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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.
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Overview
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 h/2π is reduced Planck's constant). They attract or repel each other by exchanging bosons.
The interaction of any pair of matter particles can then be modeled this way:
- two fermions go in
interaction by boson exchange two changed fermions go out.
The exchange of bosons always carries energy and momentum 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 of reduced 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 modern physics are directed at explaining every observed physical phenomenon by these interactions. Moreover, one tries to reduce the number of different interaction types (like unifying the electromagnetic interaction and the weak interaction into the electroweak interaction, see below). For an introductory explanation, four fundamental interactions (forces) may be assumed: gravitation, electromagnetism, the weak interaction, and the strong 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.
| Interaction | Current Theory | Mediators | Relative Strength1 | Long-Distance Behavior | Range(m) |
|---|---|---|---|---|---|
| Strong | Quantum chromodynamics (QCD) |
gluons | 1038 | 1 (see discussion below) |
10-15 |
| Electromagnetic | Quantum electrodynamics (QED) |
photons | 1036 |  |
infinite |
| Weak | Electroweak Theory | W and Z bosons | 1025 |  |
10-18 |
| Gravitation | General Relativity (GR) |
gravitons (not yet discovered) | 1 | |
infinite |
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 exchange virtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons are the mediators of the interaction of electric charges; and gluons are the mediators of the interaction of color charges.
The interactions
Gravitation
Gravitation is by far the weakest of the four interactions. Nevertheless, it is important for macroscopic objects and over long distances for the following reasons. Gravitational force:
- Has, unlike the strong and weak forces, an infinite range;
- Is the only interaction that acts universally on all matter;
- Is permanent. It can neither be absorbed nor transformed.
There are elementary particles, such as neutrons and neutrinos, lacking electrostatic charge. 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 eletric charge of zero. On the other hand, nothing "cancels" gravity. Hence all objects having mass are subject to gravitational force, which works in only one direction: attraction.
Because of its long range, gravity is responsible for such large-scale phenomena as the structure of galaxies, black holes, and the expansion of the universe. Gravity 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; animals and humans can jump only so high.
Gravitation was the first interaction to be described mathematically. In ancient times, Aristotle theorized that objects of different masses fall at different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this was not the case — neglecting the friction due to air resistance, all objects accelerate toward the Earth at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation to the behaviour of gravity. Our present-day understanding of gravity stems from Albert Einstein's General Theory of Relativity of 1915, a more accurate (especially for cosmological masses and distances) description of gravity in terms of the geometry of space-time.
Merging general relativity and quantum mechanics into a more general theory of quantum gravity is an area of active research. It is widely believed that in a theory of quantum gravity, gravitational force would be mediated by a hypothetical massless spin 2 particle called the graviton. Gravitons have yet to be observed.
Although general relativity has been experimentally confirmed as an accurate theory of gravity except on all but the smallest scales, there are rival theories of gravity. Those taken seriously 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 charged particles at rest, and the combined effect of electric and magnetic forces acting between charge particles moving relative to each other.
Electromagnetism is infinite-ranged like gravity, but vastly stronger, and therefore describes almost all macroscopic phenomena of everyday experience, ranging from the impenetrability of solids, friction, rainbows, lightning, and all human-made devices using electric current, such as television, lasers, and computers. Electromagnetism fundamentally determines all macroscopic, and many atomic level, properties of the chemical elements, including all chemical bonding.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 19th century that it was 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 move away from this classical theory began with Einstein's 1905 theory of special relativity, which showed that the speed of light is constant no matter how fast the observer is moving. In other work, Einstein also explained the photoelectric effect by hypothesizing that light was transmitted in quanta, 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 received theory of electromagnetism.
Weak interaction
The weak interaction or weak nuclear force is responsible for some nuclear phenomena such as beta decay. Electromagnetism and 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 which does not conserve parity; it is left-right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT.
Strong interaction
The strong interaction, or strong nuclear force, is the most complicated interaction, mainly because of the way it varies with distance. At distances greater than 10 femtometers, the strong force is practically unobservable. Moreover, it holds only inside the nucleus.
After the nucleus was discovered in 1908, 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. The pion 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, 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, the rho and omega vector bosons were proposed by Sakurai to be force carrying particles for approximate symmetries of Isospin and hypercharge, and the heavier particles were grouped by Geoffrey Chew, Edward K. Burdett and Steven 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 with George Zweig first proposed fractionally charged quarks in 1961. The first to suggest the gluons of QCD explicitly were the Korean physicist Moo-Young Han and Japanese Yoichiro 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 and Harald 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, and David Politzer discovered asymptotic 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, 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. First-principles computer calculations by Kenneth 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, the SM does not tie them together. Howard Georgi, Sheldon Glashow, and Abdus Salam discovered that under the Standard Model, particles can arise from a single interaction, known as a grand unified theory. Grand unified theories (GUTs) predict relationships among constants of nature that are unrelated in the Standard Model. GUTs predict gauge coupling unification for the relative strengths of the electromagnetic, weak, and strong forces, a prediction verified at the LEP in 1991 for supersymmetric theories.
An offshoot of a GUT would be a theory of quantum gravity. At present, while there are several candidate theories of quantum gravity, including string theory, loop quantum gravity, and twistor theory, none is widely accepted.
Some theories beyond the Standard Model include a hypothetical fifth force, and the search for such a force is an ongoing line of experimental research in physics. In supersymmetric theories, there are particles that acquire their masses only through supersymmetry breaking effects and these particles, known as moduli can mediate new forces. Another reason to look for new forces is the recent discovery that the expansion of the universe is accelerating, giving rise to a need to explain a nonzero cosmological constant, and possibly to other modifications of general relativity.
See also
- 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
- ^ Approximate. The exact strengths depend on the particles and energies involved.
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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