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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 01:53:57 +0000Many Worlds, the Born Rule, and Self-Locating Uncertainty
https://resolver.caltech.edu/CaltechAUTHORS:20141216-203110170
Authors: {'items': [{'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}, {'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2014
DOI: 10.1007/978-88-470-5217-8_10
We provide a derivation of the Born Rule in the context of the Everett (Many-Worlds) approach to quantum mechanics. Our argument is based on the idea of self-locating uncertainty: in the period between the wave function branching via decoherence and an observer registering the outcome of the measurement, that observer can know the state of the universe precisely without knowing which branch they are on. We show that there is a uniquely rational way to apportion credence in such cases, which leads directly to the Born Rule. [Editors note: for a video of the talk given by Prof. Carroll at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-14.]https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/9cdj7-axw89Quantum Mechanics as Classical Physics
https://resolver.caltech.edu/CaltechAUTHORS:20180614-114551564
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2015
DOI: 10.1086/680190
Here I explore a novel no-collapse interpretation of quantum mechanics that combines aspects of two familiar and well-developed alternatives, Bohmian mechanics and the many-worlds interpretation. Despite reproducing the empirical predictions of quantum mechanics, the theory looks surprisingly classical. All there is at the fundamental level are particles interacting via Newtonian forces. There is no wave function. However, there are many worlds.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/a3gk2-gz633Quantum Ontology: A Guide to the Metaphysics of Quantum Mechanics [Book Review]
https://resolver.caltech.edu/CaltechAUTHORS:20180620-082321863
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2016
The revolution in physics that brought us to a quantum picture of the world was so radical that it does not merely force a rethinking of physics, but metaphysics as well. Quantum physics may imply that the world is fundamentally indeterministic, that it is fundamentally indeterminate, that causes are not always local to their effects, that there are many more than three spatial dimensions, that wholes are not simply sums of their parts, that our world is just one among many, etc. According to Peter J. Lewis, "we can say quite confidently that quantum mechanics is metaphysically revisionary even if it is not clear what form the revisions should take." (p. xvii) The reason it is so hard to say what the implications of quantum mechanics are for metaphysics is that physicists and philosophers have developed a variety of alternative theories that all attempt to explain why quantum experiments turn out the way they do. These theories disagree on what exists and what rules that stuff obeys. That is, they disagree about the ontology of quantum mechanics and the physical laws that govern that ontology. Understanding the various alternatives and their metaphysical implications is not easy, but Lewis makes it remarkably accessible. Quantum Ontology is an outstanding guide to contemporary philosophy of quantum mechanics.
The book's intended audience is philosophers who are knowledgeable about metaphysics and curious about the relevance of quantum mechanics to metaphysical debates. For this audience, the book is ideal. Lewis relentlessly avoids overly technical presentations of the physics, using simple examples and helpful visualizations to get to the heart of the matter at hand. The questions that drive the text are questions of metaphysics and the metaphysical motivation for whatever is being discussed is always made clear.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/wcaaa-hv825Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics
https://resolver.caltech.edu/CaltechAUTHORS:20141216-201958532
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}, {'id': 'Carroll-S-M', 'name': {'family': 'Carroll', 'given': 'Sean M.'}, 'orcid': '0000-0002-4226-5758'}]}
Year: 2018
DOI: 10.1093/bjps/axw004
A longstanding issue in attempts to understand the Everett (many-worlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of self-locating uncertainty during the period between the branches of the wave function splitting via decoherence and the observer registering the outcome of the measurement. In this period, it is tempting to regard each branch as equiprobable, but we argue that the temptation should be resisted. Applying lessons from this analysis, we demonstrate (using methods similar to those of Zurek's envariance-based derivation) that the Born rule is the uniquely rational way of apportioning credence in Everettian quantum mechanics. In doing so, we rely on a single key principle: changes to the environment alone do not affect the probabilities one ought to assign to measurement outcomes in a local subsystem. We arrive at a method for assigning probabilities in cases that involve both classical and quantum self-locating uncertainty. This method provides unique answers to quantum Sleeping Beauty problems, as well as a well-defined procedure for calculating probabilities in quantum cosmological multiverses with multiple similar observers.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/a0vv6-h2t23Constructing and Constraining Wave Functions for Identical Quantum Particles
https://resolver.caltech.edu/CaltechAUTHORS:20180613-082631497
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2018
DOI: 10.48550/arXiv.1608.08590
I address the problem of explaining why wave functions for identical particles must be either symmetric or antisymmetric (the symmetry dichotomy) within two interpretations of quantum mechanics which include particles following definite trajectories in addition to, or in lieu of, the wave function: Bohmian mechanics and Newtonian quantum mechanics (a.k.a. many interacting worlds). In both cases I argue that, if the interpretation is formulated properly, the symmetry dichotomy can be derived and need not be postulated.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/6ap8g-4e106Forces on fields
https://resolver.caltech.edu/CaltechAUTHORS:20180612-133231822
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2018
DOI: 10.1016/j.shpsb.2017.09.005
In electromagnetism, as in Newton's mechanics, action is always equal to reaction. The force from the electromagnetic field on matter is balanced by an equal and opposite force from matter on the field. We generally speak only of forces exerted by the field, not forces exerted upon the field. But, we should not be hesitant to speak of forces acting on the field. The electromagnetic field closely resembles a relativistic fluid and responds to forces in the same way. Analyzing this analogy sheds light on the inertial role played by the field's mass, the status of Maxwell's stress tensor, and the nature of the electromagnetic field.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/7xfz7-9y693What's everything made of?
