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At least 1,063 records · Page 59Linked to original sources
arXiv2020

Teaching gauge theory to first year students

One of the biggest revelations of 20th century physics, is virtually unheard of outside the inner circles of particle physics. This is the gauge theory, the foundation for how all physical interactions are described and a guiding principle for almost all work on new physics theories. Is it not our duty as physicists to try and spread this knowledge to a wider audience? Here, two simple gauge theory models are presented that should be understandable without any advanced mathematics or physics and it is demonstrated how they can be used to show how gauge symmetries are used to construct the standard model of particle physics. This is also used to describe the real reason we need the Higgs field. Though these concepts are complicated and abstract, it seems possible for at least first year students to understand the main ideas. Since they typically are very interested in cutting edge physics, they do appreciate the effort and enjoy the more detail insight into modern particle physics. These results are certainly encouraging more efforts in this direction.

physics.ed-ph↗
arXiv2020

One time, two times, or no time?

Contemporary research programs in fundamental physics appear to suggest that there could be two (physical) times---or none at all. This essay articulates these possibilities in the context of quantum gravity, and in particular of cosmological models developed in an approach called `loop quantum gravity', and explains how they could nevertheless underwrite our manifestly temporal world. A proper interpretation of these models requires a negotiation of an atemporal and a temporal sense of the emergence of (space)time.

physics.hist-ph↗
arXiv2023

Physical grounds for causal perspectivalism

We ground the asymmetry of causal relations in the internal physical states of a special kind of open and irreversible physical system, a causal agent. A causal agent is an autonomous physical system, maintained in a steady state, far from thermal equilibrium, with special subsystems: sensors, actuators, and learning machines. Using feedback, the learning machine, driven purely by thermodynamic constraints, changes its internal states to learn probabilistic functional relations inherent in correlations between sensor and actuator records. We argue that these functional relations just are causal relations learned by the agent, and so such causal relations are simply relations between the internal physical states of a causal agent. We show that learning is driven by a thermodynamic principle: the error rate is minimised when the dissipated power is minimised. While the internal states of a causal agent are necessarily stochastic, the learned causal relations are shared by all machines with the same hardware embedded in the same environment. We argue that this dependence of causal relations on such `hardware' is a novel demonstration of causal perspectivalism.

physics.hist-ph↗
arXiv2020

Kant and Hegel in Physics

Kant and Hegel are among the philosophers who are guiding the way in which we reason these days. It is thus of interest to see how physical theories have been developed along the line of Kant and Hegel. Einstein became interested in how things appear to moving observers. Quantum mechanics is also an observer-dependent science. The question then is whether quantum mechanics and relativity can be synthesized into one science. The present form of quantum field theory is a case in point. This theory however is based on the algorithm of the scattering matrix where all participating particles are free in the remote past and in the remote future. We thus need, in addition, a Lorentz-covariant theory of bound state which will address the question of how the hydrogen atom would look to moving observers. The question is then whether this Lorentz-covariant theory of bound states can be synthesized with the field theory into a Lorentz-covariant quantum mechanics. This article reviews the progress made along this line. This integrated Kant-Hegel process is illustrated in terms of the way in which Americans practice their democracy.

physics.hist-ph↗
arXiv2020

Legerdemain in Mathematical Physics: Structure, Tricks, and Lacunae in Derivations of the Partition Function of the Two-Dimensional Ising Model and in Proofs of The Stability of Matter

We review various derivations of the partition function of the two-dimensional Ising Model of ferromagnetism and proofs of the stability of matter, paying attention to passages where there would appear to be a lacuna between steps or where the structure of the argument is not so straightforward. Authors cannot include all the intermediate steps, but sometimes most readers and especially students will be mystified by such a transition. Moreover, careful consideration of such lacunae points to interesting physics and not only mathematical technology. Also, when reading the original papers, the structure of the physics argument may be buried by the technical moves. Improvements in the derivations, in subsequent papers by others, may well be clearer and more motivated. But, there is remarkably little written and published about how to read some of the original papers, and the subsequent ones, yet students and their teachers would often benefit from such guidance. I should note that much of the discussion below will benefit from having those papers in front of you.

