# Fundamental interactions

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## Fundamental interactions

Fundamental forces that act between elementary particles, of which all matter is assumed to be composed.

At present, four fundamental interactions are distinguished. Their properties are summarized in the table.

Interaction | Range | Exchanged quanta |
---|---|---|

Gravitational | Long-range | Gravitons (g) |

Electromagnetic | Long-range | Photons (γ) |

Weak nuclear | Short-range ≈10^{-18} m | W^{+}, Z^{0}, W^{-} |

Strong nuclear | Short-range ≈10^{-15} m | Gluons (G) |

The gravitational interaction manifests itself as a long-range force of attraction between all elementary particles.

The electromagnetic interaction is responsible for the long-range force of repulsion of like, and attraction of unlike, electric charges. At comparable distances, the ratio of gravitational to electromagnetic interactions (as determined by the strength of respective forces between an electron and a proton) is approximately 4 × 10^{-37}. *See* Coulomb's law, Electrostatics, Gravitation

In modern quantum field theory, the electromagnetic interaction and the forces of attraction or repulsion between charged particles are pictured as arising secondarily as a consequence of the primary process of emission of one or more photons (particles or quanta of light) emitted by an accelerating electric charge (in accordance with Maxwell's equations) and the subsequent reabsorption of these quanta by a second charged particle. A similar picture may also be valid for the gravitational interaction.

The third fundamental interaction is the weak nuclear interaction, which is responsible for the decay of a neutron into a proton, an electron, and an antineutrino. Unlike electromagnetism and gravitation, weak interactions are short-range, the range of the force being of the order of 10^{-18} m.

An important question was finally answered in 1983: Is the weak interaction similar to electromagnetism in being mediated primarily by intermediate objects, the *W*^{+} and *W*^{-} particles. The experimental answer, discovered at the CERN laboratory at Geneva, is that *W*^{+} and *W*^{-} do exist, with a mass of 80.4 GeV/*c*^{2}. Each carries a spin of magnitude ℏ, where ℏ is Planck's constant divided by 2&pgr;, just as does the photon (γ). The mass of these particles gives the range of the weak interaction. *See* Intermediate vector boson

Another crucial discovery in weak interaction physics was the neutral current phenomenon in 1973, that is, the discovery of new types of weak interactions where (as in the case of electromagnetism or gravity) the nature of the interacting particles is not changed during the interaction. The 1983 experiments at CERN also gave evidence for the existence of an intermediate particle *Z*^{0}, with a mass of 91.2 GeV/*c*^{2}, which is believed to mediate such reactions. *See* Neutral currents, Weak nuclear interactions

The fourth fundamental interaction is the strong nuclear interaction between protons and neutrons, which resembles the weak nuclear interaction in being short-range, although the range is of the order of 10^{-15} m rather than 10^{-18} m. Within this range of distances the strong force overshadows all other forces between protons and neutrons.

Protons and neutrons are themselves believed to be made up of yet more fundamental entities, the up (*u*) and down (*d*) quarks (*P* = *uud*, *N* = *udd*). Each quark is assumed to be endowed with one of three color quantum numbers [conventionally labeled red (*r*), yellow (*y*), and blue (*b*)]. The strong nuclear force can be pictured as ultimately arising through an exchange of zero rest-mass color-carrying quanta of spin ℏ called gluons (*G*) [analogous to photons in electromagnetism], which are exchanged between quarks (contained inside protons and neutrons). Since neutrinos, electrons, and muons (the so-called leptons) do not contain quarks, their interactions among themselves or with protons and neutrinos do not exhibit the strong nuclear force. *See* Color (quantum mechanics), Gluons, Lepton, Quantum chromodynamics, Quarks, Strong nuclear interactions

Three of the four fundamental interactions (electromagnetic, weak nuclear, and strong nuclear) appear to be mediated by intermediate quanta (photons γ; *W*^{+}, *Z*^{0}, and *W*^{-}; and gluons *G*, respectively), each carrying spin of magnitude ℏ. This is characteristic of the gauge interactions, whose general theory was given by H. Weyl, C. N. Yang, R. Mills, and R. Shaw. This class of interactions is further characterized by the fact that the force between any two particles (produced by the mediation of an intermediate gauge particle) is universal in the sense that its strength is (essentially) proportional to the product of the intrinsic charges (electric, or weak-nuclear, or strong-color) carried by the two interacting particles concerned.

