arXiv subjects

Helge Kragh

Publications and source records attributed to Helge Kragh.

At least 37 records · Page 2Linked to original sources

arXiv2014

Historical Aspects of Post-1850 Cosmology

Cosmology as an exact physical science is of new date, but it has long roots in the past. This essay is concerned with four important themes in the history of cosmological thought which, if taken together, offer a fairly comprehensive account of some of the key developments that have led to the modern understanding of the universe. Apart from the first section, dealing with early views of curved space, it focuses on mainstream cosmology from the expanding universe about 1930 to the emergence of the standard big bang model in the 1960s. This development includes theories we would not today consider "mainstream," such as the steady state model of the universe. The last section outlines what might be called the prehistory of the concept of dark energy, that is, ideas that were discussed before dark energy was actually inferred from supernovae observations in the late 1990s.

physics.hist-ph↗

arXiv2014

Galileo in early modern Denmark, 1600-1650

The scientific revolution in the first half of the seventeenth century, pioneered by figures such as Harvey, Galileo, Gassendi, Kepler and Descartes, was disseminated to the northernmost countries in Europe with considerable delay. In this essay I examine how and when Galileo's new ideas in physics and astronomy became known in Denmark, and I compare the reception with the one in Sweden. It turns out that Galileo was almost exclusively known for his sensational use of the telescope to unravel the secrets of the heavens, meaning that he was predominantly seen as an astronomical innovator and advocate of the Copernican world system. Danish astronomy at the time was however based on Tycho Brahe's view of the universe and therefore hostile to Copernican and, by implication, Galilean cosmology. Although Galileo's telescope attracted much attention, it took about thirty years until a Danish astronomer actually used the instrument for observations. By the 1640s Galileo was generally admired for his astronomical discoveries, but no one in Denmark drew the consequence that the dogma of the central Earth, a fundamental feature of the Tychonian world picture, was therefore incorrect.

physics.hist-ph↗

arXiv2014

The True (?) Story of Hilbert's Infinite Hotel

What is known as "Hilbert's hotel" is a story of an imaginary hotel with infinitely many rooms that illustrates the bizarre consequences of assuming an actual infinity of objects or events. Since the 1970s it has been used in a variety of arguments, some of them relating to cosmology and others to philosophy and theology. For a long time it has remained unknown whether David Hilbert actually proposed the thought experiment named after him, or whether it was merely a piece of mathematical folklore. It turns out that Hilbert introduced his hotel in a lecture of January 1924, but without publishing it. The counter-intuitive hotel only became better known in 1947, when George Gamow described it in a book, and it took nearly three more decades until it attracted wide interest in scientific, philosophical, and theological contexts. The paper outlines the origin and early history of Hilbert's hotel paradox. At the same time it retracts the author's earlier conclusion that the paradox was originally due to Gamow.

physics.hist-ph↗

arXiv2014

Photon: New light on an old name

After G. N. Lewis (1875-1946) proposed the term "photon" in 1926, many physicists adopted it as a more apt name for Einstein's light quantum. However, Lewis' photon was a concept of a very different kind, something few physicists knew or cared about. It turns out that Lewis' name was not quite the neologism that it has usually been assumed to be. The same name was proposed or used earlier, apparently independently, by at least four scientists. Three of the four early proposals were related to physiology or visual perception, and only one to quantum physics. Priority belongs to the American physicist and psychologist L. T. Troland (1889-1932), who coined the word in 1916, and five years later it was independently introduced by the Irish physicist J. Joly (1857-1933). Then in 1925 a French physiologist, Rene Wurmser (1890-1993), wrote about the photon, and in July 1926 his compatriot, the physicist F. Wolfers (ca. 1890-1971), did the same in the context of optical physics. None of the four pre-Lewis versions of "photon" was well known and they were soon forgotten.