cond-mat.stat-mech↗
arXiv2020

The founding of Arcetri Observatory in Florence

The first idea of establishing a public astronomical observatory in Florence, Capital of the Grand Duchy of Tuscany, dates to the mid of the 18th century. Initially, the use of a low building on a high ground was proposed, and the hill of Arcetri suggested as a proper location. At the end of the century, the Florence Observatory - or Specola - was built instead on a tower at the same level as the city's centre. As soon as astronomers started to use this observatory, they recognized all its flaws and struggled to search for a better location. Giovanni Battista Donati, director of the Specola of Florence from the eve of the Italian Unification in 1859, finally succeeded in creating a new observatory: first, he obtained funds from the Parliament of the Kingdom of Italy to build an equatorial mount for the Amici 28-cm refractor, which could not be installed conveniently in the tower of the Specola; then, he went through the process of selecting a proper site, seeking funds and finally building Arcetri Observatory. Although Donati was a pioneer of spectroscopy and astrophysics, his intent was to establish a modern observatory for classical astronomy, as the Italian peninsula did not have a national observatory like those located in many foreign capitals - Florence was the capital of the Kingdom of Italy from 1865 to 1871. To promote the project, Donati made use of writings by one of the most authoritative European astronomers, Otto Wilhelm Struve. The paper describes all these steps, eventually leading to the final inauguration of the Arcetri Observatory in 1872, almost 150 years ago.

physics.hist-ph↗
arXiv2020

Time-Symmetry Breaking in Hamiltonian Mechanics. II. A Memoir for Berni Julian Alder [1925-2020]

This memoir honors the late Berni Julian Alder, who inspired both of us with his pioneering development of molecular dynamics. Berni's work with Tom Wainwright, described in the 1959 Scientific American[1], brought Bill to interview at Livermore in 1962. Hired by Berni, Bill enjoyed over 40 years' research at the Laboratory. Berni, along with Edward Teller, founded UC's Department of Applied Science in 1963. Their motivation was to attract bright students to use the laboratory's unparalleled research facilities. In 1972 Carol was offered a joint LLNL employee-DAS student appointment at Livermore. Bill, thanks to Berni's efforts, was already a Professor at DAS. Carol became one of Bill's best students. Berni's influence was directly responsible for our physics collaboration and our marriage in 1989. The present work is devoted to two early interests of Berni's, irreversibility and shockwaves. Berni and Tom studied the irreversibility of Boltzmann's "H function" in the early 1950s[2]. Berni called shockwaves the "most irreversible" of hydrodynamic processes[3]. Just this past summer, in simulating shockwaves with time-reversible classical mechanics, we found that reversed Runge-Kutta shockwave simulations yielded nonsteady rarefaction waves, not shocks. Intrigued by this unexpected result we studied the exponential Lyapunov instabilities in both wave types. Besides the Runge-Kutta and Leapfrog algorithms, we developed a precisely-reversible manybody algorithm based on trajectory storing, just changing the velocities' signs to generate the reversed trajectories. Both shocks and rarefactions were precisely reversed. Separate simulations, forward and reversed, provide interesting examples of the Lyapunov-unstable symmetry-breaking models supporting the Second Law of Thermodynamics. We describe promising research directions suggested by this work.

physics.hist-ph↗
arXiv2021

Are mental states nonlocal?

I show that if mental states are function of physical states, then they are nonlocal, in a sense that will be explained. I argue that, if mental states are reducible to brain physics, and if they are integrated experiences, this nonlocality implies that Classical Physics is not enough, in particular the computationalist thesis does not hold. I illustrate the argument with a thought experiment. The proof of nonlocality is straightforward and general, but the result is counterintuitive, so I spend a large part of the article discussing possible objections, alternatives, and implications. I discuss the possibility that Quantum Physics allows this kind of nonlocality.

physics.hist-ph↗
arXiv2020

The network structure of scientific revolutions

Philosophers of science have long postulated how collective scientific knowledge grows. Empirical validation has been challenging due to limitations in collecting and systematizing large historical records. Here, we capitalize on the largest online encyclopedia to formulate knowledge as growing networks of articles and their hyperlinked inter-relations. We demonstrate that concept networks grow not by expanding from their core but rather by creating and filling knowledge gaps, a process which produces discoveries that are more frequently awarded Nobel prizes than others. Moreover, we operationalize paradigms as network modules to reveal a temporal signature in structural stability across scientific subjects. In a network formulation of scientific discovery, data-driven conditions underlying breakthroughs depend just as much on identifying uncharted gaps as on advancing solutions within scientific communities.

cs.DL↗
arXiv2021

Do symmetries "explain" conservation laws? The modern converse Noether theorem vs pragmatism

Noether's first theorem does not establish a one-way explanatory arrow from symmetries to conservation laws, but such an arrow is widely assumed in discussions of the theorem in the physics and philosophy literature. It is argued here that there are pragmatic reasons for privileging symmetries, even if they do not strictly justify explanatory priority. To this end, some practical factors are adduced as to why Noether's direct theorem seems to be more well-known and exploited than its converse, with special attention being given to the sometimes overlooked nature of Noether's converse result and to its strengthened version due to Luis Martínez Alonso in 1979 and Peter Olver in 1986.