https://resolver.caltech.edu/CaltechAUTHORS:20230224-181800409
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2019
To answer whether the fundamental building blocks of reality are particles, fields or both means thinking beyond physics.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/4762y-et079Electromagnetism as Quantum Physics
https://resolver.caltech.edu/CaltechAUTHORS:20190515-135120517
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2019
DOI: 10.1007/s10701-019-00253-3
One can interpret the Dirac equation either as giving the dynamics for a classical field or a quantum wave function. Here I examine whether Maxwell's equations, which are standardly interpreted as giving the dynamics for the classical electromagnetic field, can alternatively be interpreted as giving the dynamics for the photon's quantum wave function. I explain why this quantum interpretation would only be viable if the electromagnetic field were sufficiently weak, then motivate a particular approach to introducing a wave function for the photon (following Good in Phys Rev 105(6):1914–1919, 1957). This wave function ultimately turns out to be unsatisfactory because the probabilities derived from it do not always transform properly under Lorentz transformations. The fact that such a quantum interpretation of Maxwell's equations is unsatisfactory suggests that the electromagnetic field is more fundamental than the photon.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/2ve7w-w6833How Electrons Spin
https://resolver.caltech.edu/CaltechAUTHORS:20180612-074244101
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2019
DOI: 10.1016/j.shpsb.2019.04.007
There are a number of reasons to think that the electron cannot truly be spinning. Given how small the electron is generally taken to be, it would have to rotate superluminally to have the right angular momentum and magnetic moment. Also, the electron's gyromagnetic ratio is twice the value one would expect for an ordinary classical rotating charged body. These obstacles can be overcome by examining the flow of mass and charge in the Dirac field (interpreted as giving the classical state of the electron). Superluminal velocities are avoided because the electron's mass and charge are spread over sufficiently large distances that neither the velocity of mass flow nor the velocity of charge flow need to exceed the speed of light. The electron's gyromagnetic ratio is twice the expected value because its charge rotates twice as fast as its mass.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/xjs66-atp16Putting Positrons into Classical Dirac Field Theory
https://resolver.caltech.edu/CaltechAUTHORS:20191223-154016646
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2020
DOI: 10.1016/j.shpsb.2019.10.003
One way of arriving at a quantum field theory of electrons and positrons is to take a classical theory of the Dirac field and then quantize. Starting with the standard classical field theory and quantizing in the most straightforward way yields an inadequate quantum field theory. It is possible to fix this theory by making some modifications (such as redefining the operators for energy and charge). Here I argue that we ought to make these modifications earlier, revising the classical Dirac field theory that serves as the starting point for quantization (putting positrons into that theory and removing negative energies). Then, quantization becomes straightforward. Also, the physics of the Dirac field is made more similar to the physics of the electromagnetic field and we are able to better understand electron spin.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/wy587-5xt76Possibility of small electron states
https://resolver.caltech.edu/CaltechAUTHORS:20201201-104227498
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2020
DOI: 10.1103/PhysRevA.102.052225
Some authors have claimed that there exists a minimum size (on the order of the Compton radius) for electron states composed entirely of positive-frequency solutions to the free Dirac equation. Other authors have put forward counterexamples to such claims. This article asks how the counterexamples of A. J. Bracken and G. F. Melloy [J. Phys. A. 32, 6127 (1999)] bypass two arguments against their possibility. The first is an old argument that, because of the prohibition on faster-than-light motion, the electron must be larger than a certain minimum size if it is to have the correct angular momentum and magnetic moment. This challenge can be addressed by analyzing the flow of energy and charge for the counterexample states. The second argument is an explicit proof (presented in C.-P. Chuu et al. [Solid State Commun. 150, 533 (2010)]) that there is a minimum size for purely positive-frequency electron states. This proof hinges on the assumption of a small spread in momentum space, which is violated by the counterexamples that have been put forward.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/wegsx-8sn96Electron Charge Density: A Clue from Quantum Chemistry for Quantum Foundations
https://resolver.caltech.edu/CaltechAUTHORS:20210727-173040478
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2021
DOI: 10.1007/s10701-021-00480-7
