In physics , the fundamental interactions or fundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist:
88-523: Mu3e is a particle physics experiment at the Paul Scherrer Institute , searching for decays of anti - muons ( Mu ) to an electron and two positrons ( 3e ). This decay is extremely unlikely in the Standard Model of particle physics, as it changes the lepton number . Several new theories, especially supersymmetric ones, predict a much more frequent decay. Searching for this decay allows
176-508: A central force without a communicating medium. Thus Newton's theory violated the tradition, going back to Descartes , that there should be 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 1873, James Clerk Maxwell unified electricity and magnetism as effects of an electromagnetic field whose third consequence
264-487: A Hilbert space , which is also treated in quantum field theory . Following the convention of particle physicists, the term elementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles. Ordinary matter is made from first- generation quarks ( up , down ) and leptons ( electron , electron neutrino ). Collectively, quarks and leptons are called fermions , because they have
352-414: A fifth force might exist, but these hypotheses remain speculative. 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
440-498: A quantum spin of half-integers (−1/2, 1/2, 3/2, etc.). This causes the fermions to obey the Pauli exclusion principle , where no two particles may occupy the same quantum state . Quarks have fractional elementary electric charge (−1/3 or 2/3) and leptons have whole-numbered electric charge (0 or 1). Quarks also have color charge , which is labeled arbitrarily with no correlation to actual light color as red, green and blue. Because
528-1055: A " Theory of Everything ", or "TOE". There are also other areas of work in theoretical particle physics ranging from particle cosmology to loop quantum gravity . In principle, all physics (and practical applications developed therefrom) can be derived from the study of fundamental particles. In practice, even if "particle physics" is taken to mean only "high-energy atom smashers", many technologies have been developed during these pioneering investigations that later find wide uses in society. Particle accelerators are used to produce medical isotopes for research and treatment (for example, isotopes used in PET imaging ), or used directly in external beam radiotherapy . The development of superconductors has been pushed forward by their use in particle physics. The World Wide Web and touchscreen technology were initially developed at CERN . Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating
616-653: A backbone, M-theory . Theories beyond the Standard Model remain highly speculative, lacking great experimental support. 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: The exchange of bosons always carries energy and momentum between
704-404: 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 full theory includes perturbations beyond simply fermions exchanging bosons; these additional perturbations can involve bosons that exchange fermions, as well as
792-448: 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). 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
880-400: 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 implied there to be instant interaction among all objects. As conventionally interpreted, Newton's theory of motion modelled
968-558: A field set to special relativity , altogether relativistic quantum field theory (QFT). 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,
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#17328725293741056-452: A fourth generation of fermions does not exist. Bosons are the mediators or carriers of fundamental interactions, such as electromagnetism , the weak interaction , and the strong interaction . Electromagnetism is mediated by the photon , the quanta of light . The weak interaction is mediated by the W and Z bosons . The strong interaction is mediated by the gluon , which can link quarks together to form composite particles. Due to
1144-501: 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. Conventionally, the Higgs interaction is not counted among the four fundamental forces. Nonetheless, although not a gauge interaction nor generated by any diffeomorphism symmetry,
1232-1011: A long and growing list of beneficial practical applications with contributions from particle physics. Major efforts to look for physics beyond the Standard Model include the Future Circular Collider proposed for CERN and the Particle Physics Project Prioritization Panel (P5) in the US that will update the 2014 P5 study that recommended the Deep Underground Neutrino Experiment , among other experiments. Fundamental interaction 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 at subatomic scales and govern nuclear interactions inside atoms . Some scientists hypothesize that
1320-406: 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. 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
1408-420: 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
1496-474: A single force at very high energies on a minuscule scale, the Planck scale , but particle accelerators cannot produce the enormous energies required to experimentally probe this. Devising a common theoretical framework that would explain the relation between the forces in a single theory is perhaps the greatest goal of today's theoretical physicists . The weak and electromagnetic forces have already been unified with
1584-511: A test of these theories, even if they cannot be tested directly in other experiments like at the LHC . It has also been shown the experiment is sensitive to probe new light dark sector particles such as dark photon. Mu3e is constructed at the Paul Scherrer Institute. It is planned to create the world's most intense muon beam which will allow to analyze two billion decays per second. This rate
1672-442: 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. The weak interaction or weak nuclear force is responsible for some nuclear phenomena such as beta decay . Electromagnetism and
1760-435: A wide range of exotic particles . All particles and their interactions observed to date can be described almost entirely by the Standard Model. Dynamics of particles are also governed by quantum mechanics ; they exhibit wave–particle duality , displaying particle-like behaviour under certain experimental conditions and wave -like behaviour in others. In more technical terms, they are described by quantum state vectors in