physics.hist-ph↗
arXiv2022

Non-Accessible Mass and the Ontology of GRW

The Mass Density approach to GRW (GRWm for short) has been widely discussed in the quantum foundations literature. A crucial feature of GRWm is the introduction of a relation of accessibility for mass, which allows to explain the determinacy of experimental outcomes thus also addressing the tails problem of GRW. However, the relation of accessibility leaves the ontological meaning of the non-accessible portion of mass utterly unexplained. In this paper I discuss two viable approaches to non-accessible mass, which I call anti-realist and realist, and will defend the latter. First, I show that the anti-realist approach suffers from various objections. Second, I develop an account of non-accessible mass density states as objectively indeterminate states of affairs.

quant-ph↗
arXiv2020

Local Quantum Reality

Unperformed measurements have no results. Unobserved results can affect future measurements.

quant-ph↗
arXiv2023

Indeterminacy in classical cosmology with dark matter

We describe a case of indeterminacy in general relativity for homogeneous and isotropic cosmologies for a class of dark energy fluids. The cosmologies are parametrized by an equation of state variable, with one instance giving the same solution as Norton's mechanical dome. Our example goes beyond previously studied cases in that indeterminacy lies in the evolution of spacetime itself: the onset of the Big Bang is indeterminate. We show further that the indeterminacy is resolved if the dynamics is viewed relationally.

physics.hist-ph↗
arXiv2020

The solar eclipse of August 30, 1905 at Alcala de Chisvert

Solar eclipses play an important role in the history of astronomy because their observation allowed astrophysics to expand. The eclipse of August 30th, 1905 is particularly interesting since the line of totality passes through Spain and Algeria, and that it will last 3 minutes and 45 seconds. Consequently, all the French observatories send astronomers to observe it. About a hundred amateur astronomers of the French Astronomical Society also go to various sites, as well as quite a few individuals who are simply curious, like Camille Saint-Saëns. Count Aymar de la Baume Pluvinel and his two young assistants settle in at Alcala de Chisvert, between Tarragona and Valencia.

physics.hist-ph↗
arXiv2020

About soft photon resummation

The first time one of us (G.P.) encountered Earle was in Summer 1966, when she was directed to study Earle's papers on radiative corrections to quasi-elastic electron scattering. The suggestion had come from Bruno Touschek, at the time head of the theoretical physics group at the Frascati National Laboratories near Rome. About the same time, Earle came from MIT to visit University of Rome and Frascati. G.P. was a young post-graduate, who had studied Earle's papers and was awed by his already impressive scientific figure. After almost 40 years had passed, Earle visited Italy with his wife Ruth, making Frascati their base for an extended visit of almost a month. They were housed in what was then the laboratory hostel for foreign visitors, a small villa higher up above the hill, toward the town of Frascati. Since then, we became close friends, a friendship which included both his family and ours, and which has been very important for us. In memory of that first visit and in gratitude for the many years of friendship, we will tell here a story of infrared radiative corrections to charged particle scattering, to which Earle's papers gave an important contribution.

physics.hist-ph↗
arXiv2020

Hamilton, Hamiltonian Mechanics, and Causation

I show how Hamilton's philosophical commitments led him to a causal interpretation of classical mechanics. I argue that Hamilton's metaphysics of causation was injected into his dynamics by way of a causal interpretation of force. I then detail how forces remain indispensable to both Hamilton's formulation of classical mechanics and what we now call Hamiltonian mechanics (i.e., the modern formulation). On this point, my efforts primarily consist of showing that the orthodox interpretation of potential energy is the interpretation found in Hamilton's work. Hamilton called the potential energy function the force-function because he believed that it represents forces at work in the world. Multifarious non-historical arguments for this orthodox interpretation of potential energy are provided, and matters are concluded by showing that in classical Hamiltonian mechanics, facts about potential energies of systems are grounded in facts about forces. Thus, if one can tolerate the view that forces are causes of motions, then Hamilton provides one with a road map for transporting causation into one of the most mathematically sophisticated formulations of classical mechanics, viz., Hamiltonian mechanics.

physics.hist-ph↗
arXiv2022

Sidney Coleman's Dirac Lecture "Quantum Mechanics in Your Face"

This is a write-up of Sidney Coleman's classic lecture first given as a Dirac Lecture at Cambridge University and later recorded when repeated at the New England sectional meeting of the American Physical Society (April 9, 1994). My sources have been this recording and a copy of the slides Sidney send to me after he gave the lecture as a Physics Colloquium at Stanford University some time between 1995 and 1998. To preserve both the scientific content and most of the charm, I have kept the editing to a minimum, but did add a bibliography containing the references Sidney mentioned.--Martin Greiter

physics.hist-ph↗