Quantum Theory and the Structure of Matter

The nuclear model of the atom, however, if it is thought of as a system obeying Newton’s mechanics, could not explain the stability of the atom.

A carbon atom, after having interacted with other atoms or after emitting radiation, always remains a carbon atom with the same electronic shells as before.

This stability could be explained simply by quantum theory that prevents a simple objective description in space and time of the structure of the atom.

In this way, one finally had a first basis for the understanding of matter.

The chemical and other properties of the atoms could be accounted for by applying the mathematical scheme of quantum theory to the electronic shells.

From this basis, one could try to extend the analysis of the structure of matter in two opposite directions.

One could either study the interaction of atoms, their relation to larger units like molecules or crystals or biological objects;

or one could try through the investigation of the atomic nucleus and its components to penetrate to the final unity of matter.

Research has proceeded on both lines.

The forces between neighboring atoms are primarily electric forces, as the attraction of opposite and the repulsion of equal charges.

The electrons are attracted by the nuclei and repelled from each other.

But these forces act not according to the laws of Newtonian mechanics but those of quantum mechanics.

This leads to 2 different types of binding between atoms.

1. In the one type the electron of one atom passes over to the other one, for example, to fill up a nearly closed electronic shell.

In this case, both atoms are finally charged and form what the physicist calls ions, and since their charges are opposite they attract each other.

2. In the second type, one electron belongs in a way characteristic of quantum theory to both atoms.

The electron goes around both nuclei spending a comparable amount of time in the one and in the other atom.

This second type of binding corresponds to what the chemists call a valency bond.

These 2 types of forces, which may occur in any mixture, cause the formation of various groupings of atoms and seem to be ultimately responsible for all the complicated structures of matter in bulk that are studied in physics and chemistry.

The formation of chemical compounds takes place through the formation of small closed groups of different atoms, each group being one molecule of the compound.

The formation of crystals is due to the arrangement of the atoms in regular lattices.

Metals are formed when the atoms are so tightly packed that their outer electrons can leave their shells and wander through the whole crystal. Magnetism is due to the spinning motion of the electron, and so on.

In all these cases the dualism between matter and force can still be retained, since one may consider nuclei and electrons as the fragments of matter that are kept together by means of the electromagnetic forces.

While in this way physics and chemistry have come to an almost complete union in their relations to the structure of matter, biology deals with structures of a more complicated and somewhat different type.

In spite of the wholeness of the living organism a sharp distinction between animate and inanimate matter can certainly not be made.

The development of biology has supplied us with a great number of examples where one can see that specific biological functions are carried by special large molecules or groups or chains of such molecules, and there has been an increasing tendency in modern biology to explain biological processes as consequences of the laws of physics and chemistry. But the kind of stability that is displayed by the living organism is of a nature somewhat different from the stability of atoms or crystals.

It is a stability of process or function rather than a stability of form. There can be no doubt that the laws of quantum theory play a very important role in the biological phenomena.

For instance, those specific quantum-theoretical forces that can be described only inaccurately by the concept of chemical valency are essential for the understanding of the big organic molecules and their various geometrical patterns; the experiments on biological mutations produced by radiation show both the relevance of the statistical quantum-theoretical laws and the existence of amplifying mechanisms.

The close analogy between the working of our nervous system and the functioning of modern electronic computers stresses again the importance of single elementary processes in the living organism.

Still all this does not prove that physics and chemistry will, together with the concept of evolution, someday offer a complete description of the living organism.

The biological processes must be handled by the experimenting scientist with greater caution than processes of physics and chemistry.

As Bohr has pointed out, it may well be that a description of the living organism that could be called complete from the standpoint of the physicist cannot be given, since it would require experiments that interfere too strongly with the biological functions. Bohr has described this situation by saying that in biology we are concerned with manifestations of possibilities in that nature to which we belong rather than with outcomes of experiments which we can ourselves perform.

The situation of complementarity to which this formulation alludes is represented as a tendency in the methods of modern biological research which, on the one hand, makes full use of all the methods and results of physics and chemistry and, on the other hand, is based on concepts referring to those features of organic nature that are not contained in physics or chemistry, like the concept of life itself.

So far we have followed the analysis of the structure of matter in one direction: from the atom to the more complicated structures consisting of many atoms; from atomic physics to the physics of solid bodies, to chemistry and to biology. Now we have to turn to the opposite direction and follow the line of research from the outer parts of the atom to the inner parts and from the nucleus to the elementary particles. It is this line which will possibly lead to an understanding of the unity of matter.

Here we need not be afraid of destroying characteristic structures by our experiments.

When the task is set to test the final unity of matter we may expose matter to the strongest possible forces, to the most extreme conditions, in order to see whether any matter can ultimately be transmuted into any other matter.

The first step in this direction was the experimental analysis of the atomic nucleus. In the initial period of these studies, which filled approximately the first three decades of our century, the only tools available for experiments on the nucleus were the a-particles emitted from radioactive bodies. With the help of these particles Rutherford succeeded in 1919 in transmuting nuclei of light elements; he could, for instance, transmute a nitrogen nucleus into an oxygen nucleus by adding the a-particle to the nitrogen nucleus and at the same time knocking out one proton. This was the first example of processes on a nuclear scale that reminded one of chemical processes, but led to the artificial transmutation of elements.