physics.hist-ph↗

arXiv2013

The many faces of the Bohr atom

The atomic model that Niels Bohr suggested in 1913 celebrated its greatest victories in connection with one-electron atoms. Among them were the isotopic spectral effect and what became known as Rydberg atoms, insights that were fully recognized only many years later. He considered the original ring model a first step towards an understanding of atomic structure, and during the following years he developed it into more ambitious models that, he hoped, would also describe many-electron atoms. His theory of the periodic system marked the culmination of the orbital atom within the framework of the old quantum theory. However, the theory would soon be replaced by more symbolic models that heralded the coming of the quantum-mechanical atom.

physics.hist-ph↗

arXiv2013

Cyclic models of the relativistic universe: the early history

Within the framework of relativistic cosmology oscillating or cyclic models of the universe were introduced by A. Friedmann in his seminal paper of 1922. With the recognition of evolutionary cosmology in the 1930s this class of closed models attracted considerable interest and was investigated by several physicists and astronomers. Whereas the Friedmann-Einstein model exhibited only a single maximum value, R. Tolman argued for an endless series of cycles. After World War II, cyclic or pulsating models were suggested by W. Bonnor and H. Zanstra, among others, but they remained peripheral to mainstream cosmology. The paper reviews the development from 1922 to the 1960s, paying particular attention to the works of Friedmann, Einstein, Tolman and Zanstra. It also points out the role played by bouncing models in the emergence of modern big-bang cosmology.

physics.hist-ph↗

arXiv2013

The rise and fall of cosmical physics: notes for a history, ca. 1850-1920

In the period from about 1890 to 1915 an interdisciplinary and unifying research programme known as "cosmical physics" attracted much scientific and public attention. It typically included aspects of the earth sciences (such as magnetic storms and atmospheric electricity) combined with astronomical subjects (such as the solar corona and cometary tails), but there was no unanimity as to the precise meaning of cosmical physics, which collapsed after World War I. The essay covers the history of cosmical physics as it unfolded in particular in Germany, Austria, England and Scandinavia. Among the scientists who contributed to the development of cosmical physics were Wilhelm Foerster from Germany, Wilhelm Trabert from Austria, Kristian Birkeland from Norway and Svante Arrhenius from Sweden. While cosmical physics did not usually involve a cosmological dimension, both Birkeland and Arrhenius constructed cosmologies based on their work on auroral and other phenomena.

physics.hist-ph↗

arXiv2013

What's in a Name: History and Meanings of the Term "Big Bang"

The name "big bang" introduced by Fred Hoyle in 1949 is one of the most successful scientific neologisms ever. How did the name originate and how was it received by physicists and astronomers in the period leading up to the hot big bang consensus model in the late 1960s? How did it reflect the meanings of the big bang, a concept that predates the name by nearly two decades? The paper gives a detailed account of names and concepts associated with finite-age cosmological models from the 1920s to the 1970s. It turns out that Hoyle's celebrated name has a richer and more surprising history than commonly assumed and also that the literature on modern cosmology and its history includes many common mistakes and errors. By following the story of "big bang" a new dimension is added to the historical understanding of the emergence of modern cosmology.

physics.hist-ph↗

arXiv2012

Cosmology and Science Education: Problems and Promises

Cosmology differs in some respects significantly from other sciences, primarily because of its intimate association with issues of a conceptual and philosophical nature. Because cosmology in the broader sense relates to the world views held by students, it provides a means for bridging the gap between the teaching of science and the teaching of humanistic subjects. Students should of course learn to distinguish between what is right and wrong about the science of the universe. No less importantly, they should learn to recognize the limits of science and that there are questions about nature that may forever remain unanswered. Cosmology, more than any other science, is well suited to illuminate issues of this kind.

