On the nuclear forces and the magnetic moments of the neutron and the proton

It was first suggested by Heisenberg that the forces between a proton and a neutron are connected with an exchange of charge between the two heavy particles. This exchange nature of the neutron-proton forces is now generally accepted. It would follow from this assumption that in suitable circumstances a proton (neutron) could emit a positively (negatively) charged particle transforming itself into a neutron (proton). At first sight it seemed that the emission of positive or negative electrons in the /3-decay could in this way be made responsible for the nuclear forces. This was, in fact, suggested by Iwanenko (I934) and Tamm (I934). It has also been pointed out by Wick (I935) that the virtual emission of fl-electrons might explain the values of the magnetic moments of the proton and the neutron. These theories, however, were not successful. The nuclear forces, for instance, turn out to be too small by a factor of more than 1010 and have far too small a range; this is due to the fact that the /3-decay constant is extremely small. Since the /3-decay is a process which, in nuclear dimensions, takes "geological ages", one might think that the ordinary properties of the heavy particles have no direct connexion with this process and that an approximate theory of the nuclear forces should be possible without the inclusion of the /3-decay. A new hope for such an "exchange theory" of the properties of nuclei is offered by the probable existence of a hitherto unknown type of particle constituting the hard component of cosmic radiation. Since these particles do not lose much energy by radiation, it has been suggested by Neddermeyer and Anderson (I937) that they are (positive and negative) "heavy electrons" with a mass between that of an electron and a proton. From cosmic-ray data the mass of these particles can hardly be determined yet, [ 154 ]