Earth is subject to a constant bombardment of subatomic particles that can reach energies far higher than the largest machines. In August , Austrian physicist Victor Hess made a historic balloon flight that opened a new window on matter in the universe. As he ascended to metres, he measured the rate of ionisation in the atmosphere and found that it increased to some three times that at sea level. He concluded that penetrating radiation was entering the atmosphere from above. He had discovered cosmic rays. When they arrive at Earth, they collide with the nuclei of atoms in the upper atmosphere, creating more particles, mainly pions.
The charged pions can swiftly decay, emitting particles called muons. Unlike pions, these do not interact strongly with matter, and can travel through the atmosphere to penetrate below ground. Studies of cosmic rays opened the door to a world of particles beyond the confines of the atom: the first particle of antimatter , the positron the antielectron was discovered in , the muon in , followed by the pion, the kaon and several more.
Until the advent of high-energy particle accelerators in the early s, this natural radiation provided the only way to investigate the growing particle "zoo". 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 , , ] 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 main challenge here is the unification of gravity and quantum mechanics. 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 [ 72 ], whether it has actually ever existed [ 72 , 95 ] and what advantages and disadvantages either unity or disunity of the sciences would bring [ 95 ].
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? 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.
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 [ , ], 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? Most certainly these questions can only be answered after a profound historical analysis has been made. What role do experiments play in the course of the development of astroparticle physics?
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. Possible constructivist aspects of experiments and the conditions under which they are conducted [ , , ] 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 [ , 69 ] might be fueled with new arguments when looking at the case of astroparticle physics. 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 [ 38 ]. 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. This article has tried to give an overview of the history of astroparticle physics, taking into account its components, with some being much older than this relatively young field, which is only around twenty years old.
Admittedly, the limited extent of such an article is by no means sufficient to give an exhaustive account of such an intricate and so far rather unexplored matter. Yet, it was possible to describe all the major developments that, according to our knowledge so far, were later to become astroparticle physics, while taking into account that our present view of this field may bias the historical findings a bit.
Apart from this, this article reveals quite a number of open questions that remain. As this article has shown, astroparticle physics — being at least a very dominant interdisciplinary field, if not a discipline of its own see Section 5. Still, astroparticle physics has more deep-reaching roots, starting with the discovery of ionizing rays of cosmic origin see Sections 2. Right from the very beginning early cosmic-ray studies have been strongly influenced by technical developments, e. But during the s and s further progress in the technical aspects of experimental set-ups, from the invention of Geiger counting tubes to better equipment for unmanned balloons, not only helped to find new ways of analyzing penetrating rays see Sections 3.
Thus cosmic-ray studies became the forerunner of modern particle physics see Sections 2. After World War II cosmic-ray studies appear to have declined in favor of particle physics using man-made accelerators, the reasons for which are so far unclear see Sections 4 and 4. Nevertheless, different types of astrophysics using cosmic rays were conducted in new scientific contexts see Sections 4.
This formation was mainly made possible by the interconnections that were established between particle physics, astrophysics, and cosmology see Section 5 through the s and s. The cooperative dealing with problems finally led to the establishment of the interdisciplinary field of astroparticle physics see Section 5. Yet, though this field of study does seem to fulfill many of the common characteristics of a discipline of its own, the status of astroparticle physics as a discipline is still somewhat ambiguous see Section 5.
Thus, astroparticle physics provides good reason to invest more historical and philosophical energy on this topic see Sections 6. The main tasks for the near future are:. To thoroughly define astroparticle physics beyond simple working definitions and to clarify how far it differs from other fields in physics, like astrophysics and to define which other fields and other disciplines are really, in a strict historical sense, linked to astroparticle physics and which ones have similar scientific contents, but have developed independently.
To establish a more complete historical picture of astroparticle physics and its related fields. Here two questions are of major interest. First, the individual fields that may have been predecessors of astroparticle physics, must be looked at more closely. Second, those events that led to the founding of modern astroparticle physics in need to be analyzed thoroughly.
What was the reason that scientists felt inclined to found a new discipline? What role did the different means of scientific communication like conferences, meetings, publications etc… play? To take astroparticle physics into consideration in the field of philosophy of science, in order to overcome the problems caused by a lack of definition when talking about astroparticle physics, but also to give new impetus to relatively old philosophical questions, as in the problem of the relationship of theory to experiment and others.
To investigate how astroparticle physics manages to cover the gap between the Standard Model of particle physics and the one of cosmology. Making visible those mechanisms in this field might help in finding tools for establishing a unified theory in physics and help philosophers to tackle the problem of the disunity of the sciences. Of course, there are many more points on the agenda, especially concerning the open questions physicists are trying to cope with.
