Scientists suggest that the four forces appear to be different because we are observing them at a time when the universe has been around for a long time and is at a relatively low temperature. The situation is somewhat analogous to freezing water. When water freezes it can adopt many apparently different forms--powdered white snow, solid ice blocks, delicate hard frost on tree branches, or a slippery layer on the sidewalk. You might interpret these forms of frozen H20 as very different things, and in some respects they are distinct, but heat them up and they are all simple water.
Similarly, the four forces look different at the relatively low temperatures of our present existence, but heat matter up to trillions of degrees and the different forces are not really different at all. Theories in which fundamental forces are seen as different aspects of one force are called unified field theories.
The first unified field theory in history was Isaac Newton's synthesis of earthly gravity and the circular motion observed in the heavens. To medieval scientists, earthly and heavenly motions seemed as different as the strong and electromagnetic forces do to us. nevertheless, they were unified in Newton's theory of universal gravitation. In the same way, scientists today are working to unify the four fundamental forces.
The general idea of these theories is that the temperature can be raised high enough---that is, enough energy can be pumped into an elementary particle---the underlying unity of these forces will become clear. At a few laboratories around the world, it is possible to take protons and antiprotons (or electrons and positrons), accelerate them to extremely high energies, and let them collide. (As we have noted, protons-antiproton collisions involve the process of annihilation between particle and antiparticle.) When these collisions occur, for a brief moment the temperature in the volume of space about the size of a proton is raised to a temperatures that have not been seen in the universe since it was less than a second old. In the resulting maelstrom, particles are produced that can be accounted for only if the electromagnetic and weak forces become unified.
In 1983, experiments at the European Center for Nuclear Research and the Stanford Linear Accelerator Center demonstrated that this kind of unification does occur. When the protons and antiprotons (at the former laboratory) or electrons and positrons (at the latter) were accelerated and allowed to collide head-on, W and Z particles were seen in the debris of the collisions. Not only were the reactions seen, but the properties of the resulting particles and their rate of productions were exactly those predicted by the first unified field theory.
The expectations of today's physicists regarding the unification of forces can be illustrated in a simple flow diagram. Scientists have already seen the unification of the electromagnetic and weak force in their laboratories. The resulting force, which physicists call theElectroweak, will be studied in great detail by the next generation of particle accelerators. At much higher energies, we expect the strong forces to unify with the Electroweak. The theories that make this prediction constitute the "standard model" the best model we have today of the elementary particles. Physicists have accumulated a fair amount of experimental evidence supporting the standard model.
Finally, at still higher energies, we hope to see gravity unify with the strong-electroweak force. No theory yet describes this unification successfully, and attempts to develop a successful model are very much a frontier issue.
The unification of the four forces has no obvious effect on our everyday world, because it requires such exotic-high energy conditions to see it happen. Nevertheless, the unification of forces played a critial role in the formation of the universe