trading empire. While the home timeline appears to be the same in each of the books there is no overlap in characters or repetition of the alternative worlds.

Rival paratime worlds

The concept of a cross-time version of a world war, involving rival paratime empires, was developed in Fritz Leiber's Change War series, starting with the Hugo Award winning The Big Time (1958); followed by Richard C. Meredith's Timeliner trilogy in the 1970s, Michael McCollum's A Greater Infinity (1982) and John Barnes' Timeline Wars trilogy in the 1990s.

Such "paratime" stories may include speculation that the laws of nature can vary from one universe to the next, providing a science fictional explanation—or veneer—for what is normally fantasy. Aaron Allston's Doc Sidhe and Sidhe Devil take place between our world, the "grim world" and an alternate "fair world" where the Sidhe retreated to. Although technology is clearly present in both worlds, and the "fair world" parallels our history, about fifty years out of step, there is functional magic in the fair world. Even with such explanation, the more explicitly the alternate world resembles a normal fantasy world, the more likely the story is to be labelled fantasy, as in Poul Anderson's "House Rule" and "Loser's Night". In both science fiction and fantasy, whether a given parallel universe is an alternate history may not be clear. The writer might allude to a POD only to explain the existence and make no use of the concept, or may present the universe without explanation of its existence.

Major writers explore alternate histories

Isaac Asimov's short story "What If—" (1952) is about a couple who can explore alternate realities by means of a television-like device. This idea can also be found in Asimov's novel The End of Eternity (1955), in which the "Eternals" can change the realities of the world, without people being aware of it. Poul Anderson's Time Patrol stories feature conflicts between forces intent on changing history and the Patrol who work to preserve it. One story, Delenda Est, describes a world in which Carthage triumphed over the Roman Republic. The Big Time, by Fritz Leiber, describes a Change War ranging across all of history.

Keith Laumer's Worlds of the Imperium is one of the earliest alternate history novels; it was published by Fantastic Stories of the Imagination in 1961, in magazine form, and reprinted by Ace Books in 1962 as one half of an Ace Double. Besides our world, Laumer describes a world ruled by an Imperial aristocracy formed by the merger of European empires, in which the American Revolution never happened, and a third world in post-war chaos ruled by the protagonist's doppelganger.

A map of the United States as depicted in The Man in the High Castle TV series, based on Philip K. Dick's The Man in the High Castle

Philip K. Dick's novel, The Man in the High Castle (1962), is an alternate history in which Nazi Germany and Imperial Japan won World War II. This book contains an example of "alternate-alternate" history, in that one of its characters authored a book depicting a reality in which the Allies won the war, itself divergent from real-world history in several aspects. The several characters live within a divided United States, in which the Empire of Japan takes the Pacific states, governing them as a puppet, Nazi Germany takes the East Coast of the United States and parts of the Midwest, with the remnants of the old United States' government as the Neutral Zone, a buffer state between the two superpowers. The book has inspired an Amazon series of the same name.

Vladimir Nabokov's novel, Ada or Ardor: A Family Chronicle (1969), is a story of incest that takes place within an alternate North America settled in part by Czarist Russia and that borrows from Dick's idea of "alternate-alternate" history (the world of Nabokov's hero is wracked by rumors of a "counter-earth" that apparently is ours). Some critics believe that the references to a counter-earth suggest that the world portrayed in Ada is a delusion in the mind of the hero (another favorite theme of Dick's novels). Strikingly, the characters in Ada seem to acknowledge their own world as the copy or negative version, calling it "Anti-Terra", while its mythical twin is the real "Terra". Like history, science has followed a divergent path on Anti-Terra: it boasts all the same technology as our world, but all based on water instead of electricity; e.g., when a character in Ada makes a long-distance call, all the toilets in the house flush at once to provide hydraulic power.

Guido Morselli described the defeat of Italy (and subsequently France) in World War I in his novel, Past Conditional (1975; Contro-passato prossimo), wherein the static Alpine front line which divided Italy from Austria during that war collapses when the Germans and the Austrians forsake trench warfare and adopt blitzkrieg twenty years in advance.

Kingsley Amis set his novel, The Alteration (1976), in the 20th century, but major events in the Reformation did not take place, and Protestantism is limited to the breakaway Republic of New England. Martin Luther was reconciled to the Roman Catholic Church and later became Pope Germanian I.

In Nick Hancock and Chris England's 1997 book What Didn't Happen Next: An Alternative History of Football it is suggested that, had Gordon Banks been fit to play in the 1970 FIFA World Cup quarter-final, there would have been no Thatcherism and the post-war consensus would have continued indefinitely.

Kim Stanley Robinson's novel, The Years of Rice and Salt (2002), starts at the point of divergence with Timur turning his army away from Europe, and the Black Death has killed 99% of Europe's population, instead of only a third. Robinson explores world history from that point in AD 1405 (807 AH) to about AD 2045 (1467 AH). Rather than following the great man theory of history, focusing on leaders, wars, and major events, Robinson writes more about social history, similar to the Annales School of history theory and Marxist historiography, focusing on the lives of ordinary people living in their time and place.

Philip Roth's novel, The Plot Against America (2004), looks at an America where Franklin D. Roosevelt is defeated in 1940 in his bid for a third term as President of the United States, and Charles Lindbergh is elected, leading to a US that features increasing fascism and anti-Semitism.

Michael Chabon, occasionally an author of speculative fiction, contributed to the genre with his novel The Yiddish Policemen's Union (2007), which explores a world in which the State of Israel was destroyed in its infancy and many of the world's Jews instead live in a small strip of Alaska set aside by the US government for Jewish settlement. The story follows a Jewish detective solving a murder case in the Yiddish-speaking semi-autonomous city state of Sitka. Stylistically, Chabon borrows heavily from the noir and detective fiction genres, while exploring social issues related to Jewish history and culture. Apart from the alternate history of the Jews and Israel, Chabon also plays with other common tropes of alternate history fiction; in the book, Germany actually loses the war even harder than they did in reality, getting hit with a nuclear bomb instead of just simply losing a ground war (subverting the common "what if Germany won WWII?" trope).