physics.hist-ph↗

arXiv2012

Criteria of Science, Cosmology, and Lessons of History

Perhaps more than any other of the physical sciences, cosmology exemplifies the inevitable contact between science and philosophy, including the problem of the demarcation criteria that distinguish science from non-science. Although modern physical cosmology is undoubtedly scientific, it is not obvious why it has this status, and nor is it obvious that all branches of theoretical cosmology satisfy ordinarily assumed criteria for science. While testability is generally admitted as an indispensable criterion for a theory being scientific, there is no agreement among cosmologists what testability means, more precisely. For example, should testability be taken to imply falsifiability in the sense of Popper? I discuss this and related questions by referring to two episodes of controversy in the history of modern cosmology, the debate over the steady state theory in the 1950s and the recent debate concerned with the anthropic multiverse. In addition, I draw attention to the use of historical analogies in cosmological and other scientific arguments, suggesting that such use is often misuse or otherwise based on distortions of the history of science.

physics.hist-ph↗

arXiv2012

Eight Physicists and Astronomers: Biographical Portraits

This essay provides concise biographical information about eight physicists, astronomers, astrophysicists, and cosmologists from the twentieth century. The portrayed scientists are Hermann Bondi (1919-2005), Charles L. Critchfield (1910-1994), Arthur E. Haas (1884-1941), Chushiro Hayashi (1920-2010, Gustave Le Bon (1841-1931), Wilhelm Lenz (1888-1957), Franz Selety (1893-1933?), and S. Elis Strömgren (1870-1947). Because the entries were written for a biographical dictionary of astronomers, the emphasis is on the astronomical and astrophysical contributions of the scientists rather than their work in physics. While some of them are well known, others (such as Selety, Haas and Le Bon) will probably be unknown to most physicists.

physics.hist-ph↗

arXiv2012

A Note on the Prehistory of Superheavy Elements

Artificially produced chemical elements heavier than uranium have been known for more than seventy years and the number of superheavy elements continues to grow. Presently 26 transuranic elements are known. This paper examines the earliest scientific interest in the very heavy elements and the related question of an upper limit of the periodic system. In the period from the 1880s to the early 1930s, three kinds of questions appealed to a minority of physicists, chemists and astronomers: (1) Why is uranium the heaviest known element? (2) Do there exist transuranic or superheavy elements elsewhere in the universe, such as in stellar interiors? (3) Is there a maximum number of elements, corresponding to a theoretical limit for the periodic system? The early attempts to answer or clarify these questions lacked a foundation in nuclear physics, not to mention the total lack of experimental evidence, which explains why most of them were of a speculative nature. Although the speculations led no nothing, they are interesting in their own right and deserve a place in the history of the physical sciences.

physics.hist-ph↗

arXiv2012

A Sense of Crisis: Physics in the fin-de-siecle Era

Whereas physics in the period from about 1880 to 1910 experienced a steady growth, it was also a revolutionary period in which the foundations of the physical world picture were criticized and reconsidered. Generally speaking, from about 1890 mechanics and materialism came under increasing attack and sought replaced by new formulations based on either energy, the ether, or the electromagnetic field. Fin-de-siecle physics was in many ways a chapter of turmoil in the history of science. I review the main developments and alternatives to the established physics, in particular energetics, ether physics, the electromagnetic world view, and also the role played by radioactivity and other new rays discovered in the years around 1900. In the end the anticipated revolution based on the "matter is dead" catchword did not succeed. A revolution did take place in the period, but it was a different one that did not derive from the Zeitgeist of fin de siecle.

physics.hist-ph↗

arXiv2012

Geometry and Astronomy: Pre-Einstein Speculations of Non-Euclidean Space

The recognition that physical space (or space-time) is curved is a product of the general theory of relativity, such as dramatically shown by the 1919 solar eclipse measurements. However, the mathematical possibility of non-Euclidean geometries was recognized by Gauss more than a century earlier, and during the nineteenth century mathematicians developed the pioneering ideas of Gauss, Lobachevsky, Bolyai and Riemann into an elaborate branch of generalized geometry. Did the unimaginative physicists and astronomers ignore the new geometries? Were they considered to be of mathematical interest only until Einstein entered the scene? This paper examines in detail the attempts in the period from about 1830 to 1910 to establish links between non-Euclidean geometry and the physical and astronomical sciences, including attempts to find observational evidence for curved space. Although there were but few contributors to "non-Euclidean astronomy," there were more than usually supposed. The paper looks in particular on a work of 1872 in which the Leipzig physicist K. F. Zoellner argued that the universe is closed in accordance with Riemann's geometry.