Within quantum chemistry, the electron clouds that surround nuclei in atoms and molecules are sometimes treated as clouds of probability and sometimes as clouds of charge. These two roles, tracing back to Schrödinger and Born, are in tension with one another but are not incompatible. Schrödinger's idea that the nucleus of an atom is surrounded by a spread-out electron charge density is supported by a variety of evidence from quantum chemistry, including two methods that are used to determine atomic and molecular structure: the Hartree-Fock method and density functional theory. Taking this evidence as a clue to the foundations of quantum physics, Schrödinger's electron charge density can be incorporated into many different interpretations of quantum mechanics (and extensions of such interpretations to quantum field theory).https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/vktac-ka932Particles, fields, and the measurement of electron spin
https://resolver.caltech.edu/CaltechAUTHORS:20200922-132154675
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}}]}
Year: 2021
DOI: 10.1007/s11229-020-02843-5
This article compares treatments of the Stern–Gerlach experiment across different physical theories, building up to a novel analysis of electron spin measurement in the context of classical Dirac field theory. Modeling the electron as a classical rigid body or point particle, we can explain why the entire electron is always found at just one location on the detector (uniqueness) but we cannot explain why there are only two locations where the electron is ever found (discreteness). Using non-relativistic or relativistic quantum mechanics, we can explain both uniqueness and discreteness. Moving to more fundamental physics, both features can be explained within a quantum theory of the Dirac field. In a classical theory of the Dirac field, the rotating charge of the electron can split into two pieces that each hit the detector at a different location. In this classical context, we can explain a feature of electron spin that is often described as distinctively quantum (discreteness) but we cannot explain another feature that could be explained within any of the other theories (uniqueness).https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/b4k65-6tp08The Mass of the Gravitational Field
https://resolver.caltech.edu/CaltechAUTHORS:20190409-111728426
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2022
DOI: 10.1093/bjps/axz002
By mass–energy equivalence, the gravitational field has a relativistic mass density proportional to its energy density. I seek to better understand this mass of the gravitational field by asking whether it plays three traditional roles of mass: the role in conservation of mass, the inertial role, and the role as source for gravitation. The difficult case of general relativity is compared to the more straightforward cases of Newtonian gravity and electromagnetism by way of gravitoelectromagnetism, an intermediate theory of gravity that resembles electromagnetism.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/q3bhn-7bn74Absorbing the Arrow of Electromagnetic Radiation
https://resolver.caltech.edu/CaltechAUTHORS:20220707-204126218
Authors: {'items': [{'id': 'Hubert-Mario-G', 'name': {'family': 'Hubert', 'given': 'Mario'}, 'orcid': '0000-0002-1170-6020'}, {'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2022
DOI: 10.48550/arXiv.arXiv.2205.14233
We argue that the asymmetry between diverging and converging electromagnetic waves is just one of many asymmetries in observed phenomena that can be explained by a past hypothesis and statistical postulate (together assigning probabilities to different states of matter and field in the early universe). The arrow of electromagnetic radiation is thus absorbed into a broader account of temporal asymmetries in nature. We give an accessible introduction to the problem of explaining the arrow of radiation and compare our preferred strategy for explaining the arrow to three alternatives: (i) modifying the laws of electromagnetism by adding a radiation condition requiring that electromagnetic fields always be attributable to past sources, (ii) removing electromagnetic fields and having particles interact directly with one another through retarded action-at-a-distance, (iii) adopting the Wheeler-Feynman approach and having particles interact directly through half-retarded half-advanced action-at-a-distance. In addition to the asymmetry between diverging and converging waves, we also consider the related asymmetry of radiation reaction.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/a4ead-0e652The Fundamentality of Fields
https://resolver.caltech.edu/CaltechAUTHORS:20220707-170614632
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2022
DOI: 10.48550/arXiv.2202.09425
There is debate as to whether quantum field theory is, at bottom, a quantum theory of fields or particles. One can take a field approach to the theory, using wave functionals over field configurations, or a particle approach, using wave functions over particle configurations. This article argues for a field approach, presenting three advantages over a particle approach: (1) particle wave functions are not available for photons, (2) a classical field model of the electron gives a superior account of both spin and self-interaction as compared to a classical particle model, and (3) the space of field wave functionals appears to be larger than the space of particle wave functions. The article also describes two important tasks facing proponents of a field approach: (1) legitimize or excise the use of Grassmann numbers for fermionic field values and in wave functional amplitudes, and (2) describe how quantum fields give rise to particle-like behavior.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/msw1n-tkx40The fundamentality of fields