The world of 1942, as depicted at the start of S. M. Stirling's The Domination series
World War I from Harry Turtledove's Southern Victory ("Timeline 191") series

The late 1980s and the 1990s saw a boom in popular-fiction versions of alternate history, fueled by the emergence of the prolific alternate history author Harry Turtledove, as well as the development of the steampunk genre and two series of anthologies—the What Might Have Been series edited by Gregory Benford and the Alternate ... series edited by Mike Resnick. This period also saw alternate history works by S. M. Stirling, Kim Stanley Robinson, Harry Harrison, Howard Waldrop, Peter Tieryas, and others.

In 1986, a sixteen-part epic comic book series called Captain Confederacy began examining a world where the Confederate States of America won the American Civil War. In the series, the Captain and others heroes are staged government propaganda events featuring the feats of these superheroes.

Since the late 1990s, Harry Turtledove has been the most prolific practitioner of alternate history and has been given the title "Master of Alternate History" by some. His books include those of Timeline 191 (a.k.a. Southern Victory, also known as TL-191), in which, while the Confederate States of America won the American Civil War, the Union and Imperial Germany defeat the Entente Powers in the two "Great War"s of the 1910s and 1940s (with a Nazi-esque Confederate government attempting to exterminate its black population), and the Worldwar series, in which aliens invaded Earth during World War II.[citation needed] Other stories by Turtledove include A Different Flesh, in which the Americas were not populated from Asia during the last ice age; In the Presence of Mine Enemies, in which the Nazis won World War II;[citation needed] and Ruled Britannia, in which the Spanish Armada succeeded in conquering England in the Elizabethan era, with William Shakespeare being given the task of writing the play that will motivate the Britons to rise up against their Spanish conquerors. He also co-authored a book with actor Richard Dreyfuss, The Two Georges, in which the United Kingdom retained the American colonies, with George Washington and King George III making peace. He did a two-volume series in which the Japanese not only bombed Pearl Harbor but also invaded and occupied the Hawaiian Islands.

Perhaps the most incessantly explored theme in popular alternate history focuses on the aftermath of an Axis victory in World War II. In some versions, the Nazis and/or Axis Powers win; or in others, they conquer most of the world but a "Fortress America" exists under siege; while in others, there is a Nazi/Japanese Cold War comparable to the US/Soviet equivalent in 'our' timeline. Fatherland (1992), by Robert Harris, is set in Europe following the Nazi victory. The novel Dominion by C.J. Sansom (2012) is similar in concept but is set in England, with Churchill the leader of an anti-German Resistance and other historic persons in various fictional roles. In the Mecha Samurai Empire series (2016), Peter Tieryas focuses on the Asian-American side of the alternate history, exploring an America ruled by the Japanese Empire while integrating elements of Asian pop culture like mechas and videogames.

Several writers have posited points of departure for such a world but then have injected time splitters from the future. For instance James P. Hogan's The Proteus Operation. Norman Spinrad wrote The Iron Dream in 1972, which is intended to be a science fiction novel written by Adolf Hitler after fleeing from Europe to North America in the 1920s.

In Jo Walton's "Small Change" series, the United Kingdom made peace with Hitler before the involvement of the United States in World War II, and slowly collapses due to severe economic depression. Former House Speaker Newt Gingrich and William R. Forstchen have written a novel, 1945, in which the US defeated Japan but not Germany in World War II, resulting in a Cold War with Germany rather than the Soviet Union. Gingrich and Forstchen neglected to write the promised sequel; instead, they wrote a trilogy about the American Civil War, starting with Gettysburg: A Novel of the Civil War, in which the Confederates win a victory at the Battle of Gettysburg - however, after Lincoln responds by bringing Grant and his forces to the eastern theater, the Army of Northern Virginia is soon trapped and destroyed in Maryland, and the war ends within weeks.

While World War II has been a common point of divergence in alternate history literature, several works have been based on other points of divergence. For example, Martin Cruz Smith, in his first novel, posited an independent American Indian nation following the defeat of Custer in The Indians Won (1970). Beginning with The Probability Broach in 1980, L. Neil Smith wrote several novels that postulated the disintegration of the US Federal Government after Albert Gallatin joins the Whiskey Rebellion in 1794 and eventually leads to the creation of a libertarian utopia. In the 2022 novel Poutine and Gin by Steve Rhinelander, the point of divergence is the Battle of the Plains of Abraham of the French and Indian War. That novel is a mystery set in 1940 of that time line.

A recent time traveling splitter variant involves entire communities being shifted elsewhere to become the unwitting creators of new time branches. These communities are transported from the present (or the near-future) to the past or to another timeline via a natural disaster, the action of technologically advanced aliens, or a human experiment gone wrong. S. M. Stirling wrote the Island in the Sea of Time trilogy, in which Nantucket Island and all its modern inhabitants are transported to Bronze Age times to become the world's first superpower. In Eric Flint's 1632 series, a small town in West Virginia is transported to 17th century central Europe and drastically changes the course of the Thirty Years' War, which was then underway. John Birmingham's Axis of Time trilogy deals with the culture shock when a United Nations naval task force from 2021 finds itself back in 1942 helping the Allies against the Empire of Japan and the Germans (and doing almost as much harm as good in spite of its advanced weapons). The series also explores the cultural impacts of people with 2021 ideals interacting with 1940s culture. Similarly, Robert Charles Wilson's Mysterium depicts a failed US government experiment which transports a small American town into an alternative version of the US run by Gnostics, who are engaged in a bitter war with the "Spanish" in Mexico (the chief scientist at the laboratory where the experiment occurred is described as a Gnostic, and references to Christian Gnosticism appear repeatedly in the book).

Although not dealing in physical time travel, in his alt-history novel Marx Returns, Jason Barker introduces anachronisms into the life and times of Karl Marx, such as when his wife Jenny sings a verse from the Sex Pistols's song "Anarchy in the U.K.", or in the games of chess she plays with the Marxes' housekeeper Helene Demuth, which on one occasion involves a Caro–Kann Defence. In her review of the novel, Nina Power writes of "Jenny's 'utopian' desire for an end to time", an attitude which, according to Power, is inspired by her husband's co-authored book The German Ideology. However, in keeping with the novel's anachronisms, the latter was not published until 1932. By contrast, the novel's timeline ends in 1871.

In the 2022 novel Hydrogen Wars: Atomic Sunrise by R.M. Christianson, a small change in post-war Japanese history leads to the election of General Douglas MacArthur as President of the United States. This minor change ultimately leads to all-out atomic war between the major Cold War powers.

