title: Top of the Pops

text: Starting in 1968 and carrying on through the 1970s a rival series of "Top of the Pops" albums were produced, however these had no connection with the television series except for its name. They were a series of budget cover albums of current chart hits recorded by anonymous session singers and musicians released on the Hallmark record label. They had initially reached the charts but were later disallowed due to a change in the criteria for entering the charts. These albums continued to be produced until the early 1980s, when the advent of compilation albums featuring the original versions of hits, such as the "Now That's What I Call Music!" series, led to a steep decline in their popularity.


title: Top of the Pops

text: In the 1990s, the BBC "Top of the Pops" brand was again licensed for use in a tie-in compilation series. Starting in 1995 with Sony Music's Columbia Records label, these double disc collections moved to the special marketing arm of PolyGram / Universal Music Group TV, before becoming a sister brand of the "Now That's What I Call Music!" range in the EMI / Virgin / Universal joint venture.


title: Top of the Pops

text: Similarly to the roles of "Top of the Pops" on BBC One and BBC Two in the late 1990s and early 2000s, the compilation albums range featured current hits for the main series and classic hits (such as '70s Rock) for the "Top of the Pops 2" spin-offs.


title: Top of the Pops

text: The "Top of the Pops" brand has now been licensed by EMI who released a compilation series in 2007–08, with one CD for each year that "Top of the Pops" was running. The boxset for the entire series of 43 discs was released 7July 2008. A podcast supporting the release of the boxset featuring interviews with Mark Goodier, Miles Leonard, Malcolm McLaren and David Hepworth is available.


title: Top of the Pops

text: "Top of the Pops" magazine has been running since February 1995, and filled the void in the BBC magazine portfolio where "Number One" magazine used to be. It began much in the mould of "Q" magazine, then changed its editorial policy to directly compete with popular teen celebrity magazines such as "Smash Hits" and "Big", with free sticker giveaways replacing Brett Anderson covers.


title: Top of the Pops

text: A July 1996 feature on the Spice Girls coined the famous "Spice" nicknames for each member (Baby, Ginger, Posh, Scary and Sporty) that stayed with them throughout their career as a group and beyond.


title: Top of the Pops

text: The BBC announced that the magazine would continue in publication despite the end of the television series, and is still running.


title: Top of the Pops

text: An earlier "Top of the Pops" magazine appeared briefly in the mid-1970s. Mud drummer Dave Mount sat reading an edition throughout a 1975 appearance on the show.


title: Top of the Pops

text: In May 2006, following a special Red Hot Chili Peppers concert recorded in the car park of BBC Television Centre, Hammersmith and Fulham Council (which governs the area the centre is located) informed the BBC that it lacked the necessary public entertainment license (as required by the Licensing Act 2003). Until the BBC could obtain the license, BBC staff stood-in as audience members for live music programmes.


title: Top of the Pops

text: In 2004 there was a DVD released called "Top of the Pops 40th Anniversary 1964–2004 DVD". It features live performances, containing one song for each year, except 1966. (Two tracks from 1965 are featured instead). Also included as extras are seven opening titles, most notably the one with the flying coloured LP's from 1981. This title sequence had Phil Lynott's song "Yellow Pearl" as the theme. The 1986 and 1989 titles are also featured, with Paul Hardcastle's hit "The Wizard" as the theme. This DVD was to celebrate 40 years since the show started.


title: Top of the Pops

text: There was also a DVD quiz released in 2007 called "The Essential Music Quiz". There was also a DVD in 2001 called "Summer 2001", a sister DVD to the album of the same name.


title: Hoplite

text: Hoplites ( ) ( : hoplítēs) were citizen-soldiers of Ancient Greek city-states who were primarily armed with spears and shields. Hoplite soldiers used the phalanx formation to be effective in war with fewer soldiers. The formation discouraged the soldiers from acting alone, for this would compromise the formation and minimize its strengths. The hoplites were primarily represented by free citizens – propertied farmers and artisans – who were able to afford a linen armour or a bronze armour suit and weapons (estimated at a third to a half of its able-bodied adult male population). Most hoplites were not professional soldiers and often lacked sufficient military training. Some states maintained a small elite professional unit, known as the "epilektoi" ("chosen") since they were picked from the regular citizen infantry. These existed at times in Athens, Argos, Thebes, and Syracuse, among other places. Hoplite soldiers made up the bulk of ancient Greek armies.


title: Hoplite

text: In the 8th or 7th century BC, Greek armies adopted the phalanx formation. The formation proved successful in defeating the Persians when employed by the Athenians at the Battle of Marathon in 490 BC during the First Greco-Persian War. The Persian archers and light troops who fought in the Battle of Marathon failed because their bows were too weak for their arrows to penetrate the wall of Greek shields that comprised the phalanx formation. The phalanx was also employed by the Greeks at the Battle of Thermopylae in 480 BC and at the Battle of Plataea in 479 BC during the Second Greco-Persian War.


title: Hoplite

text: The word "hoplite" (Greek: "hoplítēs"; pl. "hoplĩtai") derives from "hoplon" ( : "hóplon"; plural "hópla" ), referring to the hoplite's equipment. In the modern Hellenic Army, the word "hoplite" (Greek: : "oplítîs") is used to refer to an infantryman.


title: Hoplite

text: The fragmented political structure of Ancient Greece, with many competing city-states, increased the frequency of conflict, but at the same time limited the scale of warfare. Limited manpower did not allow most Greek city-states to form large armies which could operate for long periods because they were generally not formed from professional soldiers. Most soldiers had careers as farmers or workers and returned to these professions after the campaign. All hoplites were expected to take part in any military campaign when called for duty by leaders of the state. The Lacedaemonian citizens of Sparta were renowned for their lifelong combat training and almost mythical military prowess, while their greatest adversaries, the Athenians, were exempted from service only after the age of 60. This inevitably reduced the potential duration of campaigns and often resulted in the campaign season being restricted to one summer.


title: Hoplite

text: Armies generally marched directly to their destination, and in some cases the battlefield was agreed to by the contestants in advance. Battles were fought on level ground, and hoplites preferred to fight with high terrain on both sides of the phalanx so the formation could not be flanked. An example of this was the Battle of Thermopylae, where the Spartans specifically chose a narrow coastal pass to make their stand against the massive Persian army. The vastly outnumbered Greeks held off the Persians for seven days.


title: Hoplite

text: When battles occurred, they were usually set piece and intended to be decisive. The battlefield would be flat and open to facilitate phalanx warfare. These battles were usually short and required a high degree of discipline. At least in the early classical period, when cavalry was present, its role was restricted to protection of the flanks of the phalanx, pursuit of a defeated enemy, and covering a retreat if required. Light infantry and missile troops took part in the battles but their role was less important. Before the opposing phalanxes engaged, the light troops would skirmish with the enemy's light forces, and then protect the flanks and rear of the phalanx.


title: Hoplite

text: The military structure created by the Spartans was a rectangular phalanx formation. The formation was organized from eight to ten rows deep and could cover a front of a quarter of a mile or more if sufficient hoplites were available. The two lines would close to a short distance to allow effective use of their spears, while the threw stones and javelins from behind their lines. The shields would clash and the first lines (protostates) would stab at their opponents, at the same time trying to keep in position. The ranks behind them would support them with their own spears and the mass of their shields gently pushing them, not to force them into the enemy formation but to keep them steady and in place. The soldiers in the back provided motivation to the ranks in the front being that most hoplites were close community members. At certain points, a command would be given to the phalanx or a part thereof to collectively take a certain number of steps forward (ranging from half to multiple steps). This was the famed .


title: Hoplite

text: At this point, the phalanx would put its collective weight to push back the enemy line and thus create fear and panic among its ranks. There could be multiple such instances of attempts to push, but it seems from the accounts of the ancients that these were perfectly orchestrated and attempted organized "en masse". Once one of the lines broke, the troops would generally flee from the field, sometimes chased by psiloi, peltasts, or light cavalry.


title: Hoplite

text: If a hoplite escaped, he would sometimes be forced to drop his cumbersome , thereby disgracing himself to his friends and family (becoming a , one who threw his shield). To lessen the number of casualties inflicted by the enemy during battles, soldiers were positioned to stand shoulder to shoulder with their aspis. The hoplites' most prominent citizens and generals led from the front. Thus, the war could be decided by a single battle. Victory was enforced by ransoming the fallen back to the defeated, called the "Custom of the Greeks".


title: Hoplite

text: Individual hoplites carried their shields on their left arm, protecting themselves and the soldier to the left. This meant that the men at the extreme right of the phalanx were only half-protected. In battle, opposing phalanxes would exploit this weakness by attempting to overlap the enemy's right flank. It also meant that, in battle, a phalanx would tend to drift to the right (as hoplites sought to remain behind the shield of their neighbour). The most experienced hoplites were often placed on the right side of the phalanx, to counteract these problems. According to Plutarch's "Sayings of Spartans", "a man carried a shield for the sake of the whole line".


title: Hoplite

text: The phalanx is an example of a military formation in which single combat and other individualistic forms of battle were suppressed for the good of the whole. In earlier Homeric, dark age combat, the words and deeds of supremely powerful heroes turned the tide of battle. Instead of having individual heroes, hoplite warfare relied heavily on the community and unity of soldiers. With friends and family pushing on either side and enemies forming a solid wall of shields in front, the hoplite had little opportunity for feats of technique and weapon skill, but great need for commitment and mental toughness. By forming a human wall to provide a powerful defensive armour, the hoplites became much more effective while suffering fewer casualties. The hoplites had a lot of discipline and were taught to be loyal and trustworthy. They had to trust their neighbours for mutual protection, so a phalanx was only as strong as its weakest elements. Its effectiveness depended on how well the hoplites could maintain this formation in combat, and how well they could stand their ground, especially when engaged against another phalanx. The more disciplined and courageous the army, the more likely it was to win. Often engagements between various city-states of Greece would be resolved by one side fleeing after their phalanx had broken formation.


