Fundamental force

Force name	Relative Magnitude	Behavior
Strong nuclear force	10⁴⁰	1/r⁷
Electromagnetic force	10³⁸	1/r²
Weak nuclear force	10¹⁵	1/r⁵ to 1/r⁷
Gravity	10⁰	1/r²

Currently there are believed to be four fundamental forces in Nature that are responsible for all known interactions in the Universe. These are gravity, electromagnetism, the weak nuclear force and the strong nuclear force. Every observed physical phenomenon, from galaxies colliding with each other, to spacecraft orbiting the Earth, to protons jiggling around inside the nucleus of an atom, to quarks jiggling around inside a proton, can be ultimately explained in terms of one or more of these fundamental forces. Understandably therefore, the understanding of these fundamental forces has occupied the attention of physicists for over half a century, and continues to do so.

Table of contents

1 The forces

1.1 Gravity
1.2 Electromagnetism
1.3 Weak nuclear force
1.4 Strong nuclear force

2 Current status

3 Further reading

4 Related articles

The forces

Gravity

Gravity is by far the weakest force, but it is the force that has the large range. The term "long range" refers technically to the falling off of a force with distance r at a rate equal to 1/r². Unlike the other forces, gravity works universally on all matter and energy, and is (so far as we know) universally attractive. Because of its long range, and property of attraction between objects depending only on their mass, and independent of their charge etc., most interactions between objects separated by length scales larger than that of a planet, for example, are predominantly due to gravity.

Gravitation was the first kind of interaction which was explained by a mathematical theory. Isaac Newton's law of Universal Gravitation was a good approximation of the behaviour of the gravitational force. In 1916, Albert Einstein published the General Theory of Relativity, a more accurate description of gravity in terms of the geometry of space-time. According to General Relativity, any matter or energy anywhere and at any time in the universe attracts all other matter and energy in the universe, as long as it is inside its light cone. An active area of 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.

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 phenomena closer to everyday experience such as the orbits of planets and falling apples.

Other, experimentally unverified, theories of gravity that attempt to go beyond General Relativity include theories such as negative gravity and quintessence.

Main article : Gravity

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.

Electromagnetic phenomena are described at the classical level by Maxwell's equations, known since the latter half of the 19th century. The quantum theory of electromagnetism is known as quantum electrodynamics (QED). In QED, charged particles are understood as exerting forces on each other due to the exchange of photons.

Main article: Electromagnetism

Weak nuclear force

The weak nuclear force is responsible for some phenomena at the scale of the atomic nucleus, such as beta decay. Electromagnetism and the weak force were theoretically understood to be two aspects of a unified electroweak force - this 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 force is an example of a physical theory in which parity is not conserved i.e., which is left-right asymmetric.

Main article: Weak interaction

Strong nuclear force

The strong nuclear force is the force holding together nucleons inside the atomic nucleus. The strong force is independent of electric charge, and holds together, for example, two protons inside the minute volume of a Helium nucleus in spite of their tremendous electromagnetic repulsion.

The quantum theory of the strong force is called quantum chromodynamics or QCD. In QCD, the strong force is carried by particles called gluons and it acts between particles that carry a "color charge", i.e. quarks and gluons. Composite particles such as nucleons or mesons are made up out of quarks.

Main article: Strong interaction

Current status

The Standard Model is a unified quantum mechanical theory of three of the four fundamental forces - electromagnetism, weak interactions and strong interactions. Currently, there is no accepted candidate for a theory of quantum gravity, mostly due to the experimental inaccessibility of quantum gravitational phenomena. The search for an acceptable theory of quantum gravity, and a quantum mechanical grand unified theory of all the fundamental forces, are important areas of current physics research.

It is currently believed that all interactions can be explained in terms of only these four forces. However, 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. As of 2004, all of the experiments which seem to indicate a fifth force have been explainable in terms of experimental errors.