Tag Archives: History and Philosophy of Physics

Gamow's Calculation of the Neutron Star Critical Mass Revised [SSA]

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http://arxiv.org/abs/1402.3691


We revise Gamow’s calculation of the neutron star critical mass \cite{gamow} regarding calculational and conceptual aspects and discuss the implications on its importance for the history of astrophysics. We compare his approach to other early and modern approaches to the problem.

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H. Ludwig and R. Ruffini
Tue, 18 Feb 14
45/72

The Figure of the Sun from Ground-based Experiments [SSA]

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http://arxiv.org/abs/1402.0497


The figure of the Sun reflects its inner structure and dynamics, influencing also the perihelion precession of close orbiting bodies, like Mercury. To study the solar figure from ground, the deformation on the solar image induced by the atmosphere has to be known up to one part over a million, and this is done through differential refraction models: a historical review of them is drafted. The solar oblateness has been investigated in order to validate alternative theories to General Relativity and to understand the internal dynamics of our star. The solar figure should possess only micro departures from sphericity according to the standard stellar structure theory and helioseismology data, though variations along the cycle has been observed. Ground-based and satellite data show contrasting observational results. The oblateness measured onboard RHESSI satellite, the one of SDS onboard a stratospheric ballon and that one of the Astrolabe of Rio de Janeiro are presented with their implications in classical and relativistic gravitation. The perspectives offered by the reflecting heliometer in the future measurement of the oblateness are depicted.

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Wed, 5 Feb 14
6/61

In the footsteps of Ebenezer Porter Mason and his nebulae [CL]

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http://arxiv.org/abs/1401.7960


In 1839 Ebenezer Porter Mason (1819-1840) produced detailed drawings of the Omega Nebula (M17), the Trifid Nebula (M20) and the eastern part of the Veil Nebula (NGC 6992 and 6995). He used a 12-inch (30 cm) reflector that he and his friends had built at Yale College, which at the time was the largest telescope in the USA. The drawings were remarkable for their accuracy and for his adoption of a new technique for delineating gradients in nebulosity using isophotes, or lines of equal brightness. This paper reviews his life and his observations, comparing his results with those of the modern amateur astronomer.

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Fri, 31 Jan 14
14/55

Water-filled telescopes [CL]

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http://arxiv.org/abs/1401.5585


In this short note we discuss the case of the thought experiments on water-filled telescopes and their realizations during 18th and 19th century. The story of those instruments shows that the scientific progress occurs in a curious way, since there was no stringent reason for the construction of a water-filled telescope.

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Thu, 23 Jan 14
34/70

Reconstructing Past Solar Activity using Meridian Solar Observations: the Case of the Royal Observatory of the Spanish Navy (1833-1840) [SSA]

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http://arxiv.org/abs/1401.4501


Solar meridian observations have been used to evaluate the solar activity of the past. Some important examples are the solar meridian observations made at the Basilica of San Petronio in Bologna by several astronomers and the observations made by Hevelius published in his book Machina Coelestis. However, we do not know whether these observations, which were not aimed to estimate the solar activity, are reliable for evaluating solar activity. In this paper, we present the marginal notes about sunspots that are included in the manuscripts of the meridian solar observations made at the Royal Observatory of the Spanish Navy during the period 1833-1840. We compare these observations with other solar activity indices such as sunspot area and number. Our conclusion is that solar meridian observations should be used with extreme caution to evaluate past solar activity.

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Tue, 21 Jan 14
67/91

La meridiana di Egnazio Danti nella Torre dei Venti in Vaticano: un'icona della riforma Gregoriana del calendario [CL]

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http://arxiv.org/abs/1401.3577


The meridian line traced by Egnazio Danti in the Torre dei Venti in Vatican demostrated the need of the Gregorian Reformation of the Calendar. A review on this instrument is presented, along with the astrometric recognition. This meridian line, dated 1580, is deviated of 72 arcminutes Westward from the true North.

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Thu, 16 Jan 14
1/53

Cultural Astronomy and Archaeoastronomy: an Italian Experience [CL]

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http://arxiv.org/abs/1312.2332


A brief review is given of some recent positive developments regarding the reception of archaeoastronomy by the archaeological institutions in Italy. Discussions and problems that are currently going on in this field are also mentioned, such as the separation of the scientific and humanistic disciplines (i.e. the two cultures problem). Suggestions based on contemporary philosophy are also reported. Finally, sky-gazing is proposed as the place where the two cultures could meet, since, taking Plato into account, sky-gazing could be considered the mom of the human knowledge, and of the scientific and humanistic disciplines.

