Fundamental interaction

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Fundamental interaction
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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. Every observed physical phenomenon can be explained by these interactions. The apparent irreducible nature of these interactions leads physicists to study the properties of these forces in great detail. In modern physics, there are four fundamental interactions (forces): gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behavior vary greatly, as described in the table below.
Interaction Current Theory Mediators Relative Strength[1] Long-Distance Behavior Range(m)
Strong Quantum chromodynamics
(QCD) gluons 1038 1
(see discussion below) 10-15
Electromagnetic Quantum electrodynamics
(QED) photons 1036 \frac{1}{r^2} infinite
Weak Electroweak Theory W and Z bosons 1025 \frac{e^{-m_{W,Z}r}}{r} 10-18
Gravitation General Relativity
(GR) gravitons 1 \frac{1}{r^2} 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.
Contents
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* 1 The interactions
o 1.1 Gravitation
o 1.2 Electromagnetism
o 1.3 Weak interaction
o 1.4 Strong interaction
* 2 Current developments
* 3 See also
* 4 Notes
* 5 References

[edit] The interactions

[edit] Gravitation

Main article: Gravitation

Gravitation is by far the weakest interaction, but at long distances is the most important force. There are two reasons why gravity's strength relative to other forces become important at long distances. The first is that gravity has an infinite range like electromagnetism. The second reason why gravity is important at long distances is because all masses are positive and therefore gravity's interaction can not be screened like in electromagnetism. Thus large bodies such as planets, stars and galaxies dominantly feel gravitational forces. In comparison, the total electric charge of these bodies is zero because half of all charges are negative. In addition, unlike the other interactions, gravity acts universally on all matter. There are no objects that lack a gravitational "charge".

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, as well as more elementary astronomical phenomena like the orbits of planets, and everyday experience: objects fall; heavy objects act as if they were glued to the ground; people are limited in how high they can jump.

Gravitation was the first kind of interaction which was described by a mathematical theory. 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 — if friction due to air resistance is neglected, all objects accelerate toward the ground at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation of the general behaviour of gravity. In 1915, Albert Einstein completed the General Theory of Relativity, a more accurate description of gravity in terms of the geometry of space-time.

An area of active research today involves merging the theories of general relativity and quantum mechanics into a more general theory of quantum gravity. It is widely believed that in a theory of quantum gravity, gravity would be mediated by a massless spin 2 particle which is known as the graviton. Gravitons are hypothetical particles not yet observed.

Although general relativity appears to present an accurate theory of gravity in the non-quantum mechanical limit, there are a number of alternate theories of gravity. Those under any serious consideration 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.

[edit] Electromagnetism

Main article: Electromagnetism

Electromagnetism is the force that acts between electrically charged particles. This phenomenon includes the electrostatic force, acting between charges at rest, and the combined effect of electric and magnetic forces acting between charges moving relative to each other.

Electromagnetism is also an infinite-ranged force, but it is much stronger than gravity, and therefore describes almost all phenomena of our everyday experience, ranging from lasers and radios to the structure of atoms and metals to phenomena such as friction and rainbows.

Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that scientists discovered that electricity and magnetism are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified the unified phenomenon. In 1905, Einstein's theory of special relativity resolved the issue of the constancy of the speed of light, and explained the photoelectric effect by theorizing that light was transmitted in quanta, which we now call photons. Starting around 1927, Paul Dirac unified quantum mechanics with the relativistic theory of electromagnetism; the theory of quantum electrodynamics was completed in the 1940s.

[edit] Weak interaction

Main article: Weak interaction

The weak interaction or weak nuclear force is responsible for some phenomena at the scale of the atomic nucleus, such as beta decay. Electromagnetism and the weak force are theoretically understood to be two aspects of a unified electroweak interaction — this realization was the first step toward the unified theory known as the Standard Model. In electroweak theory, the carriers of the weak force are massive gauge bosons called the W and Z bosons. The weak interaction is the only known interaction in which parity is not conserved; it is left-right asymmetric. It even breaks CP symmetry. However, it does conserve CPT.

[edit] Strong interaction

Main article: Strong interaction

The strong interaction is the most complicated force because it takes on several different behaviors depending on the distance that is being tested. At distances larger than 10 femtometers, the strong force is incredibly weakly interacting, which is why it wasn't hypothesized to exist until the beginning of the 20th century. When protons and neutrons were discovered to be the constituents of the nucleus, it was necessary to postulate that there was an additional force that was stronger than electricity and magnetism so that the protons would be bound together in a 10-15 fraction of the volume of an atom. Hideki Yukawa postulated the existence of a particle with a mass of 100 MeV to explain this force. The pion was discovered in 1947 and ushered in the era of nuclear physics. An extremely complicated theory of the strongly interacting particles, known as hadrons, was developed. Hundreds of hadrons were discovered from the 1940s to 1960s.

In 1973 David Gross, Frank Wilczek, and David Politzer proposed asymptotic freedom as the theory of the strong force and put forth quantum chromodynamics or QCD, as a force mediated by gluons that act between particles that carry "color charge", quarks and gluons. A characteristic of the strong interaction is that gluons interact with each other.

[edit] Current developments

The Standard Model is a theory of three fundamental forces — electromagnetism, weak interactions and strong interactions; however, these three forces are not tied together. Howard Georgi and Sheldon Glashow discovered that the Standard Model particles can arise from a single interaction, known as a grand unified theory. Grand unified theories predict relationships between otherwise unrelated constants of nature in the Standard Model. Gauge coupling unification is the prediction from grand unified theories for the relative strengths of the electromagnetic, weak and strong forces and this prediction was verified at LEP in 1991 for supersymmetric theories.

Currently, there is no complete theory of quantum gravity. The leading candidate for a framework to fit quantum gravity is string theory.

In theories Beyond the Standard Model, there are frequently fifth forces and the search for these forces is an on-going line of experimental research in physics. In supersymmetric theories, there are particles that only acquire their masses through supersymmetry breaking effects and these particles, known as moduli can mediate new forces. Another possible motivation for new forces is related to the accelerating expansion of the universe. The most concrete examples of new forces from the cosmological expansion result from modifications of General Relativity.

[edit] See also

* Standard Model
o Strong interaction
o Electroweak interaction
o Weak interaction

* Gravity
o Quantum gravity
o String Theory
o Theory of Everything

* Grand Unified Theory
o Gauge coupling unification
o Unified Field Theory

* Quintessence the proposed fifth (or fourth if the 'electroweak' unification allowed) force.

* People: Isaac Newton, James Clerk Maxwell, Albert Einstein, Sheldon Glashow, Abdus Salam, Steven Weinberg, Gerardus 't Hooft, David Gross, Edward Witten, Howard Georgi

[edit] Notes

1. ^ Approximate. The exact strengths depend on the particles and energies involved.

[edit] 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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v • d • e
The Four Fundamental Interactions of Physics

Strong interaction · Electromagnetism · Weak interaction · Gravitation
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Categories: Cleanup from December 2006 | All pages needing cleanup | Interaction | Force
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