The fourth interaction (the gravitational) can also be considered as a gauge interaction, with the intrinsic charge in this case being the mass; the gravitational force between any two particles is proportional to the product of their masses. The only difference between gravitation and the other three interactions is that the gravitational gauge quantum (the graviton) carries spin 2ℏ rather than ℏ. It is an open question whether all fundamental interactions are gauge interactions.

Ever since the discovery and clear classification of these four interactions, particle physicists have attempted to unify these interactions as aspects of one basic interaction between all matter. A unification of weak and electromagnetic interactions, employing the gauge ideas was suggested by S. Glashow and by A. Salam and J. C. Ward in 1959. Following this initial attempt, Glashow (and independently Salam and Ward) noted that such a unification could be effected only if neutral current weak interactions were postulated to exist.

There were two major problems with this unified electroweak gauge theory considered as a fundamental theory. Yang and Mills had shown that masslessness of gauge quanta is the hallmark of unbroken gauge theories. The origin of the masses of the weak interaction quanta *W*^{+}, *W*^{-}, and *Z*^{0} (or equivalently the short-range of weak interactions), as contrasted with the masslessness of the photon (or equivalently the long-range character of electromagnetism), therefore required explanation. The second problem concerned the possibility of reliably calculating higher-order quantum effects with the new unified electroweak theory, on the lines of similar calculations for the “renormalized” theory of electromagnetism elaborated by S. Tomonaga, Schwinger, Feynman, and F. J. Dyson around 1949. The first problem was solved by S. Weinberg and Salam and the second by G. t'Hooft and by B. W. Lee and J. Zinn-Justin. *See* Renormalization

Weinberg and Salam considered the possibility of the electroweak interaction being a “spontaneously broken” gauge theory. By introducing an additional self-interacting Higgs-Englert-Brout-Kibble particle into the theory, they were able to show that the *W*^{+}, *W*^{-}, and *Z*^{0} would acquire well-defined masses through the so-called Higgs mechanism. The predicted theoretical mass values of the *W* and *Z* particles are in good accord with the experimental values found by the CERN 1983 experiments.

The Weinberg-Salam electroweak theory contains an additional neutral particle (the Higgs) but does not predict its mass. A search for this particle will be undertaken when the large hadron collider (LHC) at CERN comes into commission. *See* Electroweak interaction, Higgs boson, Particle accelerator, Symmetry breaking

The gauge unification of weak and electromagnetic interactions, which started with the observation that the relevant mediating quanta (*W*^{+}, *W*^{-}, *Z*^{0}, and γ) possess intrinsic spin ℏ, can be carried further to include strong nuclear interactions as well, if these strong interactions are also mediated through quanta (gluons) carrying spin ℏ. The resulting theory, which appears to explain all known low-energy phenomena, is called the standard model. (It is a model based on three similarly constituted generations of quarks and leptons plus the mediating quanta *W*^{+}, *W*^{-}, *Z*^{0}, photons, and gluons plus the Higgs particle.) A complete gauge unification of all three forces (electromagnetic, weak nuclear, and strong nuclear) into a single electronuclear interaction seems plausible. Such a (so-called grand) unification necessarily means that the distinction between quarks on the one hand and neutrinos, electrons, and muons (leptons) on the other, must disappear at sufficiently high energies, with all interactions (weak, electromagnetic, and strong) clearly manifesting themselves then as facets of one universal gauge force. The fact that at low energies presently available, these interactions exhibit vastly different effective strengths is ascribed to differing renormalizations due to successive spontaneous symmetry breakings. A startling consequence of the eventual universality and the disappearance of distinction between quarks and leptons is the possibility of protons transforming into leptons and pions. Contrary to the older view, protons would therefore decay into leptons and pions and not live forever. *See* Grand unification theories, Proton, Standard model

Research in unification theories of fundamental interactions is now concerned with uniting the gauge theories of gravity and of the electronuclear interactions. The most promising approach appears to be that of superstring theories. Such theories appear to describe the only possible theory of gravity which is finite and suffers from no ultraviolet infinities. A closed string is a (one-dimensional) loop which may exist in a *d*-dimensional space-time (where *d* must equal 10 to completely eliminate all ultraviolet infinities). The quantum oscillations of the string correspond to particles of higher spins and higher masses. The theory has a unique built-in gauge symmetry. *See* Superstring theory