title: Hoplite

text: As important as unity among the ranks was in phalanx warfare, individual fighting skill played a role in battle. Hoplites' shields were not locked all of the time. Throughout many points of the fight there were periods where the hoplites separated as far as two to three feet apart in order to have room to swing their shields and swords at the enemy. This led to individual prowess being more important than previously realized by some historians.


title: Hoplite

text: It would have been near impossible to swing both shield and sword if the man next to you is practically touching. One piece of evidence of this is the picking of individual champions after each battle was fought. This is most evident in Herodotus' account of the Battle of Thermopylae. "Although great valor was displayed by the entire corps of Spartans and Thespians, the man who proved himself best was a Spartan Officer named Dienekes ". The brothers Alpheos and Maron were also honored for their battlefield prowess as well. This is just one example of an ancient historian giving credit to a few individual soldiers and the individuality of phalanx warfare.


title: Hoplite

text: Each hoplite provided his own equipment. Thus, only those who could afford such weaponry fought as hoplites. As with the Roman Republican army it was the middle classes who formed the bulk of the infantry. Equipment was not standardized, although there were doubtless trends in general designs over time, and between city-states. Hoplites had customized armour, the shield was decorated with family or clan emblems, although in later years these were replaced by symbols or monograms of the city states. The equipment might be passed down in families, as it was expensive to manufacture.


title: Hoplite

text: The hoplite army consisted of heavy infantrymen. Their armour, also called panoply, was sometimes made of full bronze for those who could afford it, weighing nearly , although linen armor now known as linothorax was more common since it was cost-effective and provided decent protection. The average farmer-peasant hoplite could not afford any armor and typically carried only a shield, a spear, and perhaps a helmet plus a secondary weapon. The richer upper-class hoplites typically had a bronze cuirass of either the bell or muscled variety, a bronze helmet with cheekplates, as well as greaves and other armour. The design of helmets used varied through time. The Corinthian helmet was at first standardized and was a successful design. Later variants included the Chalcidian helmet, a lightened version of the Corinthian helmet, and the simple Pilos helmet worn by the later hoplites. Often the helmet was decorated with one, sometimes more horsehair crests, and/or bronze animal horns and ears. Helmets were often painted as well. The Thracian helmet had a large visor to further increase protection. In later periods, "linothorax" was also used, as it is tougher and cheaper to produce. The linen was thick.


title: Hoplite

text: By contrast with hoplites, other contemporary infantry (e.g., Persian) tended to wear relatively light armour, wicker shields, and were armed with shorter spears, javelins, and bows. The most famous are the Peltasts, light-armed troops who wore no armour and were armed with a light shield, javelins and a short sword. The Athenian general Iphicrates developed a new type of armour and arms for his mercenary army, which included light linen armour, smaller shields and longer spears, whilst arming his Peltasts with larger shields, helmets and a longer spear, thus enabling them to defend themselves more easily against hoplites. With this new type of army he defeated a Spartan army in 392 BC. The arms and armour described above were most common for hoplites.


title: Hoplite

text: Hoplites carried a large concave shield called an (sometimes incorrectly referred to as a ), measuring between 80–100 centimetres (31–39 in) in diameter and weighing between 6.5–8 kilograms (14–18 lbs). This large shield was made possible partly by its shape, which allowed it to be supported on the shoulder. The shield was assembled in three layers: the center layer was made of thick wood, the outside layer facing the enemy was made of bronze, and leather comprised the inside of the shield. The revolutionary part of the shield was the grip. Known as an Argive grip, it placed the handle at the edge of the shield, and was supported by a leather fastening (for the forearm) at the centre. These two points of contact eliminated the possibility of the shield swaying to the side after being struck, and as a result soldiers rarely lost their shields. This allowed the hoplite soldier more mobility with the shield, as well as the ability to capitalize on its offensive capabilities and better support the phalanx. The large shields, designed for pushing ahead, were the most essential equipment for the hoplites.


title: Hoplite

text: The main offensive weapon used was a long and in diameter spear called a . It was held with the right hand, with the left hand holding the hoplite's shield. Soldiers usually held their spears in an underhand position when approaching but once they came into close contact with their opponents, they were held in an overhand position ready to strike. The spearhead was usually a curved leaf shape, while the rear of the spear had a spike called a ("lizard-killer") which was used to stand the spear in the ground (hence the name). It was also used as a secondary weapon if the main shaft snapped, or for the rear ranks to finish off fallen opponents as the phalanx advanced over them. In addition to being used as a secondary weapon, the doubled to balance the spear, but not for throwing purposes. It is a matter of contention, among historians, whether the hoplite used the spear overarm or underarm. Held underarm, the thrusts would have been less powerful but under more control, and vice versa. It seems likely that both motions were used, depending on the situation. If attack was called for, an overarm motion was more likely to break through an opponent's defence. The upward thrust is more easily deflected by armour due to its lesser leverage. When defending, an underarm carry absorbed more shock and could be 'couched' under the shoulder for maximum stability. An overarm motion would allow more effective combination of the "aspis" and "doru" if the shield wall had broken down, while the underarm motion would be more effective when the shield had to be interlocked with those of one's neighbours in the battle-line. Hoplites in the rows behind the lead would almost certainly have made overarm thrusts. The rear ranks held their spears underarm, and raised their shields upwards at increasing angles. This was an effective defence against missiles, deflecting their force.


title: Hoplite

text: Hoplites also carried a sword, mostly a short sword called a "xiphos", but later also longer and heavier types. The short sword was a secondary weapon, used if or when their spears were broken or lost, or if the phalanx broke rank. The xiphos usually had a blade around long; however, those used by the Spartans were often only 30–45 centimetres long. This very short xiphos would be very advantageous in the press that occurred when two lines of hoplites met, capable of being thrust through gaps in the shieldwall into an enemy's unprotected groin or throat, while there was no room to swing a longer sword. Such a small weapon would be particularly useful after many hoplites had started to abandon body armour during the Peloponnesian War. Hoplites could also alternatively carry the "kopis", a heavy knife with a forward-curving blade.


title: Hoplite

text: Dark age warfare transitioned into hoplite warfare in the 8th century BC. Historians and researchers have debated the reason and speed of the transition for centuries. So far 3 popular theories exist:


title: Hoplite

text: Developed by Anthony Snodgrass, the Gradualist Theory states that the hoplite style of battle developed in a series of steps as a result of innovations in armour and weaponry. Chronologically dating the archeological findings of hoplite armour and using the findings to approximate the development of the phalanx formation, Snodgrass claims that the transition took approximately 100 years to complete from 750–650 BC. The progression of the phalanx took time because as the phalanx matured it required denser formations that made the elite warriors recruit Greek citizens. The large amounts of hoplite armour needed to then be distributed to the populations of Greek citizens only increased the time for the phalanx to be implemented. Snodgrass believes, only once the armour was in place that the phalanx formation became popular.


title: Hoplite

text: The Rapid Adaptation model was developed by historians Paul Cartledge and Victor Hanson. The historians believe that the phalanx was created individually by military forces, but was so effective that others had to immediately adapt their way of war to combat the formation. Rapid Adoptionists propose that the double grip shield that was required for the phalanx formation was so constricting in mobility that once it was introduced, dark age, free flowing warfare was inadequate to fight against the hoplites only escalating the speed of the transition. Quickly, the phalanx formation and hoplite armour became widely used throughout Ancient Greece. Cartledge and Hanson estimate the transition took place from 725–675 BC.


title: Hoplite

text: Developed by Hans Van Wees, the Extended Gradualist theory is the most lengthy of the three popular transition theories. Van Wees depicts iconography found on pots of the Dark Ages believing that the foundation of the phalanx formation was birthed during this time. Specifically, he uses an example of the Chigi Vase to point out that hoplite soldiers were carrying normal spears as well as javelins on their backs. Matured hoplites did not carry long-range weapons including javelins. The Chigi vase is important for our knowledge of the hoplite soldier because it is one if not the only representation of the hoplite formation, known as the phalanx, in Greek art. This led Van Wees to believe that there was a transitional period from long-range warfare of the Dark Ages to the close combat of hoplite warfare. Some other evidence of a transitional period lies within the text of Spartan poet Tyrtaios, who wrote, "…will they draw back for the pounding [of the missiles, no,] despite the battery of great hurl-stones, the helmets shall abide the rattle [of war unbowed]". At no point in other texts does Tyrtaios discuss missiles or rocks, making another case for a transitional period in which hoplite warriors had some ranged capabilities. Extended Gradualists argue that hoplite warriors did not fight in a true phalanx until the 5th century BC. Making estimations of the speed of the transition reached as long as 300 years, from 750–450 BC.


title: Hoplite

text: The exact time when hoplite warfare was developed is uncertain, the prevalent theory being that it was established sometime during the 8th or 7th century BC, when the "heroic age was abandoned and a far more disciplined system introduced" and the Argive shield became popular. Peter Krentz argues that "the ideology of hoplitic warfare as a ritualized contest developed not in the 7th century [BC], but only after 480, when non-hoplite arms began to be excluded from the phalanx". Anagnostis Agelarakis, based on recent archaeo-anthropological discoveries of the earliest monumental polyandrion (communal burial of male warriors) at Paros Island in Greece, unveils a last quarter of the 8th century BC date for a hoplitic phalangeal military organization.


