4,762 research outputs found
The End of Time?
I discuss J. Barbour's Machian theories of dynamics, and his proposal that a
Machian perspective enables one to solve the problem of time in quantum
geometrodynamics (by saying that there is no time). I concentrate on his recent
book 'The End of Time' (1999).Comment: 48 pages Latex. A shortened version will appear in 'The British
Journal for Philosophy of Science
Surveys of facilities for the potential effects from the fallout of airborne graphite fibers
The impact of the entry of graphite fibers into workplaces in the United States is discussed. Areas where an electrical failure could cause major problems include process and production systems, hospitals, and police/fire emergency communication systems
A topos perspective on the Kochen-Specker theorem: II. Conceptual Aspects, and Classical Analogues:
In a previous paper, we have proposed assigning as the value of a physical
quantity in quantum theory, a certain kind of set (a sieve) of quantities that
are functions of the given quantity. The motivation was in part physical---such
a valuation illuminates the Kochen-Specker theorem; and in part
mathematical---the valuation arises naturally in the topos theory of
presheaves.
This paper discusses the conceptual aspects of this proposal. We also
undertake two other tasks. First, we explain how the proposed valuations could
arise much more generally than just in quantum physics; in particular, they
arise as naturally in classical physics. Second, we give another motivation for
such valuations (that applies equally to classical and quantum physics). This
arises from applying to propositions about the values of physical quantities
some general axioms governing partial truth for any kind of proposition.Comment: Small changes and correction
On the Emergence of Time in Quantum Gravity
We discuss from a philosophical perspective the way in which the normal
concept of time might be said to `emerge' in a quantum theory of gravity. After
an introduction, we briefly discuss the notion of emergence, without regard to
time (Section 2). We then introduce the search for a quantum theory of gravity
(Section 3); and review some general interpretative issues about space, time
and matter Section 4). We then discuss the emergence of time in simple quantum
geometrodynamics, and in the Euclidean approach (Section 5). Section 6
concludes.Comment: To appear in ``The Arguments of Time'', ed. J. Butterfield, Oxford
University Press, 199
A topos perspective on the Kochen-Specker theorem: I. Quantum States as Generalized Valuations
The Kochen-Specker theorem asserts the impossibility of assigning values to
quantum quantities in a way that preserves functional relations between them.
We construct a new type of valuation which is defined on all operators, and
which respects an appropriate version of the functional composition principle.
The truth-values assigned to propositions are (i) contextual; and (ii)
multi-valued, where the space of contexts and the multi-valued logic for each
context come naturally from the topos theory of presheaves.
The first step in our theory is to demonstrate that the Kochen-Specker
theorem is equivalent to the statement that a certain presheaf defined on the
category of self-adjoint operators has no global elements. We then show how the
use of ideas drawn from the theory of presheaves leads to the definition of a
generalized valuation in quantum theory whose values are sieves of operators.
In particular, we show how each quantum state leads to such a generalized
valuation.Comment: Clarification of situation for situation for operators with
continuous spectr
Some Aspects of Modality in Analytical Mechanics
This paper discusses some of the modal involvements of analytical mechanics.
I first review the elementary aspects of the Lagrangian, Hamiltonian and
Hamilton-Jacobi approaches. I then discuss two modal involvements; both are
related to David Lewis' work on modality, especially on counterfactuals.
The first is the way Hamilton-Jacobi theory uses ensembles, i.e. sets of
possible initial conditions. The structure of this set of ensembles remains to
be explored by philosophers.
The second is the way the Lagrangian and Hamiltonian approaches' variational
principles state the law of motion by mentioning contralegal dynamical
evolutions. This threatens to contravene the principle that any actual truth,
in particular an actual law, is made true by actual facts. Though this threat
can be avoided, at least for simple mechanical systems, it repays scrutiny; not
least because it leads to some open questions.Comment: 36 pages, no figures. Delivered at a Philosophy of Science
Association Symposium in memory of the distinguished philosopher David Lewis,
Milwaukee, November 2002. This version includes significant additions to
Section 5.1. This version is forthcoming in `Formal Teleology and Causality',
ed. M. Stoeltzner, P. Weingartner, Paderborn, Germany: Mentis. A precis of
the first half of the paper is forthcoming in the journal Philosophy of
Scienc
Surveys for sensitivity to fibers and potential impacts from fiber induced failures
The surveys for sensitivities to fibers and potential impacts from fiber induced failures begins with a review of the survey work completed to date and then describes an impact study involving four industrial installations located in Virginia. The observations and results from both the surveys and the study provide guidelines for future efforts. The survey work was done with three broad objectives: (1) identify the pieces of potentially vulnerable equipment as candidates for test; (2) support the transfer function work by gaining an understanding of how fibers could get into a building; and (3) support the economic analysis by understanding what would happen if fibers precipitated a failure in an item of equipment
A study of mass production and installation of small solar thermal electric power systems
Technological constraints, materials availability, production capacity, and manufacturing and installations plans and costs at different production levels are included in a study of concentrating collector industrialization. As cobalt for the engine and receiver is supply limited, alternative lower temperature alloys and higher temperature materials such as ceramics are discussed. Economics and production efficiency favor co-location of cellular and thin glass production for reflectors. Assembly and installation are expensive for small sites and few alternatives exist to apply mass production techniques to lower these costs for the selected design. Stepping motors in the size and quantities required are not commercially available today but could be in the future
- âŠ