HS-102 Readings




There have been basically three related areas of scientific progress which have established the foundations for the release of nuclear energy by mankind. These are as follows:

1. A theoretical breakthrough involving a mathematical explanation of the idea that matter and energy are interchangeable.

2. A physical breakthrough involving observations concerning the behavior of different materials, which made it necessary to develop different theories about the fundamental composition of matter.

3. Observations about the behavior of energy which led to new theories about the transmission of energy.

Over 2,000 years ago, philosophers in Ancient Greece speculated about the nature of matter and energy. Anaxagoras explained the universe as consisting of four "seeds"; fire, air, water, and earth.(1) Democritus believed that all material things were made up of atoms, infinitely tiny building blocks of the universe, which were indestructible and which, in various combinations took the diverse forms observable in nature. These, and other ideas of the Greeks revealed a curious and inquiring mind, however, their explanations could not be supported scientifically because they lacked the the tools and equipment needed for measurement.

Their writings, re-discovered centuries later, would help to stimulate more continuous and more precise work in Europe. The observations and experiments of several people working in the fields of astronomy, mathematics, physics, and chemistry would result by the end of the 18th century, in a comprehensive understanding of the physical world. The Newtonian world view, named after the English physicist and mathematician, Sir Isaac Newton, provided a detailed, orderly explanation of the physical laws governing nature as they could be observed and supported by the experiments and mathematics of that time. This explanation was satisfactory to the needs of the technology and practical engineering of the 19th century.


The Newtonian view prevailed until Albert Einstein, as a part of his general theories of relativity (1905-16), demonstrated that matter and energy were interchangeable, and that the transformation followed a precise formula. E = mC2, where E was Energy, m was the amount of matter or mass, and C was the speed of light, is Einstein's famous formula. Given the fact that C is a very large number (186,000 miles/second), and that number is squared, in order for the two sides of the equation to be equal, m must be very small or E must be very large. In other words, a small amount of mass converted to a very large amount of energy.

Einstein's theories were not immediately accepted, and Einstein himself did not see any practical application since the difference between his and the Newtonian concepts only became obvious under extraordinary conditions not affecting ordinary life. There were, however, a number of experiments, and new discoveries in physics and chemistry which had cast doubt upon some of the 19th century theories about the nature of matter.


The most widely accepted idea of the nature of matter held by scientists in the 19th century was an atomic theory, which had been re-stated by the Englishman, John Dalton, in 1803. Dalton likened the atom to a billiard ball; a hard, indestructible ball of matter, out of which all material things were made. Some atoms were different from others in weight and other physical characteristics, thus explaining some of the different elements found in nature.

Some materials, however, were made up of combinations of atoms which we refer to as molecules. Robert Boyle had formulated this concept as early as 1661. Molecules could be broken into their atomic parts in a process referred to as chemical reactions. The true nature of combustion (fire) had been discovered by the French scientist Lavoisier in 1789. It was learned that water consisted of two parts of hydrogen and one part of oxygen. Physicists and chemists were involved in a gradual process of discovery of new elements, and the chemical combinations of many molecular materials.

The work of the Russian scientist Dmitri Mendeleev was particularly important because he was able, in 1869, to formulate a law based upon atomic weights, by which all of the known elements could be catalogued in a periodic table. The value of this table was proven by the fact that it enabled him to accurately predict the existence of other elements not yet discovered. The atomic theory gained additional support as it proved to be useful in understanding and explaining the results of new discoveries and experiments.

There were, however, a number of new discoveries around the turn of the century, which could not be explained in terms of Dalton's billiard ball concept of the atom. In 1887, Heinrich Hertz demonstrated the propagation of electro-magnetic (radio) waves which traveled at the speed of light. In 1895, another German scientist, Wilhelm Roentgen discovered a short-wave ray, which was called the X-ray. In 1897, an English physicist, J.J. Thomson, working at the Cavendish Laboratory in Cambridge, England, discovered the electron. There were already, at the time of that discovery, many applications of electricity. Yet Dalton's atomic theory provided no satisfactory explanation of electricity. With Thomson's discovery, electricity could be explained as a transmission of energy from within the atom.