Through crowdfunding on Kickstarter, Alan Jenkins and Gan Golan produced a graphic novel series called 1/6 depicting a dystopian alternate reality in which the January 6 United States Capitol attack was successful. What follows is the burning down of the Capitol building and the hanging of Vice President Mike Pence. Under Donald Trump's second term as president, a solid gold statue of him is erected and armed thugs patrol the streets of Washington, D.C. suppressing civilian resistance with brutal violence under the banner of the Confederate flag.

In fantasy genre

The Angevin Empire in 1172, before the point of divergence of Randall Garrett's Lord Darcy series

Many works of straight fantasy and science fantasy take place in historical settings, though with the addition of, for example, magic or mythological beasts. Some present a secret history in which the modern day world no longer believes that these elements ever existed. Many ambiguous alternate/secret histories are set in Renaissance or pre-Renaissance times, and may explicitly include a "retreat" from the world, which would explain the current absence of such phenomena. Other stories make plan a divergence of some kind.

In Poul Anderson's Three Hearts and Three Lions in which the Matter of France is history and the fairy folk are real and powerful. The same author's A Midsummer Tempest occurs in a world in which the plays of William Shakespeare (called here "the Great Historian"), presented the literal truth in every instance. The novel itself takes place in the era of Oliver Cromwell and Charles I. Here, the English Civil War had a different outcome, and the Industrial Revolution has occurred early.

Randall Garrett's "Lord Darcy" series presents a point of divergence: a monk systemizes magic rather than science, so the use of foxglove to treat heart disease is regarded as superstition. Another point of divergence occurs in 1199, when Richard the Lionheart survives the Siege of Chaluz and returns to England and makes the Angevin Empire so strong that it survives into the 20th century.

Jonathan Strange & Mr Norrell by Susanna Clarke takes place in an England where a separate Kingdom ruled by the Raven King and founded on magic existed in Northumbria for over 300 years. In Patricia Wrede's Regency fantasies, Great Britain has a Royal Society of Wizards.

The Tales of Alvin Maker series by Orson Scott Card (a parallel to the life of Joseph Smith, founder of the Latter Day Saint movement) takes place in an alternate America, beginning in the early 19th century. Prior to that time, a POD occurred: England, under the rule of Oliver Cromwell, had banished "makers", or anyone else demonstrating "knacks" (an ability to perform seemingly supernatural feats) to the North American continent. Thus the early American colonists embraced these gifts as perfectly ordinary, and counted on them as a part of their daily lives. The political division of the continent is considerably altered, with two large English colonies bookending a smaller "American" nation, one aligned with England, and the other governed by exiled Cavaliers. Actual historical figures are seen in a much different light: Ben Franklin is revered as the continent's finest "maker", George Washington was executed after being captured, and "Tom" Jefferson is the first president of "Appalachia", the result of a compromise between the Continentals and the British Crown.

On the other hand, when the "Old Ones" (fairies) still manifest themselves in England in Keith Roberts's Pavane, which takes place in a technologically backward world after a Spanish assassination of Elizabeth I allowed the Spanish Armada to conquer England, the possibility that the fairies were real but retreated from modern advances makes the POD possible: the fairies really were present all along, in a secret history.

Again, in the English Renaissance fantasy Armor of Light by Melissa Scott and Lisa A. Barnett, the magic used in the book, by Dr. John Dee and others, actually was practiced in the Renaissance; positing a secret history of effective magic makes this an alternate history with a point of departure. Sir Philip Sidney survives the Battle of Zutphen in 1586, and shortly thereafter saving the life of Christopher Marlowe.

When the magical version of our world's history is set in contemporary times, the distinction becomes clear between alternate history on the one hand and contemporary fantasy, using in effect a form of secret history (as when Josepha Sherman's Son of Darkness has an elf living in New York City, in disguise) on the other. In works such as Robert A. Heinlein's Magic, Incorporated where a construction company can use magic to rig up stands at a sporting event and Poul Anderson's Operation Chaos and its sequel Operation Luna, where djinns are serious weapons of war—with atomic bombs—the use of magic throughout the United States and other modern countries makes it clear that this is not secret history—although references in Operation Chaos to degaussing the effects of cold iron make it possible that it is the result of a POD. The sequel clarifies this as the result of a collaboration of Einstein and Planck in 1901, resulting in the theory of "rhea tics". Henry Moseley applies this theory to "degauss the effects of cold iron and release the goetic forces." This results in the suppression of ferromagnetism and the re-emergence of magic and magical creatures.

Alternate history shades off into other fantasy subgenres when the use of actual, though altered, history and geography decreases, although a culture may still be clearly the original source; Barry Hughart's Bridge of Birds and its sequels take place in a fantasy world, albeit one clearly based on China, and with allusions to actual Chinese history, such as the Empress Wu. Richard Garfinkle's Celestial Matters incorporates ancient Chinese physics and Greek Aristotelian physics, using them as if factual.

Alternate history has long been a staple of Japanese speculative fiction with such authors as Futaro Yamada and Ryō Hanmura writing novels set in recognizable historical settings with added supernatural or science fiction elements. Ryō Hanmura's 1973 Musubi no Yama Hiroku which recreated 400 years of Japan's history from the perspective of a secret magical family with psychic abilities. The novel has since come to be recognized as a masterpiece of Japanese speculative fiction. Twelve years later, author Hiroshi Aramata wrote the groundbreaking Teito Monogatari which reimagined the history of Tokyo across the 20th century in a world heavily influenced by the supernatural.

Disney's Pirates of the Caribbean series takes place in an alternate history. The filmmakers of The Curse of the Black Pearl made no secret about taking liberties with the time period in which their story takes place. Producer Jerry Bruckheimer explained that the film is a fantasy, but did want to be true to the overall feel of the era, paying particular attention to the years between 1720 and 1750 "in an effort to find an approximation." Director Gore Verbinski asserted that it takes place "roughly at the tail end of the Golden Age of Piracy, when the Morgans lived. Maybe the late 1720s." The crew went to great lengths to maintain authenticity, such as Jack Sparrow's sword being an original that dates from the 1750s. Ann C. Crispin knew about the Pirates universe being an alternate history writing the prequel novel The Price of Freedom, with Disney's instructions for Crispin being to "stick to historical fact, unless it conflicts with established Pirates of the Caribbean continuity." Crispin made a faithful effort to do this, having done plenty of research, with Under the Black Flag by David Cordingly being one of the four pirate-related books she found herself using the most consistently. According to production designer John Myhre, the filmmakers of the fourth film, On Stranger Tides, picked the date of 1750, or in the range of the mid-1700s. The film also featured Blackbeard, based on the historical figure and an element retained from the novel On Stranger Tides by Tim Powers. The history prior to On Stranger Tides is also slightly different from real-world history, with Blackbeard's death at Ocracoke Inlet in 1718 was considered a legend in the film, with Jack Sparrow saying he was beheaded, and that his headless body swam three times around his ship before climbing back on board. The fifth film, Dead Men Tell No Tales, also took place in the 1750s, with an early draft taking place sometime the Seven Years' War.

