Fundamental interaction
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In physics, the fundamental interactions or fundamental forces are the interactions that do not appear 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 at minuscule, subatomic distances and govern nuclear interactions inside atoms.
Some scientists hypothesize that a
Each of the known fundamental interactions can be described mathematically as a
Within the Standard Model, the strong interaction is carried by a particle called the
Many theoretical physicists believe these fundamental forces to be related and to become unified into a single force at very high energies on a minuscule scale, the
History
Classical theory
In his 1687 theory,
In 1873,
The Standard Model
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
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 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 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 the Standard Model as science's most experimentally confirmed theory.
Overview of the fundamental interactions
In the
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
According to the present understanding, there are four fundamental interactions or forces:
- magnetic forceinto electromagnetism;
- The electromagnetic interactionand the weak interaction into the electroweak interaction; see below.
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.
Interaction | Current theory | Mediators | Relative strength[20] | Long-distance behavior (potential) | Range (m)[21] |
---|---|---|---|---|---|
Weak | Electroweak theory (EWT) |
W and Z bosons | 1033 | 10−18 | |
Strong | Quantum chromodynamics (QCD) |
gluons | 1038 | (Color confinement, see discussion below) |
10−15 |
Gravitation |
General relativity (GR) |
gravitons (hypothetical) | 1 | ∞ | |
Electromagnetic | Quantum electrodynamics (QED) |
photons | 1036 | ∞ |
The modern (perturbative)
The interactions
Gravity
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.[22]
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, 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,
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, 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 is to establish limits on what deviations from general relativity are possible.
Proposed extra dimensions could explain why the gravity force is so weak.[23]
Electroweak interaction
The electroweak theory 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 1015 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 the Nobel Prize in Physics in 1979.[24][25]
Electromagnetism
Electromagnetism is the force that acts between
Electromagnetism has an infinite range like gravity, but is vastly stronger than it, and therefore describes several macroscopic phenomena of everyday experience such as
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. 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 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 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
Weak interaction
The weak interaction or weak nuclear force is responsible for some nuclear phenomena such as
Strong interaction
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 10−15 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 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 whose diameter 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 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 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 insights, no approach led directly to a fundamental theory.
In 1971, Murray Gell-Mann and
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 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.
Higgs interaction
Conventionally, the Higgs interaction is not counted among the four fundamental forces.[26][27]
Nonetheless, although not a
- ,
with Yukawa coupling , particle mass (in eV), and Higgs vacuum expectation value 246.22 GeV. Hence coupled particles can exchange a virtual Higgs boson, yielding classical potentials of the form
- ,
with Higgs mass 125.18 GeV. Because the
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
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
See also
- Quintessence, a hypothesized fifth force
- Gerardus 't Hooft
- Edward Witten
- Howard Georgi
References
- ISBN 978-2863320549.
- ^ Weisstein, Eric W. (2007). "Fifth Force". World of Science. Wolfram Research. Retrieved September 14, 2017.
- ISBN 978-3319284125.
- ^ "The Standard Model of Particle Physics | symmetry magazine". www.symmetrymagazine.org. Retrieved 2018-10-30.
- ^ Shivni, Rashmi (2016-05-16). "The Planck scale". symmetry magazine. Fermilab/SLAC. Retrieved 2018-10-30.
- ^ "The Nobel Prize in Physics 1979". NobelPrize.org. Retrieved 2018-10-30.
- ^ "The Nobel Prize in Physics 1979". NobelPrize.org. Retrieved 2018-10-30.
- ^ "The Nobel Prize in Physics 1979". NobelPrize.org. Retrieved 2018-10-30.
- ^ "Newton's Laws of Motion". www.grc.nasa.gov. NASA.
- ^ "Newton's law of gravitation | Definition, Formula, & Facts". Encyclopedia Britannica. Retrieved 22 March 2021.
- S2CID 125197336.
- ^ Newton's absolute space was a medium, but not one transmitting gravitation.
- .
- )
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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
- S2CID 119258859.
- ^ Meinard Kuhlmann, "Physicists debate whether the world is made of particles or fields—or something else entirely", Scientific American, 24 Jul 2013.
- ^ Krauss, Lawrence M. (2017-03-16). "A Brief History of the Grand Unified Theory of Physics". Nautilus.
- ^ Approximate. See Coupling constant for more exact strengths, depending on the particles and energies involved.
- .
- Starts With a Bang. Retrieved 22 March 2021.
- ^ CERN (20 January 2012). "Extra dimensions, gravitons, and tiny black holes".
- ISBN 978-0-674-01967-6p.84
- ^ "The Nobel Prize in Physics 1979". The Nobel Foundation. Retrieved 2008-12-16.
- ^ "fundamental force | Definition, List, & Facts". Encyclopedia Britannica. Retrieved 22 March 2021.
- ^ "The Standard Model". CERN. Retrieved 22 March 2021.
Bibliography
- Davies, Paul (1986), The Forces of Nature, Cambridge Univ. Press 2nd ed.
- ISBN 978-0-262-56003-0
- Schumm, Bruce A. (2004), Deep Down Things, Johns Hopkins University Press While all interactions are discussed, discussion is especially thorough on the weak.
- ISBN 978-0-465-02437-7
- ISBN 978-0-679-74408-5
- Padmanabhan, T. (1998), After The First Three Minutes: The Story of Our Universe, Cambridge Univ. Press, ISBN 978-0-521-62972-0
- Perkins, Donald H. (2000), Introduction to High Energy Physics (4th ed.), Cambridge Univ. Press, ISBN 978-0-521-62196-0
- Riazuddin (December 29, 2009). "Non-standard interactions" (PDF). NCP 5th Particle Physics Sypnoisis. 1 (1): 1–25. Retrieved March 19, 2011.