Meanwhile, in 1896, a Frenchman, Antoine Becquerel had discovered that uranium, a very heavy metal, radiated a form of energy which was observable on a photographic plate. Pierre and Marie Curie collaborated with Becquerel, and working with pitchblende, which contained uranium, discovered and isolated radium and polonium, which also radiated energy. The three received Nobel prizes in 1903 for their discovery of radioactivity. It was also in 1903, that Ernest Rutherford, a new Zealand physicist who had worked with Thomson at Cambridge, and Frederick Soddy, an English scientist, collaborated at McGill University in Montreal to produce a new theory about the atom which could explain radioactivity.

Thus it was, exactly one century after the pronouncement of Dalton's atomic theory, and a few years before Einstein developed his famous equation, that a new theory of the structure of the atom was devised. The new theory took into account the spontaneous emission of energy from the atom. No mere billiard ball could do that. It further postulated that the mass of the atom changed when the energy radiated. More precisely, as the atom radiated energy it lost atomic weight. It was said to have disintegrated into another lighter form. Thus, the atom was destructible, and one element could be changed into another.

Rutherford's new concept of the atom was like a miniature solar system. A nucleus of protons, (positively-charged particles) was surrounded by clouds of electrons (negatively-charged) particles which orbitted around the nucleus. Radioactivity was caused by unstable atoms which emitted protons or neutrons, (the neutron, unknown to Rutherford when he formulated his theory, was discovered in 1932 by Chadwick, a scientist working with Rutherford.) The atoms continued to disintegrate until a stable, non-radioactive element was formed. Thus, the atom would be transmuted from one element to another of lesser weight, or more likely, to a lighter version of the same element. That is, the lighter version had less atomic weight, but if it retained all of the other (chemical) characteristics of the original element, it was said to be an isotope.

Atoms, therefore, were not indestructible, and were not the smallest particles of matter. Protons or neutrons radiated from unstable (radioactive) atoms, and when an atom lost a proton or neutron, it lost mass and emitted energy. The amount of energy emitted was determined by Einstein's formula. Since the mass of a proton was very small, the energy released was also small. This kind of nuclear energy (from the nucleus of the atom), known as radioactivity, has also sometimes been called "weak" energy, to differentiate it from the nuclear energy of fission which will be discussed later.

Molecules could now be visualized as combinations of atoms which were linked together by the bonding of the electron clouds surrounding each atom. Since the nuclei of these atoms were unaffected by this bonding, the energy associated with molecular combinations was not nuclear energy, but chemical energy.

People were familiar with chemical energy because it was a daily part of their lives and had innumerable practical applications (electricity, combustion, etc.) Nuclear energy was a mysterious new form of energy which only concerned scientists engaged in pure research. But after Rutherford's new formulation of the nature of the atom, scientific experimentation could be directed along more promising paths.


Ever since Christiaan Huygens, in 1690, had propounded the theory that light traveled in waves, other forms of energy were conceived of in similar fashion. It was a useful concept which fitted many experimental results. Different colors of light (the spectrum) were seen as being different because they had different wave lengths. Non-visual energy transmissions; electromagnetic, X-ray, gamma ray, infra-red (heat), and ultra-violet were also different because their wave lengths were different. However, transmission of energy was also sometimes expressed in terms of particles, as in the flow of electrons (electricity), and this, too, was useful in explaining experimental results.

In the year 1900, the physicist Max Planck postulated that electromagnetic energy is emitted in discrete bundles which he called quanta. This also fit certain experimental results, and would lead to the development of quantum mechanics. There seemed to be a contradiction between wave and quantum theory. In 1913, the Danish physicist, Neils Bohr, using Rutherford's planetary concept of the atom, explained the emission of radiation which occurred when an electron shifted its orbit closer to the nucleus in what he referred to as a quantum jump. Both the wave and particle theories continued to be used as visualizations of what was occurring inside the atom.

All of these new ideas concerning energy, mass, and the atom had completely overturned the world of physics. At the turn of the century, scientists were on the brink of startling new discoveries and understandings of the physical world, which even they could not comprehend. Physicists were making observations which could be only explained mathematically, or in terms of a world so infinitesimally small (the inner structure of the atom), that it could not be seen, but only visualized in ways that fit the experimental results. It was clear that there was no longer a distinct separation between matter and energy. They were apparently two different forms of the same thing.


HS-12 Readings