Television

1983 is set on a world where the Iron Curtain never fell and the Cold War continues until the present (2003).

An Englishman's Castle tells the story of the writer of a soap opera in a 1970s England which lost World War II. England is run by a collaborator government which strains to maintain a normal appearance of British life. Slowly, however, the writer begins to uncover the truth.

In the Community episode "Remedial Chaos Theory," each of the six members of the study group rolls a die to decide who has to go downstairs to accept a pizza delivery for the group, creating 6 different alternative worlds. Characters from the worst universe, "darkest timeline," would later appear in the "prime universe".

Confederate was a planned HBO series set on a world where the south won the US Civil War. Social media backlash during pre-production led to the series being cancelled with no episodes produced.

Counterpart tells of a United Nations agency that is responsible for monitoring passage between alternative worlds. Two of the worlds, Alpha and Prime, are locked in a cold war.

The Court-Martial of George Armstrong Custer is a 1977 telemovie where George Custer survives the Battle of Little Bighorn and faces a court martial hearing over his incompetence.

C.S.A.: The Confederate States of America presents itself as a British TV documentary uncovering some of the dark secrets of the Confederacy on a world where the south won the US Civil War.

Dark Skies tells that much of history having been shaped since the 1940s by a government conspiracy with aliens. One race of aliens can take over humans, while those immune to the alien's control fight back.

Doctor Who's main character has visited two alternative worlds in the TV show and several in its spin off media. The Third Doctor visits a world with a fascist Great Britain on the brink of destruction in Inferno, while the Tenth Doctor visits a Britain that has a President and blimps are a common form of transportation beset by Cybermen in Rise of the Cybermen / The Age of Steel. The Seventh Doctor faces a threat from an alternative world in Battlefield, where magic is real and the alternative version of The Doctor is hinted to be that reality's Merlin.

The U.S. flag in the Fallout (franchise)

Fallout shows a 1950s retro-future world that suffers a global nuclear war on the Amazon streaming service.

Fatherland is a TV movie set in a 1960 alternative world where US President Joseph Kennedy and Adolf Hitler have agreed to meet to discuss an end to their country's Cold War 15 years after the Axis victory in World War II. However, an American reporter has discovered proof of the long denied Final Solution threatens the meeting.

The anime Fena: Pirate Princess featured an alternate 18th century.

For All Mankind depicts an alternate timeline in which the Soviet crewed lunar program successfully lands on the Moon before the US Apollo program, resulting in a continued and intensified Space Race.

Fringe has the father of one of the main characters cross into another reality to steal that world's version of his son after his son dies. The second world has a slightly different history, with a few different states in the United States, such as only one Carolina and Upper Michigan as a state. In addition, the 9/11 attack didn't take down the Twin Towers but the White House. Also, several major DC Comics events are different, such as Superman not Supergirl dying during Crisis on Infinite Earths. The incursion to steal the son has many negative effects on that world, and while the realities start out as antagonist, they eventually work together to repair the damage.

The Man in the High Castle, an adaptation of the novel of the same name, showed a world where the Axis Powers won World War II.

Motherland: Fort Salem explores a female-dominated world in which witchcraft is real. Its world diverged from our timeline when the Salem witch trials are resolved by an agreement between witches and ungifted humans.

Noughts + Crosses is a British TV show set on a world where a powerful West African empire colonizes Europe 700 years before the start of the series.

Parallels was a planned TV show whose pilot was later released as a Netflix movie. The plot concerns a building which can shift realities every 36 hours and those who use the building to travel to other realities.

The Plot Against America is an HBO miniseries where Charles Lindbergh wins the 1940 US presidential election as an anti-war candidate who moves the country toward fascism.

Primal features Spear and Fang from the Prehistoric encountering with Ancient Egypt and Vikings era, and in episode The Primal Theory where Charles Darwin is alive in 1890 instead of 1882.

The TV show Sliders explores different possible alternate realities by having the protagonist "slide" into different parallel dimensions of the same planet Earth.

The Great Martian War 1913-1917 is an alternate history mockumentary where giant martians with machines invaded the Earth during WW1, causing huge technological upgrades and the entente and central powers fighting alongside each other.

SS-GB shows a world where the Axis Powers quickly win World War II, killing Churchill and installing a puppet government. However, British resistance fights back.

In the various Star Trek TV shows and spin off media a Mirror Universe has been encountered where Earth has an empire that subjugates other planets. Doppelgängers of the main cast of many the TV shows appear in that reality.

The Watchmen series is set on a world where costumed heroes were initially welcomed but later outlawed. It is set 34 years after the events of the comic book on which the series shares a name.

The Marvel Cinematic Universe series, Loki (2021 & 2023), on Disney+, shows an agency which prevents alterations to the timeline. Alternate versions of Loki from various universes appear.

The Marvel Cinematic Universe series, What If...? (2021–2024), on Disney+, shows alternate universes that depict alternate events from the MCU films.

Online

Fans of alternate history have made use of the internet from a very early point to showcase their own works and provide useful tools for those fans searching for anything alternate history, first in mailing lists and usenet groups, later in web databases and forums. The "Usenet Alternate History List" was first posted on 11 April 1991, to the Usenet newsgroup rec.arts.sf-lovers. In May 1995, the dedicated newsgroup soc.history.what-if was created for showcasing and discussing alternate histories. Its prominence declined with the general migration from unmoderated usenet to moderated web forums, most prominently AlternateHistory.com, the self-described "largest gathering of alternate history fans on the internet" with over 10,000 active members.

In addition to these discussion forums, in 1997 Uchronia: The Alternate History List was created as an online repository, now containing over 2,900 alternate history novels, stories, essays, and other printed materials in several different languages. Uchronia was selected as the Sci Fi Channel's "Sci Fi Site of the Week" twice.