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Tue, 10 Dec 13
50/67

On the Possibility of an Astronomical Perspective in the Study of Human Evolution [CL]

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http://arxiv.org/abs/1312.1561


The Sapient Paradox is the apparently unexplainable time delay of several ten thousand years following the arrival of Homo sapiens in Asia and Europe and before the introduction of impressive innovations with the agricultural revolution. Renfrew (2007) has suggested that the solution of the paradox has to do with changes in modes of thought that occurred with sedentism. According to Renfrew, this is a subject of study for cognitive archaeology where the final goal would be to understand the formation of the human mind. Several scholars, however, affirm that climatic change was crucial to such a revolution as it would have been very difficult to develop agriculture during the Palaeolithic. In other words, sedentism was not justified during the ice age, and that may be the solution to the paradox. It is widely accepted that climate variations were due to so-called orbital forcing, the slow periodic changes of orbital parameters of the Earth (known also as the Milankovitch theory). These and other astronomical effects on the climate are discussed along with the consequent impact on human evolution. The question then rises as to whether or not it is possible to adopt an astronomical perspective instead of (or complementary to) the cognitive archaeological one. Such would be possible by adopting a different point of reference (that is, from outside), and a non-anthropocentric approach.

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Fri, 6 Dec 13
39/55

Solar motion correction in the early extragalactic astrophysics [CL]

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http://arxiv.org/abs/1312.0472


Redshift observations of galaxies outside the Local Group are fairly common in extragalactic astrophysics. If redshifts are interpreted as arising from radial velocities, these must be corrected by the contamination of the solar motion. We discuss the details of such correction in the way it was performed by the American astronomer Edwin Hubble in his 1929 seminal paper. The investigations of spiral nebulae undertaken by the Swedish astronomer Knut Lundmark, in 1924, are also considered in this context.

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Tue, 3 Dec 13
44/69

Non-standard Models and the Sociology of Cosmology [CL]

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http://arxiv.org/abs/1311.6324


I review some theoretical ideas in cosmology different from the standard “Big Bang”: the quasi-steady state model, the plasma cosmology model, non-cosmological redshifts, alternatives to non-baryonic dark matter and/or dark energy, and others. Cosmologists do not usually work within the framework of alternative cosmologies because they feel that these are not at present as competitive as the standard model. Certainly, they are not so developed, and they are not so developed because cosmologists do not work on them. It is a vicious circle. The fact that most cosmologists do not pay them any attention and only dedicate their research time to the standard model is to a great extent due to a sociological phenomenon (the “snowball effect” or “groupthink”). We might well wonder whether cosmology, our knowledge of the Universe as a whole, is a science like other fields of physics or a predominant ideology.

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Tue, 26 Nov 13
8/58

Einstein's equivalence principle in cosmology [CL]

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http://arxiv.org/abs/1311.4912


We study physical consequences of the Einstein equivalence principle (EEP) for a Hubble observer in FLRW universe. We introduce the local inertial coordinates with the help of a special conformal transformation. The local inertial metric is Minkowski flat and materialized by a congruence of time-like geodesics of static observers. The static observers are equipped with the ideal clocks measuring the proper time that is synchronized with the clocks of the Hubble observer. The local inertial metric is used for physical measurements of spacetime intervals with the ideal clocks and rulers. The special conformal transformation preserves null geodesics but does not keep invariant time-like geodesics. Moreover, it makes the rate of the local time coordinate dependent on velocity of the particle which makes impossible to rich the uniform parameterization of the world lines of static observers and light geodesics with a single parameter – they differ by the conformal factor of FLRW metric. The most convenient way to study the local propagation of light in cosmology is in terms of the optical metric induced by projection of FLRW metric on the light cone characteristics parameterized by the proper time of the Hubble observer. The local optical metric is not flat but depends on the scale factor of FLRW universe and it can be interpreted as a weak violation of EEP for photons. The importance of this violation is that some of local experiments conducted with freely-propagating electromagnetic waves may be sensitive to the Hubble expansion. We show that the Hubble constant H can be measured within the solar system by means of high-precision spacecraft Doppler tracking as a blue shift of frequency of radio waves circulating in the Earth-spacecraft radio link. We also analyze the behavior of the standing wave in a microwave resonator and show that the standing wave is insensitive to the Hubble expansion.

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Thu, 21 Nov 13
13/50

Einstein's conversion from his static to an expanding universe [CL]

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http://arxiv.org/abs/1311.2763


In 1917 Einstein initiated modern cosmology by postulating, based on general relativity, a homogeneous, static, spatially curved universe. To counteract gravitational contraction he introduced the cosmological constant. In 1922 Alexander Friedman showed that Einstein’s fundamental equation also allowed dynamical worlds, and in 1927 Geroges Lemaitre, backed by observational evidence, concluded that our universe was expanding. Einstein impetuously rejected Friedman’s as well as Lemaitre’s findings. However, in 1931 he retracted his former static model in favour of a dynamic solution. This investigation follows Einstein on his hesitating path from a static to the expanding universe. Contrary to an often repeated belief the primary motive for his switch was not observational evidence, but the realisation that his static model was unstable.

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Wed, 13 Nov 13
7/56

Discovery of the Hot Big Bang: What happened in 1948

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The idea that the universe is filled with the thermal radiation now termed the Cosmic Microwave Background was first discussed in eleven publications in the year 1948. These papers offer a detailed example of the process of development of a new and now very productive line of research, and of the confusion that can attend new ideas. The confusion in this case left a common misunderstanding of the considerations that motivated the idea of the sea of radiation.

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Date added: Wed, 9 Oct 13