MODERN IDEAS ON THE STRUCTURE OF MATTER. ATOMS, NUCLEI, AND ELEMENTARY PARTICLES Let us now pause to examine the structure of "ordinary" matter, atoms, nuclei, and elementary particles, leaving out gravitation for the moment. In the interaction of particles of any accessible energy, up to energies of about or ev detected in cosmic rays, at all the small distances attained, down to distances of about cm, the gravitational forces are vanishingly small with respect to the electromagnetic forces, and even more so as compared with nuclear forces. The question as to the effect gravita tional forces have on the inner structure, i. e. , the core of particles, is still a moot point; there, it seems, they have to be taken into account.
Before tackling the classification of particles and the construction of what may be termed a unified "atomic" picture of matter, let us recall briefly the current concepts of the structure of atoms and nuclei, and the main events in the discovery of elementary particles.
The contemporary attempts at producing a quantum atomic picture of matter constitute a fourth stage, following the attempts at a universal mechanical, electromagnetic, and geometrical world picture.
As is known, all substances occurring in nature are basically made up of atoms of various chemical elements. The systematization of the elements was achieved by D. I. Mendeleev, who arranged them in a table by increasing order of atomic weight. The serial number of an element in Mendeleev's periodic table is equal to the charge of the atomic nucleus or to the number of electrons in the neutral atom. The structure of the atoms of all the elements is in a sense similar to the solar system: at the center is located the positively charged nucleus, around which revolve the negatively charged electrons. The parallel with the solar system, which was demonstrated by Rutherford in 1911, is not accidental: it derives from the fact that Newton's law of attraction in the solar system on the one hand, and the Coulomb law of electrical attraction between the electrons and the nucleus in atoms on the other, are expressed by the same dependence on the distance. In both
cases the force of attraction varies in inverse ratio to the square of the distance: The magnitude of the force of electrical attraction between the electrons and the nucleus is, however, immensely larger than that of the force of gravitation between them, by a factor of about And this is where the similarity ends. The most important difference between an atom and the solar system resides in the fact that the electrons and atomic nuclei do not obey the laws of classical mechanics but those of quantum mechanics. It is possible to speak of the orbits of electrons only as a rough approximation, in the sense of their being the most probable trajectories, as the motion of the electron cannot really be visualized in classical terms; the electron may be seen to move everywhere within the atom.
What is involved in essence is that the state of an atom is defined by a probability wave, viz. , the wave. The atom settles into one of a set of stationary states, in each of which the electrons may be described as revolving about the nucleus along definite orbits (i. e. , the most probable ones) containing an integral number of waves.
The energies of the atomic electrons can assume only a discrete (discontinuous) series of values. When an electron "jumps" from one state to another in the atom, it either emits or absorbs a definite amount of electromagnetic energy (a quantum of light or a photon): The fact that the electron exhibits wave properties, and all the other remarkable predictions of the quantum (wave) mechanics founded by Bohr, Sommerfeld, de Broglie, Heisenberg, Schrodinger, Dirac, and Born, have been brilliantly verified by many experiments. We will not delve into the interpretation of quantum mechanics, however, since we are interested at this point only in the structure of matter.