Uchronia

In Spanish, French, German, Portuguese, Italian, Catalan, and Galician, the words uchronie, ucronia, and ucronía are native versions of alternate history, from which comes the English loanword uchronia. The English term uchronia is a neologism that is sometimes used in its original meaning as a straightforward synonym for alternate history. However, it may also now refer to other concepts, namely an umbrella genre of fiction that encompasses alternate history, parallel universes in fiction, and fiction based in futuristic or non-temporal settings.

Second

From Wikipedia, the free encyclopedia
 
A pendulum-governed escapement of a clock, ticking every second
General information
Unit systemSI
Unit oftime
Symbols

The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and lastly to 60 seconds each, for a total of 24 × 60 × 60 = 86,400 seconds per day. The current and formal definition in the International System of Units (SI) is more precise:

The second [...] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks. As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation.

The definition that is based on 186400 of a rotation of the earth is still used by the Universal Time 1 (UT1) system.

Etymology

"Minute" comes from the Latin pars minuta prima, meaning "first small part" i.e. first division of the hour – dividing into sixty, and "second" comes from the pars minuta secunda, "second small part", dividing again into sixty.

Uses

Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a two-digit seconds counter.

SI prefixes are frequently combined with the word second to denote subdivisions of the second: milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. Multiples of seconds are usually counted in hours and minutes. Though SI prefixes may also be used to form multiples of the second such as kiloseconds (thousands of seconds), such units are rarely used in practice. An everyday experience with small fractions of a second is a 1-gigahertz microprocessor that has a cycle time of 1 nanosecond. Camera shutter speeds are often expressed in fractions of a second, such as 130 second or 11000 second.

Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today. Small divisions of time could not be measured back then, so such divisions were mathematically derived. The first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s, atomic clocks became better timekeepers than Earth's rotation, and they continue to set the standard today.

Clocks and solar time

A mechanical clock, which does not depend on measuring the relative rotational position of the Earth, keeps uniform time called mean time, within whatever accuracy is intrinsic to it. That means that every second, minute and every other division of time counted by the clock has the same duration as any other identical division of time. A sundial, which measures the relative position of the Sun in the sky called apparent time, does not keep uniform time. The time kept by a sundial varies by time of year, meaning that seconds, minutes and every other division of time is a different duration at different times of the year. The time of day measured with mean time versus apparent time may differ by as much as 15 minutes, but a single day differs from the next by only a small amount; 15 minutes is a cumulative difference over a part of the year. The effect is due chiefly to the obliqueness of Earth's axis with respect to its orbit around the Sun.

The difference between apparent solar time and mean time was recognized by astronomers since antiquity, but prior to the invention of accurate mechanical clocks in the mid-17th century, sundials were the only reliable timepieces, and apparent solar time was the only generally accepted standard.

Events and units of time in seconds

Fractions of a second are usually denoted in decimal notation, for example 2.01 seconds, or two and one hundredth seconds. Multiples of seconds are usually expressed as minutes and seconds, or hours, minutes and seconds of clock time, separated by colons, such as 11:23:24, or 45:23 (the latter notation can give rise to ambiguity, because the same notation is used to denote hours and minutes). It rarely makes sense to express longer periods of time like hours or days in seconds, because they are awkwardly large numbers. For the metric unit of second, there are decimal prefixes representing 10−30 to 1030 seconds.

Some common units of time in seconds are: a minute is 60 seconds; an hour is 3,600 seconds; a day is 86,400 seconds; a week is 604,800 seconds; a year (other than leap years) is 31,536,000 seconds; and a (Gregorian) century averages 3,155,695,200 seconds; with all of the above excluding any possible leap seconds. In astronomy, a Julian year is precisely 31,557,600 seconds.

Some common events in seconds are: a stone falls about 4.9 meters from rest in one second; a pendulum of length about one meter has a swing of one second, so pendulum clocks have pendulums about a meter long; the fastest human sprinters run 10 meters in a second; an ocean wave in deep water travels about 23 meters in one second; sound travels about 343 meters in one second in air; light takes 1.3 seconds to reach Earth from the surface of the Moon, a distance of 384,400 kilometers.

Other units incorporating seconds

A second is directly part of other units, such as frequency measured in hertz (inverse seconds or s−1), speed in meters per second, and acceleration in meters per second squared. The metric system unit becquerel, a measure of radioactive decay, is measured in inverse seconds and higher powers of second are involved in derivatives of acceleration such as jerk. Though many derivative units for everyday things are reported in terms of larger units of time, not seconds, they are ultimately defined in terms of the SI second; this includes time expressed in hours and minutes, velocity of a car in kilometers per hour or miles per hour, kilowatt hours of electricity usage, and speed of a turntable in rotations per minute.

Moreover, most other SI base units are defined by their relationship to the second: the meter is defined by setting the speed of light (in vacuum) to be 299 792 458 m/s, exactly; definitions of the SI base units kilogram, ampere, kelvin, and candela also depend on the second. The only base unit whose definition does not depend on the second is the mole, and only two of the 22 named derived units, radian and steradian, do not depend on the second either.

Timekeeping standards

A set of atomic clocks throughout the world keeps time by consensus: the clocks "vote" on the correct time, and all voting clocks are steered to agree with the consensus, which is called International Atomic Time (TAI). TAI "ticks" atomic seconds.

Civil time is defined to agree with the rotation of the Earth. The international standard for timekeeping is Coordinated Universal Time (UTC). This time scale "ticks" the same atomic seconds as TAI, but inserts or omits leap seconds as necessary to correct for variations in the rate of rotation of the Earth.

A time scale in which the seconds are not exactly equal to atomic seconds is UT1, a form of universal time. UT1 is defined by the rotation of the Earth with respect to the Sun, and does not contain any leap seconds. UT1 always differs from UTC by less than a second.

Optical lattice clock

While they are not yet part of any timekeeping standard, optical lattice clocks with frequencies in the visible light spectrum now exist and are the most accurate timekeepers of all. A strontium clock with frequency 430 THz, in the red range of visible light, during the 2010s held the accuracy record: it gains or loses less than a second in 15 billion years, which is longer than the estimated age of the universe. Such a clock can measure a change in its elevation of as little as 2 cm by the change in its rate due to gravitational time dilation.

History of definition

There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock, which have each realized a sexagesimal division of the day from ancient astronomical calendars.

