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arXiv2015

Causa Efficiens versus Causa Formalis: origens da discussão moderna sobre a dimensionalidade do espaço

Metascientific criteria used for explaining or constraining physical space dimensionality and their historical relationship to prevailing causal systems are discussed. The important contributions by Aristotle, Kant and Ehrenfest to the dimensionality of space problem are considered and shown to be grounded on different causal explanations: {\it causa materialis} for Aristotle, {\it causa efficiens} for young Kant and an ingenious combination of {\it causa efficiens} and {\it causa formalis} for Ehrenfest. The prominent and growing rôle played by {\it causa formalis} in modern physical approaches to this problem is emphasized.

physics.hist-ph↗
arXiv2015

Is Time's Arrow Perspectival?

We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depends on coarse-graining. Therefore the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. Low past entropy may be due to the fact that this coupling (rather than microstate of the universe) is non-generic. I argue that for any generic microstate of a sufficiently rich system there are always special subsystems defining a coarse graining for which the entropy of the rest is low in one time direction (the "past"). These are the subsystems allowing creatures that "live in time" ---such as those in the biosphere--- to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.

physics.hist-ph↗
arXiv2015

Dynamical and Hamiltonian formulation of General Relativity

This is a substantially expanded version of a chapter-contribution to "The Springer Handbook of Spacetime", edited by Abhay Ashtekar and Vesselin Petkov, published by Springer Verlag in 2014. This contribution introduces the reader to the reformulation of Einstein's field equations of General Relativity as a constrained evolutionary system of Hamiltonian type and discusses some of its uses, together with some technical and conceptual aspects. Attempts were made to keep the presentation self contained and accessible to first-year graduate students. This implies a certain degree of explicitness and occasional reviews of background material.

gr-qc↗
arXiv2015

No information or horizon paradoxes for Th. Smiths

'Th'e 'S'tatistical 'm'echanician 'i'n 'th'e 's'treet (our Th. Smiths) must be surprised upon hearing popular versions of some of today's most discussed paradoxes in astronomy and cosmology. In fact, rather standard reminders of the meaning of thermal probabilities in statistical mechanics appear to answer the horizon problem (one of the major motivations for inflation theory) and the information paradox (related to black hole physics), at least as they are usually presented. Still the paradoxes point to interesting gaps in our statistical understanding of (quantum) gravitational effects.

physics.pop-ph↗
arXiv2015

Greeks were right : critical comments on Qbism

In this short essay we reject the interpretation of quantum theory called quantum Bayesianism (Qbism) which has been promoted recently by David Mermin in his essay published in Nature. According to Qbism quantum states are personal judgements of human agents. Physicists are not verifying their personal beliefs about their observations but search for the mathematical abstract description allowing to explain and to predict in a quantitative way the regularities observed (and those to be discovered) in physical phenomena which exist independently of the presence of any agent. We reject also the claim that Qbism explains properly the quantum nonlocality. The distant long range correlations can be correctly explained using a contextual statistical interpretation of quantum theory. We conclude that the Greeks were right to remove a perceiving subject out of Science.

quant-ph↗
arXiv2016

Categories and the Foundations of Classical Field Theories

I review some recent work on applications of category theory to questions concerning theoretical structure and theoretical equivalence of classical field theories, including Newtonian gravitation, general relativity, and Yang-Mills theories.

physics.hist-ph↗
arXiv2015

On Einstein Algebras and Relativistic Spacetimes

In this paper, we examine the relationship between general relativity and the theory of Einstein algebras. We show that according to a formal criterion for theoretical equivalence recently proposed by Halvorson (2012, 2015) and Weatherall (2015), the two are equivalent theories.

physics.hist-ph↗
arXiv2015

The little-hierarchy problem is a little problem: understanding the difference between the big- and little-hierarchy problems with Bayesian probability

Experiments are once again under way at the LHC. This time around, however, the mood in the high-energy physics community is pessimistic. There is a growing suspicion that naturalness arguments that predict new physics near the weak scale are faulty and that prospects for a new discovery are limited. We argue that such doubts originate from a misunderstanding of the foundations of naturalness arguments. In spite of the first run at the LHC, which aggravated the little-hierarchy problem, there is no cause for doubting naturalness or natural theories. Naturalness is grounded in Bayesian probability logic - it is not a scientific theory and it makes no sense to claim that it could be falsified or that it is under pressure from experimental data. We should remain optimistic about discovery prospects; natural theories, such as supersymmetry, generally predict new physics close to the weak scale. Furthermore, from a Bayesian perspective, we briefly discuss 't Hooft's technical naturalness and a contentious claim that the little-hierarchy problem hints that the Standard Model is a fundamental theory.

hep-ph↗
arXiv2015

On the Analysis of Bell's 1964 Paper by Wiseman, Cavalcanti, and Rieffel

In a recent series of papers Wiseman, Cavalcanti, and Rieffel have outlined and contrasted two different views about what we now call Bell's theorem. They also assert that Bell presented these two different versions at different times. This view is clearly at odds with the detailed explanation that Bell himself gave in his later writings. A careful examination of the historic 1964 paper in context shows clearly that Bell's own later interpretation is the correct one.