1848-425: Is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions. A third major effort in theoretical particle physics is string theory . String theorists attempt to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings, and branes rather than particles. If the theory is successful, it may be considered
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#17328725293741936-426: 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, i.e. not black holes) on all but the smallest scales, there are alternatives to general relativity . These theories must reduce to general relativity in some limit, and the focus of observational work
2024-440: 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 utilizing Max Planck's discovery that light was transmitted in 'quanta' of specific energy content based on
2112-508: Is fundamentally composed of elementary particles dates from at least the 6th century BC. In the 19th century, John Dalton , through his work on stoichiometry , concluded that each element of nature was composed of a single, unique type of particle. The word atom , after the Greek word atomos meaning "indivisible", has since then denoted the smallest particle of a chemical element , but physicists later discovered that atoms are not, in fact,
2200-493: Is in a magnetic field of 1 Tesla to determine the energy of the particles based on their curvature radius. As of November 2021 data taking is expected to begin in 2024. The full rate of two billion muons per second will not be reached before 2028. The experiment is expected to either find the decay or to set an upper limit of 10 on the branching fraction , a factor 10,000 better than previous experiments. Particle physics Particle physics or high-energy physics
2288-591: Is in model building where model builders develop ideas for what physics may lie beyond the Standard Model (at higher energies or smaller distances). This work is often motivated by the hierarchy problem and is constrained by existing experimental data. It may involve work on supersymmetry , alternatives to the Higgs mechanism , extra spatial dimensions (such as the Randall–Sundrum models ), Preon theory, combinations of these, or other ideas. Vanishing-dimensions theory
2376-466: Is left–right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT . The strong interaction , or strong 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 metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm,
2464-521: Is made only from the first fermion generation. The first generation consists of up and down quarks which form protons and neutrons , and electrons and electron neutrinos . The three fundamental interactions known to be mediated by bosons are electromagnetism , the weak interaction , and the strong interaction . Quarks cannot exist on their own but form hadrons . Hadrons that contain an odd number of quarks are called baryons and those that contain an even number are called mesons . Two baryons,
2552-652: Is necessary to study more than 10 muon decays in total. Important backgrounds are the decays μ + → e + e − e + ν μ ¯ ν e {\displaystyle \mu ^{+}\to e^{+}e^{-}e^{+}{\bar {\nu _{\mu }}}\nu _{e}} and μ + → e + ν μ ¯ ν e {\displaystyle \mu ^{+}\to e^{+}{\bar {\nu _{\mu }}}\nu _{e}} . To distinguish between signal and background,
2640-431: Is the gluon , traversing minuscule distance among quarks, is modeled in quantum chromodynamics (QCD). EWT, QCD, and the Higgs mechanism 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
2728-407: Is the classical theory of electromagnetism, suitable for most technological purposes. The constant speed of light in vacuum (customarily denoted with a lowercase letter c ) can be derived from Maxwell's equations, which are consistent with the theory of special relativity. Albert Einstein 's 1905 theory of special relativity , however, which follows from the observation that the speed of light
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2816-510: Is the study of fundamental particles and forces that constitute matter and radiation . The field also studies combinations of elementary particles up to the scale of protons and neutrons , while the study of combination of protons and neutrons is called nuclear physics . The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). There are three generations of fermions, although ordinary matter
2904-483: Is the study of these particles in radioactive processes and in particle accelerators such as the Large Hadron Collider . Theoretical particle physics is the study of these particles in the context of cosmology and quantum theory . The two are closely interrelated: the Higgs boson was postulated by theoretical particle physicists and its presence confirmed by practical experiments. The idea that all matter
2992-441: Is to establish limits on what deviations from general relativity are possible. Proposed extra dimensions could explain why the gravity force is so weak. 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 100 GeV , they would merge into a single electroweak force. The electroweak theory
3080-600: Is used to extract the parameters of the Standard Model with less uncertainty. This work probes the limits of the Standard Model and therefore expands scientific understanding of nature's building blocks. Those efforts are made challenging by the difficulty of calculating high precision quantities in quantum chromodynamics . Some theorists working in this area use the tools of perturbative quantum field theory and effective field theory , referring to themselves as phenomenologists . Others make use of lattice field theory and call themselves lattice theorists . Another major effort
3168-435: Is vastly stronger. It is the force that binds electrons to atoms, and it holds molecules together . It is responsible for everyday phenomena like light , magnets , electricity , and friction . Electromagnetism fundamentally determines all macroscopic, and many atomic-level, properties of the chemical elements . In a four kilogram (~1 gallon) jug of water, there is of total electron charge. Thus, if we place two such jugs
3256-518: Is very important for modern cosmology , particularly on how the universe evolved. This is because shortly after the Big Bang, when the temperature was still above approximately 10 K , 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 between elementary particles , Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded
3344-464: The Higgs boson were originally mixed components of a different set of ancient pre-symmetry-breaking fields. As the early universe cooled, these fields split into the long-range electromagnetic interaction, the short-range weak interaction, and the Higgs boson. In the Higgs 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
3432-473: The Higgs field 's cubic Yukawa coupling produces a weakly attractive fifth interaction. After spontaneous symmetry breaking via the Higgs mechanism , Yukawa terms remain of the form with Yukawa coupling λ i {\displaystyle \lambda _{i}} , particle mass m i {\displaystyle m_{i}} (in eV ), and Higgs vacuum expectation value 246.22 GeV . Hence coupled particles can exchange
3520-504: The Nobel Prize in Physics in 1979. 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 an infinite range, as gravity does, but
3608-522: 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)
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3696-440: 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 quarks binding 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
3784-544: The atomic nuclei are baryons – the neutron is composed of two down quarks and one up quark, and the proton is composed of two up quarks and one down quark. A baryon is composed of three quarks, and a meson is composed of two quarks (one normal, one anti). Baryons and mesons are collectively called hadrons . Quarks inside hadrons are governed by the strong interaction, thus are subjected to quantum chromodynamics (color charges). The bounded quarks must have their color charge to be neutral, or "white" for analogy with mixing
3872-420: The electroweak theory of Sheldon Glashow , Abdus Salam , and Steven Weinberg , for which they received the 1979 Nobel Prize in physics. Some physicists seek to unite the electroweak and strong fields within what is called a Grand Unified Theory (GUT). An even 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
3960-417: The proton and the neutron , make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of a microsecond . They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays . Mesons are also produced in cyclotrons or other particle accelerators . Particles have corresponding antiparticles with
4048-421: The quantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model . The reconciliation of gravity to the current particle physics theory is not solved; many theories have addressed this problem, such as loop quantum gravity , string theory and supersymmetry theory . Practical particle physics
4136-441: The reduced Planck constant ). Since such interactions result in a change in momentum, they can give rise to classical Newtonian forces . 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,
4224-417: 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: Both magnitude ("relative strength") and "range" of
4312-402: The 1940s to 1960s, and an extremely complicated theory of hadrons as strongly interacting particles was developed. Most notably: While each of these approaches offered 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
4400-454: The 1950s and 1960s, a bewildering variety of particles was found in collisions of particles from beams of increasingly high energy. It was referred to informally as the " particle zoo ". Important discoveries such as the CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance . After the formulation of the Standard Model during the 1970s, physicists clarified
4488-439: 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
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#17328725293744576-571: The aforementioned color confinement, gluons are never observed independently. The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism – the gluon and photon are expected to be massless . All bosons have an integer quantum spin (0 and 1) and can have the same quantum state . Most aforementioned particles have corresponding antiparticles , which compose antimatter . Normal particles have positive lepton or baryon number , and antiparticles have these numbers negative. Most properties of corresponding antiparticles and particles are
4664-416: 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
4752-536: The constituents of all matter . Finally, the Standard Model also predicted the existence of a type of boson known as the Higgs boson . On 4 July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson. The Standard Model, as currently formulated, has 61 elementary particles. Those elementary particles can combine to form composite particles, accounting for
4840-422: The creation or destruction of particles: see Feynman 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
4928-420: The detector has a spatial resolution better than 200 μm , a time resolution better than 100 ps and an energy resolution better than 0.5 MeV for the individual electrons. To minimize multiple scattering , the detector is built as light as possible. Semiconductor detectors are used for the spatial and energy resolution, scintillator fibers provide a good timing resolution. The whole experiment
5016-483: The electromagnetic field's force carrier, which if unimpeded traverse potentially infinite distance. Electromagnetism's QFT is quantum electrodynamics (QED). The force carriers of the weak interaction are the massive W and Z bosons . Electroweak theory (EWT) covers both electromagnetism and the weak interaction. At the high temperatures shortly after the Big Bang , the weak interaction, the electromagnetic interaction, and
5104-597: 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.) 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
5192-412: 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. Many theoretical physicists believe these fundamental forces to be related and to become unified into
5280-558: 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 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 ,
5368-412: 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
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#17328725293745456-478: The first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model. Modern particle physics research is focused on subatomic particles , including atomic constituents, such as electrons , protons , and neutrons (protons and neutrons are composite particles called baryons , made of quarks ), that are produced by radioactive and scattering processes; such particles are photons , neutrinos , and muons , as well as
