The Fundamental
Forces of Nature

There are 4 fundamental forces that have been identified. In our present Universe they have rather different properties.

Properties of the Fundamental Forces

The Tortoise and the Hare: Gravity Always Wins

The four fundamental forces all play central roles in making the Universe what it is today, but with respect to the large-scale issues that are of interest to cosmology it is gravitation that is most important. This is because of two of its basic properties that set it apart from the other forces: (1) it is long-ranged and thus can act over cosmological distances, and (2) it always supplies an attractive force between any two pieces of matter in the Universe.

Thus, although gravitation is extremely weak, it always wins over cosmological distances and therefore is the most important force for the understanding of the large scale structure and evolution of the Universe.

Unification of the Forces of Nature

Although the above discussion indicates that the fundamental forces in our present Universe are distinct and have very different characteristics, the current thinking in theoretical physics is that this was not always so. 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 separating to leave us with the 4 distinct forces that we see in our present Universe. The process of the forces separating from each other is called spontaneous symmetry breaking.

There is further speculation, which is even less firm than that above, that at even higher temperatures (the Planck Scale) all four forces were unified into a single force. Then, as the temperature dropped, gravitation separated first and then the other 3 forces separated as described above. The time and temperature scales for this proposed sequential loss of unification are illustrated in the following table.

Loss of Unity in the Forces of Nature
Characterization Forces Unified Time Since Beginning Temperature (GeV)*
All 4 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

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, or sometimes just the Standard Model.

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.

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