title: Hoplite

text: The rise and fall of hoplite warfare was tied to the rise and fall of the city-state. As discussed above, hoplites were a solution to the armed clashes between independent city-states. As Greek civilization found itself confronted by the world at large, particularly the Persians, the emphasis in warfare shifted. Confronted by huge numbers of enemy troops, individual city-states could not realistically fight alone. During the Greco-Persian Wars (499–448 BC), alliances between groups of cities (whose composition varied over time) fought against the Persians. This drastically altered the scale of warfare and the numbers of troops involved. The hoplite phalanx proved itself far superior to the Persian infantry at such conflicts as the Battle of Marathon, Thermopylae, and the Battle of Plataea.


title: Hoplite

text: During this period, Athens and Sparta rose to a position of political eminence in Greece, and their rivalry in the aftermath of the Persian wars brought Greece into renewed internal conflict. The Peloponnesian War was on a scale unlike conflicts before. Fought between leagues of cities, dominated by Athens and Sparta respectively, the pooled manpower and financial resources allowed a diversification of warfare. Hoplite warfare was in decline. There were three major battles in the Peloponnesian War, and none proved decisive. Instead there was increased reliance on navies, skirmishers, mercenaries, city walls, siege engines, and non-set piece tactics. These reforms made wars of attrition possible and greatly increased the number of casualties. In the Persian war, hoplites faced large numbers of skirmishers and missile-armed troops, and such troops (e.g., peltasts) became much more commonly used by the Greeks during the Peloponnesian War. As a result, hoplites began wearing less armour, carrying shorter swords, and in general adapting for greater mobility. This led to the development of the ekdromos light hoplite.


title: Hoplite

text: According to Nefiodkin, fighting against Greek heavy infantry during the Greco-Persian Wars inspired the Persians to introduce scythed chariots.


title: Hoplite

text: Sparta is one of the most famous city-states, along with Athens, which had a unique position in ancient Greece. Contrary to other city states, the free citizens of Sparta served as hoplites their entire lives, training and exercising in peacetime, which gave Sparta a professional standing army. Often small, numbering around 6000 at its peak to no more than 1000 soldiers at lowest point, divided into six mora or battalions, the Spartan army was feared for its discipline and ferocity. Military service was the primary duty of Spartan men, and Spartan society was organized around its army.


title: Hoplite

text: Military service for hoplites lasted until the age of 40, and sometimes until 60 years of age, depending on a man's physical ability to perform on the battlefield.


title: Hoplite

text: Later in the hoplite era, more sophisticated tactics were developed, in particular by the Theban general Epaminondas. These tactics inspired the future king Philip II of Macedon, who was at the time a hostage in Thebes, to develop a new type of infantry, the Macedonian phalanx. After the Macedonian conquests of the 4th century BC, the hoplite was slowly abandoned in favour of the phalangite, armed in the Macedonian fashion, in the armies of the southern Greek states. Although clearly a development of the hoplite, the Macedonian phalanx was tactically more versatile, especially used in the combined arms tactics favoured by the Macedonians. These forces defeated the last major hoplite army, at the Battle of Chaeronea (338 BC), after which Athens and its allies joined the Macedonian empire.


title: Hoplite

text: While Alexander's army mainly fielded "Pezhetairoi" (= Foot Companions) as his main force, his army also included some classic hoplites, either provided by the League of Corinth or from hired mercenaries. Beside these units, the Macedonians also used the so-called "Hypaspists", an elite force of units possibly originally fighting as hoplites and used to guard the exposed right wing of Alexander's phalanx.


title: Hoplite

text: Hoplite-style warfare was influential, and influenced several other nations in the Mediterranean. Hoplite warfare was the dominant fighting style on much of the Italian Peninsula until the early 3rd century BC, employed by both the Etruscans and the Early Roman army, though scutum infantry had existed for centuries and some groups fielded both. The Romans later standardized their fighting style to a more flexible maniple organization, which was more versatile on rough terrain like that of the Apennines. Roman equipment also changed, trading spears for swords and heavy javelins ("pilum"). In the end only the "triarii" would keep a long spear ("hasta") as their main weapon. The triarii would still fight in a traditional phalanx formation. Though this combination or similar was popular in much of Italy, some continued to fight as hoplites. Mercenaries serving under Pyrrhus of Epirus or Hannibal (namely Lucanians) were equipped and fought as hoplites.


title: Hoplite

text: Early in its history, Ancient Carthage also equipped its troops as Greek hoplites, in units such as the Sacred Band of Carthage. Many Greek hoplite mercenaries fought in foreign armies, such as Carthage and Achaemenid Empire, where it is believed by some that they inspired the formation of the Cardaces. Some hoplites served under the Illyrian king Bardylis in the 4th century. The Illyrians were known to import many weapons and tactics from the Greeks.


title: Hoplite

text: The Diadochi imported the Greek phalanx to their kingdoms. Though they mostly fielded Greek citizens or mercenaries, they also armed and drilled local natives as hoplites or rather Macedonian phalanx, like the Machimoi of the Ptolemaic army.


title: Hoplite

text: The Greek armies of the Hellenistic period mostly fielded troops in the fashion of the Macedonian phalanx. Many armies of mainland Greece retained hoplite warfare. Besides classical hoplites Hellenistic nations began to field two new types of hoplites, the "Thureophoroi" and the "Thorakitai". They developed when Greeks adopted the Galatian "Thureos" shield, of an oval shape that was similar to the shields of the Romans, but flatter. The Thureophoroi were armed with a long thrusting spear, a short sword and, if needed, javelins. While the Thorakitai were similar to the Thureophoroi, they were more heavily armoured, as their name implies, usually wearing a mail shirt. These troops were used as a link between the light infantry and the phalanx, a form of medium infantry to bridge the gaps.


title: Vapor pressure

text: Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as "volatile". The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules also increases. As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.


title: Vapor pressure

text: The vapor pressure of any substance increases non-linearly with temperature according to the Clausius–Clapeyron relation. The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher temperature due to the higher fluid pressure, because fluid pressure increases above the atmospheric pressure as the depth increases. More important at shallow depths is the higher temperature required to start bubble formation. The surface tension of the bubble wall leads to an overpressure in the very small, initial bubbles.


title: Vapor pressure

text: The vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial pressure. For example, air at sea level, and saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2 kPa, making the basis for standard atmospheric pressure.


title: Vapor pressure

text: Vapor pressure is measured in the standard units of pressure. The International System of Units (SI) recognizes pressure as a derived unit with the dimension of force per area and designates the pascal (Pa) as its standard unit. One pascal is one newton per square meter (N·m or kg·m·s).


title: Vapor pressure

text: Experimental measurement of vapor pressure is a simple procedure for common pressures between 1 and 200 kPa. Most accurate results are obtained near the boiling point of substances and large errors result for measurements smaller than . Procedures often consist of purifying the test substance, isolating it in a container, evacuating any foreign gas, then measuring the equilibrium pressure of the gaseous phase of the substance in the container at different temperatures. Better accuracy is achieved when care is taken to ensure that the entire substance and its vapor are at the prescribed temperature. This is often done, as with the use of an isoteniscope, by submerging the containment area in a liquid bath.


title: Vapor pressure

text: In a medical context, vapor pressure is sometimes expressed in other units, specifically millimeters of mercury (mmHg). This is important for volatile anesthetics, most of which are liquids at body temperature, but with a relatively high vapor pressure.


title: Vapor pressure

text: The Antoine equation is a pragmatic mathematical expression of the relation between the vapor pressure and the temperature of pure liquid or solid substances. It is obtained by curve-fitting and is adapted to the fact that vapor pressure is usually increasing and concave as a function of temperature. The basic form of the equation is:


title: Vapor pressure

text: Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures. Each parameter set for a specific compound is only applicable over a specified temperature range. Generally, temperature ranges are chosen to maintain the equation's accuracy of a few up to 8–10 percent. For many volatile substances, several different sets of parameters are available and used for different temperature ranges. The Antoine equation has poor accuracy with any single parameter set when used from a compound's melting point to its critical temperature. Accuracy is also usually poor when vapor pressure is under 10 Torr because of the limitations of the apparatus used to establish the Antoine parameter values.


title: Vapor pressure

text: The Wagner equation gives "one of the best" fits to experimental data but is quite complex. It expresses reduced vapor pressure as a function of reduced temperature.


title: Vapor pressure

text: As a general trend, vapor pressures of liquids at ambient temperatures increase with decreasing boiling points. This is illustrated in the vapor pressure chart (see right) that shows graphs of the vapor pressures versus temperatures for a variety of liquids. At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure defined as 1 atmosphere, 760Torr, 101.325kPa, or 14.69595psi.


title: Vapor pressure

text: For example, at any given temperature, methyl chloride has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point at , which is where the vapor pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapor pressure.


title: Vapor pressure

text: Although the relation between vapor pressure and temperature is non-linear, the chart uses a logarithmic vertical axis to produce slightly curved lines, so one chart can graph many liquids. A nearly straight line is obtained when the logarithm of the vapor pressure is plotted against 1/(T + 230) where T is the temperature in degrees Celsius. The vapor pressure of a liquid at its boiling point equals the pressure of its surrounding environment.


title: Vapor pressure

text: Raoult's law gives an approximation to the vapor pressure of mixtures of liquids. It states that the activity (pressure or fugacity) of a single-phase mixture is equal to the mole-fraction-weighted sum of the components' vapor pressures:


title: Vapor pressure

text: where formula_14 is the mixture's vapor pressure, formula_15 is the mole fraction of component formula_16 in the liquid phase and formula_17 is the mole fraction of component formula_16 in the vapor phase respectively. formula_19 is the vapor pressure of component formula_16. Raoult's law is applicable only to non-electrolytes (uncharged species); it is most appropriate for non-polar molecules with only weak intermolecular attractions (such as London forces).