Sexagesimal divisions of calendar time and day

Civilizations in the classic period and earlier created divisions of the calendar as well as arcs using a sexagesimal system of counting, so at that time the second was a sexagesimal subdivision of the day (ancient second = day/60×60), not of the hour like the modern second (= hour/60×60). Sundials and water clocks were among the earliest timekeeping devices, and units of time were measured in degrees of arc. Conceptual units of time smaller than realisable on sundials were also used.

There are references to "second" as part of a lunar month in the writings of natural philosophers of the Middle Ages, which were mathematical subdivisions that could not be measured mechanically.

Fraction of solar day

The earliest mechanical clocks, which appeared starting in the 14th century, had displays that divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60. In fact, the hour was not commonly divided in 60 minutes as it was not uniform in duration. It was not practical for timekeepers to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. Mechanical clocks kept the mean time, as opposed to the apparent time displayed by sundials. By that time, sexagesimal divisions of time were well established in Europe.

The earliest clocks to display seconds appeared during the last half of the 16th century. The second became accurately measurable with the development of mechanical clocks. The earliest spring-driven timepiece with a second hand that marked seconds is an unsigned clock depicting Orpheus in the Fremersdorf collection, dated between 1560 and 1570. During the third quarter of the 16th century, Taqi al-Din built a clock with marks every 15 minute. In 1579, Jost Bürgi built a clock for William of Hesse that marked seconds. In 1581, Tycho Brahe redesigned clocks that had displayed only minutes at his observatory so they also displayed seconds, even though those seconds were not accurate. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds.

In 1656, Dutch scientist Christiaan Huygens invented the first pendulum clock. It had a pendulum length of just under a meter, giving it a swing of one second, and an escapement that ticked every second. It was the first clock that could accurately keep time in seconds. By the 1730s, 80 years later, John Harrison's maritime chronometers could keep time accurate to within one second in 100 days.

In 1832, Gauss proposed using the second as the base unit of time in his millimeter–milligram–second system of units. The British Association for the Advancement of Science (BAAS) in 1862 stated that "All men of science are agreed to use the second of mean solar time as the unit of time." BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units. Both the CGS and MKS systems used the same second as their base unit of time. MKS was adopted internationally during the 1940s, defining the second as 186,400 of a mean solar day.

Fraction of an ephemeris year

Sometime in the late 1940s, quartz crystal oscillator clocks with an operating frequency of ~100 kHz advanced to keep time with accuracy better than 1 part in 108 over an operating period of a day. It became apparent that a consensus of such clocks kept better time than the rotation of the Earth. Metrologists also knew that Earth's orbit around the Sun (a year) was much more stable than Earth's rotation. This led to proposals as early as 1950 to define the second as a fraction of a year.

The Earth's motion was described in Newcomb's Tables of the Sun (1895), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. This resulted in adoption of an ephemeris time scale expressed in units of the sidereal year at that epoch by the IAU in 1952. This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion. In 1955, the tropical year, considered more fundamental than the sidereal year, was chosen by the IAU as the unit of time. The tropical year in the definition was not measured but calculated from a formula describing a mean tropical year that decreased linearly over time.

In 1956, the second was redefined in terms of a year relative to that epoch. The second was thus defined as "the fraction 131,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time". This definition was adopted as part of the International System of Units in 1960.

Atomic definition

Even the best mechanical, electric motorized and quartz crystal-based clocks develop discrepancies from environmental conditions; far better for timekeeping is the natural and exact "vibration" in an energized atom. The frequency of vibration (i.e., radiation) is very specific depending on the type of atom and how it is excited. Since 1967, the second has been defined as exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom". This length of a second was selected to correspond exactly to the length of the ephemeris second previously defined. Atomic clocks use such a frequency to measure seconds by counting cycles per second at that frequency. Radiation of this kind is one of the most stable and reproducible phenomena of nature. The current generation of atomic clocks is accurate to within one second in a few hundred million years. Since 1967, atomic clocks based on atoms other than caesium-133 have been developed with increased precision by a factor of 100. Therefore a new definition of the second is planned.

Atomic clocks now set the length of a second and the time standard for the world.

Table

Evolution of the Second
Decisions of the CIPM Resolution of the CGPM Information
That according to the decisions of the 8th General Assembly of the International Astronomical Union (Rome, 1952), the second of ephemeris time (ET) is the fraction

of the tropical year for 1900 January 0 at 12 h ET.

The second is the fraction of the tropical year for 1900 January 0 at 12 hours ephemeris time. 1956 CIPM

11th CGPM 1960 Resolution 9

The standard to be employed is the transition between the hyperfine levels F=4, M=0 and F=3, M=0 of the ground state of the caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9192631770 hertz. The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom 13th CGPM Resolution 1

CIPM 1967

This definition implies that the caesium atom is at rest and unperturbed. In consequence, in its practical realization, measurements must be corrected for velocity of the atoms with respect to the clock reference frame, for magnetic and electric fields including ambient black-body radiation, for spin-exchange effects and for other possible perturbations. At its 1997 meeting, the CIPM affirmed that: This definition refers to a caesium atom at rest at a temperature of 0 K. This note was intended to make it clear that the definition of the SI second is based on a Cs atom unperturbed by black-body radiation, that is, in an environment whose temperature is 0 K, and that the frequencies of primary frequency standards should therefore be corrected for the shift due to ambient radiation, as stated at the meeting of the CCTF in 1999. footnote added by the 14th meeting of the Consultative Committee for Time and Frequency in 1999

the footnote was added at the 86th (1997) meeting of the CIPM GCPM 1998 7th Edition SI Brochure

The definition of a unit refers to an idealized situation that can be reached in the practical realization with some uncertainty only. In this spirit, the definition of the second has to be understood as referring to atoms free of any perturbation, at rest and in the absence of electric and magnetic fields.

A future re-definition of the second would be justified if these idealized conditions can be achieved much easier than with the current definition.

The definition of the second should be understood as the definition of the unit of proper time: it applies in a small spatial domain that shares the motion of the caesium atom used to realize the definition.

In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory. It is wrong to correct for the local gravitational field.

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1.

The reference to an unperturbed atom is intended to make it clear that the definition of the SI second is based on an isolated caesium atom that is unperturbed by any external field, such as ambient black-body radiation.

The second, so defined, is the unit of proper time in the sense of the general theory of relativity. To allow the provision of a coordinated time scale, the signals of different primary clocks in different locations are combined, which have to be corrected for relativistic caesium frequency shifts (see section 2.3.6).

