Inphysics, thefundamental interactions orfundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist:[1]
The gravitational and electromagnetic interactions produce long-range forces whose effects can be seen directly in everyday life. The strong and weak interactions produce forces atsubatomic scales and govern nuclear interactions insideatoms.
Some scientists hypothesize that afifth force might exist, but these hypotheses remain speculative.
Within the Standard Model, the strong interaction is carried by a particle called thegluon and is responsible forquarks binding together to formhadrons, such asprotons andneutrons. As a residual effect, it creates thenuclear force that binds the latter particles to formatomic nuclei. The weak interaction is carried by particles calledW and Z bosons, and also acts on the nucleus ofatoms, mediatingradioactive decay. The electromagnetic force, carried by thephoton, createselectric andmagnetic fields, which are responsible for the attraction betweenorbitalelectrons and atomic nuclei which holds atoms together, as well aschemical bonding andelectromagnetic waves, includingvisible 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 large (astronomical) distances gravity tends to be the dominant force, and is responsible for holding together the large scale structures in the universe, such as planets, stars, and galaxies.
The historical success of models that show relationships between fundamental interactions have led to efforts to go beyond the Standard Model and combine all four forces in to atheory of everything.
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, thusabsolute 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. Hislaw of universal gravitation implied there to be instant interaction among all objects.[3][4] As conventionally interpreted, Newton's theory of motion modelled acentral force without a communicating medium.[5][6] Thus Newton's theory violated the tradition, going back toDescartes, that there should be noaction at a distance.[7] Conversely, during the 1820s, when explaining magnetism,Michael Faraday inferred afield filling space and transmitting that force. Faraday conjectured that ultimately, all forces unified into one.[8]
In 1873,James Clerk Maxwell unified electricity and magnetism as effects of an electromagnetic field whose third consequence was light, travelling at constant speed in vacuum. If hiselectromagnetic field theory held true in allinertial frames of reference, this would contradict Newton's theory of motion, which relied onGalilean relativity.[9] If, instead, his field theory only applied to reference frames at rest relative to a mechanicalluminiferous aether—presumed to fill all space whether within matter or in vacuum and to manifest the electromagnetic field—then it could be reconciled with Galilean relativity and Newton's laws. (However, such a "Maxwell aether" was later disproven; Newton's laws did, in fact, have to be replaced.)[10]
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 withelementary particles, whose behaviours are modelled inquantum mechanics (QM). For predictive success with QM'sprobabilistic outcomes,particle physics conventionally models QMevents across a field set tospecial relativity, altogether relativistic quantum field theory (QFT).[11] Force particles, calledgauge bosons—force carriers ormessenger particles of underlying fields—interact with matter particles, calledfermions.
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, formmolecules, 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 isquantum electrodynamics (QED).
The force carriers of the weak interaction are the massiveW and Z bosons. Electroweak theory (EWT) covers both electromagnetism and the weak interaction. At the high temperatures shortly after theBig Bang, the weak interaction, the electromagnetic interaction, and theHiggs boson were originally mixed components of a different set of ancient pre-symmetry-breaking fields. As the early universe cooled, these fieldssplit into the long-range electromagnetic interaction, the short-range weak interaction, and the Higgs boson. In theHiggs mechanism, the Higgs field manifests Higgs bosons that interact with some quantum particles in a way that endows those particles with mass. The strong interaction, whose force carrier is thegluon, traversing minuscule distance among quarks, is modeled inquantum chromodynamics (QCD). EWT, QCD, and the Higgs mechanism compriseparticle physics'Standard Model (SM). Predictions are usually made using calculational approximation methods, although suchperturbation theory is inadequate to model some experimental observations (for instancebound states andsolitons). 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 andstrong interactions within aGrand Unified Theory[12] (GUT). Some attempts at GUTs hypothesize "shadow" particles, such that every knownmatter particle associates with an undiscoveredforce particle, and vice versa, altogethersupersymmetry (SUSY). Other theorists seek to quantize the gravitational field by the modelling behaviour of its hypothetical force carrier, thegraviton and achieve quantum gravity (QG). One approach to QG isloop quantum gravity (LQG). Still other theorists seek both QG and GUT within one framework, reducing all four fundamental interactions to aTheory of Everything (ToE). The most prevalent aim at a ToE isstring theory, although to modelmatter particles, it addedSUSY toforce particles—and so, strictly speaking, becamesuperstring 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.
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.