title: Vapor pressure

text: Systems that have vapor pressures higher than indicated by the above formula are said to have positive deviations. Such a deviation suggests weaker intermolecular attraction than in the pure components, so that the molecules can be thought of as being "held in" the liquid phase less strongly than in the pure liquid. An example is the azeotrope of approximately 95% ethanol and water. Because the azeotrope's vapor pressure is higher than predicted by Raoult's law, it boils at a temperature below that of either pure component.


title: Vapor pressure

text: There are also systems with negative deviations that have vapor pressures that are lower than expected. Such a deviation is evidence for stronger intermolecular attraction between the constituents of the mixture than exists in the pure components. Thus, the molecules are "held in" the liquid more strongly when a second molecule is present. An example is a mixture of trichloromethane (chloroform) and 2-propanone (acetone), which boils above the boiling point of either pure component.


title: Vapor pressure

text: The negative and positive deviations can be used to determine thermodynamic activity coefficients of the components of mixtures.


title: Vapor pressure

text: Equilibrium vapor pressure can be defined as the pressure reached when a condensed phase is in equilibrium with its own vapor. In the case of an equilibrium solid, such as a crystal, this can be defined as the pressure when the rate of sublimation of a solid matches the rate of deposition of its vapor phase. For most solids this pressure is very low, but some notable exceptions are naphthalene, dry ice (the vapor pressure of dry ice is 5.73 MPa (831 psi, 56.5 atm) at 20 °C, which causes most sealed containers to rupture), and ice. All solid materials have a vapor pressure. However, due to their often extremely low values, measurement can be rather difficult. Typical techniques include the use of thermogravimetry and gas transpiration.


title: Vapor pressure

text: There are a number of methods for calculating the sublimation pressure (i.e., the vapor pressure) of a solid. One method is to estimate the sublimation pressure from extrapolated liquid vapor pressures (of the supercooled liquid), if the heat of fusion is known, by using this particular form of the Clausius–Clapeyron relation:


title: Vapor pressure

text: This method assumes that the heat of fusion is temperature-independent, ignores additional transition temperatures between different solid phases, and it gives a fair estimation for temperatures not too far from the melting point. It also shows that the sublimation pressure is lower than the extrapolated liquid vapor pressure (Δ"H" > 0) and the difference grows with increased distance from the melting point.


title: Vapor pressure

text: Like all liquids, water boils when its vapor pressure reaches its surrounding pressure. In nature, the atmospheric pressure is lower at higher elevations and water boils at a lower temperature. The boiling temperature of water for atmospheric pressures can be approximated by the Antoine equation:


title: Vapor pressure

text: where the temperature formula_32 is the boiling point in degrees Celsius and the pressure formula_3 is in Torr.


title: Vapor pressure

text: Dühring's rule states that a linear relationship exists between the temperatures at which two solutions exert the same vapor pressure.


title: Vapor pressure

text: The following table is a list of a variety of substances ordered by increasing vapor pressure (in absolute units).


title: Vapor pressure

text: Several empirical methods exist to estimate the vapor pressure from molecular structure for organic molecules. Some examples are SIMPOL.1 method, the method of Moller et al., and EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature, Intramolecular, and Non-additivity effects).


title: Vapor pressure

text: In meteorology, the term vapor pressure means the partial pressure of water vapor in the atmosphere, even if it is not in equilibrium. This differs from its meaning in other sciences.


title: Vapor pressure

text: According to the American Meteorological Society "Glossary of Meteorology", saturation vapor pressure properly refers to the equilibrium vapor pressure of water above a flat surface of liquid water or solid ice, and is a function only of temperature and whether the condensed phase is liquid or solid.


title: Vapor pressure

text: Equilibrium vapor pressure does not require the condensed phase to be a flat surface; it might consist of tiny droplets possibly containing solutes (impurities), such as a cloud. Equilibrium vapor pressure may differ significantly from saturation vapor pressure depending on the size of droplets and presence of other particles which act as cloud condensation nuclei.


title: Vapor pressure

text: However, these terms are used inconsistently, and some authors use ""saturation vapor pressure"" outside the narrow meaning given by the AMS "Glossary". For example, a text on atmospheric convection states, "The Kelvin effect causes the saturation vapor pressure over the curved surface of the droplet to be greater than that over a flat water surface" (emphasis added).


title: Vapor pressure

text: The still-current term "saturation vapor pressure" derives from the obsolete theory that water vapor dissolves into air, and that air at a given temperature can only hold a certain amount of water before becoming "saturated". Actually, as stated by Dalton's law (known since 1802), the partial pressure of water vapor or any substance does not depend on air at all, and the relevant temperature is that of the liquid. Nevertheless, the erroneous belief persists among the public and even meteorologists, aided by the misleading terms "saturation pressure" and "supersaturation" and the related definition of "relative humidity".


Egyptian pyramid construction techniques are the controversial subject of many hypotheses. These techniques seem to have developed over time; later pyramids were not constructed in the same way as earlier ones. Most of the construction hypotheses are based on the belief that huge stones were carved from quarries with copper chisels, and these blocks were then dragged and lifted into position. Disagreements chiefly concern the methods used to move and place the stones.


title: Egyptian pyramid construction techniques

text: In addition to the many unresolved arguments about the construction techniques, there have been disagreements as to the kind of workforce used. The Greeks, many years after the event, believed that the pyramids must have been built by slave labor. Archaeologists now believe that the Great Pyramid of Giza (at least) was built by tens of thousands of skilled workers who camped near the pyramids and worked for a salary or as a form of tax payment (levy) until the construction was completed, pointing to workers' cemeteries discovered in 1990. For the Middle Kingdom pyramid of Amenemhat II, there is evidence from the annal stone of the king that foreigners from Canaan were employed.


text: During the earliest period, pyramids were constructed wholly of stone. Locally quarried limestone was the material of choice for the main body of these pyramids, while a higher quality of limestone quarried at Tura (near modern Cairo) was used for the outer casing. Granite, quarried near Aswan, was used to construct some architectural elements, including the portcullis (a type of gate) and the roofs and walls of the burial chamber. Occasionally, granite was used in the outer casing as well, such as in the Pyramid of Menkaure. In the early pyramids, the layers of stone (called "courses") forming the pyramid body were laid sloping inwards; however, this configuration was found to be less stable than simply stacking the stones horizontally on top of each other. The Bent Pyramid at Dahshur seems to indicate acceptance of a new technique at a transition between these two building techniques. Its lower section is built of sloping courses while in its upper section the stones are laid horizontally.


text: During the Middle Kingdom, pyramid construction techniques changed again. Most pyramids built then were little more than mountains of mud-brick encased in a veneer of polished limestone. In several cases, later pyramids were built on top of natural hills to further reduce the volume of material needed in their construction. The materials and methods of construction used in the earliest pyramids have ensured their survival in a generally much better state of preservation than for the pyramid monuments of the later pharaohs.


text: One of the major problems faced by the early pyramid builders was the need to move huge quantities of stone. The Twelfth Dynasty tomb of Djehutihotep has an illustration of 172 men pulling an alabaster statue of him on a sledge. The statue is estimated to weigh 60 tons and Denys Stocks estimated that 45 workers would be required to start moving a lubricated block, or eight workers to move a block. Dr. R. H. G. Parry has suggested a method for rolling the stones, using a cradle-like machine that had been excavated in various new kingdom temples. Four of those objects could be fitted around a block so it could be rolled easily. Experiments done by the Obayashi Corporation, with concrete blocks square by long and weighing , showed how 18 men could drag the block over a 1-in-4 incline ramp, at a rate of . This idea was previously described by John Bush in 1977, and is mentioned in the "Closing Remarks" section of Parry's book. Vitruvius in "De architectura" described a similar method for moving irregular weights. It is still not known whether the Egyptians used this method but the experiments indicate it could have worked using stones of this size. Egyptologists generally accept this for the 2.5 ton blocks mostly used but do not agree over the methods used for the 15+ ton and several 70 to 80 ton blocks.


text: As the stones forming the core of the pyramids were roughly cut, especially in the Great Pyramid, the material used to fill the gaps was another problem. Huge quantities of gypsum and rubble were needed. The filling has almost no binding properties, but it was necessary to stabilize the construction. To make the gypsum mortar, it had to be dehydrated by heating which requires large quantities of wood. According to Egyptologists, the findings of both the 1984 and 1995 David H. Koch Pyramids Radiocarbon Projects may suggest that Egypt had to strip its forest and scrap every bit of wood it had to build the pyramids of Giza and other even earlier 4th Dynasty pyramids. Carbon dating samples from core blocks and other materials revealed that dates from the 1984 study averaged 374 years earlier than currently accepted and the 1995 dating averaging 100–200 years. As suggested by team members, "We thought that it was unlikely that the pyramid builders consistently used centuries-old wood as fuel in preparing mortar. The 1984 results left us with too little data to conclude that the historical chronology of the Old Kingdom was wrong by nearly 400 years, but we considered this at least a possibility". To explain this discrepancy, Egyptologists proposed the "old wood" theory claiming the earlier dates were possibly derived from recycling large amounts of centuries-old wood and other earlier materials.


