Fundamental interaction

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A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. Every observed physical phenomenon, from galaxies colliding with each other to quarks jiggling around inside a proton, can thus be explained by these interactions. Because of their fundamental importance, understanding of these interactions has occupied the attention of physicists for over half a century and continues to do so.

Traditionally, modern physicists have counted four fundamental interactions: gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behavior vary greatly, as can be seen in the table below. Yet, it is strongly believed that three of these interactions are manifestations of a single, more fundamental, interaction, just as electricity and magnetism are now understood as two aspects of the electromagnetic interaction. Electromagnetism and the weak nuclear forces have been shown to be two aspects of a single electroweak interaction at low energy limit. Grand unified theories seek to unify the electroweak force and the strong nuclear interaction, but none have passed experimental muster as of 2006. The topic of unifying gravitation with the other three into an interaction that is completely universal is called quantum gravity.

Interaction	Current Theory	Mediators	Relative Strength¹	Long-Distance Behavior
Strong	Quantum chromodynamics (QCD)	gluons	10³⁸	$1$ (see discussion below)
Electromagnetic	Quantum electrodynamics (QED)	photons	10³⁶	$\frac{1}{r^2}$
Weak	Electroweak Theory (G W S theory)	W and Z bosons	10²⁵	$\frac{e^{-m_{W,Z}r}}{r}$
Gravity	General Relativity (GR, not a quantum theory.)	gravitons	1	$\frac{1}{r^2}$

These interactions are sometimes called fundamental forces, although many find this terminology misleading because they cannot be described by classical potentials and forces in the Newtonian sense. For example, according to general relativity, no gravitational force is acting at a distance to cause a body to accelerate. Instead, a body is moving in inertial motion along a straight line in the curved spacetime (such line in curved spacetime is called geodesics). In addition, the weak interaction need not even result in the same outgoing particles as those that entered the interaction.

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 interaction mediators. Thus, for example, photons are the mediators of the interaction of electric charges; and gluons are the mediators of the interaction of color charges. This coupling of matter (charged fermions) with force mediating particles (gauge bosons) is the result of fundamental symmetries of nature. Mathematically, the coupling is captured by Noether's theorem.

1 The interactions
2 Current developments
3 Notes
4 References
5 See also

[edit] The interactions

[edit] Gravitation

Main article: Gravitation

Gravitation is by far the weakest interaction. However, because it has an infinite range and because all masses are positive, it is nevertheless very important in the universe. Because all masses are positive, large bodies such as planets, stars and galaxies have large total masses and therefore exert large 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 works universally on all matter and energy. 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 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 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 a long-ranged force that is relatively strong, and therefore describes almost all phenomena of our everyday experience—phenomena ranging all the way from lasers and radios to the structure of atoms and the structure of metals to friction and rainbows.

Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that it was discovered that these are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified the unified phenomenon. In 1905, special relativity resolved the issue of the constancy of the speed of light, and Einstein 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 special relativity; quantum electrodynamics was completed in the 1940s.

Theodor Kaluza in 1919 noticed a curious property of electromagnetism, namely that Maxwell's classical (non-quantum) theory of electromagnetism arises naturally from the equations of general relativity with the assumption that there is an extra fourth dimension of space. This property is the basis of Kaluza-Klein theories which have been used to formulate a theory of quantum gravity.

[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

In quantum chromodynamics or QCD, the strong interaction is carried by particles called gluons and acts between particles that carry "color charge", quarks and gluons. A unique characteristic of the strong interaction is the fact that these gluons interact with each other. This causes the strong interaction's strength to be independent of distance. This can be interpreted to mean that the force has an infinite range. However, in actuality, since the energy stored in the field increases with separation between the interacting particles, at large distances the field contains enough energy to produce particle-antiparticle pairs. When this occurs, the field lines are cut in half. By this mechanism, strong forces never act over distances much larger than the proton's radius.

Hadrons are held together by the strong force. These include familiar particles such as protons and neutrons as well as many other baryons and mesons.

Nucleons are held to each other in the atomic nucleus by the nuclear force, which is a residual effect of the strong interaction. This force is unrelated to electric charge. Because the strong force is so much stronger than the electromagnetic force, it can easily hold many protons together in the nucleus despite their tremendous electric repulsion. This nuclear force does not have constant strength for different particle separations, but rather goes as 1/r⁷^{[citation needed]} with an effective range of 1.4 x 10⁻¹⁵ m.

[edit] Current developments

The Standard Model is a unified quantum mechanical theory of three fundamental forces—electromagnetism, weak interactions and strong interactions. Currently, there is no accepted candidate for a theory of quantum gravity. The search for an acceptable theory of quantum gravity, and a quantum mechanical grand unified theory, are important areas of current physics research. Until such a search is successful, the gravitational interaction cannot be considered as a force because it is of a geometrical rather than dynamical nature. Particles are thought to be moving as they do because the curvature of spacetime directs their movement, and not because they are pushed or pulled by forces resulting from the exchange of gravitons.

One important aspect of Quantum Mechanics, however, is that it allows for different ways of looking at things, such as gravity. One way of looking at it is as a force field, another way of looking at it is as curvature of spacetime and a last way of looking at it is as the exchange of gravitons. The equations can be rearranged to represent all three different points of view.

An exotic fifth force has been proposed by some physicists from time to time, mostly to explain discrepancies between predicted and measured values of the gravitational constant.

[edit] Notes

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