Abstract
The greater part of this paper is concerned with a historical discussion of the development of the search for the origins of the highest-energy cosmic-rays together with a few remarks about future prospects.Additionally, in section 6, the situation with regard to the mass composition and energy spectrum at the highest energies is discussed. It is shown that the change of the depth of shower maximum with energy above 1 EeV, measured using the Telescope Array, is in striking agreement with similar results from the Auger Observatory. This implies that either the mean mass of cosmic rays is becoming heavier above ~4 EeV or that there is a change in details of the hadronic interactions in a manner such that protons masquerade as heavier nuclei. A long-standing controversy is thus resolved: the belief that pure protons dominate the mass distribution at the highest energies is no longer tenable.
Highlights
The situation with regard to the mass composition and energy spectrum at the highest energies is discussed
What I find remarkable is that the present flourishing of activity on the highest-energy cosmicrays relies almost entirely on concepts, both analytical and technical, that were proposed more than 40 years ago and by a handful of people
What is perhaps less wellknown is that it was a Scottish physicist, C T R Wilson, who in 1901 was the first to suggest that the problem of the discharge of well-insulated charged objects – a problem that challenged some of the best physicists of the late nineteenth century – might be explained by invoking a non-terrestrial agency
Summary
V L Fitch has pointed out that those interested in cosmic rays tended, in earlier times, to be rugged individualists, to be iconoclasts, and to march to the drummer in their own heads rather than some distant one [1]. Wilson moved from cosmic-ray research to perfect his cloud chamber and study the causes of lightning. Auger’s assistant, Roland Maze, a highly talented electronics expert and a fine physicist, constructed improved Geiger counters and, more importantly, a coincidence circuit with a resolving time of ~5 μs [10].
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