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Basketball positions
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The five basketball positions normally employed by organized basketball teams are the point guard (PG or 1), the shooting guard (SG or 2), the small forward (SF or 3), the power forward (PF or 4), and the center (C or 5).
Typically, the point guard is the leader of the team when on the court. This position requires substantial ball handling skills and the ability to facilitate the team during a play. The shooting guard, as the name implies, is often the best shooter and is probably capable of shooting accurately from longer distances. Generally, they also have good ball-handling skills. The small forward often has an aggressive approach to the basket when handling the ball. The power forward and the center are usually called "low post" players, who play with their back to the basket, often acting as their team's primary rebounders or shot blockers, or receiving passes to take inside shots. The center is typically the larger of the two.
Historically, only three positions were recognized (two guards, two forwards and one center) based on where they played on the court: guards generally played outside and away from the hoop, forwards played outside and near the baseline, with the center usually positioned in the key. During the 1980s, as team strategy evolved, more specialized roles developed, resulting in the five position designations used today. Team strategy and available personnel, however, still dictate the positions used by a particular team. For example, the dribble drive motion offense and the Princeton offense use four interchangeable guards and one center. Other combinations are also prevalent.
Besides the five basic positions, some teams use non-standard or hybrid positions, such as point forward, a hybrid of a small forward and a point guard; swingman (hybrid small forward-shooting guard); the big (hybrid power forward-center); and the stretch four, a power forward with the shooting range of typical shooting guards.
Basketball positions
Contents
[hide]
1 Point guard
2 Shooting guard
3 Small forward
4 Power forward
5 Center
6 See also
7 References
8 External links
Point guard [edit]
Main article: Point guard
The point guard (PG),[1] also known as the one, is typically the team's best ball handler and passer. Therefore, they often lead their team in assists and steals. They are often quick and are able to hit shots either outside the three-point line or "in the paint", largely depending on the player's skill level. Point guards are looked upon as the "floor general" or the "coach on the floor". They should study the game and game film to be able to recognize the weaknesses of the defense, and the strengths of their own offense. They are responsible for directing plays, making the position equivalent to that of quarterback in American football, playmaker in Association football, center in ice hockey, or setter in volleyball. Good point guards increase team efficiency and generally have a high number of assists. They are often referred to as dribblers or play-makers. In the NBA, point guards are usually the shortest players on the team and are mostly 6 feet 5 inches (1.96 m) or shorter.
Shooting guard[edit]
Main article: Shooting guard
The shooting guard (SG) is also known as the two or the off guard. Along with the small forward, a shooting guard is often referred to as a wing because of its use in common positioning tactics. As the name suggests, most shooting guards are good shooters from three-point range. Besides being able to shoot the ball, shooting guards tend to have good ball handling skills and the ability to drive the ball to the basket, often creating their own shots off the dribble. A versatile shooting guard will have good passing skills, allowing them to assume point guard responsibilities known as combo guards. In the NBA, shooting guards usually range from 6 feet 4 inches (1.93 m) to 6 feet 7 inches (2.01 m).
Small forward [edit]
Main article: Small forward
The small forward (SF), also known as the three, is considered to be perhaps the most versatile of the main five basketball positions. Versatility is key for small forwards because of the nature of their role, which resembles that of a shooting guard more often than that of a power forward. This is why the small forward and shooting guard positions are often interchangeable and referred to as bird wings.
Small forwards have a variety of assets, such as quickness and strength inside. One common thread among all kinds of small forwards is an ability to "get to the line" and draw fouls by aggressively attempting (post up) plays, lay-ups, or slam dunks. As such, accurate foul shooting is a common skill for small forwards, many of whom record a large portion of their points from the foul line. Besides being able to drive to the basket, they are also good shooters from long-range. Some small forwards have good passing skills, allowing them to assume point guard responsibilities known as point forwards. Small forwards should be able to do a little bit of everything on the court, typically playing roles such as swingmen and defensive specialists. In the NBA, small forwards usually range from 6 feet 6 inches (1.98 m) to 6 feet 9 inches (2.06 m).
Power forward[edit]
Main article: Power forward (basketball)
The power forward (PF), also known as the four, often plays a role similar to that of the center, down in the "post" or "low blocks". The power forward is often the team's most versatile scorer, being able to score close to the basket while also being able to shoot mid-range jump shots from 12 to 18 feet from the basket. Some power forwards, known as stretch fours, have since extended their shooting range to three-pointers. On defense, they are required to have the strength to guard bigger players close to the basket, and to have athleticism to guard quick players away from the basket. Most power forwards tend to be more versatile than centers since they can be part of plays and are not always in the low block. In the NBA, power forwards usually range from 6 feet 8 inches (2.03 m) to 6 feet 11 inches (2.11 m).
Center[edit]
Main article: Center (basketball)
The center (C), also known as the five, nicknamed the "pivot", usually plays near the baseline, close to the basket (referred to as the "low post"). They are usually the tallest player on the floor. The center usually scores "down low, in the paint" (near the basket, in the key), but some can be good perimeter shooters. They are typically skilled at gathering rebounds, contesting shots and setting screens on plays.
