General survey of theory and experiment for high-energy electrons
Observations on the passage of cosmic-ray electrons through matter have aroused considerable interest in the problem of collisions. Prior to the discovery of these electrons by Skobelzyn the fastest particles available for the investigation of this problem were the electrons emitted by radioactive bodies. These have velocities up to about 0.990 of that of light, which is of course well in the relativistic region. Their behaviour is in general accord with present theory1. The existence and availability of cosmic ray electrons has extended the field of investigation to what may be called an “ultra-relativistic” region, where the velocities extend up to within 0.0001 % of that of light. In this region the requirements of theory diverge seriously from the existing experimental results. Some investigators were at first inclined to regard this divergence as not at all unexpected because the de Broglie wavelength of the cosmic-ray electrons concerned is less than the classical electron-radius (15). However this circumstance has no significance as such. The perturbed electron in the system in which it is initially at rest attains no very high energies and its de Broglie wavelength remains comparatively large. Actually the “ultra-relativistic” nature of the phenomena is represented by the extreme Lorentz contraction of the perturbing field. The present situation would not be unsatisfactory if the discrepancies between theory and experiment in the different phenomena could be directly related to this contraction. Unfortunately however this does not appear to be possible, and for this reason the hypotheses which have so far been suggested regarding the limitation of existing theory can certainly not lead to a solution of the difficulties in all the phenomena. One is led to question the experimental results and more particularly the supposition that the cosmic ray particles concerned are in all cases electrons. If, as previously suggested by the writer in this connection, we suppose the higher energy particles to be protons then some of the more serious discrepancies are removed. This however entails the assumption that negative protons are also present. In view of this and certain other objections this possibility will not be considered until the problems have first been fully discussed on the usual supposition that the particles are electrons.
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