Unification of the Fundamental Forces

There are four fundamental forces that have been identified. In our present Universe they have rather different properties, as summarized below.

The Fundamental Forces of Nature

1. The strong interaction is very strong but only of range 10-13 centimeters. It is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.
2. The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.
3. The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.
4. The gravitational force is the weakest of all, but very long ranged. Furthermore, it is always attractive (except in the presence of dark energy, when it can become repulsive), and acts between any two pieces of matter in the Universe since mass is its source. As noted in the right panel, these features make gravity the most important force determining structure over large distances.

Although the above list indicates that the fundamental forces in our present Universe are distinct and have very different characteristics, we think that this was not always so.

Evidence for Unification

Grand unified and superunified theories remain theoretical speculations that are as yet unproven, but there is strong experimental evidence for the unification of the electromagnetic and weak interactions in the Standard Electroweak Theory. Furthermore, although GUTs are not proven experimentally, there is strong circumstantial evidence to suggest that a theory at least like a grand unified theory is required to make sense of the Universe.

Unified Theories
There is a rather strong belief (although it is yet to be confirmed experimentally) that in the very early Universe when temperatures were very high compared with today, the weak, electromagnetic, and strong forces were unified into a single force. Only when the temperature dropped did these forces separate from each other, with the strong force separating first and then at a still lower temperature the electromagnetic and weak forces going their different ways to leave us with the four distinct forces that we see in our present Universe. The process of the forces separating from each other is called spontaneous symmetry breaking.

The Planck Scale
There is less firm speculation that at even higher temperatures (the Planck scale, to be discussed shortly) all four forces were unified into a single force. As the temperature dropped, gravitation separated first and then the other three forces separated as described above. The unification of forces is illustrated in the adjacent left figure and the time and temperature scales for the sequential loss of unification as temperature is lowered are illustrated in the table below.

Grand Unified and Superunified Theories
Theories that postulate the unification of the strong, weak, and electromagnetic forces are called grand unified theories (often known by the acronym GUTs). Theories that add gravity to the mix and try to unify all four fundamental forces into a single force are called superunified theories. The theory that describes the unified electromagnetic and weak interactions is called the Standard Electroweak Theory.

Loss of Unity in the Forces of Nature
Characterization Forces Unified Time Since Beginning Temperature (GeV)*
All four forces unified Gravity, Strong, Electromagnetic, Weak ~ 0 ~ infinite
Gravity separates (Planck Scale) Strong, Electromagnetic, Weak 10-43 s 1019
Strong force separates (GUTs Scale) Electromagnetic, Weak 10-35 s 1014
Split of weak and electromagnetic forces None 10-11 s 100
Present Universe None 1010 y 10-12
*Temperature Conversion: 1 GeV = 1.2 x 1013 K