The most urgent one is certainly the problem of dark matter, but also other findings beyond the Standard Model of particle physic are to be made. For historians there is a rich corpus of material to write some very interesting chapters in the history of physics. All in all one might say that astroparticle physics still has a lot of potential to keep people busy for quite a while, no matter whether they are physicists, philosophers or historians. National Center for Biotechnology Information , U.
Living Reviews in Relativity. Living Rev Relativ. Published online May 7. Vanessa Cirkel-Bartelt 1, 2. Author information Article notes Copyright and License information Disclaimer. Vanessa Cirkel-Bartelt, Email: ude. Corresponding author. Accepted Mar Associated Data Supplementary Materials mov-Movie 5.
Abstract This article gives an outline of the historical events that led to the formation of contemporary astroparticle physics. Electronic Supplementary Material Supplementary material is available for this article at Open in a separate window. Table 1 Experiments in Astroparticle Physics Status: [ 10 ]. Figure 1. The spectrum of cosmic rays [ 96 ].
Table 2 Sequence of development of cosmic-ray physics a [ 34 ]. Discovery —14 and exploration —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 and continued at intervals to about , in the atmosphere, under water, earth, etc.
- 1. First stars, proto-galaxies and cosmic reionization.
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Particle physics, later —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. State of research: a brief survey The early times of cosmic-ray studies have been partly examined in different books that are mainly concerned with either the history of particle physics or astrophysics. Ionization of air From the initial points described above, experimental work led to the problem of ionization of air.
Wilson chamber The cloud chamber, invented by Wilson, made the tracks of particles visible by means of condensing water vapor. Figure 3. Rockoon with Deacon rocket [ 47 ]. Figure 2. Showers of cosmic rays One of the most important findings in early cosmic-ray studies was the fact that not all the ionizing particles that had been found were the original — or primary as they are called today — particles of cosmic radiation, but often consisted of their products of decay.
Detection of new particles From that basis scientific work focused on the more complex phenomena of cosmic-ray physics. Theoretical approaches Another such entanglement of astroparticle and particle physics that mirrored the growing tendency towards favoring the latter was the first steps of the discovery of pions and muons. Balloons for cosmic rays: the beginning of spaceflight?
High altitude observatories But balloon flights were not the only means of high altitude research; a number of observatories were erected and existing stations were extended for the special needs of cosmic-ray research. Bubble chambers, spark-chamber detectors and drift chambers Particle detectors, as we have already seen in the case of the cloud chamber, are important in particle physics and astroparticle physics alike. Figure 4. Computers Today the usage of computers in general is so common among physicists that this method might be seen as self-evident, but the application of ever-better computer programs to various scientific problems must have been quite a relief for researchers in the s and s, both for the mathematical analysis of collected data and the calculation of theoretical predictions.
Particle accelerators Particle accelerators do not, of course, belong to cosmic-ray physics.
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Figure 6. Figure 5. X-rays and gamma-rays of cosmic origin Of course, cosmic-ray studies had always been related to x-rays, as the very early beginnings were fostered by an interest in the various aspects of radioactivity at the turn of the century. Dark matter and dark energy As mentioned in the introduction, dark matter is high on the agenda of modern astroparticle physicists. Figure 7. Black holes Already in the 18th century, Michell [ ] and Laplace [ ] had thought about the possibility of objects that would not allow light to escape their surface.
Figure 8. Figure 9. Figure Neutrinos The idea of neutrinos goes back to Pauli who postulated them in , together with the neutron, which he, according to Pontecorvo [ ], for a while mistook for being identical to the remnants of beta decay. Astroparticle physics and its current scientific standing Current experiments in astroparticle physics span almost the whole spectrum of cosmic rays, as even the most superficial look at recent articles shows [ 1 , 97 , , , ].
Theory formation Though the external factors for reaching the status of a scientific discipline are all more or less fulfilled by astroparticle physics, so that one might speak of a discipline in the making, or at least of something more than just an interdisciplinary field of study, this is exactly the term used in the latest publications [ 17 ].
Historical Questions and Philosophical Implications: a Brief Overview The following section will first have a look at the historical questions that have been left unanswered so far, in order to analyze secondly which philosophical problems can be found in this field.
Historical questions Not many historians have considered the field of astroparticle physics, until now. Which scientists have worked on the phenomena of cosmic rays? What institutions were involved in cosmic-ray studies? What role did technical improvement play? How did modern astroparticle physics come into being? Philosophical implications Many of the philosophical problems that can be found in the field of astroparticle physics are closely linked to its history.