text: There is good information concerning the location of the quarries, some of the tools used to cut stone in the quarries, transportation of the stone to the monument, leveling the foundation, and leveling the subsequent tiers of the developing superstructure. Workmen probably used copper chisels, drills, and saws to cut softer stone, such as most of the limestone. The harder stones, such as granite, granodiorite, syenite, and basalt, cannot be cut with copper tools alone; instead, they were worked with time-consuming methods like pounding with dolerite, drilling, and sawing with the aid of an abrasive, such as quartz sand. Blocks were transported by sledge likely lubricated by water. Leveling the foundation may have been accomplished by use of water-filled trenches as suggested by Mark Lehner and I. E. S. Edwards or through the use of a crude square level and experienced surveyors.


text: The diary of Merer, logbooks written more than 4,500 years ago by an Egyptian official and found in 2013 by a French archeology team under the direction of Pierre Tallet in a cave in Wadi al-Jarf, describes the transportation of limestone from the quarry in Tora to Giza.


text: The unknowns of pyramid construction chiefly center on the question of how the blocks were moved up the superstructure. There is no known accurate historical or archaeological evidence that definitively resolves the question. Therefore, most discussion on construction methods involves functional possibilities that are supported by limited historical and archaeological evidence.


text: Historical accounts for the construction of the Egyptian pyramids do little to point definitively to methods to lift the blocks; yet most Egyptologists refer to these accounts when discussing this portion of pyramid construction. Thales, according to the philosopher Hieronymus (3rd century BC), visited the Egyptian pyramids during the 7th century BC and by using the intercept theorem, also known as Thales's theorem, measured their height and thus their volume. The first historical accounts of the construction of these monuments came centuries after the era of pyramid construction, by Herodotus in the 5th century BC and Diodorus Siculus in the 1st century BC. Herodotus's account states:


text: Diodorus Siculus's description of the shipment of the stone from Arabia is correct since the term "Arabia" in those days implied the land between the Nile and the Red Sea where the limestone blocks have been transported from quarries across the river Nile.


text: Most Egyptologists acknowledge that ramps are the most tenable of the methods to raise the blocks, yet they acknowledge that it is an incomplete method that must be supplemented by another device. Archaeological evidence for the use of ramps has been found at the Great Pyramid of Giza and other pyramids. The method most accepted for assisting ramps is levering (Lehner 1997: 222). The archaeological record gives evidence of only small ramps and inclined causeways, not something that could have been used to construct even a majority of the monument. To add to the uncertainty, there is considerable evidence demonstrating that non-standardized or "ad hoc" construction methods were used in pyramid construction (Arnold 1991: 98, Lehner 1997: 223).


text: Therefore, there are many proposed ramps and there is a considerable amount of discrepancy regarding what type of ramp was used to build the pyramids. One of the widely discredited ramping methods is the large straight ramp, and it is routinely discredited on functional grounds for its massive size, lack of archaeological evidence, huge labor cost, and other problems (Arnold 1991: 99, Lehner 1997: 215, Isler 2001: 213).


text: Other ramps serve to correct these problems of ramp size, yet either run into critiques of functionality and limited archaeological evidence. There are zig-zagging ramps, straight ramps using the incomplete part of the superstructure (Arnold 1991), spiraling ramps supported by the superstructure and spiraling ramps leaning on the monument as a large accretion are proposed. Mark Lehner speculated that a spiraling ramp, beginning in the stone quarry to the southeast and continuing around the exterior of the pyramid, may have been used. The stone blocks may have been drawn on sleds along the ramps lubricated by water.


text: Levering methods are considered to be the most tenable solution to complement ramping methods, partially due to Herodotus's description; and partially to the shadoof, a lever-enabled irrigation device first depicted in Egypt during the New Kingdom and found concomitantly with the Old Kingdom in Mesopotamia. In Lehner's (1997: 222) point of view, levers should be employed to lift the top 3% of the material of the superstructure. It is important to note that the top 4% of this material comprises of the total height of the monument. In other words, in Lehner's view, levers should be employed to lift a small amount of material and a great deal of vertical height of the monument.


text: In the milieu of levering methods, there are those that lift the block incrementally, as in repeatedly prying up alternating sides of the block and inserting wooden or stone shims to gradually move the stone up one course; and there are other methods that use a larger lever to move the block up one course in one lifting procedure. Since the discussion of construction techniques to lift the blocks attempts to resolve a gap in the archaeological and historical record with a plausible functional explanation, the following examples by Isler, Keable, and Hussey-Pailos list experimentally tested methods. Isler's method (1985, 1987) is an incremental method and, in the Nova experiment (1992), used wooden shims or cribbing. Isler was able to lift a block up one tier in approximately one hour and 30 minutes. Peter Hodges's and Julian Keable's method is similar to Isler's method and instead used small manufactured concrete blocks as shims, wooden pallets, and a pit where their experimental tests were performed. Keable was able to perform his method in approximately 2 minutes. Scott Hussey-Pailos's (2005) method uses a simple levering device to lift a block up a course in one movement. This method was tested with materials of less strength than historical analogs (tested with materials weaker than those available in ancient Egypt), a factor of safety of 2, and lifted a 2500-pound block up one course in under a minute. This method is presented as a levering device to work complementary with Mark Lehner's idea of a combined ramp and levering techniques.


text: Houdin's father was an architect who, in 1999, thought of a construction method that, it seemed to him, made more sense than any existing method proposed for building pyramids. To develop this hypothesis, Jean-Pierre Houdin, also an architect, gave up his job and set about drawing the first fully functional CAD architectural model of the Great Pyramid of Giza. His/their scheme involves using a regular external ramp to build the first 30% of the pyramid, with an "internal ramp" taking stones up beyond that height. The stones of the external ramp are re-cycled into the upper stories, thus explaining the otherwise puzzling lack of evidence for ramps.


text: After four years working alone, Houdin was joined by a team of engineers from the French 3D software company Dassault Systèmes, who used the most modern computer-aided design technology available to further refine and test the hypothesis, making it (according to Houdin) the only one proven to be a viable technique. In 2006 Houdin announced it in a book: "Khufu: The Secrets Behind the Building of the Great Pyramid", and in 2008 he and Egyptologist Bob Brier wrote a second book: "The Secret of the Great Pyramid".


text: In Houdin's method, each ramp inside the pyramid ended at an open space, a notch temporarily left open in the edge of the construction. This 10-square-meter clear space housed a crane that lifted and rotated each 2.5-ton block, to ready it for eight men to drag up the next internal ramp. There is a notch of sorts in one of the right places, and in 2008 Houdin's co-author Bob Brier, with a National Geographic film crew, entered a previously unremarked chamber that could be the start of one of these internal ramps. In 1986 a member of the French team (see below) saw a desert fox at this notch, rather as if it had ascended internally.


text: Houdin's thesis remains unproven and in 2007, Egyptologist David Jeffreys from the University College London described the internal spiral hypothesis as "far-fetched and horribly complicated", while Oxford University's John Baines, declared he was "suspicious of any theory that seeks to explain only how the Great Pyramid was built".


text: Houdin has another hypothesis developed from his architectural model, one that could finally explain the internal "Grand Gallery" chamber that otherwise appears to have little purpose. He believes the gallery acted as a trolley chute/guide for counterbalance weights. It enabled the raising of the five 60-ton granite beams that roof the King's Chamber. Houdin and Brier and the Dassault team are already credited with proving for the first time that cracks in beams appeared during construction, were examined and tested at the time and declared relatively harmless.


text: Materials scientist Joseph Davidovits has claimed that the blocks of the pyramid are not carved stone, but mostly a form of limestone concrete and that they were "cast" as with modern concrete. According to this hypothesis, soft limestone with a high kaolinite content was quarried in the wadi on the south of the Giza Plateau. The limestone was then dissolved in large, Nile-fed pools until it became a watery slurry. Lime (found in the ash of cooking fires) and natron (also used by the Egyptians in mummification) were mixed in. The pools were then left to evaporate, leaving behind a moist, clay-like mixture. This wet "concrete" would be carried to the construction site where it would be packed into reusable wooden moulds and in a few days would undergo a chemical reaction similar to the curing of concrete. New blocks, he suggests, could be cast in place, on top of and pressed against the old blocks. Proof-of-concept tests using similar compounds were carried out at a geopolymer institute in northern France and it was found that a crew of five to ten, working with simple hand tools, could agglomerate a structure of five, 1.3 to 4.5 ton blocks in a couple of weeks. He also claims that the Famine Stele, along with other hieroglyphic texts, describe the technology of stone agglomeration.


text: Davidovits's method is not accepted by the academic mainstream. His method does not explain the granite stones, weighing well over 10 tons, above the King's Chamber, which he agrees were carved. Geologists have carefully scrutinized Davidovits's suggested technique and concluded his concrete came from natural limestone quarried in the Mokattam Formation. However, Davidovits alleges that the bulk of the soft limestone came from the same natural Mokkatam Formation quarries found by geologists, and insists that ancient Egyptians used the soft marly layer instead of the hard layer to re-agglomerate stones.


text: Davidovits's hypothesis gained support from Michel Barsoum, a materials science researcher. Michel Barsoum and his colleagues at Drexel University published their findings supporting Davidovits's hypothesis in the "Journal of the American Ceramic Society" in 2006. Using scanning electron microscopy, they discovered in samples of the limestone pyramid blocks mineral compounds and air bubbles that do not occur in natural limestone.


text: Dipayan Jana, a petrographer, made a presentation to the ICMA (International Cement Microscopy Association) in 2007 and gave a paper in which he discusses Davidovits's and Barsoum's work and concludes "we are far from accepting even as a remote possibility a 'man-made' origin of pyramid stones."