The center position has been traditionally considered[by whom?] one of the most important positions, if not the most important. The range of players used in the position has transitioned from relatively slower but much taller 'back to the basket' players to players who would normally be classified as power forwards but can dominate the position with their defensive skills, or mismatch ability to shoot from the high post. This has been due to the scarcity of players possessing the combination of great skill, ideal height, and durability. This has been matched by the development of more fast-paced and athletic basketball play which calls for less traditional center play and a more up-and-down the court style. In the NBA, centers are usually 6 feet 10 inches (2.08 m) or taller.
See also[edit]
Starting lineup
References[edit]
Jump up ^ ^ a b Rose, Lee H. (2016). The Basketball Handbook. Point guards pass the ball on offense and then head down the floor to set up and start the play. On defense, they guard the other point guard or one of the wings. Once they receive the ball they sprint towards the basket if it is open. If not open they stand back and wait til all of their team's players are down the floor and pass the ball. If open for a shot they will shoot to score a 2 or 3 points depending on where they standing. Human Kinetics.
External links[edit]
"Basketball players" at BBC Sport Academy
"How Basketball Works" at HowStuffWorks.com
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Pearl Harbor
From Wikipedia, the free encyclopedia
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For its current operations as a military base, see Joint Base Pearl Harbor–Hickam. For the attack in 1941, see Attack on Pearl Harbor. For other uses, see Pearl Harbor (disambiguation).
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Aerial view of Pearl Harbor, Ford Island in center. The Arizona memorial is the small white dot on the left side above Ford Island
Coordinates: 21°22′04″N 157°58′38″W / 21.3679°N 157.9771°W / 21.3679; -157.9771
Pearl Harbor is a lagoon harbor on the island of Oahu, Hawaii, west of Honolulu. Much of the harbor and surrounding lands is a United States Navy deep-water naval base. It is also the headquarters of the United States Pacific Fleet. The U.S. government first obtained exclusive use of the inlet and the right to maintain a repair and coaling station for ships here in 1887.[1] The attack on Pearl Harbor by the Empire of Japan on December 7, 1941, was the immediate cause of the United States' entry into World War II.[2][3][4]
Contents
[hide]
1 History
1.1 19th century
1.2 Naval presence (1899–present)
1.2.1 Post-World War II
2 See also
3 References
4 External links
History
See also: History of Hawaii
Pearl Harbor was originally an extensive shallow embayment called Wai Momi (meaning, “Waters of Pearl”)[5] or Puʻuloa (meaning, “long hill”) by the Hawaiians. Puʻuloa was regarded as the home of the shark goddess, Kaʻahupahau, and her brother (or son), Kahiʻuka, in Hawaiian legends. According to tradition, Keaunui, the head of the powerful Ewa chiefs, is credited with cutting a navigable channel near the present Puʻuloa saltworks, by which he made the estuary, known as "Pearl River," accessible to navigation. Making due allowance for legendary amplification, the estuary already had an outlet for its waters where the present gap is; but Keaunui is typically given the credit for widening and deepening it.[6]
19th century
See also: Kingdom of Hawaii
Pearl Harbor in the 1880s.
During the early 19th century, Pearl Harbor was not used for large ships due to its shallow entrance. The interest of United States in the Hawaiian Islands grew as a result of its whaling, shipping and trading activity in the Pacific. As early as 1820, an "Agent of the United States for Commerce and Seamen" was appointed to look after American business in the Port of Honolulu. These commercial ties to the American continent were accompanied by the work of the American Board of Commissioners for Foreign Missions. American missionaries and their families became an integral part of the Hawaiian political body.
Throughout the 1820s and 1830s, many American warships visited Honolulu. In most cases, the commanding officers carried letters from the U.S. Government giving advice on governmental affairs and of the relations of the island nation with foreign powers. In 1841, the newspaper Polynesian, printed in Honolulu, advocated that the U.S. establish a naval base in Hawaii for protection of American citizens engaged in the whaling industry. The British Hawaiian Minister of Foreign Affairs Robert Crichton Wyllie, remarked in 1840 that "... my opinion is that the tide of events rushes on to annexation to the United States."
From the conclusion of the Civil War, to the purchase of Alaska, to the increased importance of the Pacific states, the projected trade with countries in Asia and the desire for a duty-free market for Hawaiian staples, Hawaiian trade expanded. In 1865, the North Pacific Squadron was formed to embrace the western coast and Hawaii. Lackawanna in the following year was assigned to cruise among the islands, "a locality of great and increasing interest and importance." This vessel surveyed the Northwestern Hawaiian Islands toward Japan. As a result, the United States claimed Midway Island. The Secretary of the Navy was able to write in his annual report of 1868, that in November 1867, 42 American flags flew over whaleships and merchant vessels in Honolulu to only six of other nations. This increased activity caused the permanent assignment of at least one warship to Hawaiian waters. It also praised Midway Island as possessing a harbor surpassing Honolulu's. In the following year, Congress approved an appropriation of $50,000 on March 1, 1869, to deepen the approaches to this harbor.