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 carriesenergy andmomentum 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 thereduced Planck constant). Since such interactions result in a change in momentum, they can give rise to classical Newtonianforces. In quantum mechanics, physicists often use the terms "force" and "interaction" interchangeably; for example, the weak interaction is sometimes referred to as the "weak force".
According to the present understanding, there are four fundamental interactions or forces:gravitation, electromagnetism, theweak interaction, and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observedphysical phenomenon by these fundamental interactions. Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are theunification of:
Both magnitude ("relative strength") and "range" of the associated potential, as given in the table, are meaningful only within a rather complex theoretical framework. The table below lists properties of a conceptual scheme that remains the subject of ongoing research.
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 exchangevirtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons mediate the interaction ofelectric charges, and gluons mediate the interaction ofcolor charges. The full theory includes perturbations beyond simply fermions exchanging bosons; these additional perturbations can involve bosons that exchange fermions, as well as the creation or destruction of particles: seeFeynman diagrams for examples.
Gravitation is the weakest of the four interactions at the atomic scale, where electromagnetic interactions dominate.
Gravitation is the most important of the four fundamental forces for astronomical objects over astronomical distances for two reasons. First, gravitation has an infinite effective range, like electromagnetism but unlike the strong and weak interactions. Second, gravity always attracts and never repels; in contrast, astronomical bodies tend toward a near-neutral net electric charge, such that the attraction to one type of charge and the repulsion from the opposite charge mostly cancel each other out.[15]
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 andblack holes and, being only attractive, it retards theexpansion of the universe. Gravitation also explains astronomical phenomena on more modest scales, such asplanetaryorbits, 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 theScientific 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'slaw of Universal Gravitation (1687) was a good approximation of the behaviour of gravitation. Present-day understanding of gravitation stems from Einstein'sGeneral Theory of Relativity of 1915, a more accurate (especially forcosmological masses and distances) description of gravitation in terms of thegeometry ofspacetime.
Although general relativity has been experimentally confirmed (at least for weak fields, i.e. not black holes) on all but the smallest scales, there arealternatives to general relativity. These theories must reduce to general relativity in some limit, and the focus of observational work is to establish limits on what deviations from general relativity are possible.
Electromagnetism and weak interaction appear to be very different at everyday low energies. They can be modeled using two different theories. However, above unification energy, on the order of 100GeV, they would merge into a single electroweak force.
The electroweak theory is very important for moderncosmology, particularly on how theuniverse evolved. This is because shortly after the Big Bang, when the temperature was still above approximately 1015K, the electromagnetic force and the weak force were still merged as a combined electroweak force.
For contributions to the unification of the weak and electromagnetic interaction betweenelementary particles, Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded theNobel Prize in Physics in 1979.[17][18]
Electromagnetism is the force that acts betweenelectrically charged particles. This phenomenon includes theelectrostatic force acting between charged particles at rest, and the combined effect of electric andmagnetic forces acting between charged particles moving relative to each other.
Electromagnetism has an infinite range, as gravity does, but is vastly stronger. It is the force that binds electrons to atoms, and itholds molecules together. It is responsible for everyday phenomena likelight,magnets,electricity, andfriction. Electromagnetism fundamentally determines all macroscopic, and many atomic-level, properties of thechemical elements.
In a four kilogram (~1 gallon) jug of water, there is
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
This force is many times larger than the weight of the planet Earth. Theatomic 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 tend to cancel out so that for astronomical-scale bodies, gravity dominates.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 19th centuryJames 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 usingvector calculus, is the classical theory of electromagnetism, suitable for most technological purposes.
The constantspeed of light in vacuum (customarily denoted with a lowercase letterc) can be derived from Maxwell's equations, which are consistent with the theory of special relativity.Albert Einstein's 1905 theory ofspecial relativity, however, which follows from the observation that thespeed 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 thephotoelectric effect by utilizing Max Planck's discovery that light was transmitted in 'quanta' of specific energy content based on the frequency, which we now callphotons. Starting around 1927,Paul Dirac combinedquantum mechanics with the relativistic theory ofelectromagnetism. Further work in the 1940s, byRichard Feynman,Freeman Dyson,Julian Schwinger, andSin-Itiro Tomonaga, completed this theory, which is now calledquantum electrodynamics, the revised theory of electromagnetism. Quantum electrodynamics and quantum mechanics provide a theoretical basis for electromagnetic behavior such asquantum 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 astransistors to function.