text: In 1992, Mark Lehner and stonemason Roger Hopkins conducted a three-week pyramid-building experiment for a "NOVA" television episode. They built a pyramid high by wide, consisting of a total of , or about 405 tons. It was made out of 186 stones weighing an average of 2.2 tons each. Twelve quarrymen carved 186 stones in 22 days, and the structure was erected using 44 men. They used iron hammers, chisels and levers (this is a modern shortcut, as the ancient Egyptians were limited to using copper and later bronze and wood). But Lehner and Hopkins did experiments with copper tools, noting that they were adequate for the job in hand, provided that additional manpower was available to constantly resharpen the ancient tools. They estimated they would have needed around 20 extra men for this maintenance. Another shortcut taken was the use of a front-end loader or fork lift truck, but modern machinery was not used to finish the construction. They used levers to lift the capstone to a height of . Four or five men were able to use levers on stones less than one ton to flip them over and transport them by rolling, but larger stones had to be towed. Lehner and Hopkins found that by putting the stones on wooden sledges and sliding the sledges on wooden tracks, they were able to tow a two-ton stone with 12 to 20 men. The wood for these sledges and tracks would have to have been imported from Lebanon at great cost since there was little, if any, wood in ancient Egypt. While the builders failed to duplicate the precise jointing created by the ancient Egyptians, Hopkins was confident that this could have been achieved with more practice.


text: The so-called fluvial-port-complex hypothesis proposes that ancient Egyptians must have used the now-disappeared parts of the Nile to transport the tons of construction materials. A 2012 study led by geographer Hader Sheisha at Aix-Marseille University proposed that the former waterscapes and higher river levels around 4,500 years ago facilitated the construction of the Giza Pyramid Complex. The Nile's present waterways have receded too far from the pyramid sites to be of use.


text: Some research suggests other estimates to the accepted workforce size. For instance, physicist Kurt Mendelssohn calculated that the workforce may have been 50,000 men at most, while Ludwig Borchardt and Louis Croon placed the number at 36,000. According to Miroslav Verner, a workforce of no more than 30,000 was needed in the Great Pyramid's construction. Evidence suggests that around 5,000 were permanent workers on salaries with the balance working three- or four-month shifts in lieu of taxes while receiving subsistence "wages" of ten loaves of bread and a jug of beer per day. Zahi Hawass believes that the majority of workers may have been volunteers. Most archaeologists agree that only about 4,000 of the total workforce were labourers who quarried the stone, hauled blocks to the pyramid, and set the blocks in place. The vast majority of the workforce provided support services such as scribes, toolmakers, and other backup services. The tombs of supervisors contain inscriptions regarding the organisation of the workforce. There were two crews of approximately 2,000 workers sub-divided into named gangs of 1,000. The gangs were divided into five phyles of 200 which were in turn split into groups of around 20 workers grouped according to their skills, with each group having their own project leader and a specific task.


text: A construction management study carried out by the firm Daniel, Mann, Johnson, & Mendenhall in association with Mark Lehner, and other Egyptologists, estimates that the total project required an average workforce of 14,567 people and a peak workforce of 40,000. Without the use of pulleys, wheels, or iron tools, they used critical path analysis to suggest the Great Pyramid was completed from start to finish in approximately 10 years. Their study estimates the number of blocks used in construction was between 2 and 2.8 million (an average of 2.4 million), but settles on a reduced finished total of 2 million after subtracting the estimated volume of the hollow spaces of the chambers and galleries. Most sources agree on this number of blocks somewhere above 2.3 million. Their calculations suggest the workforce could have sustained a rate of 180 blocks per hour (3 blocks/minute) with ten-hour workdays for putting each individual block in place. They derived these estimates from modern third-world construction projects that did not use modern machinery, but conclude it is still unknown exactly how the Great Pyramid was built. As Dr. Craig Smith of the team points out:


text: The entire Giza Plateau is believed to have been constructed over the reign of five pharaohs in less than a hundred years, which generally includes: the Great Pyramid, Khafre and Menkaure's pyramids, the Great Sphinx, the Sphinx, and Valley Temples, 35 boat pits cut out of solid bedrock, and several causeways, as well as paving nearly the entire plateau with large stones. This does not include Khafre's brother Djedefre's northern pyramid, Abu Rawash, which would have also been built during this time frame of 100 years. In the hundred years prior to Giza—beginning with Djoser, who ruled from 2687 to 2667 BC, and amongst dozens of other temples, smaller pyramids, and general construction projects—four other massive pyramids were built: the Step pyramid of Saqqara (believed to be the first Egyptian pyramid), the pyramid of Meidum, the Bent Pyramid, and the Red Pyramid. Also during this period (between 2686 and 2498 BC) the Sadd el-Kafara dam, which used an estimated 100,000 cubic meters of rock and rubble, was built.


text: In October 2018, a team of archaeologists from the Institut Français d'Archéologie Orientale and University of Liverpool had announced the discovery of the remains of a 4,500-year-old ramp contraption at Hatnub, excavated since 2012. This method which aided in lifting the heavy alabaster stones up from their quarries may have been used to build Egypt's Great Pyramid as well. Yannis Gourdon, co-director of the joint mission at Hatnub, said:


title: Gravitational wave

text: Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as waves similar to electromagnetic waves but the gravitational equivalent. Gravitational waves were later predicted in 1916 by Albert Einstein on the basis of his general theory of relativity as ripples in spacetime. Later he refused to accept gravitational waves. Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics, does not provide for their existence, since that law is predicated on the assumption that physical interactions propagate instantaneously (at infinite speed)showing one of the ways the methods of Newtonian physics are unable to explain phenomena associated with relativity.


title: Gravitational wave

text: The first indirect evidence for the existence of gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar, which matched the decay predicted by general relativity as energy is lost to gravitational radiation. In 1993, Russell A. Hulse and Joseph Hooton Taylor Jr. received the Nobel Prize in Physics for this discovery. The first direct observation of gravitational waves was not made until 2015, when a signal generated by the merger of two black holes was received by the LIGO gravitational wave detectors in Livingston, Louisiana, and in Hanford, Washington. The 2017 Nobel Prize in Physics was subsequently awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the direct detection of gravitational waves. Where General Relativity is accepted, gravitational waves as detected are attributed to ripples in spacetime, otherwise the gravitational waves can be thought of simply as a product of the orbit of binary systems (a binary orbit causes the binary system's geometry to change through 180 degrees and also causes the distance between each body of the binary system and the observer to change through 180 degrees causing a gravitational wave frequency of two times the orbital frequency).


title: Gravitational wave

text: In gravitational-wave astronomy, observations of gravitational waves are used to infer data about the sources of gravitational waves. Sources that can be studied this way include binary star systems composed of white dwarfs, neutron stars, and black holes; events such as supernovae; and the formation of the early universe shortly after the Big Bang.


title: Gravitational wave

text: In Einstein's general theory of relativity, gravity is treated as a phenomenon resulting from the curvature of spacetime. This curvature is caused by the presence of mass. Generally, the more mass that is contained within a given volume of space, the greater the curvature of spacetime will be at the boundary of its volume. As objects with mass move around in spacetime, the curvature changes to reflect the changed locations of those objects. In certain circumstances, accelerating objects generate changes in this curvature which propagate outwards at the speed of light in a wave-like manner. These propagating phenomena are known as gravitational waves.


title: Gravitational wave

text: As a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of strain. Distances between objects increase and decrease rhythmically as the wave passes, at a frequency equal to that of the wave. The magnitude of this effect is inversely proportional to the distance from the source. Inspiraling binary neutron stars are predicted to be a powerful source of gravitational waves as they coalesce, due to the very large acceleration of their masses as they orbit close to one another. However, due to the astronomical distances to these sources, the effects when measured on Earth are predicted to be very small, having strains of less than 1 part in 10. Scientists have demonstrated the existence of these waves with ever more sensitive detectors. The most sensitive detector accomplished the task possessing a sensitivity measurement of about one part in () provided by the LIGO and VIRGO observatories. In 2019, the Japanese detector KAGRA was completed and made its first joint detection with LIGO and VIRGO in 2021. A space based observatory, the Laser Interferometer Space Antenna, is currently under development by ESA. Another European ground based detector, the Einstein Telescope, is also being developed.


title: Gravitational wave

text: Gravitational waves can penetrate regions of space that electromagnetic waves cannot. They allow the observation of the merger of black holes and possibly other exotic objects in the distant Universe. Such systems cannot be observed with more traditional means such as optical telescopes or radio telescopes, and so gravitational wave astronomy gives new insights into the working of the Universe. In particular, gravitational waves could be of interest to cosmologists as they offer a possible way of observing the very early Universe. This is not possible with conventional astronomy, since before recombination the Universe was opaque to electromagnetic radiation. Precise measurements of gravitational waves will also allow scientists to test more thoroughly the general theory of relativity.


title: Gravitational wave

text: In principle, gravitational waves could exist at any frequency. However, very low frequency waves would be impossible to detect, and there is no credible source for detectable waves of very high frequency as well. Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10 Hz up to 10 Hz.


title: Gravitational wave

text: The speed of gravitational waves in the general theory of relativity is equal to the "speed of light" in a vacuum, . Within the theory of special relativity, the constant is not only about light; instead it is the highest possible speed for any interaction in nature. Formally, is a conversion factor for changing the unit of time to the unit of space. This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves, and further the speed of any massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light (hence carrier of electromagnetic force), and the hypothetical gravitons (which are the presumptive field particles associated with gravity; however, an understanding of the graviton, if any exist, requires an as-yet unavailable theory of quantum gravity).


title: Gravitational wave

text: In August 2017, the LIGO and Virgo detectors received gravitational wave signals within 2 seconds of gamma ray satellites and optical telescopes seeing signals from the same direction. This confirmed that the speed of gravitational waves was the same as the speed of light.


