All the isotopes of the 103 known elements of the periodic system are made up of only three kinds of elementary particles —protons, neutrons, and electrons.
Let us illustrate the point by means of two familiar examples, which may be helpful in our subsequent discussion. The neutral hydrogen atom consists of a single electron revolving around a proton (light hydrogen), and that of deuterium consists of an electron revolving around a deuteron (D=H;), i. e., the hydrogen isotope known as heavy hydrogen, which consists of a proton and a neutron. Lastly, in the radioactive isotope of hydrogen, tritium (T=H; ), the electron revolves around a triton nucleus, made up of a proton and two neutrons.
If hypernuclei are taken into consideration, further types of hydrogen are possible: the hypertriton A—HL in which one of the neutrons is replaced by a lambda hyperon, or even a hypernucleus of hydrogen of atomic weight 4, in which the electron revolves around a nucleus made up of one proton, two neutrons, and one hyperon.
In regard to the various possible types of hydrogen it is also relevant to point out that not only electrons but also negative muons, pions, or K mesons may also revolve around ordinary nuclei (or around hypernuclei), thus forming mesic hydrogen, or, correspondingly, other cc-, p-, and K-mesic atoms. The p -and cc-mesic atoms were discovered at the beginning of the fifties by Rainwater and Fitch; these atoms are characterized by distinctive spectra and they have been fairly well investigated by now throughout the range of elements from mesic hydrogen to mesic lead.
A significant aspect of mesic atoms is the fact that the motion of the meson is appreciably affected by the distribution of protonic charge over the nucleus. This is because the mesons have larger mass, and their orbits are much closer to the nucleus than electron orbits. The mesons in heavy mesic atoms move very close to the surface and sometimes even jump through the nucleus. Clearly, the study of mesic atoms can yield valuable information on the structure, size and shape of nuclei, and some important data have already been obtained in this way. In particular, the use of mesic atoms has provided the most accurate measurement of the size of nuclei.
Now since the atoms of all the chemical elements occurring in nature turn out to be built of three kinds of elementary particles, the electrons, protons, and neutrons, it might be assumed that other forms of matter, that is to say, other particles, play no part in the atoms or nuclei and are produced only by various nuclear reactions in cosmic rays or in the laboratory. In point of fact, such is not the case. Aside from the three types of particles actually present in atoms, the latter also contain (in a latent or virtual state, so to speak) fields responsible for the coupling forces between the particles. The electron, for instance, is attracted to
the nucleus mainly because of electrostatic forces (other electromagnetic forces play an insignificant part). The atoms emit this field in bundles taking the form of photons, when the electrons pass from a higher to a lower energy level or when atoms collide with each other. The photon, which is the field quantum of energy E=hv, is quite similar to any other particle: it possesses a definite angular momentum (spin), S = 1 • an aggregate of photons, like other particles with integral spin, obeys the Bose statistics. Such "bosons" are also the a and K mesons, which have no spin, and the as yet hypothetical gravitons of spin 2. The basic difference between the photon and other particles such as the electron is that the photon has no rest mass. Furthermore, the electrons, protons, µ mesons, neutrons, neutrinos, and all the hyperons, possess half-integral spin (in units of h/2ic) and obey the Fermi-Dirac statistics. All such "fermions" are subject to Pauli's exclusion principle, which states that any energy state of given momentum can contain only two fermions with antiparallel spin momenta.
The electromagnetic field is also present in a virtual, unradiated state in nuclei, and causes the electrostatic repulsion between protons and the magnetic interaction between protons and neutrons (since both these nucleons possess magnetic moments). These facts have actually been known for quite a while, and until recently only some very fine quantum corrections, often difficult to calculate or to measure, have been found. In particular, one of the major events in postwar physics was Lamb's discovery in 1947 of the small additional shift in the energy levels of the electron in the hydrogen atom. The Lamb shift, which is a correction of the one obtained by Dirac's relativistic theory generalizing the results of Bohr, Sommerfeld, and Schriidinger, proved to be the result of the interaction of the electron with the fluctuations (deviations) in the "zero-point" oscillations of the photonic field and the electron-positron field, which had not been taken into account before (the calculation of Bethe, Schwinger, Feynman, and Dyson). During the same period, in 1947, Kusch, working with radio electronics at the Rabi Laboratory of Colombia University, discovered a small correction to the magnetic moment of the electron, i. e. , to the usual Bohr magneton. He found that the magnetic moment of the electron is: This "anomalous" correction turned out to be due to the so-called "vacuum" effects, which may be visualized as the electron being shaken from its position under the impact of the photons continually produced by random fluctuation, even though the average electromagnetic field may be zero, and also partly by the fluctuation of the electron-positron pairs whose field surrounds the electron.