Quantum Theory and the Structure of Matter

The concept of matter has undergone many changes in the history of human thinking.

The early Greek philosophy from Thales to the Atomists, in seeking the unifying principle in the universal mutability of all things, had formed the concept of cosmic matter, a world substance which experiences all these transformations, from which all individual things arise and into which they become again transformed.

This matter was partly identified with some specific matter like water was air or fire; only partly, because it had no other attribute but to be the material from which all things are made.

Aristotle thought of matter in the relation between form and matter.

All that we perceive in the world of phenomena around us is formed matter. Matter is in itself not a reality but only a possibility, a `potentia’; it exists only by means of form.

In the natural process the ’essence,’ as Aristotle calls it, passes over from mere possibility through form into actuality.

The matter of Aristotle is not:

• a specific matter like water or air
• empty space

It is a kind of indefinite corporeal substratum, embodying the possibility of passing over into actuality by means of the form.

The typical examples of this relation between matter and form in the philosophy of Aristotle are the biological processes in which matter is formed to become the living organism, and the building and forming activity of man.

The statue is potentially in the marble before it is cut out by the sculptor.

Then, much later, starting from the philosophy of Descartes, matter was primarily thought of as opposed to mind.

There were the 2 complementary aspects of the world:

1. Matter
2. Mind

As Descartes put it, the res extensa' and the res cogitans.'

Since the new methodical principles of natural science, especially of mechanics, excluded all tracing of corporeal phenomena back to spiritual forces, matter could be considered as a reality of its own independent of the mind and of any supernatural powers.

The matter' of this period is formed matter,’ the process of formation being interpreted as a causal chain of mechanical interactions; it has lost its connection with the vegetative soul of Aristotelian philosophy, and therefore the dualism between matter and form is no longer relevant.

It is this concept of matter which constitutes by far the strongest component in our present use of the word `matter.'

The 19th century had another dualism – the dualism between matter and force.

Matter is that on which forces can act; or matter can produce forces.

Matter, for instance, produces the force of gravity, and this force acts on matter.

Matter and force are two distinctly different aspects of the corporeal world. In so far as the forces may be formative forces this distinction comes closer to the Aristotelian distinction of matter and form.

On the other hand, in the most recent development of modern physics this distinction between matter and force is completely lost, since every field of force contains energy and in so far constitutes matter.

To every field of force there belongs a specific kind of elementary particles with essentially the same properties as all other atomic units of matter.

When natural science investigates the problem of matter it can do so only through a study of the forms of matter. The infinite variety and mutability of the forms of matter must be the immediate object of the investigation and the efforts must be directed toward finding some natural laws, some unifying principles that can serve as a guide through this immense field.

Therefore, natural science — and especially physics — has concentrated its interest for a long time analyzing:

• the structure of matter, and
• the forces responsible for this structure

Since Galileo’s time, the fundamental method of natural science had been the experiment.

This method made it possible to pass from general experience to specific experience, to single out characteristic events in nature from which its `laws’ could be studied more directly than from general experience.

If one wanted to study the structure of matter one had to do experiments with matter.

One had to expose matter to extreme conditions in order to study its transmutations there, in the hope of finding the fundamental features of matter which persist under all apparent changes.

In the early days of modern natural science, this was the object of chemistry, and this endeavor led rather early to the concept of the chemical element. A substance that could not be further dissolved or disintegrated by any of the means at the disposal of the chemist — boiling, burning, dissolving, mixing with other substances, etc. — was called an element.

The introduction of this concept was a first and most important step toward an understanding of the structure of matter.

The enormous variety of substances was at least reduced to a comparatively small number of more fundamental substances, the `elements,’ and thereby some order could be established among the various phenomena of chemistry.

The word `atom’ was consequently used to designate the smallest unit of matter belonging to a chemical element, and the smallest particle of a chemical compound could be pictured as a small group of different atoms.

The smallest particle of the element iron, e.g., was an iron atom, and the smallest particle of water, the water molecule, consisted of one oxygen atom and two hydrogen atoms.

The next and almost equally important step was the discovery of the conservation of mass in the chemical process. For instance, when the element carbon is burned into carbon dioxide the mass of the carbon dioxide is equal to the sum of the masses of the carbon and the oxygen before the process.

It was this discovery that gave a quantitative meaning to the concept of matter: independent of its chemical properties matter could be measured by its mass.

During the following period, mainly the nineteenth century, a number of new chemical elements were discovered; in our time this number has reached one hundred.

This development showed clearly that the concept of the chemical element had not yet reached the point where one could understand the unity of matter. It was not satisfactory to believe that there are very many kinds of matter, qualitatively different and without any connection between one another.

In the beginning of the nineteenth century some evidence for a connection between the different elements was found in the fact that the atomic weights of different elements frequently seemed to be integer multiples of a smallest unit near to the atomic weight of hydrogen.

The similarity in the chemical behavior of some elements was another hint leading in the same direction.

But only the discovery of forces much stronger than those applied in chemical processes could really establish the connection between the different elements and thereby lead to a closer unification of matter.

These forces were actually found in the radioactive process discovered in 1896 by Becquerel.

Successive investigations by Curie, Rutherford and others revealed the transmutation of elements in the radioactive process. The a-particles are emitted in these processes as fragments of the atoms with an energy about a million times greater than the energy of a single atomic particle in a chemical process.

Therefore, these particles could be used as new tools for investigating the inner structure of the atom. The result of Rutherford’s experiments on the scattering of a-rayswas the nuclear model of the atom in 1911.

The most important feature of this well-known model was the separation of the atom into two distinctly different parts, the atomic nucleus and the surrounding electronic shells.

The nucleus in the middle of the atom occupies only an extremely small fraction of the space filled by the atom (its radius is about a hundred thousand times smaller than that of the atom), but contains almost its entire mass.

Its positive electric charge, which is an integer multiple of the so-called elementary charge, determines the number of the surrounding electrons – the atom as a whole must be electrically neutral – and the shapes of their orbits.

This distinction between the atomic nucleus and the electronic shells at once gave a proper explanation of the fact that for chemistry the chemical elements are the last units of matter and that very much stronger forces are required to change the elements into each other.

The chemical bond between neighboring atoms is due to an inter-action of the electronic shells, and the energies of this interaction are comparatively small. An electron that is accelerated in a discharge tube by a potential of only several volts has sufficient energy to excite the electronic shells to the emission of radiation, or to destroy the chemical bond in a molecule.

But the chemical behavior of the atom, though it consists of the behavior of its electronic shells, is determined by the charge of the nucleus. One has to change the nucleus if one wants to change the chemical properties, and this requires energies about a million times greater.