5544-454: 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
5632-435: The frequency, 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
5720-424: The fundamental particles of nature, but are conglomerates of even smaller particles, such as the electron . The early 20th century explorations of nuclear physics and quantum physics led to proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn ), and nuclear fusion by Hans Bethe in that same year; both discoveries also led to the development of nuclear weapons . Throughout
5808-579: 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
5896-538: The gravitational interaction, but it has not been detected or completely reconciled with current theories. Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably, supersymmetric particles aim to solve the hierarchy problem , axions address the strong CP problem , and various other particles are proposed to explain the origins of dark matter and dark energy . The world's major particle physics laboratories are: Theoretical particle physics attempts to develop
5984-424: The hundreds of other species of particles that have been discovered since the 1960s. The Standard Model has been found to agree with almost all the experimental tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature and that a more fundamental theory awaits discovery (See Theory of Everything ). In recent years, measurements of neutrino mass have provided
6072-433: The interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is called color confinement . There are three known generations of quarks (up and down, strange and charm , top and bottom ) and leptons (electron and its neutrino, muon and its neutrino , tau and its neutrino ), with strong indirect evidence that
6160-456: The mid-1970s after experimental confirmation of the existence of quarks . It describes the strong , weak , and electromagnetic fundamental interactions , using mediating gauge bosons . The species of gauge bosons are eight gluons , W , W and Z bosons , and the photon . The Standard Model also contains 24 fundamental fermions (12 particles and their associated anti-particles), which are
6248-497: The models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments (see also theoretical physics ). There are several major interrelated efforts being made in theoretical particle physics today. One important branch attempts to better understand the Standard Model and its tests. Theorists make quantitative predictions of observables at collider and astronomical experiments, which along with experimental measurements
6336-504: 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. 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
6424-416: 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 the electrostatic repulsion , a manifestation of electromagnetism, of the positively charged protons. Otherwise,
6512-461: 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 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 force 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
6600-406: 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
6688-426: The origin of the particle zoo. The large number of particles was explained as combinations of a (relatively) small number of more fundamental particles and framed in the context of quantum field theories . This reclassification marked the beginning of modern particle physics. The current state of the classification of all elementary particles is explained by the Standard Model , which gained widespread acceptance in
6776-483: The photon or gluon, have no antiparticles. Quarks and gluons additionally have color charges, which influences the strong interaction. Quark's color charges are called red, green and blue (though the particle itself have no physical color), and in antiquarks are called antired, antigreen and antiblue. The gluon can have eight color charges , which are the result of quarks' interactions to form composite particles (gauge symmetry SU(3) ). The neutrons and protons in
6864-426: The primary colors . More exotic hadrons can have other types, arrangement or number of quarks ( tetraquark , pentaquark ). An atom is made from protons, neutrons and electrons. By modifying the particles inside a normal atom, exotic atoms can be formed. A simple example would be the hydrogen-4.1 , which has one of its electrons replaced with a muon. The graviton is a hypothetical particle that can mediate
6952-402: 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
7040-438: The quarks are permanently confined : the strong force increases indefinitely with distance, trapping quarks inside the hadrons. 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
7128-470: 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
7216-412: The same mass but with opposite electric charges . For example, the antiparticle of the electron is the positron . The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called antimatter . Some particles, such as the photon , are their own antiparticle. These elementary particles are excitations of
7304-444: The same, with a few gets reversed; the electron's antiparticle, positron, has an opposite charge. To differentiate between antiparticles and particles, a plus or negative sign is added in superscript . For example, the electron and the positron are denoted e and e . When a particle and an antiparticle interact with each other, they are annihilated and convert to other particles. Some particles, such as
7392-543: 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. 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
7480-533: The structure of galaxies and black holes and, being only attractive, it retards the expansion of the universe . 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
7568-407: 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 that does not conserve parity ; it
7656-427: Was a good approximation of the behaviour of gravitation. Present-day understanding of gravitation stems from 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
7744-422: Was light, travelling at constant speed in vacuum. If his electromagnetic field theory held true in all inertial frames of reference , this would contradict Newton's theory of motion, which relied on Galilean relativity . If, instead, his field theory only applied to reference frames at rest relative to a mechanical luminiferous aether —presumed to fill all space whether within matter or in vacuum and to manifest
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