Astronaut photograph of Pearl Harbor from October 2009
After 1868, when the Commander of the Pacific Fleet visited the islands to look after American interests, naval officers played an important role in internal affairs. They served as arbitrators in business disputes, negotiators of trade agreements and defenders of law and order. Periodic voyages among the islands and to the mainland aboard U.S. warships were arranged for members of the Hawaiian royal family and important island government officials. When King Lunalilo died in 1873, negotiations were underway for the cessation of Pearl Harbor as a port for the duty-free export of sugar to the U.S.[citation needed] With the election of King Kalākaua in March 1874, riots prompted landing of sailors from USS Tuscarora and Portsmouth. The British warship, HMS Tenedos, also landed a token force. During the reign of King Kalākaua the United States was granted exclusive rights to enter Pearl Harbor and to establish "a coaling and repair station."
Although this treaty continued in force until August 1898, the U.S. did not fortify Pearl Harbor as a naval base. The shallow entrance constituted a formidable barrier against the use of the deep protected waters of the inner harbor as it had for 60 years.
The United States and the Hawaiian Kingdom signed the Reciprocity Treaty of 1875 as supplemented by Convention on December 6, 1884, the Reciprocity Treaty was made by James Carter and ratified it in 1887. On January 20, 1887, the United States Senate allowed the Navy to exclusive right to maintain a coaling and repair station at Pearl Harbor. (The US took possession on November 9 that year). The Spanish–American War of 1898 and the desire for the United States to have a permanent presence in the Pacific both contributed to the decision.
Naval presence (1899–present)
Main article: Naval Station Pearl Harbor
USS Arizona, during the Japanese attack on Pearl Harbor, December 7, 1941
Following the overthrow of the Hawaiian Kingdom, the United States Navy established a base on the island in 1899. On December 7, 1941, the base was attacked by the Imperial Japanese Navy airplanes and midget submarines, causing the American entry into World War II. One of the main reasons that Pearl Harbor happened was because the United States had major communication breakdowns among several branches of the U.S. armed services and departments of the U.S. government. This led to the surprise Japanese attack at the Hawaiian air base.[7] There was no meaningful plan for the air defense of Hawaii, for American commanders had no understanding of the capabilities and proper employment of air power. As it was, had the Pacific Fleet acted on the war warnings it undoubtedly would have sortied and been at sea on December 7, where the major ships would have been sunk in deep water, making salvage impossible.[8] Shortly after the devastating Japanese surprise attack at Pearl Harbor two American military commanders, Lt. Gen. Walter Short and Adm. Husband Kimmel were demoted of their full ranks. The two American commanders later sought to restore their reputations and full ranks.[9]
Post-World War II
Over the years, Pearl Harbor remained a main base for the US Pacific Fleet after World War II along with Naval Base San Diego. In 2010, the Navy and the Air Force merged their two nearby bases; Pearl Harbor joined with Hickam Air Force Base to create Joint Base Pearl Harbor-Hickam.
On February 18, 2016, a tourist helicopter fell into Pearl Harbor, injuring four people; one person was missing.[10][11]
In December 2016, Japanese Prime Minister Shinzo Abe made a joint visit to Pearl Harbor with US President Barack Obama. This trip marked the 75 year anniversary of the attack, and was the first official visit by a sitting Japanese leader.[12]
See also
Ford Island
Admiral Clarey Bridge
Pearl Harbor National Wildlife Refuge
References
Jump up ^ http://www.history.navy.mil: The U.S. Navy in Hawaii, 1826-1945: An Administrative History
Jump up ^ FDR Pearl Harbor Speech. December 8, 1941. Retrieved 2011-02-05. December 7th, 1941, a day that will live in infamy.
Jump up ^ Apple, Russell A.; Benjamin Levy (February 8, 1974). "Pearl Harbor" (pdf). National Register of Historic Places - Nomination and Inventory. National Park Service. Retrieved 25 May 2012.
Jump up ^ "Pearl Harbor" (pdf). Photographs. National Park Service. Retrieved 25 May 2012.
Jump up ^ "Places - The History of Pearl Harbor". National Park Service, U.S. Department of the Interior. Retrieved December 22, 2014.
Jump up ^ "Cold Spots - Pearl Harbor - Dread Central". Dread Central.
Jump up ^ Burtness, Paul; Warren, Ober (2013). "Communication Lapses Leading to the Pearl Harbor Disaster". 75 (4): 20.
Jump up ^ Smith, Dale (1997). "Pearl Harbor: A lesson in air power". Air Power History. 44 (1): 46–53.
Jump up ^ "Remember Pearl Harbor". Christian Science Monitor: 2. January 6, 1996.
Jump up ^ Molly Roecker. "Honolulu Tourist Helicopter Crashes in Pearl Harbor". NBC News.
Jump up ^ Steve Almasy, CNN (19 February 2016). "Pearl Harbor helicopter crash: 1 critically injured". CNN.
Jump up ^ Ito, Shingo (5 December 2016). "We did our jobs: Japanese participant remembers Pearl Harbor". www.atimes.com. Retrieved 7 December 2016.
External links
Wikimedia Commons has media related to:
Pearl Harbor (category)
Wikisource has the text of the 1921 Collier's Encyclopedia article Pearl Harbor.
