Not many historians have considered the field of astroparticle physics, until now. One reason for this lack of interest in the topic by historians of science might the fact that any historical approach to astroparticle physics will have to face some problems first. One is the missing definition of what this field’s contents are and were, a topic that has been mentioned in this article before. The blurred image of the historical and current matters of astroparticle physics is problematic, as it tempts one to analyze events from a modern point of view, that is with modern astroparticle physics in mind. But this may lead to the omission of certain developments, which may have been dead end streets from a physicist’s point of view, but might be most elucidating for historians and philosophers of science. From a modern scientific standpoint the Phlogiston theory or the idea of the existence of the luminiferous aether are simply wrong, but from a historical perspective they played a decisive role in the establishment of current science. Therefore efforts should be made to investigate the history of all the different disciplines that came to influence astroparticle physics to some extent. Thus one might be able to give a more complex picture of the history of this field and may even come closer to a generally accepted definition of it.
The state of research (see Section 2) so far is rather limited to scientific reviews and a number of biographical notes from famous physicists. The most important of these selected few books are certainly by Hillas , Sekido and Elliot . The first is a rather complete scientific review from the experiments of Hess up to the astrophysical problems of the 1970s. It shows that in those days cosmic-ray studies was as heterogeneous as it appears to be today, uniting different aspects of cosmology and astrophysics. Sekido and Elliot , on the other hand, try to mingle autobiographical aspects with a scientific review and therefore come closest to a complete survey of the history of astroparticle physics. The main difficulty of such a mixed approach between the fields of history,physics and personal reminiscence has been paraphrased as a “violation of parity between physicists and historians in the study of the past, [that] resulted, of course, in weak interactions” . Many others [131, 192, 34, 82, 33, 133, 83, 70, 138] have contributed to the history of physics in general and/or the history of particle physics in particular; these works have pieced together various parts of the development of astroparticle physics and its related fields, thus becoming the basis for this article. Still, there has been no systematic historical work done on the subject. Any such systematic approach should take into account a number of historical questions that are still unanswered, some of which I will try to specify in the following.
Some books about famous physicists who were involved in (early) cosmic-ray studies, have already been written, such as about Millikan . Yet there is still a lot of historical work to be invested in this field, for a mere biographical approach does not give a very detailed account of the history of scientific developments, even though the two are usually closely linked. The relationships between individual people, between people and institutions, as well as people and their respective work fields especially have not been analyzed properly so far. If we try to establish a picture of the early community we soon reach the limitations of this approach, for, as we have seen, the people involved form a rather heterogeneous group. The difficulty lies in the fact that we access the history of these people according to the division of physical disciplines that prevailed in the 1950s. For example, the famous chemical physicist Walther Nernst was the one to revive the debate of the cosmic origin of penetrating rays after World War I, though it did not touch even one of his general subjects . So we see that scientists from the fields of radio chemistry, theoretical physics and astrophysics were involved in cosmic-ray studies. But what if it was the other way around and cosmic-ray physics was the starting point for many researchers to become more deeper involved in other disciplines? I am not saying that cosmic-ray physics is the mother of all the others, but it might be worth it to take a look at how the different “disciplines” of physics were more united in the very early stages of 20th century physics than they are today and why and when this unity broke apart.
The question of which scientists were involved in cosmic-ray studies is also linked to the historiographical problem of how to deal properly with the numerous people who worked behind the scenes. It would far exceed the limits of this article to give even a brief overview of the various problems of prosopography, especially for the history of science. Prosopography, a special tool in the historical and social sciences that is used to analyze the history of groups of people, could turn out to be a quite fruitful approach to the history of astroparticle physics.
Whereas for modern astroparticle physics it is difficult to find out how the different subjects of the discipline are linked, we have easy access to the knowledge of which institutions are involved in this field. But for early cosmic-ray research this is far more difficult, because investigative work has not been done so far on any institutional aspect of this field. We do not yet know, for example, how the experiments at high altitude observatories were managed on an administrative level. To understand the mechanisms of institutional cooperation in the very early stages of cosmic-ray studies is essential in order to discover the standing this field had and what this means for its modern successor.
As shown in Section 3 the different stages of cosmic-ray physics were dependent upon a number of inventions and technical improvements that led to new findings as well as new theories. But what exactly were the mechanisms that gave way to progress in this scientific field? This question connects the historical question of the influence of technical development on science  to general philosophical questions concerning the internal and external conditions of the constitution of scientific theories. Moreover, the analysis of technical improvements may provide some clues as to how early cosmic-ray studies and modern astroparticle physics might be linked when it comes to joint technical devices or mathematical means being used.
The overly simple answer would be that in the late 1980s physicists from different disciplines met in order to “found” this field . The conceptual links established throughout the 1960s and 1970s had made it possible for physicists from different fields to meet in interdisciplinary workshops and try to tackle the problems at hand by working together. The mechanisms behind these conceptual links as well as the question as to why the need for an interdisciplinary solution to these problems was felt – instead of an inner-disciplinary approach on them – have not been investigated so far. But it seems very unlikely that the reason the researchers involved had in mind was mere practical necessity. Another interesting starting point would be to spell out those special scientific problems that fostered interdisciplinary work. Likewise, one might cast an eye on the formation of common means of publication, like journals and others, and analyze what role they played in the establishment of astroparticle physics. Apart from this, just looking at the events of the 1980s neglects the fact that some fields, like cosmic-ray studies, though no one has yet defined what exactly divides them from astroparticle physics as such, have been on the physicists’ agenda ever since the first discoveries in this field. It seems more reasonable to assume that the various historical threads I have tried to disentangle above were closely connected even before the 1980’s, though these connections have not been analyzed so far. It would be interesting to discover those inherent connections, so that one might establish a complete history of astroparticle physics from its beginning till now.
