According to the standard model of elementary particle physics, all forces acting in nature can be traced back to four fundamental interactions. These are gravitation, the weak, the strong, and the electromagnetic force. Each one of these four forces has an individual coupling strength and distance dependence. Furthermore, they differ with respect to the properties of the elementary particles they act on. It was among the very first acts of nature to set up the fundamental forces. Within only 10-12 seconds after the beginning of space and time presumably unified one single force was successively split up into the four forces that are known today.
In the modern quantum field theory frame, the fundamental forces are described by the exchange of mediatorial particles, so called gauge bosons. The electromagnetic repulsion of two identically charged particles, for example, is mediated by interchanged photons in this theoretical framework.
The electromagnetic force acts between particles that carry electric charge. There are two types of electric charge: “positive” and “negative.” Charges of identical signature repel one another, while those of different signature attract. The electromagnetic force is responsible for the structure of matter above the molecular level and dominates the physics of our macroscopic environment. It determines chemistry, that is, the physics of the electron sheath and its interactions with atoms and molecules. It causes the impenetrability of solid bodies and allows for action potentials that initiate and regulate muscular motion in the human body. The electromagnetic force between two charges declines with the square of their distance (1/r2).
The strong interaction or force is acting in atomic nuclei consisting of more than one proton (which carries a positive electrical charge); the chargeless neutrons have to be stabilized against electromagnetic repulsion. This is the main task of the strong interaction, which acts around 100 times stronger than the electromagnetic force at very short distances. The strong force is also responsible for the inner coherence of the nucleons (protons and neutrons, the particles constituting atomic nuclei) as well as all particles that are composed of quarks. The strong force has an extremely short range (10-15 m) and is mediated by so-called gluons.
The weak interaction or force, like the strong force, has a very short special range of influence (10-18 m). It plays an essential role in radioactive decay and the complex fusion processes acting as energy sources within the stars. During the first minutes of the universe it was a major regulating factor in establishing the number density of neutrons, which again determines the cosmic abundance of hydrogen and helium. The weak force has a relative strength of 10-13 compared with the strong force. The mediator particles are the very massive bosons W+, W-, and Z0, first detected in 1983 at one of CERN’s (European Organization for Nuclear Research’s) particle accelerators.
Finally, gravitation, with a relative strength of 10-40, is by far the weakest of the four fundamental interactions. It is nevertheless responsible for the orbits of the moon and the earth, the dynamics of the solar system, and the large-scale structure of the universe. This apparent contradiction—the weakest force dominating on the largest scales—can be explained by two facts: the first is that gravitation, like the electromagnetic force, has an infinite spatial range. Second, gravitational forces cannot be cancelled by an opposing force. Thus the effect of gravity can in fact spread over very large distances. Today it is the only fundamental interaction that does not have a theoretical description in quantum mechanics. Its mediator particle, the graviton, is therefore still hypothetical. According to Einstein’s general theory of relativity, gravitation is understood as intrinsic to the curvature of spacetime.
Immediately after the big bang, at the beginning of time when the universe had a temperature of 1032 Kelvin, the fundamental forces are believed to have been perfectly unified into one force called quantum gravity. During only the first 10-12 seconds after the big bang, the individual interactions— first gravity, followed by the strong force and then the electromagnetic force—separated from a decreasing level of unification and then finally the weak interaction broke away, resulting in the four forces of nature known today.
Helmut Hetznecker See also Big Bang Theory; Cosmogony; Cosmology, Inflationary; Einstein, Albert; Hawking, Stephen; Light, Speed of; Quantum Mechanics; Relativity, General Theory of; Universe, Origin of
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