arXiv subjects

Helge Kragh

Publications and source records attributed to Helge Kragh.

40 records · Page 3Linked to original sources

arXiv2011

Spreading the gospel: The Bohr atom popularised

The emergence of quantum theory in the early decades of the twentieth century was accompanied by a wide range of popular science books, all of which presented in words and in images new scientific ideas about the structure of the atom. The work of physicists such as Ernest Rutherford and Niels Bohr, among others, was pivotal to the so-called planetary model of the atom, which, still today, is used in popular accounts and in science textbooks. In an attempt to add to our knowledge about the popular trajectory of the new atomic physics, this paper examines one book in particular, coauthored by Danish science writer Helge Holst and Dutch physicist and close collaborator of Niels Bohr, Hendrik A. Kramers. Translated from Danish into four European languages, the book not only presented contemporary ideas about the quantum atom, but also went into rather lengthy discussions about unresolved problems. Moreover, the book was quite explicit in identifying the quantum atom with the atom as described by Bohr's theory. We argue that Kramers and Holst's book, along with other atomic books, was a useful tool for physicists and science popularisers as they grappled with the new quantum physics.

physics.hist-ph↗

arXiv2011

The isotope effect: Prediction, discussion, and discovery

The precise position of a spectral line emitted by an atomic system depends on the mass of the atomic nucleus and is therefore different for isotopes belonging to the same element. The possible presence of an isotope effect followed from Bohr's atomic theory of 1913, but it took several years before it was confirmed experimentally. Its early history involves the childhood not only of the quantum atom, but also of the concept of isotopy. Bohr's prediction of the isotope effect was apparently at odds with early attempts to distinguish between isotopes by means of their optical spectra. However, in 1920 the effect was discovered in HCl molecules, which gave rise to a fruitful development in molecular spectroscopy. The first detection of an atomic isotope effect was no less important, as it was by this means that the heavy hydrogen isotope deuterium was discovered in 1932. The early development of isotope spectroscopy illustrates the complex relationship between theory and experiment, and is also instructive with regard to the concepts of prediction and discovery.

physics.hist-ph↗

arXiv2011

Visions of Revolutions: Microphysics and Cosmophysics in the 1930s

By 1930, at a time when the new physics based on relativity and quantum theory had reached a state of consolidation, problems of a foundational kind began to abound. Physicists began to speak of a new "crisis" and envisage a forthcoming "revolution" of a scale similar to the one in the mid-1920s. The perceived crisis was an issue not only in microphysics but also in cosmology, where it resulted in ambitious cosmophysical theories that transcended the ordinary methods of physics. The uncertain cognitive situation was, in some circles, associated to the uncertain political and moral situation. Did the problems of foundational physics demand a revolution in thinking that somehow paralleled the political revolutions of the time? I argue that although such ideas were indeed discussed in the 1930s, they were more rhetoric than reality. With the benefit of hindsight one can see that the perceived crisis was only temporary and not significantly related to social or ideological developments in the decade.

physics.hist-ph↗

arXiv2011

Preludes to dark energy: Zero-point energy and vacuum speculations

Although dark energy is a modern concept, some elements in it can be traced back to the early part of the twentieth century. This paper examines the origin of the idea of zero-point energy and in particular how it appeared in a cosmological context in a hypothesis proposed by Walther Nernst in 1916. The hypothesis of a zero-point vacuum energy attracted some attention in the 1920s, but without attempts to relate it to the cosmological constant that was discussed by Georges Lemaître in particular. Only in the late 1960s was it recognized that there is a connection between the cosmological constant and the quantum vacuum. As seen in retrospect, many of the steps that eventually led to the insight of a kind of dark energy occurred isolated and uncoordinated.

physics.hist-ph↗