The CIPM has adopted various secondary representations of the second, based on a selected number of spectral lines of atoms, ions or molecules. The unperturbed frequencies of these lines can be determined with a relative uncertainty not lower than that of the realization of the second based on the 133Cs hyperfine transition frequency, but some can be reproduced with superior stability.

Current Definition resolved in 2018 effective after the 26th GCPM approved the redefinition May 20, 2019.

SI Brochure 9

Future redefinition

In 2022, the best realisation of the second is done with caesium primary standard clocks such as IT-CsF2, NIST-F2, NPL-CsF2, PTB-CSF2, SU–CsFO2 or SYRTE-FO2. These clocks work by laser-cooling a cloud of Cs atoms to a microkelvin in a magneto-optic trap. These cold atoms are then launched vertically by laser light. The atoms then undergo Ramsey excitation in a microwave cavity. The fraction of excited atoms is then detected by laser beams. These clocks have 5×10−16 systematic uncertainty, which is equivalent to 50 picoseconds per day. A system of several fountains worldwide contribute to International Atomic Time. These caesium clocks also underpin optical frequency measurements.

Optical clocks are based on forbidden optical transitions in ions or atoms. They have frequencies around 1015 Hz, with a natural linewidth of typically 1 Hz, so the Q-factor is about 1015, or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks" faster. Optical clocks use either a single ion, or an optical lattice with 104106 atoms.

Rydberg constant

A definition based on the Rydberg constant would involve fixing the value to a certain value: . The Rydberg constant describes the energy levels in a hydrogen atom with the nonrelativistic approximation .

The only viable way to fix the Rydberg constant involves trapping and cooling hydrogen. This is difficult because it is very light and the atoms move very fast, causing Doppler shifts. The radiation needed to cool the hydrogen – 121.5 nm – is also difficult. Another hurdle involves improving the uncertainty in QED calculations, specifically the Lamb shift in the 1s-2s transition of the hydrogen atom.

Requirements

A redefinition must include improved optical clock reliability. TAI must be contributed to by optical clocks before the BIPM affirms a redefinition. A consistent method of sending signals must be developed before the second is redefined, such as fiber-optics.

SI multiples

SI prefixes are commonly used for times shorter than one second, but rarely for multiples of a second. Instead, certain non-SI units are permitted for use with SI: minutes, hours, days, and in astronomy Julian years.

SI multiples for second (s)
Submultiples
Multiples
Value SI symbol Name Value SI symbol Name Equivalent to
10−1 s ds decisecond 101 s das decasecond 10 seconds
10−2 s cs centisecond 102 s hs hectosecond 1 minute, 40 seconds
10−3 s ms millisecond 103 s ks kilosecond 16 minutes, 40 seconds
10−6 s μs microsecond 106 s Ms megasecond 1 week, 4 days, 13 hours, 46 minutes, 40 seconds
10−9 s ns nanosecond 109 s Gs gigasecond 31.7 years
10−12 s ps picosecond 1012 s Ts terasecond 31,700 years
10−15 s fs femtosecond 1015 s Ps petasecond 31.7 million years
10−18 s as attosecond 1018 s Es exasecond 31.7 billion years
10−21 s zs zeptosecond 1021 s Zs zettasecond 31.7 trillion years
10−24 s ys yoctosecond 1024 s Ys yottasecond 31.7 quadrillion years
10−27 s rs rontosecond 1027 s Rs ronnasecond 31.7 quintillion years
10−30 s qs quectosecond 1030 s Qs quettasecond 31.7 sextillion years

Crown group

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Clade T is a total group or pan-group, its extant species organized into crown groups C1 and C2. Within clade T are sister clades T1 and C2. Clade T1 comprises crown group C1 and the stem group S1 which is the set of extinct species that are closer to C1 than any other crown group of extant species.

In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. It is thus a way of defining a clade, a group consisting of a species and all its extant or extinct descendants. For example, Neornithes (birds) can be defined as a crown group, which includes the most recent common ancestor of all modern birds, and all of its extant or extinct descendants.

The concept was developed by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to their extinct relatives in his "Die Stammesgeschichte der Insekten", and the "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979. Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 by Graham Budd and Sören Jensen

Contents of the crown group

It is not necessary for a species to have living descendants in order for it to be included in the crown group. Extinct side branches on the family tree that are descended from the most recent common ancestor of living members will still be part of a crown group. For example, if we consider the crown-birds (i.e. all extant birds and the rest of the family tree back to their most recent common ancestor), extinct side branches like the dodo or great auk are still descended from the most recent common ancestor of all living birds, so fall within the bird crown group. One very simplified cladogram for birds is shown below:

Aves

Archaeopteryx




other extinct groups



Neornithes (modern birds, some recently extinct)




In this diagram, the clade labelled "Neornithes" is the crown group of birds: it includes the most recent common ancestor of all living birds and its descendants, living or not. Although considered to be birds (i.e. members of the clade Aves), Archaeopteryx and other extinct groups are not included in the crown group, as they fall outside the Neornithes clade, being descended from an earlier ancestor.

An alternative definition does not require any members of a crown group to be extant, only to have resulted from a "major cladogenesis event". The first definition forms the basis of this article.

Often, the crown group is given the designation "crown-", to separate it from the group as commonly defined. Both birds and mammals are traditionally defined by their traits, and contain fossil members that lived before the last common ancestors of the living groups or, like the mammal Haldanodon, were not descended from that ancestor although they lived later. Crown-Aves and Crown-Mammalia therefore differ slightly in content from the common definition of Aves and Mammalia. This has caused some confusion in the literature.

Other groups under the crown group concept

The cladistic idea of strictly using the topology of the phylogenetic tree to define groups necessitates other definitions than crown groups to adequately define commonly discussed fossil groups. Thus, a host of prefixes have been defined to describe various branches of the phylogenetic tree relative to extant organisms.

Pan-group

A pan-group or total group is the crown group and all organisms more closely related to it than to any other extant organisms. In a tree analogy, it is the crown group and all branches back to (but not including) the split with the closest branch to have living members. The Pan-Aves thus contain the living birds and all (fossil) organisms more closely related to birds than to crocodilians (their closest living relatives). The phylogenetic lineage leading back from Neornithes to the point where it merges with the crocodilian lineage, along with all side branches, constitutes pan-birds. In addition to non-crown group primitive birds like Archaeopteryx, Hesperornis and Confuciusornis, therefore, pan-group birds would include all dinosaurs and pterosaurs as well as an assortment of non-crocodilian animals like Marasuchus.