British Pathé Online archive of Pearl Harbor and related footage
Pearl Harbor at DMOZ
Pearl Harbor on The History Channel
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Fundamental interaction
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In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at minuscule, subatomic distances and govern nuclear interactions. Some scientists[1][2][3] speculate that a fifth force might exist, but this is not widely accepted nor proven.
Each of the known fundamental interactions can be described mathematically as a field. The gravitational force is attributed to the curvature of spacetime, described by Einstein's general theory of relativity. The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics.
Within the Standard Model, the strong interaction is carried by a particle called the gluon, and is responsible for the binding of quarks together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons, and also acts on the nucleus of atoms, mediating radioactive decay. The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for the attraction between orbital electrons and atomic nuclei which holds atoms together, as well as chemical bonding and electromagnetic waves, including visible light, and forms the basis for electrical technology. Although the electromagnetic force is far stronger than gravity, it tends to cancel itself out within large objects, so over the largest distances (on the scale of planets and galaxies), gravity tends to be the dominant force.
All four fundamental forces are believed to be related, and to unite into a single force at high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. Efforts to devise a common theoretical framework that would explain the relation between the forces are perhaps the greatest goal of theoretical physicists today. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg for which they received the 1979 Nobel Prize in physics. Progress is currently being made in uniting the electroweak and strong fields within a Grand Unified Theory (GUT). A bigger challenge is to find a way to quantize the gravitational field, resulting in a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE).
The four fundamental interactions of nature[4]
Property/Interaction
Gravitation
Weak
Electromagnetic
Strong
(Electroweak)
Fundamental
Residual
Acts on: Mass/Energy Flavor Electric charge Color charge Atomic nuclei
Particles experiencing: All particles All particles Electrically charged Quarks, Gluons Hadrons
Particles mediating: Not yet observed
(Graviton hypothesised) W+, W− and Z0 γ (photon) Gluons π, ρ and ω mesons
Strength at the scale of quarks: 6959100000000000000♠10−41(predicted) 6996100000000000000♠10−4 1 60 Not applicable
to quarks
Strength at the scale of
protons/neutrons: 6964100000000000000♠10−36(predicted) 6993100000000000000♠10−7 1 Not applicable
to hadrons 20
Contents
[hide]
1 History
1.1 Classical theory
1.2 The Standard Model
2 Overview of the fundamental interactions
3 The interactions
3.1 Gravity
3.2 Electroweak interaction
3.2.1 Electromagnetism
3.2.2 Weak interaction
3.3 Strong interaction
3.4 Beyond the Standard Model
4 See also
5 References
5.1 Bibliography
5.1.1 General
5.1.2 Texts
History[edit]
Classical theory[edit]
In his 1687 theory, Isaac Newton postulated space as an infinite and unalterable physical structure existing before, within, and around all objects while their states and relations unfold at a constant pace everywhere, thus absolute space and time. Inferring that all objects bearing mass approach at a constant rate, but collide by impact proportional to their masses, Newton inferred that matter exhibits an attractive force. His law of universal gravitation mathematically stated it to span the entire universe instantly (despite absolute time), or, if not actually a force, to be instant interaction among all objects (despite absolute space.) As conventionally interpreted, Newton's theory of motion modelled a central force without a communicating medium.[5] Thus Newton's theory violated the first principle of mechanical philosophy, as stated by Descartes, No action at a distance. Conversely, during the 1820s, when explaining magnetism, Michael Faraday inferred a field filling space and transmitting that force. Faraday conjectured that ultimately, all forces unified into one.
In the early 1870s, James Clerk Maxwell unified electricity and magnetism as effects of an electromagnetic field whose third consequence was light, travelling at constant speed in a vacuum. The electromagnetic field theory contradicted predictions of Newton's theory of motion, unless physical states of the luminiferous aether—presumed to fill all space whether within matter or in a vacuum and to manifest the electromagnetic field—aligned all phenomena and thereby held valid the Newtonian principle relativity or invariance.
The Standard Model[edit]
Main article: Standard Model
See also: Standard Model (mathematical formulation)
The Standard Model of elementary particles, with the fermions in the first three columns, the gauge bosons in the fourth column, and the Higgs boson in the fifth column
The Standard Model of particle physics was developed throughout the latter half of the 20th century. In the Standard Model, the electromagnetic, strong, and weak interactions associate with elementary particles, whose behaviours are modelled in quantum mechanics (QM). For predictive success with QM's probabilistic outcomes, particle physics conventionally models QM events across a field set to special relativity, altogether relativistic quantum field theory (QFT).[6] Force particles, called gauge bosons—force carriers or messenger particles of underlying fields—interact with matter particles, called fermions. Everyday matter is atoms, composed of three fermion types: up-quarks and down-quarks constituting, as well as electrons orbiting, the atom's nucleus. Atoms interact, form molecules, and manifest further properties through electromagnetic interactions among their electrons absorbing and emitting photons, the electromagnetic field's force carrier, which if unimpeded traverse potentially infinite distance. Electromagnetism's QFT is quantum electrodynamics (QED).
