Showing papers by "Helge Kragh published in 2012"

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

Abstract: According to modern physics and cosmology, the universe expands at an increasing rate as the result of a “dark energy” that characterizes empty space. Although dark energy is a modern concept, some elements in it can be traced back to the early part of the twentieth century. I examine 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 Lemaitre 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.

‘The Wildest Speculation of All’: Lemaître and the Primeval-Atom Universe

Abstract: Although there is no logical connection between the expanding universe and the idea of a big bang, from a historical perspective the two concepts were intimately connected. Four years after his pioneering work on the expanding universe, Lemaitre suggested that the entire universe had originated in a kind of explosive act from what he called a primeval atom and which he likened to a huge atomic nucleus. His theory of 1931 was the first realistic finite-age model based upon relativistic cosmology, but it presupposed a material proto-universe and thus avoided an initial singularity. What were the sources of Lemaitre’s daring proposal? Well aware that his new cosmological model needed to have testable consequences, he argued that the cosmic rays were fossils of the original radioactive explosion. However, this hypothesis turned out to be untenable. The first big-bang model ever was received with a mixture of indifference and hostility. Why? The answer is not that contemporary cosmologists failed to recognize Lemaitre’s genius, but rather that his model was scientifically unconvincing. Although Lemaitre was indeed the father of big-bang cosmology, his brilliant idea was only turned into a viable cosmological theory by later physicists.

Zöllner’s Universe

Abstract: The idea that space is not Euclidean by necessity, and that there are other kinds of “curved” spaces, diffused slowly to the physical and astronomical sciences. Until Einstein’s general theory of relativity, only a handful of astronomers contemplated a connection between non-Euclidean geometry and real space. One of them, the German astrophysicist Johann Carl Friedrich Zollner (1834–1882), suggested in 1872 a remarkable cosmological model describing a finite universe in closed space. I examine Zollner’s little-known contribution to cosmology and also his even more unorthodox speculations of a four-dimensional space including both physical and spiritual phenomena. I provide an overview of Zollner’s scientific work, of his status in the German scientific community, and of the controversies caused by his polemical style of science. Zollner’s cosmology was effectively forgotten, but there is no reason why it should remain an unwritten chapter in the history of science.

The isotope effect: Prediction, discussion, and discovery

Abstract: 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 led to successes as well as problems. At the end of the paper I briefly comment on the relationship between theory, experiment and prediction in this area of spectral physics.