Pan-Mammalia consists of all mammals and their fossil ancestors back to the phylogenetic split from the remaining amniotes (the Sauropsida). Pan-Mammalia is thus an alternative name for Synapsida.

Stem groups

A stem group is a paraphyletic assemblage composed of the members of a pan-group or total group, above, minus the crown group itself (and therefore minus all living members of the pan-group). This leaves primitive relatives of the crown groups, back along the phylogenetic line to (but not including) the last common ancestor of the crown group and their closest living relatives. It follows from the definition that all members of a stem group are extinct. The "stem group" is the most used and most important of the concepts linked to crown groups, as it offers a means to reify and name paraphyletic assemblages of fossils that otherwise do not fit into systematics based on living organisms.

While often attributed to Jefferies (1979), Willmann (2003) traced the origin of the stem group concept to Austrian systematist Othenio Abel (1914), and it was discussed and diagrammed in English as early as 1933 by A. S. Romer.

Alternatively, the term "stem group" is sometimes used in a narrower sense to cover any members of the traditional taxon falling outside the crown group, such as the first cynodonts in the late Triassic being the first stem mammals. Permian synapsids like Dimetrodon or Anteosaurus are stem mammals in the wider sense but not in the narrower one.

Often, an (extinct) grouping is identified as belonging together. Later, it may be realized other (extant) groupings actually emerged within such grouping, rendering them a stem grouping. Cladistically, the new groups should then be added to the group, as paraphyletic groupings are not natural. In any case, stem groupings with living descendants should not be viewed as a cohesive group, but their tree should be further resolved to reveal the full bifurcating phylogeny.

Examples of stem groups (in the wider sense)

Stem birds perhaps constitute the most cited example of a stem group, as the phylogeny of this group is fairly well known. The following cladogram, based on Benton (2005), illustrates the concept († denotes extinct taxa).

The crown group here is Neornithes, all modern bird lineages back to their last common ancestor. The closest living relatives of birds are crocodilians. If we follow the phylogenetic lineage leading to Neornithes to the left, the line itself and all side branches belong to the stem birds until the lineage merges with that of the crocodilians. In addition to non-crown group primitive birds like Archaeopteryx, Hesperornis and Confuciusornis, stem group birds include the dinosaurs and the pterosaurs. The last common ancestor of birds and crocodilians—the first crown group archosaur—was neither bird nor crocodilian and possessed none of the features unique to either. As the bird stem group evolved, distinctive bird features such as feathers and hollow bones appeared. Finally, at the base of the crown group, all traits common to extant birds were present.

Under the widely used total-group perspective, the Crocodylomorpha would become synonymous with the Crocodilia, and the Avemetatarsalia would become synonymous with the birds, and the above tree could be summarized.

An advantage of this approach is that declaring Theropoda to be birds (or Pan-aves) is more specific than declaring it to be a member of the Archosauria, which would not exclude it from the Crocodilia branch. Basal branch names such as Avemetatarsalia are usually more obscure. However, not so advantageous are the facts that "Pan-Aves" and "Aves" are not the same group, the circumscription of the concept of "Pan-Aves" (synonymous with Avemetatarsalia) is only evident by examination of the above tree, and calling both groups "birds" is ambiguous.

Stem mammals are those in the lineage leading to living mammals, together with side branches, from the divergence of the lineage from the Sauropsida to the last common ancestor of the living mammals. This group includes the synapsids as well as mammaliaforms like the morganucodonts and the docodonts; the latter groups have traditionally and anatomically been considered mammals even though they fall outside the crown group mammals.

Stem tetrapods are the animals belonging to the lineage leading to tetrapods from their divergence from the lungfish, our nearest relatives among the fishes. In addition to a series of lobe-finned fishes, they also include some of the early labyrinthodonts. Exactly what labyrinthodonts are in the stem group tetrapods rather than the corresponding crown group is uncertain, as the phylogeny of early tetrapods is not well understood. This example shows that crown and stem group definitions are of limited value when there is no consensus phylogeny.

Stem arthropods constitute a group that has seen attention in connection with the Burgess Shale fauna. Several of the finds, including the enigmatic Opabinia and Anomalocaris have some, though not all, features associated with arthropods, and are thus considered stem arthropods. The sorting of the Burgess Shale fauna into various stem groups finally enabled phylogenetic sorting of this enigmatic assemblage and also allowed for identifying velvet worms as the closest living relatives of arthropods.

Stem priapulids are other early Cambrian to middle Cambrian faunas, appearing in Chengjiang to Burgess Shale. The genus Ottoia has more or less the same build as modern priapulids, but phylogenetic analysis indicates that it falls outside the crown group, making it a stem priapulid.

Plesion-group

The name plesion has a long history in biological systematics, and plesion group has acquired several meanings over the years. One use is as "nearby group" (plesion means close to in Greek), i.e. sister group to a given taxon, whether that group is a crown group or not. The term may also mean a group, possibly paraphyletic, defined by primitive traits (i.e. symplesiomorphies). It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question.

Palaeontological significance of stem and crown groups

Placing fossils in their right order in a stem group allows the order of these acquisitions to be established, and thus the ecological and functional setting of the evolution of the major features of the group in question. Stem groups thus offer a route to integrate unique palaeontological data into questions of the evolution of living organisms. Furthermore, they show that fossils that were considered to lie in their own separate group because they did not show all the diagnostic features of a living clade, can nevertheless be related to it by lying in its stem group. Such fossils have been of particular importance in considering the origins of the tetrapods, mammals, and animals.

The application of the stem group concept also influenced the interpretation of the organisms of the Burgess Shale. Their classification in stem groups to extant phyla, rather than in phyla of their own, is thought by some to make the Cambrian explosion easier to understand without invoking unusual evolutionary mechanisms; however, application of the stem group concept does nothing to ameliorate the difficulties that phylogenetic telescoping poses to evolutionary theorists attempting to understand both macroevolutionary change and the abrupt character of the Cambrian explosion. Overemphasis on the stem group concept threatens to delay or obscure proper recognition of new higher taxa.

Stem groups in systematics

As originally proposed by Karl-Ernst Lauterbach, stem groups should be given the prefix "stem" (i.e. Stem-Aves, Stem-Arthropoda), however the crown group should have no prefix. The latter has not been universally accepted for known groups. A number of paleontologists have opted to apply this approach anyway.

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