The electromagnetic interaction was modelled with the weak interaction, whose force carriers are W and Z bosons, traversing the minuscule distance, in electroweak theory (EWT). Electroweak interaction would operate at such high temperatures as soon after the presumed Big Bang, but, as the early universe cooled, split into electromagnetic and weak interactions. The strong interaction, whose force carrier is the gluon, traversing minuscule distance among quarks, is modeled in quantum chromodynamics (QCD). EWT, QCD, and the Higgs mechanism, whereby the Higgs field manifests Higgs bosons that interact with some quantum particles and thereby endow those particles with mass comprise particle physics' Standard Model (SM). Predictions are usually made using calculational approximation methods, although such perturbation theory is inadequate to model some experimental observations (for instance bound states and solitons.) Still, physicists widely accept the Standard Model as science's most experimentally confirmed theory.
Beyond the Standard Model, some theorists work to unite the electroweak and strong interactions within a Grand Unified Theory (GUT). Some attempts at GUTs hypothesize "shadow" particles, such that every known matter particle associates with an undiscovered force particle, and vice versa, altogether supersymmetry (SUSY). Other theorists seek to quantize the gravitational field by the modelling behaviour of its hypothetical force carrier, the graviton and achieve quantum gravity (QG). One approach to QG is loop quantum gravity (LQG). Still other theorists seek both QG and GUT within one framework, reducing all four fundamental interactions to a Theory of Everything (ToE). The most prevalent aim at a ToE is string theory, although to model matter particles, it added SUSY to force particles—and so, strictly speaking, became superstring theory. Multiple, seemingly disparate superstring theories were unified on a backbone, M-theory. Theories beyond the Standard Model remain highly speculative, lacking great experimental support.
Overview of the fundamental interactions[edit]
An overview of the various families of elementary and composite particles, and the theories describing their interactions. Fermions are on the left, and Bosons are on the right.
In the conceptual model of fundamental interactions, matter consists of fermions, which carry properties called charges and spin ±1⁄2 (intrinsic angular momentum ±ħ⁄2, where ħ is the reduced Planck constant). They attract or repel each other by exchanging bosons.
The interaction of any pair of fermions in perturbation theory can then be modelled thus:
Two fermions go in → interaction by boson exchange → Two changed fermions go out.
The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process (e.g., turn them from one type of fermion to another). Since bosons carry one unit of angular momentum, the fermion's spin direction will flip from +1⁄2 to −1⁄2 (or vice versa) during such an exchange (in units of the reduced Planck's constant).
Because an interaction results in fermions attracting and repelling each other, an older term for "interaction" is force.
According to the present understanding, there are four fundamental interactions or forces: gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observed physical phenomenon by these fundamental interactions. Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are the unification of:
Electric and magnetic force into electromagnetism;
The electromagnetic interaction and the weak interaction into the electroweak interaction; see below.
Both magnitude ("relative strength") and "range", as given in the table, are meaningful only within a rather complex theoretical framework. It should also be noted that the table below lists properties of a conceptual scheme that is still the subject of ongoing research.
Interaction
Current theory
Mediators
Relative strength[7]
Long-distance behavior
Range (m)[citation needed]
Weak Electroweak Theory (EWT) W and Z bosons 1025 1 r e − m W , Z r {\displaystyle {\frac {1}{r}}\ e^{-m_{W,Z}\ r}} 10−18
Strong Quantum chromodynamics
(QCD) gluons 1038 ∼ r {\displaystyle {\sim r}}
(Color confinement, see discussion below) 10−15
Electromagnetic Quantum electrodynamics
(QED) photons 1036 1 r 2 {\displaystyle {\frac {1}{r^{2}}}} ∞
Gravitation General relativity
(GR) gravitons (hypothetical) 1 1 r 2 {\displaystyle {\frac {1}{r^{2}}}} ∞
The modern (perturbative) quantum mechanical view of the fundamental forces other than gravity is that particles of matter (fermions) do not directly interact with each other, but rather carry a charge, and exchange virtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons mediate the interaction of electric charges, and gluons mediate the interaction of color charges.
The interactions[edit]
Gravity[edit]
Main article: Gravity
Gravitation is by far the weakest of the four interactions at the atomic scale, where electromagnetic interactions dominate. But the idea that the weakness of gravity can easily be demonstrated by suspending a pin using a simple magnet (such as a refrigerator magnet) is fundamentally flawed. The only reason the magnet is able to hold the pin against the gravitational pull of the entire Earth is due to its relative proximity. There is clearly a short distance of separation between magnet and pin where a breaking point is reached, and due to the large mass of Earth this distance is disappointingly small.
Thus gravitation is very important for macroscopic objects and over macroscopic distances for the following reasons. Gravitation:
Is the only interaction that acts on all particles having mass, energy and/or momentum
Has an infinite range, like electromagnetism but unlike strong and weak interaction[citation needed]
Cannot be absorbed, transformed, or shielded against
Always attracts and never repels (see function of geodesic equation in general relativity)
Even though electromagnetism is far stronger than gravitation, electrostatic attraction is not relevant for large celestial bodies, such as planets, stars, and galaxies, simply because such bodies contain equal numbers of protons and electrons and so have a net electric charge of zero. Nothing "cancels" gravity, since it is only attractive, unlike electric forces which can be attractive or repulsive. On the other hand, all objects having mass are subject to the gravitational force, which only attracts. Therefore, only gravitation matters on the large-scale structure of the universe.