physics.hist-ph↗

arXiv2012

The Universe, the Cold War, and Dialectical Materialism

Ideological considerations have always influenced science, but rarely as directly and massively as in the Soviet Union during the early Cold War period. Cosmology was among the sciences that became heavily politicized and forced to conform to the doctrines of Marxism-Leninism. This field of science developed entirely differently in the Communist countries than in the West, in large measure because of political pressure. Certain cosmological models, in particular of the big bang type, were declared pseudo-scientific and idealistic because they implied a cosmic creation, a concept which was taken to be religious. The result of the ideological pressure was not an independent Soviet cosmology, but that astronomers and physicists abandoned cosmological research in the Western sense. Only in the 1960s did this situation change, and cosmology in the Soviet Union began to flourish. The paper examines the relationship between science and political ideology in the case of the Soviet Union from about 1947 to 1963, and it also relates this case to the later one in the People's Republic of China.

physics.hist-ph↗

arXiv2012

Rutherford, Radioactivity, and the Atomic Nucleus

Modern atomic and nuclear physics took its start in the early part of the twentieth century, to a large extent based upon experimental investigations of radioactive phenomena. Foremost among the pioneers of the new kind of physics was Ernest Rutherford, who made fundamental contributions to the structure of matter for more than three decades and, in addition, founded important research schools in Manchester and Cambridge. This paper reviews the most important aspects of Rutherford's scientific work in the period from about 1900 to 1920, and it also refers to some of his last experiments of the 1930s. The emphasis is on his theory of radioactive disintegration (1902), the discovery of the atomic nucleus (1911), and the first artificially produced element transformation (1919). Following the transmutation experiments, Rutherford developed elaborate models of the atomic nucleus, but these turned out to be unsuccessful. Other subjects could be included, but the three mentioned are undoubtedly those of the greatest importance, the nuclear atom perhaps the greatest and the one with the most far-reaching consequences.

physics.hist-ph↗

arXiv2012

Quasi-Steady-State and Related Cosmological Models: A Historical Review

Since the emergence in the late 1960s of the standard hot big-bang theory, cosmology has been dominated by finite-age models. However, the rival view that the universe has existed for an indefinite time has continued to be defended by a minority of researchers. This view has roots far back in history and in the 1950s and 1960s several models were proposed in opposition to the big-bang paradigm. The most important of the alternative models, the steady-state cosmology proposed in 1948, was uniformly expanding rather than exhibiting a cyclical behaviour. In a much revised version it was developed into the quasi-steady-state cosmological model (QSSC) of the 1990s. From a historical point of view, this model, and a few other related models, can be seen as the latest examples of a tradition in cosmological thought that goes back to ancient Greece. The paper describes the background and development of the QSSC model.

physics.hist-ph↗

arXiv2011

An early explanation of the periodic table: Lars Vegard and X-ray spectroscopy

The Norwegian physicist Lars Vegard may have been the first to propose electron configurations for all the chemical elements, from hydrogen to uranium, on the basis of quantum atomic theory. This he did in papers of 1918-1919 in which he argued that the principal quantum number corresponded to the shells in Bohr's picture of atomic structure. Vegard's theory of the periodic system, based on the methods of theoretical X-ray spectroscopy in particular, was a significant advance but exerted little influence on the further development. It presupposed atoms made up of planar electron rings, and with the abandonment of the ring atom about 1920 this and similar models lost their credibility.

physics.hist-ph↗