quant-ph↗
arXiv2016

Probabilistic Knowledge as Objective Knowledge in Quantum Mechanics: Potential Powers Instead of Actual Properties

In classical physics, probabilistic or statistical knowledge has been always related to ignorance or inaccurate subjective knowledge about an actual state of affairs. This idea has been extended to quantum mechanics through a completely incoherent interpretation of the Fermi-Dirac and Bose-Einstein statistics in terms of "strange" quantum particles. This interpretation, naturalized through a widespread "way of speaking" in the physics community, contradicts Born's physical account of Ψ as a "probability wave" which provides statistical information about outcomes that, in fact, cannot be interpreted in terms of 'ignorance about an actual state of affairs'. In the present paper we discuss how the metaphysics of actuality has played an essential role in limiting the possibilities of understating things differently. We propose instead a metaphysical scheme in terms of powers with definite potentia which allows us to consider quantum probability in a new light, namely, as providing objective knowledge about a potential state of affairs.

quant-ph↗
arXiv2020

The de Broglie universal substratum, the Lochak monopoles and the dark universe

We propose to interpret the dark universe (dark matter and dark energy) of the current cosmological standard model as emerging together with ordinary matter, from the quantum vacuum, which de Broglie called the "universal substratum". It will be shown that this interpretation may lead to a reasonable agreement between the current understanding of the quantum vacuum in quantum field theory and current evidences for the existence of dark energy and dark matter, and that dark matter can be interpreted as emerging from the QCD vacuum, as a Bose-Einstein gluon condensate, with an energy density relative to the baryonic energy density that agrees with observation.

physics.hist-ph↗
arXiv2016

The Logic of Experimental Tests, Particularly of Everettian Quantum Theory

Claims that the standard methodology of scientific testing is inapplicable to Everettian quantum theory, and hence that the theory is untestable, are due to misconceptions about probability and about the logic of experimental testing. Refuting those claims by correcting those misconceptions leads to various simplifications, notably the elimination of everything probabilistic from fundamental physics (stochastic processes) and from the methodology of testing ('Bayesian' credences).

quant-ph↗
arXiv2015

Is Weyl unified theory wrong or incomplete?

In 1918, H. Weyl proposed a unified theory of gravity and electromagnetism based on a generalization of Riemannian geometry. In spite of its elegance and beauty, a serious objection was raised by Einstein, who argued that Weyl's theory was not suitable as a physical theory . According to Einstein, the theory led to the prediction of a "second clock effect", which is not observed by experiments. We briefly revisit this point and argue that a preliminary discussion on the very notion of proper time is needed in order to consider Einstein's critical point of view.

gr-qc↗
arXiv2016

Mathemagical 2014 - An Indian Perspective

Mathematics is probably the only subject that can be classified both as art as well as science - former, because it is not constrained by the real world and latter because it is a logical system with precisely defined rules as well as primitives that lead to unambiguous nontrivial theorems. Indian (and of Indian origin) mathematicians have continued to do seminal work till present times, culminating in Manjul Bhargava receiving the Fields Medal last year. In such fabulous times, a non-mathematician ponders about the nature of mathematics, and revisits the question: why are fundamental laws of Nature inherently mathematical?

physics.hist-ph↗
arXiv2015

Time and Fermions: General Covariance vs. Ockham's Razor for Spinors

It is a commonplace attributed to Kretschmann that any local physical theory can be represented in arbitrary coordinates using tensor calculus. But the literature also claims that spinors _as such_ cannot be represented in coordinates in a curved space-time. These commonplaces are inconsistent, so what is general covariance for fermions? In fact both commonplaces are wrong. Ogievetsky and Polubarinov (OP) constructed spinors in coordinates in 1965, enhancing the unity of physics and helping to spawn nonlinear group representations. Roughly and locally, OP spinors resemble the orthonormal basis or tetrad formalism in the symmetric gauge, but they are conceptually self-sufficient and more economical. The tetrad formalism is thus de-Ockhamized, with six extra components and six compensating gauge symmetries. As developed nonperturbatively by Bilyalov, OP spinors admit any coordinates at a point, but 'time' must be listed first; 'time' is defined by an eigenvalue problem involving the metric and diag(-1,1,1,1), the product of which must have no negative eigenvalues. Thus even formal general covariance requires reconsideration; admissible coordinates should be sensitive to the types and _values_ of the fields. Apart from coordinate order and the usual spinorial two-valuedness, (densitized) Ogievetsky-Polubarinov spinors form, with the (conformal part of the) metric, a nonlinear geometric object, with Lie and covariant derivatives. The mild consequences of coordinate order are explored for the conventionality of simultaneity in Special Relativity and the Schwarzschild solution in General Relativity.

gr-qc↗
arXiv2015

Antimatter in the Direct-Action Theory of Fields

One of Feynman's greatest contributions to physics was the interpretation of negative energies as antimatter in quantum field theory. A key component of this interpretation is the Feynman propagator, which seeks to describe the behavior of antimatter at the virtual particle level. Ironically, it turns out that one can dispense with the Feynman propagator in a direct-action theory of fields, while still retaining the interpretation of negative energy solutions as antiparticles.

quant-ph↗