The long range of gravitation makes it responsible for such large-scale phenomena as the structure of galaxies and black holes and it retards the expansion of the universe.[citation needed] Gravitation also explains astronomical phenomena on more modest scales, such as planetary orbits, as well as everyday experience: objects fall; heavy objects act as if they were glued to the ground, and animals can only jump so high.
Gravitation was the first interaction to be described mathematically. In ancient times, Aristotle hypothesized that objects of different masses fall at different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this hypothesis was wrong under certain circumstances — neglecting the friction due to air resistance, and buoyancy forces if an atmosphere is present (e.g. the case of a dropped air-filled balloon vs a water-filled balloon) all objects accelerate toward the Earth at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation of the behaviour of gravitation. Our present-day understanding of gravitation stems from Albert Einstein's General Theory of Relativity of 1915, a more accurate (especially for cosmological masses and distances) description of gravitation in terms of the geometry of spacetime.
Merging general relativity and quantum mechanics (or quantum field theory) into a more general theory of quantum gravity is an area of active research. It is hypothesized that gravitation is mediated by a massless spin-2 particle called the graviton.
Although general relativity has been experimentally confirmed (at least for weak fields[which?]) on all but the smallest scales, there are rival theories of gravitation. Those taken seriously by[citation needed] the physics community all reduce to general relativity in some limit, and the focus of observational work is to establish limitations on what deviations from general relativity are possible.
Proposed extra dimensions could explain why the gravity force is so weak.[8]
Electroweak interaction[edit]
Main article: Electroweak interaction
Electromagnetism and weak interaction appear to be very different at everyday low energies. They can be modelled using two different theories. However, above unification energy, on the order of 100 GeV, they would merge into a single electroweak force.
Electroweak theory is very important for modern cosmology, particularly on how the universe evolved. This is because shortly after the Big Bang, the temperature was approximately above 1015 K, the electromagnetic force and the weak force were merged into a combined electroweak force.
For contributions to the unification of the weak and electromagnetic interaction between elementary particles, Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded the Nobel Prize in Physics in 1979.[9][10]
Electromagnetism[edit]
Main article: Electromagnetism
Electromagnetism is the force that acts between electrically charged particles. This phenomenon includes the electrostatic force acting between charged particles at rest, and the combined effect of electric and magnetic forces acting between charged particles moving relative to each other.
Electromagnetism has infinite range like gravity, but is vastly stronger, and therefore describes a number of macroscopic phenomena of everyday experience such as friction, rainbows, lightning, and all human-made devices using electric current, such as television, lasers, and computers. Electromagnetism fundamentally determines all macroscopic, and many atomic levels, properties of the chemical elements, including all chemical bonding.
In a four kilogram (~1 gallon) jug of water there are
4000 g H 2 O ⋅ 1 mol H 2 O 18 g H 2 O ⋅ 10 mol e − 1 mol H 2 O ⋅ 96 , 000 C 1 mol e − = 2.1 × 10 8 C {\displaystyle 4000\ {\mbox{g}}\,H_{2}O\cdot {\frac {1\ {\mbox{mol}}\,H_{2}O}{18\ {\mbox{g}}\,H_{2}O}}\cdot {\frac {10\ {\mbox{mol}}\,e^{-}}{1\ {\mbox{mol}}\,H_{2}O}}\cdot {\frac {96,000\ {\mbox{C}}\,}{1\ {\mbox{mol}}\,e^{-}}}=2.1\times 10^{8}C\ \,\ }
of total electron charge. Thus, if we place two such jugs a meter apart, the electrons in one of the jugs repel those in the other jug with a force of
1 4 π ε 0 ( 2.1 × 10 8 C ) 2 ( 1 m ) 2 = 4.1 × 10 26 N . {\displaystyle {1 \over 4\pi \varepsilon _{0}}{\frac {(2.1\times 10^{8}C)^{2}}{(1m)^{2}}}=4.1\times 10^{26}N.}
This force is larger than the planet Earth would weigh if weighed on another Earth. The atomic nuclei in one jug also repel those in the other with the same force. However, these repulsive forces are canceled by the attraction of the electrons in jug A with the nuclei in jug B and the attraction of the nuclei in jug A with the electrons in jug B, resulting in no net force. Electromagnetic forces are tremendously stronger than gravity but cancel out so that for large bodies gravity dominates.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 19th century James Clerk Maxwell discovered that electricity and magnetism are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified this unified interaction. Maxwell's theory, restated using vector calculus, is the classical theory of electromagnetism, suitable for most technological purposes.
The constant speed of light in a vacuum (customarily described with the letter "c") can be derived from Maxwell's equations, which are consistent with the theory of special relativity. Einstein's 1905 theory of special relativity, however, which flows from the observation that the speed of light is constant no matter how fast the observer is moving, showed that the theoretical result implied by Maxwell's equations has profound implications far beyond electromagnetism on the very nature of time and space.
