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1 Introduction

Though modern astroparticle physics dates from as recently as the 1980’s, it is commonly understood that it has a long and complex history. Whereas the first generally acknowledged proof of the existence of cosmic rays by Hess in 1912 is quite well known among historians of science and (astroparticle) physicists alike, very few members of either discipline have so far paid much attention to the fact that this “discovery” was neither a solitary event in its own historical context, nor one whose connection to the development of modern astroparticle physics has been elucidated so far. Nor has it been clarified how early developments in cosmic-ray studies are actually related to the fields of “particle physics, cosmology and astrophysics” [52Jump To The Next Citation Point], which supposedly form the basis for astroparticle physics.

One of the main challenges of writing about the history of astroparticle physics is defining the field itself. There is no such thing as a commonly acknowledged textbook definition of “astroparticle physics”. Though, of course, there are ideas of what astroparticle physics deals with. The German Committee for Astroparticle Physics (KAT Komitee für Astroteilchenphysik [6566]) has put forward the following as the most urgent topics of astroparticle physics:

These topics are also at the heart of the agenda of the ApPEC (Astroparticle European Coordination) [112] and its succeeding organization, the ASPERA (Astroparticle European Cooperation Eranet) [9]: European networks for the advancement of astroparticle physics (see Section 5.2). In a 2008 status list from ASPERA, more than 70 experiments that collaborate on an international basis with up to 65 institutions involved and up to a few hundred authors each were mentioned. All the experiments described there could be grouped into the aforementioned topics (see Table 1).


Table 1: Experiments in Astroparticle Physics (Status: 2008) [10]

Double Beta Decay and Neutrino Mass

Dark Matter Search

Neutrino Telescopes

Gamma-Ray Telescopes

NEMO-3

DAMA/LIBRA

Baikal

HESS

CUORICINO

PVLAS

ANTRES

MAGIC

TGV

HDMS

NEMO

AGILE

MANU2

CRESST

NESTOR

ARGO-YBJ

GERDA

CAST

KM2NeT

VERITAS

KATRIN

ZEPLIN III

AMANDA

GLAST

Double Chooz

GENIUS-TF

IceCube

MIBETA

DRIFT

CUOREN

EDELWEISS

COBRA

WARP

SuperNEMO

ROSEBUD

MARE

ANAIS

EXO

DRIFT 1T

EURECA

ArDM

SIMPLE

XENON

PICASSO

Charged Cosmic Ray Experiments

Gravitational Waves

Cosmic Low-Energy Neutrinos, Proton Decay

a) Low energy

AURIGA

LVD

PAMELA

ROG

CTF

AMS-02

DUAL-R&D

BOREXINO

TRACER

MiniGRAIL

MEMPHYS

CREAM

SFERA

GLACIER

ATIC

Virgo

LENA

GEO600

SNO

b) High energy

LISA-Pathfinder

KASCADE-Grande

LISA

LOPES

LIGO

CODALEMA

Advanced LIGO

NuMoon

LOFAR

EUSO

Tunka

Auger


Yet the problem remains: What exactly IS astroparticle physics? As we will see in Section 5.2, there is a considerable discrepancy between the fact that on the one hand astroparticle physics meets more or less all the demands for a scientific discipline, on the other hand even most recent publications call astroparticle physics an ‘interdisciplinary area’ [17Jump To The Next Citation Point] or ‘young field’ [43Jump To The Next Citation Point]. In the following sections, I will therefore not hesitate to use ‘discipline’, ‘sub-discipline’ or ‘field’ as equivalent terms for astroparticle physics, leaving the decision whether or not this might be correct [168Jump To The Next Citation Point] to a later discussion (see Section 5.2). At the moment we might introduce the following simplifying working definition of astroparticle physics: Astroparticle physics is an interdisciplinary field lying between particle physics and cosmology that attempts to reveal the nature and structure of matter in the universe. It evolved from various other fields, the most commonly agreed upon being early cosmic-ray studies. The question remains, as Stanev put it for cosmic-ray research: “Where does the cosmic ray field belong?” giving the answer “A better definition than an outline of its history and its ever-changing priorities is hardly possible.” ([205Jump To The Next Citation Point], page 3) This historical approach will be helpful in determining the character of astroparticle physics. Therefore, as a first step, this article will review all previous historical work and will then specify those questions that are still open.

