Artículos con la etiqueta ‘historia de la física’

Entropy is in Flux

Por • 30 mar, 2014 • Category: Crítica

The science of thermodynamics was put together in the Nineteenth Century to describe large systems in equilibrium. One part of thermodynamics defines entropy for equilibrium systems and demands an ever-increasing entropy for non-equilibrium ones. However, starting with the work of Ludwig Boltzmann in 1872, and continuing to the present day, various models of non-equilibrium behavior have been put together with the specific aim of generalizing the concept of entropy to non-equilibrium situations. This kind of entropy has been termed {\em kinetic entropy} to distinguish it from the thermodynamic variety. Knowledge of kinetic entropy started from Boltzmann’s insight about his equation for the time dependence of gaseous systems. In this paper, his result is stated as a definition of kinetic entropy in terms of a local equation for the entropy density. This definition is then applied to Landau’s theory of the Fermi liquid thereby giving the kinetic entropy within that theory.

An intuition behind quantum measurement

Por • 29 mar, 2014 • Category: Opinion

An attempt is made to give a heuristic explanation of the distinguished role of measurement in the quantum theory. We question the notion of “naive” reductionism by stressing the difference between an isolated quantum and classical object. It is argued that the transition from the micro- to the macroscopic description should be made along some parameters not characterized by the quantum theory.

Innovations in Statistical Physics

Por • 29 mar, 2014 • Category: Leyes

In 1963-71, a group of people, myself included, formulated and perfected a new approach to physics problems, which eventually came to be known under the names of scaling, universality, and renormalization. This work formed the basis of a wide variety of theories ranging from its starting point in critical phenomena, and moving out to particle physics and relativity and then into economics and biology. This work was of transcendental beauty and of considerable intellectual importance.

Quantum Humeanism,or: physicalism without properties

Por • 27 mar, 2014 • Category: Ciencia y tecnología

In recent literature, it has become clear that quantum physics does not refute Humeanism. This point has so far been made with respect to Bohms quantum theory. Against this background, this paper seeks to achieve the following four results: to generalize the option of quantum Humeanism from Bohmian mechanics to primitive ontology theories in general, to show that this option applies also to classical mechanics, to establish that it requires a commitment to matter as primitive stuff, but no commitment to natural properties (physicalism without properties, to point out that by removing the commitment to properties, the stock metaphysical objections against Humeanism from quidditism and humility no longer apply. In that way, quantum physics strengthens Humeanism instead of refuting it.

Quantum Theory and Human Perception of the Macro-World

Por • 21 mar, 2014 • Category: Filosofía

We investigate the question of ‘why customary macroscopic entities appear to us humans as they do, i.e. as bounded entities occupying space and persisting through time’, starting from our knowledge of quantum theory, how it affects the behavior of such customary macroscopic entities, and how it influences our perception of them. For this purpose, we approach the question from three perspectives. The specific and very classical perception of human seeing — light as a geometric theory — and human touching — only ruled by Pauli’s exclusion principle — plays a role in our perception of macroscopic entities as ontologically stable entities in space. To ascertain quantum behavior in such macroscopic entities, we will need measuring apparatuses capable of its detection. Future experimental research will have to show if sharp quantum effects — as they occur in smaller entities — appear to be ontological aspects of customary macroscopic entities.

Present time

Por • 21 mar, 2014 • Category: Educacion

The idea of a moving present or `now’ seems to form part of our most basic beliefs about reality. Such a present, however, is not reflected in any of our theories of the physical world. I show in this article that presentism, the doctrine that only what is present exists, is in conflict with modern relativistic cosmology and recent advances in neurosciences. I argue for a tenseless view of time, where what we call `the present’ is just an emergent secondary quality arising from the interaction of perceiving self-conscious individuals with their environment. I maintain that there is no flow of time, but just an ordered system of events.

Solving the Hard Problem of Bertrand’s Paradox

Por • 19 mar, 2014 • Category: Leyes

Bertrand’s paradox is a famous problem of probability theory, pointing to a possible inconsistency in Laplace’s principle of insufficient reason. In this article we show that Bertrand’s paradox contains two different problems: an “easy” problem and a “hard” problem. The easy problem can be solved by formulating Bertrand’s question in sufficiently precise terms, so allowing for a non ambiguous modelization of the entity subjected to the randomization. We then show that once the easy problem is settled, also the hard problem becomes solvable, provided Laplace’s principle of insufficient reason is applied not to the outcomes of the experiment, but to the different possible “ways of selecting” an interaction between the entity under investigation and that producing the randomization.

The Early Years of Condensed Matter Physics at Illinois — in Celebration of the 80th Birth Year of Charles P. Slichter — Charlie Slichter & the gang at Urbana

Por • 15 mar, 2014 • Category: Opinion

The 1950s– and perhaps also the 1960s– were very special times for the development of solid-state/condensed-matter physics. The University of Illinois at Urbana was at the center of these activities. In areas like NMR and superconductivity, methods were developed which would form the basis for the next half century of science and technology. Experimentalists, including Charlie and John Wheatley, worked hand in hand with theorists, including the incomparable John Bardeen. They worked cooperatively to develop ideas, often born in Urbana, but with godparents at Harvard and Moscow and Paris. A characteristic style of broad collaboration and spirited exchange developed and spread from Illinois.

The axiomatic deduction of the quadratic Hencky strain energy by Heinrich Hencky

Por • 23 feb, 2014 • Category: Crítica

The introduction of the quadratic Hencky strain energy based on the logarithmic strain tensor log V is a milestone in the development of nonlinear elasticity theory in the first half of the 20th century. Since the original manuscripts are written in German, they are not easily accessible today. However, we believe that the deductive approach taken by Hencky deserves to be rediscovered today.

Schwinger’s Quantum Action Principle. I. From Dirac’s Formulation through Feynman’s Path Integrals to the Schwinger-Keldysh Method

Por • 23 feb, 2014 • Category: Leyes

Starting from the earlier notions of stationary action principles, we show how Schwinger’s Quantum Action Principle descended from Dirac’s formulation, which independently led Feynman to his path-integral formulation of quantum mechanics. The connection between the two is brought out, and applications are discussed. The Keldysh-Schwinger time-cycle method of extracting matrix elements is described. Part II will discuss the variational formulation of quantum electrodynamics and the development of source theory.