- Fundamental interaction
In

physics , a**fundamental interaction**or**fundamental force**is a mechanism by which particles interact with each other, and which cannot be explained in terms of another interaction.**Overview**In the

concept ualmodel of fundamental interactions,matter consists offermion s, which carry "properties" called "charge s" and spin 1/2 (intrinsicangular momentum ±ℏ/2, where h/2π isreduced Planck's constant ). They attract or repel each other by exchangingboson s.The interaction of any pair of matter particles can then be modeled this way:

: two fermions go in $ightarrow$ "interaction" by boson exchange $ightarrow$ two changed fermions go out.

The exchange of bosons always carries

energy andmomentum between the fermions, thereby changing their directions of flight and their respective speed. It may 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 type of fermion). 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 ofreduced Planck's constant ).Because fermions can attract and repel each other due to an interaction, such an interaction is sometimes called a "

force ". Efforts of modernphysics are directed at explaining every observed physical phenomenon by these interactions. Moreover, one tries to reduce the number of different interaction types (like "unifying" theelectromagnetic interaction and theweak interaction into theelectroweak interaction , see below). For an introductory explanation, four fundamental interactions (forces) may be assumed:gravitation ,electromagnetism , theweak interaction , and thestrong interaction . Their magnitude and behavior vary greatly, as described in the table below. Both magnitude ("relative strength") and "range", as given in the table, have some meaning only within a rather complex framework of ideas.It should be noted that the table below lists properties of a conceptual model that is still subject to research in modern physics.

The modern quantum mechanical view of the three fundamental forces (all except gravity) is that particles of matter (

fermions ) do not directly interact with each other, but rather carry a charge, and exchangevirtual particles (gauge bosons ), which are the interaction carriers or force mediators. For example,photons are the mediators of the interaction ofelectric charges ; andgluons are the mediators of the interaction ofcolor charge s.**The interactions****Gravitation**"Gravitation" is by far the weakest interaction, but at long distances gravity's strength relative to other forces becomes important. There are three reasons for this. First, gravity has an infinite range, like that of electromagnetism. Secondly, all masses are positive and therefore gravity's interaction cannot be screened like in electromagnetism. Finally, gravitational force cannot be absorbed or transformed, and so is permanent. Thus large celestial bodies such as planets, stars and galaxies dominantly feel gravitational forces. In comparison, the total electric charge of these bodies is zero because half of all charges are negative. In addition, unlike the other interactions, gravity acts universally on all matter. There are no objects that lack a gravitational "charge".

Because of its long range, gravity is responsible for such large-scale phenomena as the structure of galaxies,

black hole s and the expansion of the universe, as well as more elementary astronomical phenomena like theorbit s ofplanet s, and everyday experience: objects fall; heavy objects act as if they were glued to the ground; people are limited in how high they can jump.Gravitation was the first kind of interaction described by a mathematical theory. In ancient times,

Aristotle theorized that objects of different masses fall at different rates. During theScientific Revolution ,Galileo Galilei experimentally determined that this was not the case — if friction due to air resistance is neglected, all objects accelerate toward the ground at the same rate.Isaac Newton 'slaw of Universal Gravitation (1687) was a good approximation of the general behaviour of gravity. In 1915,Albert Einstein completed theGeneral Theory of Relativity , a more accurate description of gravity in terms of thegeometry ofspace-time .An area of active research today involves merging the theories of general relativity and

quantum mechanics into a more general theory ofquantum gravity . It is widely believed that in a theory of quantum gravity, gravity would be mediated by a massless spin 2 particle which is known as thegraviton . Gravitons are hypothetical particles not yet observed.Although general relativity appears to present an accurate theory of gravity in the non-quantum mechanical limit, there are a number of alternate theories of gravity. Those under any serious consideration by 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.

**Electromagnetism**"Electromagnetism" is the force that acts between electrically charged particles. This phenomenon includes the

electrostatic force , acting between charges at rest, and the combined effect ofelectric andmagnetic forces acting between charges moving relative to each other.Electromagnetism is also an infinite-ranged force, but it is much stronger than gravity, and therefore describes almost all phenomena of our everyday experience, ranging from the impenetrability of macroscopic bodies, to

laser s andradio s, to the structure ofatoms andmetal s, to phenomena such asfriction andrainbow s.Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that scientists discovered that electricity and magnetism are two aspects of the same fundamental interaction. By 1864,