https://resolver.caltech.edu/CaltechAUTHORS:20220916-665813000
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2022
DOI: 10.1007/s11229-022-03844-2
There is debate as to whether quantum field theory is, at bottom, a quantum theory of fields or particles. One can take a field approach to the theory, using wave functionals over field configurations, or a particle approach, using wave functions over particle configurations. This article argues for a field approach, presenting three advantages over a particle approach: (1) particle wave functions are not available for photons, (2) a classical field model of the electron gives a superior account of both spin and self-interaction as compared to a classical particle model, and (3) the space of field wave functionals appears to be larger than the space of particle wave functions. The article also describes two important tasks facing proponents of a field approach: (1) legitimize or excise the use of Grassmann numbers for fermionic field values and in wave functional amplitudes, and (2) describe how quantum fields give rise to particle-like behavior.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/kn8v3-apv86The Disappearance and Reappearance of Potential Energy in Classical and Quantum Electrodynamics
https://resolver.caltech.edu/CaltechAUTHORS:20221104-609510400.2
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2022
DOI: 10.1007/s10701-022-00630-5
In electrostatics, we can use either potential energy or field energy to ensure conservation of energy. In electrodynamics, the former option is unavailable. To ensure conservation of energy, we must attribute energy to the electromagnetic field and, in particular, to electromagnetic radiation. If we adopt the standard energy density for the electromagnetic field, then potential energy seems to disappear. However, a closer look at electrodynamics shows that this conclusion actually depends on the kind of matter being considered. Although we cannot get by without attributing energy to the electromagnetic field, matter may still have electromagnetic potential energy. Indeed, if we take the matter to be represented by the Dirac field (in a classical precursor to quantum electrodynamics), then it will possess potential energy (as can be seen by examining the symmetric energy-momentum tensor of the Dirac field). Thus, potential energy reappears. Upon field quantization, the potential energy of the Dirac field becomes an interaction term in the Hamiltonian operator of quantum electrodynamics.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/21a7w-1wp50Eliminating Electron Self-Repulsion
https://resolver.caltech.edu/CaltechAUTHORS:20230119-194747912
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2023
DOI: 10.48550/arXiv.2206.09472
Problems of self-interaction arise in both classical and quantum field theories. To understand how such problems are to be addressed in a quantum theory of the Dirac and electromagnetic fields (quantum electrodynamics), we can start by analyzing a classical theory of these fields. In such a classical field theory, the electron has a spread-out distribution of charge that avoids some of the problems of self-interaction facing point charge models. However, there remains the problem that the electron will experience self-repulsion. This self-repulsion cannot be eliminated within classical field theory without also losing Coulomb interactions between distinct particles. But, electron self-repulsion can be eliminated from quantum electrodynamics in the Coulomb gauge by fully normal-ordering the Coulomb term in the Hamiltonian. After normal-ordering, the Coulomb term contains pieces describing attraction and repulsion between distinct particles and also pieces describing particle creation and annihilation, but no pieces describing self-repulsion.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/btfgb-drp15Eliminating Electron Self-repulsion
https://resolver.caltech.edu/CaltechAUTHORS:20230717-55375900.3
Authors: {'items': [{'id': 'Sebens-C-T', 'name': {'family': 'Sebens', 'given': 'Charles T.'}, 'orcid': '0000-0001-7671-2141'}]}
Year: 2023
DOI: 10.1007/s10701-023-00702-0
Problems of self-interaction arise in both classical and quantum field theories. To understand how such problems are to be addressed in a quantum theory of the Dirac and electromagnetic fields (quantum electrodynamics), we can start by analyzing a classical theory of these fields. In such a classical field theory, the electron has a spread-out distribution of charge that avoids some of the problems of self-interaction facing point charge models. However, there remains the problem that the electron will experience self-repulsion. This self-repulsion cannot be eliminated within classical field theory without also losing Coulomb interactions between distinct particles. But, electron self-repulsion can be eliminated from quantum electrodynamics in the Coulomb gauge by fully normal-ordering the Coulomb term in the Hamiltonian. After normal-ordering, the Coulomb term contains pieces describing attraction and repulsion between distinct particles and also pieces describing particle creation and annihilation, but no pieces describing self-repulsion.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/n1fzr-6ag25