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 strong interaction is very strong, but very short-ranged. It acts
only over ranges of order 10-13 centimeters and is responsible for holding
the nuclei of atoms together. It is basically attractive, but can be effectively
repulsive in some circumstances.
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.
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.
The gravitational force is weak, but very long ranged. Furthermore, it is
always attractive, and acts between any two pieces of matter in the Universe since
mass is its source.
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
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
|All 4 forces unified
||Gravity, Strong, Electromagnetic,
|Gravity separates (Planck
|Strong force separates (GUTs
|Split of weak and electromagnetic
*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.