Maxwell's equations had rigorously quantified the unified phenomenon. In 1905, Einstein's theory ofspecial relativity resolved the issue of the constancy of the speed of light, and Einstein also explained thephotoelectric effect by theorizing that light was transmitted in quanta, which we now callphoton s. Starting around 1927,Paul Dirac unified quantum mechanics with the relativistic theory ofelectromagnetism ; the theory ofquantum electrodynamics was completed in the 1940s byRichard Feynman ,Freeman Dyson ,Julian Schwinger , andSin-Itiro Tomonaga .**Weak interaction**The "weak interaction" or "weak nuclear force" is responsible for some phenomena at the scales of the atomic nucleus, such as

beta decay . Electromagnetism and the weak force are theoretically understood to be two aspects of a unifiedelectroweak interaction — this realization was the first step toward the unified theory known as theStandard Model . In electroweak theory, the carriers of the weak force are massivegauge boson s called theW and Z bosons . The weak interaction is the only known interaction in which parity is not conserved; it is left-right asymmetric. It even breaks CP symmetry.However, it does conserve CPT.**trong interaction**The "strong interaction", or "strong nuclear force", is the most complicated force because it behaves differently at different distances. At distances larger than 10

femtometers , the strong force is practically unobservable, which is why it wasn't noticed until the beginning of the 20th century.After the nucleus was discovered, it was clear that a new force was needed to keep the positive protons in the nucleus from flying out. The force had to be much stronger than electromagnetism, so that the nucleus could be stable even though the protons were so close together, squeezed down to a volume which is 10

^{-15}of the volume of an atom. From the short range of the force,Hideki Yukawa predicted that it was associated with a massive particle, whose mass is approximately 100 MeV. Thepion was discovered in 1947 and this discovery marks the beginning of the modern era of particle physics.Hundreds of

hadrons were discovered from the 1940s to 1960s. An extremely complicated theory of the strongly interacting particles, known ashadrons , was developed. Most notably, the pions were understood to be oscillations of vacuum condensates, the rho and omega vector bosons were proposed by Sakurai to be force carrying particles for approximate symmetries ofIsospin andhypercharge , and the heavier particles were grouped byGeoffrey Chew , Edward K. Burdett andSteven Frautschi into families that could be understood as vibrational and rotational excitations of strings. None of these approaches led directly to the fundamental theory, but each of these were deep insights in their own right.Throughout the sixties, different authors considered theories similar to the modern fundamental theory of QCD as simple models for the interactions of quarks, starting with

Murray Gell-Mann who along withGeorge Zweig first proposed fractionally charged quarks in 1961. The first to suggest the gluons of QCD explicitly were the Korean physicistMoo-Young Han and JapaneseYoichiro Nambu , who introduced the quark color charge and hypothesized that it might be associated with a force-carrying field. but at that time, 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 only fractionally charged on average, and they did not expect the quarks in their model to be permanently confined.In 1971,

Murray Gell-Mann andHarald Fritsch 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 , andDavid Politzer discoveredasymptotic freedom in this theory, which allowed them to make contact with experiment. They came to the conclusion 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 , andValentine 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. First-principles computer calculations byKenneth Wilson in 1980 established that QCD will confine quarks, to a level of confidence tantamount to certainty. From this point on, QCD was the established theory of the strong interactions.QCD is a theory of fractionally charged quarks interacting with 8 photon-like 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 is not only responsible for the short-distance properties of quarks, but for the long-distance string-like behavior discovered by Chew and Frautschi.

**Current developments**The

Standard Model is a theory of three fundamental forces — electromagnetism, weak interactions and strong interactions; however, these three forces are not tied together.Howard Georgi ,Sheldon Glashow andAbdus Salam discovered that the Standard Model particles can arise from a single interaction, known as agrand unified theory . Grand unified theories predict relationships between otherwise unrelated constants of nature in the Standard Model.Gauge coupling unification is the prediction from grand unified theories for the relative strengths of the electromagnetic, weak and strong forces and this prediction was verified atLEP in 1991 for supersymmetric theories.Currently, there is no complete theory of

quantum gravity . There are several candidates for a framework to fit quantum gravity, includingstring theory ,loop quantum gravity andtwistor theory .In theories

beyond the Standard Model , there are frequentlyfifth force s and the search for these forces is an on-going line of experimental research in physics. Insupersymmetric theories, there are particles that only acquire their masses through supersymmetry breaking effects and these particles, known asmoduli can mediate new forces. Another possible motivation for new forces is related to the accelerating expansion of the universe. The most concrete examples of new forces from the cosmological expansion result from modifications ofGeneral Relativity .**ee also***

Standard Model

**Strong interaction

**Electroweak interaction

**Weak interaction *

Gravity

**Quantum gravity

**String Theory

**Theory of Everything *

Grand Unified Theory

**Gauge coupling unification

**Unified Field Theory * Quintessence, the proposed fifth force.

* "People":

Isaac Newton ,James Clerk Maxwell ,Albert Einstein ,Sheldon Glashow ,Abdus Salam ,Steven Weinberg ,Gerardus 't Hooft ,David Gross ,Edward Witten ,Howard Georgi **Notes****References*** Feynman, Richard P. (1967). "The Character of Physical Law". MIT Press. ISBN 0-262-56003-8

* Weinberg, S. (1993). "The First Three Minutes: A Modern View of the Origin of the Universe". Basic Books. ISBN 0-465-02437-8

* Weinberg, S. (1994). "Dreams of a Final Theory". Vintage Books USA. ISBN 0-679-74408-8

* Padmanabhan, T. (1998). "After The First Three Minutes: The Story of Our Universe". Cambridge University Press. ISBN 0-521-62972-1

* Perkins, Donald H. (2000). "Introduction to High Energy Physics". Cambridge University Press. ISBN 0-521-62196-8

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