Fourteen Billion Years of Cosmic Evolution (eBook)

Chapter 1:

Fundamental Elements

It is a basic assumption of science that the universe in which we live is constructed according to simple principles. Though the physical phenomena that scientists observe are often complex, they invariably assume that the basic laws of nature are not. Nevertheless, it is not immediately obvious that nature is as simple as scientists like to think.

The idea of simplicity is not something that can be proved or disproved, and no one has ever devised an experiment that would tell us whether nature is fundamentally simple or complex. The reason scientists have accepted the postulate of simplicity is that it has helped physics to advance in their work.

Simplicity

By making the assumption that the underlying principles of nature are simple, scientists have been able to gain significant insights into such things as the origin of the universe, the nature of the forces that act on objects as small as electrons or as large as galaxies, and the nature of matter. In other words, it is possible to justify the postulate of simplicity on practical grounds.

It has also prompted scientists to become skeptical of theories that seemed to be too contrived and complicated to be correct, and this has frequently advanced scientific understanding. It is not hard to find examples of this. It was apparent to Galileo that the Ptolemaic system of astronomy, according to which the Sun and the planets followed elaborate orbits around the Earth, was too complicated to be true.

Consequently, Galileo championed the simpler Copernican model, which put the Sun and not the Earth at the center of the solar system. There are other examples of the rejection of complicated ideas in favor of ones that seemed simpler when looking at the attempt of scientists to understand the nature of matter.

Time and time again, they have tried to understand matter in terms of a small number of constituents. Then, as further discoveries were made, these constituents would become more numerous. Finally, it would reach the point where the feeling would become widespread that things were too complicated, and a simpler theory would be developed. In the time of the classical Greeks, it seemed that matter was not so complex a thing.

According to Aristotle, for example, all terrestrial objects were made up of only four elements: earth, air, fire, and water. By the middle of the seventeenth century, however, it had become apparent that this simple scheme was not workable. The number of basic substances that could be found on the surface of the Earth was much greater than four. If one continued to define an element as something that could not be broken into simpler components, then the elements were numerous indeed.

The Chemical Elements

By the end of the nineteenth century, scientists had discovered all of the ninety-two naturally occurring elements. The majority were solids, such as iron, nickel, sulphur and carbon. Some were gasses, such as hydrogen, oxygen and nitrogen. Finally, two were liquids at ordinary conditions of temperature and pressure: mercury and bromine. Though the discovery of the various chemical elements was a scientific advance, many scientists thought that ninety-two basic elements made the world seem unnecessarily complicated.

Fortunately, matters became much more simple when important new discoveries were made by the British physicists J. J. Thomson, Ernest Rutherford, and James Chadwick. Thomson’s discovery of the electron in 1897 was followed by Rutherford’s discovery of the proton in 1919. When Chadwick discovered the neutron in 1932, it appeared that science’s understanding of the nature of matter was complete. Atoms consisted of tiny nuclei that were surrounded by orbiting electrons.

The nuclei, in turn, were composed of protons and neutrons. So the ninety-two elements were not the basic constituents of matter after all. The elements seemed to be derived from only three fundamental particles: protons, neutrons, and electrons. Hydrogen, for example, was made of one proton and one electron, and was the simplest of the elements. Oxygen, on the other hand, was more complex. The nucleus had eight protons and eight neutrons, and eight electrons circled around it.

An atom of uranium was even more complex. Its nucleus contained 92 protons and 146 neutrons. Since the positively charged protons and the negatively charged electrons had to be equal in number if the atom was to be electrically neutral, it followed that a uranium atom contained 92 electrons also. Thus there were 330 particles in all. However, every one of them was one of the three basic varieties.

Proliferation of Particles

Almost at once, it became apparent that this simple scheme was inadequate. In 1932, the same year that the neutron was discovered, the American physicist Carl Anderson discovered another subatomic particle, the positron. The positron was similar to the electron, except that it carried a positive, rather than a negative, electric charge. It soon became obvious why positrons had not been discovered before. They do not continue to exist for very long once they encounter ordinary matter.

As soon as a positron encounters an electron, it and the electron annihilate one another, and gamma rays appear in their place. In essence, the positron is an anti-electron, because the positron is the electron’s antiparticle. If the positron had been discovered in modern times, physicists would have named it the anti-electron, for the positron is the electron’s antiparticle.

Today, the prefix anti is always part of an antiparticle’s name. The positron is the only exception, since it has had this name for so long a time that there has never been an attempt to change it. Scientists know that, for every particle, there exists an antiparticle. There are protons and antiprotons, neutrons and antineutrons.

E = MC2

Some antiparticles, such as the positron, can continue to exist for long periods of time if they happen to be traveling through space, where the density of matter is low. However, as soon as a particle and its antiparticle meet, they annihilate one another just as the electron and positron do. This process is described by Einstein’s famous equation E = mc2. Here, the E is energy, m is mass, and c is the speed of light. In the metric units used by scientists, mass may be measured in kilograms, while the speed of light is taken to be 300 million meters per second.

In the metric system, energy will be expressed in joules. A joule is defined to be one watt-second. It is equal to one four-thousandth of a food calorie. Although one joule is not a very large quantity, it is obvious that a great deal of energy (E) can be released when matter is annihilated. After all, c2 is the speed of light squared, which is 90 trillion, an enormous number; mc2 is 90 trillion meters squared per second.

Incidentally, if matter can be converted into energy when a particle and its antiparticle encounter one another, one might suspect that the reverse could take place: that matter could be created out of energy. A particle-antiparticle pair can be created in this manner, and the amount of energy required to produce them is equal to the amount that is released when a pair is annihilated. Note that particles and anti-particles are always created in pairs. It is not possible to create an electron, or a positron, or an antineutron alone.

The Muon

In 1936, just four years after the discovery of the positron, Carl Anderson found another subatomic particle, the muon. This particle resembled the electron and possessed the same negative charge, but it was 207 times as heavy. Originally, the new particle was called the mu meson, but it was later reclassified as a muon. Mu is one of the letters of the Greek alphabet.

Meson comes from a Greek word meaning intermediate. This was a reference to the fact that the new particle had a mass much greater than that of the electron, but much less than that of a proton or neutron. Protons and neutrons, by the way, are about equal in mass. They are both approximately 1,800 times as heavy as the electron. By 1936, the number of elementary particles had grown from three to five, to include the electron, proton, neutron, positron, and muon.

The discovery of the positron suggested that other antiparticles might also exist. In addition, there was yet another particle, whose existence was still hypothetical. In the 1930s, the Austrian physicist Wolfgang Pauli had pointed out that certain puzzling features of radioactive decay could be explained if one assumed that there existed a particle called the neutrino.

A Particle Zoo

If the list of fundamental particles had turned out to have no more than a handful of entries, physicists would most likely have been able to consider them all to be elementary. Unfortunately, as the years passed, the number of known particles increased beyond all reason. By 1960, scores of new particles had been discovered. By the early 1970’s, the number of elementary particles that had been seen by experimenters was in the hundreds.

Some of the subatomic particles, collectively known as baryons, seemed to resemble the neutron and the proton, except that they had bigger masses. Some of them also had unusual electrical charges. Where the neutron was electrically neutral and the proton carried a positive charge, some of the baryons had negative charges like the lighter electron, or had twice the positive charge of the...