In another work that departed from classical electro-magnetism, Einstein also explained the photoelectric effect by hypothesizing that light was transmitted in quanta, which we now call photons. Starting around 1927, Paul Dirac combined quantum mechanics with the relativistic theory of electromagnetism. Further work in the 1940s, by Richard Feynman, Freeman Dyson, Julian Schwinger, and Sin-Itiro Tomonaga, completed this theory, which is now called quantum electrodynamics, the revised theory of electromagnetism. Quantum electrodynamics and quantum mechanics provide a theoretical basis for electromagnetic behavior such as quantum tunneling, in which a certain percentage of electrically charged particles move in ways that would be impossible under the classical electromagnetic theory, that is necessary for everyday electronic devices such as transistors to function.
Weak interaction[edit]
Main article: Weak interaction
The weak interaction or weak nuclear force is responsible for some nuclear phenomena such as beta decay. Electromagnetism and the weak force are now understood to be two aspects of a unified electroweak interaction — this discovery was the first step toward the unified theory known as the Standard Model. In the theory of the electroweak interaction, the carriers of the weak force are the massive gauge bosons called the W and Z bosons. The weak interaction is the only known interaction which does not conserve parity; it is left-right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT.
Strong interaction[edit]
Main article: Strong interaction
The strong interaction, or strong nuclear force, is the most complicated interaction, mainly because of the way it varies with distance. At distances greater than 10 femtometers, the strong force is practically unobservable. Moreover, it holds only inside the atomic nucleus.
After the nucleus was discovered in 1908, it was clear that a new force, today known as the nuclear force, was needed to overcome the electrostatic repulsion, a manifestation of electromagnetism, of the positively charged protons. Otherwise, the nucleus could not exist. Moreover, the force had to be strong enough to squeeze the protons into a volume that is about 10−15 m, much smaller than that of the entire atom. From the short range of this force, Hideki Yukawa predicted that it was associated with a massive particle, whose mass is approximately 100 MeV.
The 1947 discovery of the pion ushered in the modern era of particle physics. Hundreds of hadrons were discovered from the 1940s to 1960s, and an extremely complicated theory of hadrons as strongly interacting particles was developed. Most notably:
The pions were understood to be oscillations of vacuum condensates;
Jun John Sakurai proposed the rho and omega vector bosons to be force carrying particles for approximate symmetries of isospin and hypercharge;
Geoffrey Chew, Edward K. Burdett and Steven Frautschi grouped the heavier hadrons into families that could be understood as vibrational and rotational excitations of strings.
While each of these approaches offered deep insights, no approach led directly to a fundamental theory.
Murray Gell-Mann along with George Zweig first proposed fractionally charged quarks in 1961. Throughout the 1960s, different authors considered theories similar to the modern fundamental theory of quantum chromodynamics (QCD) as simple models for the interactions of quarks. The first to hypothesize the gluons of QCD were Moo-Young Han and Yoichiro Nambu, who introduced the quark color charge and hypothesized that it might be associated with a force-carrying field. At that time, however, it was difficult to see how such a model could permanently confine quarks. Han and Nambu also assigned each quark color an integer electrical charge, so that the quarks were fractionally charged only on average, and they did not expect the quarks in their model to be permanently confined.
In 1971, Murray Gell-Mann and Harald Fritzsch proposed that the Han/Nambu color gauge field was the correct theory of the short-distance interactions of fractionally charged quarks. A little later, David Gross, Frank Wilczek, and David Politzer discovered that this theory had the property of asymptotic freedom, allowing them to make contact with experimental evidence. They concluded that QCD was the complete theory of the strong interactions, correct at all distance scales. The discovery of asymptotic freedom led most physicists to accept QCD since it became clear that even the long-distance properties of the strong interactions could be consistent with experiment if the quarks are permanently confined.
Assuming that quarks are confined, Mikhail Shifman, Arkady Vainshtein and Valentine Zakharov were able to compute the properties of many low-lying hadrons directly from QCD, with only a few extra parameters to describe the vacuum. In 1980, Kenneth G. Wilson published computer calculations based on the first principles of QCD, establishing, to a level of confidence tantamount to certainty, that QCD will confine quarks. Since then, QCD has been the established theory of the strong interactions.
QCD is a theory of fractionally charged quarks interacting by means of 8 bosonic particles called gluons. The gluons interact with each other, not just with the quarks, and at long distances the lines of force collimate into strings. In this way, the mathematical theory of QCD not only explains how quarks interact over short distances but also the string-like behavior, discovered by Chew and Frautschi, which they manifest over longer distances.
Beyond the Standard Model[edit]
Main article: Physics beyond the Standard Model
See also: Elementary particle § Beyond the Standard Model
Numerous theoretical efforts have been made to systematize the existing four fundamental interactions on the model of electroweak unification.