View Image

Figure 1: The spectrum of cosmic rays [96]

There are many different types of radiation that are of concern in modern astroparticle physics (see above). Looking at the spectrum of light (see Figure 1View Image), we see that it consists of gamma-rays, x-rays, UV light, visible light, infrared light, microwaves and radio emissions. Each of these kinds of radiation was discovered separately [106Jump To The Next Citation Point], and some are traditionally an integrative part of other scientific disciplines, e.g. radio astronomy is important for astrophysics. Similarly the spectra of charged cosmic particles or neutrinos is important for modern astroparticle physics. We can divide these spectra not only by energy level, but also by detection method, thus providing a guideline for writing the history of astroparticle physics. When in 1987 physicists from the fields of “particle physics, cosmology and astrophysics” [52Jump To The Next Citation Point] met at the “First International School on Astroparticle Physics”, thus laying the foundation for modern astroparticle physics, all these different historical “threads” became interwoven again. But how this happened, what mechanism made this possible (and even necessary) has not been analyzed by either historians or by philosophers of science so far, though this might help to solve the aforementioned problem of the scientific standing of astroparticle physics, as well as the problem of defining the actual contents of this field.

So, how to narrow down this broad range of aspects that are all incorporated into one field? A good starting point, preferred by many of the physicists that are familiar with the origins of astroparticle physics, is the “discovery” of cosmic rays by Hess in 1911/12. Apart from the fact that this event is not the sole discovery of one single scientist, as will be shown in Section 2 of this article, the history of astroparticle physics has some more components worth casting an eye on. In order not to bias the historical findings with our present-day knowledge of this scientific field, I will proceed chronologically first, showing the major developments in the history of the various segments that are the contents of present-day astroparticle physics. But I will only touch on those aspects that may be interesting in a historical sense and will try not to go too deep into scientific detail. In addition, I will follow the time table (see Table 2) Brown and Hoddeson developed for their review of the beginnings of particle physics [34Jump To The Next Citation Point], which also touches on some aspects of cosmic-ray physics.

Section 3 will deal with the major developments of early cosmic-ray studies. It will provide an overview of well-known events like the “discovery” of cosmic rays by Hess and will put them into context. It will then give a very short summary of the most important technical advances and what role they played for further developments, especially in the early phase of cosmic-ray studies, but also for the emergence of new fields, like particle physics. Section 4 will deal with the events after World War II, a time that is usually referred to as the downfall of cosmic ray studies and the heyday of high-energy-physics with man-made accelerators. In those years other fields and disciplines emerged or gained importance, parts of which had and still have an influence on astroparticle physics. Section 4 will summarize the scientific progress made in those fields and briefly describe their connection to astroparticle physics. Section 5 will cast light on the beginnings of astroparticle physics, as we know it today. Besides bringing together the known facts about the “founding act” and the attempt to explain how it came about, this chapter will deal with the problem of the scientific standing of astroparticle physics. Finally, in Section 6, the open questions concerning the history of astroparticle physics – for this article will certainly find more open questions than answers to them – and the philosophical implications that follow from them, will be shortly discussed.


Table 2: Sequence of development of cosmic-ray physicsa [34Jump To The Next Citation Point]

I.

Prehistory (to 1911, especially from 1900)

“Atmospheric electricity” during calm weather; conductivity of air measured by electrometers; connection with radioactivity of earth and atmosphere; interest was also geophysical and meteorological.

II.

Discovery (1911–14) and exploration (1922–30)

Balloons carrying observers with electrometers measured the altitude dependence of ionization and showed that there is an ionization radiation that comes from above; these measurements began in 1909 and continued (at intervals) to about 1930, in the atmosphere, under water, earth, etc.; the primaries were assumed to be high-energy photons from outer space; search for diurnal and annual intensity variations; study of energy inhomogeneity.

III.

Particle physics, early (1930–47)

Direct observation of the primaries was not yet possible, but “latitude effect” showed that they were charged particles; secondary charged-particle trajectories were observed with cloud chambers and counter telescope arrays, and momentum was measured by curvature of trajectory in a magnetic field; discovery of positron and of pair production; soft and penetrating components; radiation processes and electromagnetic cascades; meson theory of nuclear forces; discovery of mesotron (present-day muon); properties of the muon, including mass, lifetime, and penetrability; two-meson theory and the meson “paradox.”

IV.

Particle physics, later (1947–53)

Observation of particle tracks in photographic emulsion; discovery of pion and pion-muon-electron decay chain; nuclear capture of negative pions; observation of cosmic-ray primary protons and fast nuclei; extensive air showers; discovery of the strange particles; the strangeness quantum number.

V.

Astrophysics (1954 and later)

Even now the highest-energy particles are in cosmic rays, but such particles are rare; studies made with rockets and earth satellites; primary energy spectrum, isotopic composition; x-ray and γ-ray astronomy; galactic and extragalactic magnetic fields.

a In successive periods, at least one change occurred that was so significant that it required a totally new interpretation of the previous observations and theories.



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