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” [52
], 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 [65, 66]) has put forward the following as the most urgent
topics of astroparticle physics:
- Dark Matter
- Charged Cosmic Radiation
- Gamma-Ray Astronomy
- High-Energy Neutrino Astrophysics
- Low-Energy Neutrino Astrophysics
- Neutrino Properties
- Gravitational Waves
- Theoretical Astroparticle Physics
- Nuclear Astrophysics
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’ [17
] or ‘young field’ [43
]. 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 [168
] 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.” ([205
], 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.
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 1), 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 [106
], 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” [52
] 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 [34
], 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 [34 ] |
|
|
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.
|
|