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THE BEGINNING OF THE END ?
Nowadays nobody should be surprised to learn that all matter we see in the universe is made of atoms. In fact, this is known since 1808 when the English chemist, Dalton, proposed the modern atomic theory of matter. He believed that atoms were indivisible and he called them atoms from a Greek word meaning indivisible. In this way he honoured the Greek philosopher Democritus who was the first in proposing the atomic theory over 2500 years ago. Of course, Democritus had no experimental evidence for his theory, most other philosophers of the time rejected it, and his idea remained useless for 2000 years.

Since the beginning of the present century the atomic physics showed us that, as a matter of fact, atoms are not indivisible. By the 1930s their structure was finally determined. Every atom consists of a small central nucleus composed of particles called protons and neutrons with other particles called electrons moving around this nucleus. Once the question, what are things made of ?, was answered the job of physicists was apparently finished. However, the new discoveries were in fact the beginning of a new branch of physics known as particle physics. Many work has been carried out to determine whether protons, neutrons and electrons are the fundamental building blocks of matter or whether there are other particles that make up them. The result of this research is that protons and neutrons are not fundamental, they are built from other particles called quarks. Therefore, current ideas on the physics of the subatomic world state that quarks and electrons are the elementary particles that make up atoms. They are elementary in the sense that they are so small that nothing smaller is conceivable.

The task of the particle physics consists not only of discovering and classifying the elementary particles but also to formulate the laws that hold them together building up everything: atoms, molecules, ordinary matter like this piece of paper, living things like us, stars, galaxies-the universe itself. Apparently, only four fundamental forces between particles-strong, weak, electromagnetic and gravitational-are enough to account for all structures in the universe. The strong nuclear force holds quarks together, through the exchange of new elementary particles called gluons, making up protons and neutrons. It is also responsible for holding protons and neutrons together inside the nucleus. The electromagnetic force holds the nucleus and the electrons together, through the exchange of photons, making up atoms. The weak nuclear force produces the so called beta decays of the nucleus-most radioactive substances used in medicine and technology are beta radioactive-through the exchange of the intermediate bosons W and Z. Another new elementary particle called neutrino also participates in the weak interaction. Finally, gravity holds, for example, the planets to their orbits around the sun, possibly through the exchange of a postulate elementary particle called graviton.

In a sense, particle physics began with the speculations of Democritus but, fortunately, 2500 years later these new theories about the laws of nature can be tested. In the 1960s huge machines were built to accelerate particles up to nearly the velocity of light (300000 kilometers per second) in order to study their properties. Today, the largest particle accelerator in the world, built by several European countries, is located in Geneve, Switzerland. A tunnel 27 kilometers in circumference contains an accelerator in which particles move in opposite directions and collide at four points where detectors monitor their interactions. On the other hand, the energy produced in these collisions is enormous, the same that prevailed when our expanding universe was a tenth of a nanosecond ($10^{-10}$ seconds) old. These experiments as well as other theoretical discoveries are giving a new understanding of the origin and evolution of the universe.

At this stage the reader should agree that the unifying power of physics is impressive. All phenomena observed in the universe can be explained with a small bunch of elementary particles and just four fundamental forces-the messengers of the forces being themselves elementary particles. Is this the end of the adventure ?. Much progress has been made but the task is not finished, many important details remain unsolved. But the main problem are the physicists themselves. They are too ambitious. So ambitious that their task will only be finished when the complete unification of physics is achieved. The theory succeed in doing it should unify all forces and particles of nature in a common description. That the task is not simple can easily be understood just mentioning that one of the greatest scientist of the history, Einstein, spent the last 30 years of his life in search of a unified theory of gravity and electromagnetism unsuccessfully. Since then, other two fundamental forces-strong and weak nuclear-were discovered, the electromagnetic force has already been shown to be closely related with them, and several interesting theories have been proposed to unify all of them including gravity. This is the case of the so called grand unified theory, supersymmetric theory and supergravity theory. However, none of them can achieve the goal completely.

In the last years superstring theory-which in a sense contains the other theories-has emerged as the best candidate to achieve the goal of unification. In this theory there is only one fundamental object, the string, and the world is made out of nothing but tiny strings. Crudely speaking, in the same way that a violin string unifies the musical tones a fundamental string unifies the various forces and particles of nature. The musical notes are not fundamental, they are originated from the violin string. The forces and particles are not fundamental, they are simply the different modes of vibration of a fundamental string. The beauty and elegance of the superstring theory is remarkable but, unfortunately, the essence of the theory is not yet well understood. Many years of research will still be needed before stablishing whether of not it is the final theory, the theory of everything.
 
 


Carlos Munoz 2000-11-18