Rodrigo de Abreu

The principle of relativity and the indeterminacy of special relativity

This work ends a trilogy devoted to a journey into the foundations of special relativity. The first paper debated the meaning of the constancy of the twoway speed of light and its close relation to the conceptualization of time. The second one addressed the question of the possible constancy of the oneway speed of light and the trivial—but, unfortunately, even now somewhat controversial—question of the compatibility between the assumption of a special system of reference and Einstein’s special relativity. The present study deals with the principle of relativity. Its historical evolution is reviewed and a ‘weak’ formulation is defended. It is emphasized that many assertions usually associated with special relativity, such as the ‘relativity of time dilation’ and ‘relativity of space contraction’ are indeed philosophical statements, as it has been established already by several authors in the past. Nonetheless, most teachers and scientists still believe nowadays they are implied by the theory and by the group property of the Lorentz transformation. This is by no means so, as it is reviewed and elucidated with the simple example on space contraction. It is argued that the lack of knowledge of the true value of the one-way speed of light in empty space leaves the theory undetermined. Einstein’s special relativity corresponds to a simple and very elegant solution to this problem, allowing the study of relative motion without any concern with the study of absolute motion, which is considered to be superfluous. However, its standard interpretation is minimalist and even misleading. A large number of researchers have discussed this question, mostly within the conventionality of simultaneity thesis. The typical formulation of this thesis provides some new physical insight and points out the problem, but does not solve it. In contrast, it often leads to a labyrinth of difficult language which is herein clarified. (Some figures in this article are in colour only in the electronic version)

Introducing thermodynamics through energy and entropy

We suggest a simple approach to introducing thermodynamics, beginning with the concept of internal energy of deformable bodies. From a series of thought experiments involving ideal gases, we show that the internal energy depends on the volume and on a second parameter, leading to the development of the concept of entropy. By introducing entropy before the notions of temperature and heat, the proposed approach avoids some of the major conceptual difficulties with the traditional presentation. The relationship between mechanics and thermodynamics naturally emerges, mechanics corresponding to isentropic thermodynamics. The questions of evolution to equilibrium and irreversibility are studied under the light of the action of the “dynamic force” and its dissipative character, evincing the benefits of keeping in mind the microscopic picture.

Comment on “A close examination of the motion of an adiabatic piston,” by Eric A. Gislason [Am. J. Phys. 78 (10), 995–1001 (2010)]

A recent paper by Gislason treats the adiabatic piston, a system of two ideal gases in a horizontal cylinder and separated by an insulating piston that moves without friction. The analysis in this paper is comprehensive and useful as a teaching tool, but is somewhat misleading if not understood in the appropriate context. The evolution to equilibrium involves two mechanisms, a faster one leading to the equalization of pressures, and a slower one bringing the system to identical temperatures. Gislason addressed only the first mechanism. We note that the eventual final state is described by thermodynamics. Therefore, a discussion of the adiabatic piston can be enriched to promote a proper and general view of thermodynamics.

Comment on 'Symmetry of the adiabatic condition in the piston problem'

We make some observations regarding a recent paper by Anacleto and Ferreira (2011 Symmetry of the adiabatic condition in the piston problem Eur. J. Phys. 32 1625).

Time, Clocks and the Speed of Light

In this work we show that both the null result of Michelson‐Morley experiment in vacuum and the postulate of constancy of the “two‐way” speed of light in vacuum in all inertial frames independently of the state of motion of the emitting body are a direct consequence of the fundamental notions of time and clocks. They can be obtained without any mathematics under three very reasonable assumptions, namely: i) that all good clocks can be used to measure time, independently of the periodic physical phenomena they are built upon and of the machinery involved in their manufacturing; ii) that time is measured in the same way in all inertial frames, i.e., if a particular clock can be used to measure time in the “rest system”, a similar clock can be used to measure time in “moving” inertial frames; iii) that a limit speed exists in the “rest system”. The postulate of the constancy of the two‐way speed of light simply results from the construction of clocks where tic‐tacs are made by objects traveling with the limi...