Theweak interaction orweak nuclear force is responsible for some nuclear phenomena such asbeta decay. Electromagnetism and the weak force are now understood to be two aspects of a unifiedelectroweak interaction — this discovery was the first step toward the unified theory known as theStandard Model. In the theory of the electroweak interaction, the carriers of the weak force are the massivegauge bosons called theW and Z bosons. The weak interaction is the only known interaction that does not conserveparity; it is left–right asymmetric. The weak interaction evenviolates CP symmetry but doesconserve CPT.
Thestrong interaction, orstrong nuclear force, is the most complicated interaction, mainly because of the way it varies with distance. The nuclear force is powerfully attractive between nucleons at distances of about 1 femtometre (fm, or 10−15 metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm, the nuclear force becomes repulsive. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows.
After the nucleus was discovered in 1908, it was clear that a new force, today known as the nuclear force, was needed to overcome theelectrostatic 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 whose diameter is about 10−15m, 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 force particle, whose mass is approximately 100 MeV.
The 1947 discovery of thepion ushered in the modern era of particle physics. Hundreds of hadrons were discovered from the 1940s to 1960s, and anextremely complicated theory of hadrons as strongly interacting particles was developed. Most notably:
Geoffrey Chew, Edward K. Burdett andSteven Frautschi grouped the heavier hadrons into families that could be understood as vibrational and rotational excitations ofstrings.
While each of these approaches offered insights, no approach led directly to a fundamental theory.
Murray Gell-Mann along withGeorge Zweig first proposed fractionally charged quarks in 1961. Throughout the 1960s, different authors considered theories similar to the modern fundamental theory ofquantum chromodynamics (QCD) as simple models for the interactions of quarks. The first to hypothesize the gluons of QCD wereMoo-Young Han andYoichiro Nambu, who introduced thequark color charge. Han and Nambu 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 andHarald 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, andDavid Politzer discovered that this theory had the property ofasymptotic freedom, allowing them to make contact withexperimental 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 permanentlyconfined: the strong force increases indefinitely with distance, trapping quarks inside the hadrons.
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. 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 strong interactions.
QCD is a theory of fractionally charged quarks interacting by means of 8 bosonic particles called gluons. The gluons also interact with each other, not just with the quarks, and at long distances the lines of force collimate into strings, loosely modeled by a linear potential, a constant attractive force. 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.
with Higgs mass125.18 GeV. Because thereduced Compton wavelength of theHiggs boson is so small (1.576×10−18 m, comparable to theW and Z bosons), this potential has an effective range of a fewattometers. Between two electrons, it begins roughly 1011 times weaker than theweak interaction, and grows exponentially weaker at non-zero distances.
The fundamental forces may become unified into a single force at very high energies and on a minuscule scale, thePlanck scale.[21]Particle accelerators cannot produce the enormous energies required to experimentally probe this regime. The weak and electromagnetic forces have already been unified with theelectroweak theory ofSheldon Glashow,Abdus Salam, andSteven Weinberg, for which they received the 1979 Nobel Prize in physics.[22][23][24] 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 withsymmetries 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 predictinggauge coupling unification for the relative strengths of the electromagnetic, weak, and strong forces.
A so-calledtheory of everything, which would integrate GUTs with a quantum gravity theory face a greater barrier, because no quantum gravity theories, which includestring theory,loop quantum gravity, andtwistor 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 hypotheticalfifth force, and the search for such a force is an ongoing line of experimental physics research. Insupersymmetric theories, some particles acquire their masses only through supersymmetry breaking effects and these particles, known asmoduli, can mediate new forces. Another reason to look for new forces is the discovery that theexpansion of the universe is accelerating (also known asdark energy), giving rise to a need to explain a nonzerocosmological constant, and possibly to other modifications ofgeneral relativity. Fifth forces have also been suggested to explain phenomena such asCP violations,dark matter, anddark flow.
^Goldin, Gerald A.; Shtelen, Vladimir M. (February 2001). "On Galilean invariance and nonlinearity in electrodynamics and quantum mechanics".Physics Letters A.279 (5–6):321–326.arXiv:quant-ph/0006067.Bibcode:2001PhLA..279..321G.doi:10.1016/S0375-9601(01)00017-2.S2CID5398578.no fully Galilean-covariant theory of a coupled Schrödinger-Maxwell system (where the density and current of the Schrödinger field act as source of the nonrelativistic Maxwell field) is possible
Schumm, Bruce A. (2004),Deep Down Things, Johns Hopkins University Press While all interactions are discussed, discussion is especially thorough on the weak.