Grand Unified Theories (GUTs) are proposals to show that the three fundamental interactions described by the Standard Model are all different manifestations of a single interaction with symmetries that break down and create separate interactions below some extremely high level of energy. GUTs are also expected to predict some of the relationships between constants of nature that the Standard Model treats as unrelated, as well as predicting gauge coupling unification for the relative strengths of the electromagnetic, weak, and strong forces (this was, for example, verified at the Large Electron–Positron Collider in 1991 for supersymmetric theories).[specify]
Theories of everything, which integrate GUTs with a quantum gravity theory face a greater barrier, because no quantum gravity theories, which include string theory, loop quantum gravity, and twistor theory, have secured wide acceptance. Some theories look for a graviton to complete the Standard Model list of force-carrying particles, while others, like loop quantum gravity, emphasize the possibility that time-space itself may have a quantum aspect to it.
Some theories beyond the Standard Model include a hypothetical fifth force, and the search for such a force is an ongoing line of experimental research in physics. In supersymmetric theories, there are particles that acquire their masses only through supersymmetry breaking effects and these particles, known as moduli can mediate new forces. Another reason to look for new forces is the recent discovery that the expansion of the universe is accelerating (also known as dark energy), giving rise to a need to explain a nonzero cosmological constant, and possibly to other modifications of general relativity. Fifth forces have also been suggested to explain phenomena such as CP violations, dark matter, and dark flow.
In December 2015, two observations in the ATLAS and CMS detectors at the Large Hadron Collider hinted at the existence of a new particle six times heavier than the Higgs boson.[11] However, after obtaining more experimental data, the anomaly appeared to not be significant.[12]
See also[edit]
This "see also" section may contain an excessive number of suggestions. Please ensure that only the most relevant links are given, that they are not red links, and that any links are not already in this article. (May 2018) (Learn how and when to remove this template message)
Physics portal
Electroweak interaction
Quantum gravity
String Theory
Gauge coupling unification
Unified Field Theory
Quintessence, a hypothesized fifth force.
People: Isaac Newton, James Clerk Maxwell, Albert Einstein, Richard Feynman, Sheldon Glashow, Abdus Salam, Steven Weinberg, Gerardus 't Hooft, David Gross, Edward Witten, Howard Georgi.
References[edit]
Jump up ^ Fackler, Orrin; Tran, J. Thanh Van (1988). 5th Force Neutrino Physics. Atlantica Séguier Frontières. ISBN 2863320548.
Jump up ^ Weisstein, Eric W. (2007). "Fifth Force". World of Science. Wolfram Research. Retrieved September 14, 2017.
Jump up ^ Franklin, Allan; Fischbach, Ephraim (2016). The Rise and Fall of the Fifth Force: Discovery, Pursuit, and Justification in Modern Physics, 2nd Ed. Springer. ISBN 3319284126.
Jump up ^ "Standard Model of Particles and Interactions". jhu.edu. Johns Hopkins University. Archived from the original on March 4, 2016. Retrieved August 18, 2016. .gif
Jump up ^ Newton's absolute space was a medium, but not one transmitting gravitation.
Jump up ^ Meinard Kuhlmann, "Physicists debate whether the world is made of particles or fields—or something else entirely", Scientific American, 24 Jul 2013.
Jump up ^ Approximate. See Coupling constant for more exact strengths, depending on the particles and energies involved.
Jump up ^ CERN (20 January 2012). "Extra dimensions, gravitons, and tiny black holes".
Jump up ^ Bais, Sander (2005), The Equations. Icons of knowledge, ISBN 0-674-01967-9 p.84
Jump up ^ "The Nobel Prize in Physics 1979". The Nobel Foundation. Retrieved 2008-12-16.
Jump up ^ "Is the Large Hadron Collider set to reveal secrets of the universe?". Retrieved 2016-07-12.
Jump up ^ Elizabeth Gibney (August 5, 2016). "Hopes for revolutionary new LHC particle dashed". Nature. Bibcode:2016Natur.536..133G. doi:10.1038/nature.2016.20376. Retrieved August 18, 2016.
Bibliography[edit]
General[edit]
Davies, Paul (1986), The Forces of Nature, Cambridge Univ. Press 2nd ed.
Feynman, Richard (1967), The Character of Physical Law, MIT Press, ISBN 0-262-56003-8
Schumm, Bruce A. (2004), Deep Down Things, Johns Hopkins University Press While all interactions are discussed, discussion is especially thorough on the weak.
Weinberg, Steven (1993), The First Three Minutes: A Modern View of the Origin of the Universe, Basic Books, ISBN 0-465-02437-8
Weinberg, Steven (1994), Dreams of a Final Theory, Basic Books, ISBN 0-679-74408-8
Texts[edit]
Padmanabhan, T. (1998), After The First Three Minutes: The Story of Our Universe, Cambridge Univ. Press, ISBN 0-521-62972-1
Perkins, Donald H. (2000), Introduction to High Energy Physics, Cambridge Univ. Press, ISBN 0-521-62196-8
Riazuddin (December 29, 2009). "Non-standard interactions" (PDF). NCP 5th Particle Physics Sypnoisis. Islamabad: Riazuddin, Head of High-Energy Theory Group at National Center for Physics. 1 (1): 1–25. Retrieved March 19, 2011.
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