Conclusion This article has tried to give an overview of the history of astroparticle physics, taking into account its components, with some being much older than this relatively young field, which is only around twenty years old. The main tasks for the near future are: To thoroughly define astroparticle physics beyond simple working definitions and to clarify how far it differs from other fields in physics, like astrophysics and to define which other fields and other disciplines are really, in a strict historical sense, linked to astroparticle physics and which ones have similar scientific contents, but have developed independently.
Electronic supplementary material mov-Movie 5.
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A view held by many is that this is the most important reason to engage in the search for gravitational radiation. The signatures of gravitational waves may well be the most definitive means to establish the existence of black holes and to study the interactions of compact objects of all kinds with their surroundings. Thus, the detection of gravitational radiation has become an important problem in relativistic astrophysics. Estimates of the gravitational-wave spectrum incident on the Earth suffer from our limited knowledge about massive compact objects in the universe.
If the precedent set by the development of radio, in- frared, and x-ray astronomy serves as a guide, chances are excellent that the first sources of gravitational waves to be detected will not have been included in the present inventory of hypothesized sources. Several classes of known astrophysical objects have been proposed as emitters of gravitational radiation. A few of these are described below, and estimates of their strength at the Earth are shown in Figures 6.
The collapse of stellar cores in Type II supernovae may produce millisecond bursts of gravitational radiation provided there is sufficient departure from spherical symmetry in the collapse. Such a strain measurement is just barely within the capabilities of currently operating detectors.
Similarly, a strain is induced in a solid body. Thus, the strength of a gravitational wave is customarily measured by the displacement per unit separation, or strain h. This quality is also equal to the perturbation in the space-time metric accompanying the wave. Detectors having such a sensitivity would be able to detect supernovae in our own galaxy in which only of the mass is converted to gravitational radiation.
Neutron stars in binary systems gradually spiral together owing to the emission of gravitational radiation. In the final hours of its existence the binary system will emit a strong chirp of gravitational radiation sweeping from 10 Hz to 1 kHz, terminated by the tidal disruption of one or both of the stars themselves. By inferring a death rate for such binary systems from pulsar observations, one can anticipate that detectors having a strain sensitivity of , by reaching deeper into the universe, would detect several events of this type per year.
The above examples illustrate impulsive or burst sources; some periodic sources have also been posited. For these the anticipated gravitational-wave strains are much smaller; and correspondingly any practical search for them will most likely be restricted to our galaxy. A compensation, however, is that the observations can be extended over long integration times to improve strain sensitivity.
Pulsars rotating neutron stars would emit gravitational radiation as a result of any deviations from axial symmetry; the radiation frequency can be at the pulsar rotation frequency and at twice that frequency. The gravitational wave's strain amplitude is proportional to the ellipticity of the source. If the Crab or Vela pulsars had ellipticities as large as s, they would produce periodic strains at the Earth of at 60 and 22 Hz, respec- tively. These strain amplitudes could be within reach of some proposed detectors after a month of integration see Figure 6.
A final category of cosmic gravitational radiation is the stochastic background- a gravitational-wave background noise detectable as a correlated noise component in the output of a pair or more of detectors. The sources of such a background would most likely reside in the early universe, probably at epochs not accessible by electromag- netic radiation. Since a gravitational-wave background has energy density, experimental limits are usually quoted in terms of the universe's closure density Pc. At lower frequencies from 1 Hz to 1 Hz, space techniques and astro- physical observations must be used to search for gravitational waves.
This band contains the only astro- physical sources of gravitational radiation whose properties are well known the nearby binary stellar systems. The expected strain amplitude at multiples of the orbital frequency lo-4 Hz is of the order of These detectors were aluminum cylinders instrumented to detect excitations of the bar's fundamental quadrupole mode by passing gravitational waves. The bars, typically of 1-ton mass, were suspended in vacuum chambers on shock mounts to reduce acoustic and seismic noise.
They were operated at room temperature and achieved sensitivities limited only by thermal excitation of the quadrupole mode, a remarkably small noise amplitude. Coincidence detection with two separated bars was used to reduce accidental events. Experiments with such Weber bars have continued in several research groups throughout the world. Instrumentation improvements and cooling of the bars have helped to achieve a recent major improvement in sensitivity. The second main class of detectors, the laser interferometers, began development later and is less mature.
In these detectors the change in propagation time of light traversing a gravitational wave is measured. The polarization of quadrupole Einstein tensor waves causes changes in the propagation time of light with opposite sign in orthogonal directions transverse to the direction of gravitational-wave propaga- tion.
Laser-interferometer detectors exploit this polarization property by measuring the time difference of light propagating along the orthog- onal legs of an L-shaped interferometer whose mirrors are attached to three freely suspended masses. The time differences are measured. The effect grows with the time of interaction between the light and the gravitational wave, so multipass cavities are used.