title: Gravitational wave

text: The possibility of gravitational waves was discussed in 1893 by Oliver Heaviside, using the analogy between the inverse-square law of gravitation and the electrostatic force. In 1905, Henri Poincaré proposed gravitational waves, emanating from a body and propagating at the speed of light, as being required by the Lorentz transformations and suggested that, in analogy to an accelerating electrical charge producing electromagnetic waves, accelerated masses in a relativistic field theory of gravity should produce gravitational waves. When Einstein published his general theory of relativity in 1915, he was skeptical of Poincaré's idea since the theory implied there were no "gravitational dipoles". Nonetheless, he still pursued the idea and based on various approximations came to the conclusion there must, in fact, be three types of gravitational waves (dubbed longitudinal–longitudinal, transverse–longitudinal, and transverse–transverse by Hermann Weyl).


title: Gravitational wave

text: However, the nature of Einstein's approximations led many (including Einstein himself) to doubt the result. In 1922, Arthur Eddington showed that two of Einstein's types of waves were artifacts of the coordinate system he used, and could be made to propagate at any speed by choosing appropriate coordinates, leading Eddington to jest that they "propagate at the speed of thought". This also cast doubt on the physicality of the third (transverse–transverse) type that Eddington showed always propagate at the speed of light regardless of coordinate system. In 1936, Einstein and Nathan Rosen submitted a paper to "Physical Review" in which they claimed gravitational waves could not exist in the full general theory of relativity because any such solution of the field equations would have a singularity. The journal sent their manuscript to be reviewed by Howard P. Robertson, who anonymously reported that the singularities in question were simply the harmless coordinate singularities of the employed cylindrical coordinates. Einstein, who was unfamiliar with the concept of peer review, angrily withdrew the manuscript, never to publish in "Physical Review" again. Nonetheless, his assistant Leopold Infeld, who had been in contact with Robertson, convinced Einstein that the criticism was correct, and the paper was rewritten with the opposite conclusion and published elsewhere. In 1956, Felix Pirani remedied the confusion caused by the use of various coordinate systems by rephrasing the gravitational waves in terms of the manifestly observable Riemann curvature tensor.


title: Gravitational wave

text: At the time, Pirani's work was overshadowed by the community's focus on a different question: whether gravitational waves could transmit energy. This matter was settled by a thought experiment proposed by Richard Feynman during the first "GR" conference at Chapel Hill in 1957. In short, his argument known as the "sticky bead argument" notes that if one takes a rod with beads then the effect of a passing gravitational wave would be to move the beads along the rod; friction would then produce heat, implying that the passing wave had done work. Shortly after, Hermann Bondi, published a detailed version of the "sticky bead argument". This later lead to a series of articles (1959 to 1989)


title: Gravitational wave

text: After the Chapel Hill conference, Joseph Weber started designing and building the first gravitational wave detectors now known as Weber bars. In 1969, Weber claimed to have detected the first gravitational waves, and by 1970 he was "detecting" signals regularly from the Galactic Center; however, the frequency of detection soon raised doubts on the validity of his observations as the implied rate of energy loss of the Milky Way would drain our galaxy of energy on a timescale much shorter than its inferred age. These doubts were strengthened when, by the mid-1970s, repeated experiments from other groups building their own Weber bars across the globe failed to find any signals, and by the late 1970s consensus was that Weber's results were spurious.


title: Gravitational wave

text: In the same period, the first indirect evidence of gravitational waves was discovered. In 1974, Russell Alan Hulse and Joseph Hooton Taylor, Jr. discovered the first binary pulsar, which earned them the 1993 Nobel Prize in Physics. Pulsar timing observations over the next decade showed a gradual decay of the orbital period of the Hulse–Taylor pulsar that matched the loss of energy and angular momentum in gravitational radiation predicted by general relativity.


title: Gravitational wave

text: This indirect detection of gravitational waves motivated further searches, despite Weber's discredited result. Some groups continued to improve Weber's original concept, while others pursued the detection of gravitational waves using laser interferometers. The idea of using a laser interferometer for this seems to have been floated independently by various people, including M. E. Gertsenshtein and V. I. Pustovoit in 1962, and Vladimir B. Braginskiĭ in 1966. The first prototypes were developed in the 1970s by Robert L. Forward and Rainer Weiss. In the decades that followed, ever more sensitive instruments were constructed, culminating in the construction of GEO600, LIGO, and Virgo.


title: Gravitational wave

text: After years of producing null results, improved detectors became operational in 2015. On 11 February 2016, the LIGO-Virgo collaborations announced the first observation of gravitational waves, from a signal (dubbed GW150914) detected at 09:50:45 GMT on 14 September 2015 of two black holes with masses of 29 and 36 solar masses merging about 1.3 billion light-years away. During the final fraction of a second of the merger, it released more than 50 times the power of all the stars in the observable universe combined. The signal increased in frequency from 35 to 250 Hz over 10 cycles (5 orbits) as it rose in strength for a period of 0.2 second. The mass of the new merged black hole was 62 solar masses. Energy equivalent to three solar masses was emitted as gravitational waves. The signal was seen by both LIGO detectors in Livingston and Hanford, with a time difference of 7 milliseconds due to the angle between the two detectors and the source. The signal came from the Southern Celestial Hemisphere, in the rough direction of (but much farther away than) the Magellanic Clouds. The confidence level of this being an observation of gravitational waves was 99.99994%.


title: Gravitational wave

text: A year earlier, the BICEP2 claimed that they had detected the imprint of gravitational waves in the cosmic microwave background. However, they were later forced to retract this result.


title: Gravitational wave

text: In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the detection of gravitational waves.


title: Gravitational wave

text: Gravitational waves are constantly passing Earth; however, even the strongest have a minuscule effect and their sources are generally at a great distance. For example, the waves given off by the cataclysmic final merger of GW150914 reached Earth after travelling over a billion light-years, as a ripple in spacetime that changed the length of a 4 km LIGO arm by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width. This tiny effect from even extreme gravitational waves makes them observable on Earth only with the most sophisticated detectors.


title: Gravitational wave

text: The effects of a passing gravitational wave, in an extremely exaggerated form, can be visualized by imagining a perfectly flat region of spacetime with a group of motionless test particles lying in a plane, e.g., the surface of a computer screen. As a gravitational wave passes through the particles along a line perpendicular to the plane of the particles, i.e., following the observer's line of vision into the screen, the particles will follow the distortion in spacetime, oscillating in a "cruciform" manner, as shown in the animations. The area enclosed by the test particles does not change and there is no motion along the direction of propagation.


title: Gravitational wave

text: The oscillations depicted in the animation are exaggerated for the purpose of discussion in reality a gravitational wave has a very small amplitude (as formulated in linearized gravity). However, they help illustrate the kind of oscillations associated with gravitational waves as produced by a pair of masses in a circular orbit. In this case the amplitude of the gravitational wave is constant, but its plane of polarization changes or rotates at twice the orbital rate, so the time-varying gravitational wave size, or 'periodic spacetime strain', exhibits a variation as shown in the animation. If the orbit of the masses is elliptical then the gravitational wave's amplitude also varies with time according to Einstein's quadrupole formula.


title: Gravitational wave

text: The speed, wavelength, and frequency of a gravitational wave are related by the equation "c = λf", just like the equation for a light wave. For example, the animations shown here oscillate roughly once every two seconds. This would correspond to a frequency of 0.5 Hz, and a wavelength of about 600 000 km, or 47 times the diameter of the Earth.


title: Gravitational wave

text: In the above example, it is assumed that the wave is linearly polarized with a "plus" polarization, written "h". Polarization of a gravitational wave is just like polarization of a light wave except that the polarizations of a gravitational wave are 45 degrees apart, as opposed to 90 degrees. In particular, in a "cross"-polarized gravitational wave, "h", the effect on the test particles would be basically the same, but rotated by 45 degrees, as shown in the second animation. Just as with light polarization, the polarizations of gravitational waves may also be expressed in terms of circularly polarized waves. Gravitational waves are polarized because of the nature of their source.


title: Gravitational wave

text: In general terms, gravitational waves are radiated by objects whose motion involves acceleration and its change, provided that the motion is not perfectly spherically symmetric (like an expanding or contracting sphere) or rotationally symmetric (like a spinning disk or sphere). A simple example of this principle is a spinning dumbbell. If the dumbbell spins around its axis of symmetry, it will not radiate gravitational waves; if it tumbles end over end, as in the case of two planets orbiting each other, it will radiate gravitational waves. The heavier the dumbbell, and the faster it tumbles, the greater is the gravitational radiation it will give off. In an extreme case, such as when the two weights of the dumbbell are massive stars like neutron stars or black holes, orbiting each other quickly, then significant amounts of gravitational radiation would be given off.


title: Gravitational wave

text: More technically, the second time derivative of the quadrupole moment (or the "l"-th time derivative of the "l"-th multipole moment) of an isolated system's stress–energy tensor must be non-zero in order for it to emit gravitational radiation. This is analogous to the changing dipole moment of charge or current that is necessary for the emission of electromagnetic radiation.


title: Gravitational wave

text: Gravitational waves carry energy away from their sources and, in the case of orbiting bodies, this is associated with an in-spiral or decrease in orbit. Imagine for example a simple system of two masses such as the Earth–Sun system moving slowly compared to the speed of light in circular orbits. Assume that these two masses orbit each other in a circular orbit in the "x"–"y" plane. To a good approximation, the masses follow simple Keplerian orbits. However, such an orbit represents a changing quadrupole moment. That is, the system will give off gravitational waves.