Many of the philosophical problems that can be found in the field of astroparticle physics are closely linked to its history. Yet, as they are also linked to many problems that are genuinely of a philosophical nature, they will be separately listed below.
Even a rather superficial look at the contents of modern astroparticle physics, and at the problems of early cosmic-ray studies, will disclose the problem of a lack of definition. Different types of radiation at different wavelengths are all partly the topic of modern astroparticle physics, but even early on, cosmic-ray studies investigated not only charged particles of cosmic origin, but other phenomena as well, such as x-rays and gamma-rays. Finding out what exactly the fields of astroparticle physics and its predecessor, cosmic-ray studies, are and were, would help to explain “where ... the cosmic ray field belong[s]”, as Stanev  put it, thus enabling us to judge properly whether modern astroparticle physics should be granted the status of a discipline of its own right, or rather “only” to be some interdisciplinary field between more established disciplines. Maybe the blurred definition of the term “cosmic rays”, traditionally referring to charged particles, but sometimes also used for other radiation phenomena of cosmic origin, is the cause for the difficulty in defining the subject of astroparticle physics. I assume that a profound analysis of the different concepts of (cosmic) radiation , like the study of the concept of particles [61, 62], might be a great help on that point.
As we have seen above, astroparticle physics encompasses matters of particle physics and problems of cosmology. From a philosophical point of view this is quite striking, for both fields of study rely on a standard model, the first being the one of particle physics, the second the one of cosmology. Though the two models have not been unified theoretically, astroparticle physics works with both of them. How it is able to do so and what role experiments play in this has not been investigated so far, though the unification problem has been an urgent matter of physics as well as philosophy [60, 62, 85, 214, 217] throughout the 20th century, beginning with Planck in the very first years of the century, when in a lecture he developed the concept of unity as fundamental for future physics .
Today there are two aspects of the unification problem that are of major interest. The first is the search for the “Grand Unified Theory” or a “Theory of Everything”, that would combine at least three, if not all four fundamental forces of nature [18, 217]. The main challenge here is the unification of gravity and quantum mechanics. The other aspect is the question of the unity of physics or rather the world view it conveys , without discussing the difficulties of defining properly the meaning of “world view” at this point. Of course this is closely related to the first aspect, as the problem lies in the question of how a scientific approach being that fragmented could deliver one consistent world view at all. Finally it has even been questioned whether there is still a unity of the sciences today , whether it has actually ever existed [72, 95] and what advantages and disadvantages either unity or disunity of the sciences would bring .
The analysis of astroparticle physics might help to find new ways of accessing these problems, as it seems to link the two disparate standard models. But how does it accomplish this task? Is there a common theoretical basis that helps to establish this link or is it only combined appliance in experiments? Is it possible at all that there is such a thing as a common basis or do we have to interpret astroparticle physics from a mere instrumentalist point of view as being only useful “for all practical purposes”? But if that were true, how can the research on the smallest known entities of matter provide clues about the properties of the largest objects in the universe? Maybe an analysis of the semantic unity of physics, i.e. the unity of the scale of scientific measurements would be a promising approach here [60, 61, 62]? Without some semantic and methodological unity of cosmology and particle physics the establishment of astroparticle physics would not have been possible.
Taking a look at the historical facts we see that in early cosmic-ray physics and astroparticle physics we find both continuity and discontinuity. But what caused the break in cosmic-ray studies after World War II? Was it due to external factors like the two world wars and the Cold War and the following shift of financial aid in favor of satellite-based cosmology and high-energy physics using accelerators [165, 126], that cosmic-ray studies seem to have merged into other disciplines? Or did it have internal reasons caused by a lack of knowledge about certain aspects of cosmic rays? Why did physicists in 1987 feel the necessity to “found” a new discipline instead of reviving older traditions? Most certainly these questions can only be answered after a profound historical analysis has been made.
The question of the role of experiment is quite interesting in a philosophical context. First, it touches the problem of the general relationship of experiment to theory, i.e. whether or not experiments might falsify or support theories at all [72, 101]. Possible constructivist aspects of experiments and the conditions under which they are conducted [122, 134, 208] might also be analyzed by closely examining the case of astroparticle physics. Another aspect is the role of scientific models as a transitory state between theories and experiments [16, 87]. Finally, the discussion of realistic interpretations of experimental results [173, 69] might be fueled with new arguments when looking at the case of astroparticle physics. Moreover, early cosmic-ray studies have already proven to be a good example of the theory-ladenness of observation, as in the case of solar neutrinos being used to examine the Sun’s inner activities , or more generally, in the case of tracks of cosmic rays being observed with the help of emulsion techniques .
But, whereas the role of experiment in general is on the agenda of many philosophers of science, the problem has not been discussed regarding astroparticle physics so far. That is quite astonishing as experiments, as well as observations, formed and form the majority of both early cosmic-ray studies and astroparticle physics, thus becoming the basis for particle physics . On the other hand, as a complete historical survey of the field is still pending, to answer this question would be a promising task for the future.
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