title: Gravitational wave

text: In theory, the loss of energy through gravitational radiation could eventually drop the Earth into the Sun. However, the total energy of the Earth orbiting the Sun (kinetic energy + gravitational potential energy) is about 1.14 joules of which only 200 watts (joules per second) is lost through gravitational radiation, leading to a decay in the orbit by about 1 meters per day or roughly the diameter of a proton. At this rate, it would take the Earth approximately 3 times more than the current age of the Universe to spiral onto the Sun. This estimate overlooks the decrease in "r" over time, but the radius varies only slowly for most of the time and plunges at later stages, as formula_1 with formula_2 the initial radius and formula_3 the total time needed to fully coalesce.


title: Gravitational wave

text: where "r" is the separation between the bodies, "t" time, "G" the gravitational constant, "c" the speed of light, and "m" and "m" the masses of the bodies. This leads to an expected time to merger of


title: Gravitational wave

text: Compact stars like white dwarfs and neutron stars can be constituents of binaries. For example, a pair of solar mass neutron stars in a circular orbit at a separation of 1.89 m (189,000 km) has an orbital period of 1,000 seconds, and an expected lifetime of 1.30 seconds or about 414,000 years. Such a system could be observed by LISA if it were not too far away. A far greater number of white dwarf binaries exist with orbital periods in this range. White dwarf binaries have masses in the order of the Sun, and diameters in the order of the Earth. They cannot get much closer together than 10,000 km before they will merge and explode in a supernova which would also end the emission of gravitational waves. Until then, their gravitational radiation would be comparable to that of a neutron star binary.


title: Gravitational wave

text: When the orbit of a neutron star binary has decayed to 1.89 m (1890 km), its remaining lifetime is about 130,000 seconds or 36 hours. The orbital frequency will vary from 1 orbit per second at the start, to 918 orbits per second when the orbit has shrunk to 20 km at merger. The majority of gravitational radiation emitted will be at twice the orbital frequency. Just before merger, the inspiral could be observed by LIGO if such a binary were close enough. LIGO has only a few minutes to observe this merger out of a total orbital lifetime that may have been billions of years. In August 2017, LIGO and Virgo observed the first binary neutron star inspiral in GW170817, and 70 observatories collaborated to detect the electromagnetic counterpart, a kilonova in the galaxy NGC 4993, 40 megaparsecs away, emitting a short gamma ray burst (GRB 170817A) seconds after the merger, followed by a longer optical transient (AT 2017gfo) powered by r-process nuclei. Advanced LIGO detector should be able to detect such events up to 200 megaparsecs away. Within this range of the order 40 events are expected per year.


title: Gravitational wave

text: Black hole binaries emit gravitational waves during their in-spiral, merger, and ring-down phases. The largest amplitude of emission occurs during the merger phase, which can be modeled with the techniques of numerical relativity. The first direct detection of gravitational waves, GW150914, came from the merger of two black holes.


title: Gravitational wave

text: A supernova is a transient astronomical event that occurs during the last stellar evolutionary stages of a massive star's life, whose dramatic and catastrophic destruction is marked by one final titanic explosion. This explosion can happen in one of many ways, but in all of them a significant proportion of the matter in the star is blown away into the surrounding space at extremely high velocities (up to 10% of the speed of light). Unless there is perfect spherical symmetry in these explosions (i.e., unless matter is spewed out evenly in all directions), there will be gravitational radiation from the explosion. This is because gravitational waves are generated by a changing quadrupole moment, which can happen only when there is asymmetrical movement of masses. Since the exact mechanism by which supernovae take place is not fully understood, it is not easy to model the gravitational radiation emitted by them.


title: Gravitational wave

text: As noted above, a mass distribution will emit gravitational radiation only when there is spherically asymmetric motion among the masses. A spinning neutron star will generally emit no gravitational radiation because neutron stars are highly dense objects with a strong gravitational field that keeps them almost perfectly spherical. In some cases, however, there might be slight deformities on the surface called "mountains", which are bumps extending no more than 10 centimeters (4 inches) above the surface, that make the spinning spherically asymmetric. This gives the star a quadrupole moment that changes with time, and it will emit gravitational waves until the deformities are smoothed out.


title: Gravitational wave

text: Many models of the Universe suggest that there was an inflationary epoch in the early history of the Universe when space expanded by a large factor in a very short amount of time. If this expansion was not symmetric in all directions, it may have emitted gravitational radiation detectable today as a gravitational wave background. This background signal is too weak for any currently operational gravitational wave detector to observe, and it is thought it may be decades before such an observation can be made.


title: Gravitational wave

text: Water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum and by doing so they carry those away from the source. Gravitational waves perform the same function. Thus, for example, a binary system loses angular momentum as the two orbiting objects spiral towards each other—the angular momentum is radiated away by gravitational waves.


title: Gravitational wave

text: The waves can also carry off linear momentum, a possibility that has some interesting implications for astrophysics. After two supermassive black holes coalesce, emission of linear momentum can produce a "kick" with amplitude as large as 4000 km/s. This is fast enough to eject the coalesced black hole completely from its host galaxy. Even if the kick is too small to eject the black hole completely, it can remove it temporarily from the nucleus of the galaxy, after which it will oscillate about the center, eventually coming to rest. A kicked black hole can also carry a star cluster with it, forming a hyper-compact stellar system. Or it may carry gas, allowing the recoiling black hole to appear temporarily as a "naked quasar".


title: Gravitational wave

text: Like electromagnetic waves, gravitational waves should exhibit shifting of wavelength and frequency due to the relative velocities of the source and observer (the Doppler effect), but also due to distortions of spacetime, such as cosmic expansion. This is the case even though gravity itself is a cause of distortions of spacetime. Redshifting "of" gravitational waves is different from redshifting "due to" gravity (gravitational redshift).


title: Gravitational wave

text: In the framework of quantum field theory, the graviton is the name given to a hypothetical elementary particle speculated to be the force carrier that mediates gravity. However the graviton is not yet proven to exist, and no scientific model yet exists that successfully reconciles general relativity, which describes gravity, and the Standard Model, which describes all other fundamental forces. Attempts, such as quantum gravity, have been made, but are not yet accepted.


title: Gravitational wave

text: If such a particle exists, it is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. It can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress–energy tensor in the same way that the gravitational field does; therefore if a massless spin-2 particle were ever discovered, it would be likely to be the graviton without further distinction from other massless spin-2 particles. Such a discovery would unite quantum theory with gravity.


title: Gravitational wave

text: Due to the weakness of the coupling of gravity to matter, gravitational waves experience very little absorption or scattering, even as they travel over astronomical distances. In particular, gravitational waves are expected to be unaffected by the opacity of the very early universe. In these early phases, space had not yet become "transparent", so observations based upon light, radio waves, and other electromagnetic radiation that far back into time are limited or unavailable. Therefore, gravitational waves are expected in principle to have the potential to provide a wealth of observational data about the very early universe.


title: Gravitational wave

text: The difficulty in directly detecting gravitational waves means it is also difficult for a single detector to identify by itself the direction of a source. Therefore, multiple detectors are used, both to distinguish signals from other "noise" by confirming the signal is not of earthly origin, and also to determine direction by means of triangulation. This technique uses the fact that the waves travel at the speed of light and will reach different detectors at different times depending on their source direction. Although the differences in arrival time may be just a few milliseconds, this is sufficient to identify the direction of the origin of the wave with considerable precision.


title: Gravitational wave

text: Only in the case of GW170814 were three detectors operating at the time of the event, therefore, the direction is precisely defined. The detection by all three instruments led to a very accurate estimate of the position of the source, with a 90% credible region of just 60 deg, a factor 20 more accurate than before.


title: Gravitational wave

text: During the past century, astronomy has been revolutionized by the use of new methods for observing the universe. Astronomical observations were initially made using visible light. Galileo Galilei pioneered the use of telescopes to enhance these observations. However, visible light is only a small portion of the electromagnetic spectrum, and not all objects in the distant universe shine strongly in this particular band. More information may be found, for example, in radio wavelengths. Using radio telescopes, astronomers have discovered pulsars and quasars, for example. Observations in the microwave band led to the detection of faint imprints of the Big Bang, a discovery Stephen Hawking called the "greatest discovery of the century, if not all time". Similar advances in observations using gamma rays, x-rays, ultraviolet light, and infrared light have also brought new insights to astronomy. As each of these regions of the spectrum has opened, new discoveries have been made that could not have been made otherwise. The astronomy community hopes that the same holds true of gravitational waves.


title: Gravitational wave

text: Gravitational waves have two important and unique properties. First, there is no need for any type of matter to be present nearby in order for the waves to be generated by a binary system of uncharged black holes, which would emit no electromagnetic radiation. Second, gravitational waves can pass through any intervening matter without being scattered significantly. Whereas light from distant stars may be blocked out by interstellar dust, for example, gravitational waves will pass through essentially unimpeded. These two features allow gravitational waves to carry information about astronomical phenomena heretofore never observed by humans.


title: Gravitational wave

text: The sources of gravitational waves described above are in the low-frequency end of the gravitational-wave spectrum (10 to 10 Hz). An astrophysical source at the high-frequency end of the gravitational-wave spectrum (above 10 Hz and probably 10 Hz) generates relic gravitational waves that are theorized to be faint imprints of the Big Bang like the cosmic microwave background. At these high frequencies it is potentially possible that the sources may be "man made" that is, gravitational waves generated and detected in the laboratory.


title: Gravitational wave

text: A supermassive black hole, created from the merger of the black holes at the center of two merging galaxies detected by the Hubble Space Telescope, is theorized to have been ejected from the merger center by gravitational waves.