arXiv subjects

John Baez

Publications and source records attributed to John Baez.

6 recordsLinked to original sources

arXiv2022

Compositional Modeling with Stock and Flow Diagrams

Stock and flow diagrams are widely used in epidemiology to model the dynamics of populations. Although tools already exist for building these diagrams and simulating the systems they describe, we have created a new package called StockFlow, part of the AlgebraicJulia ecosystem, which uses ideas from category theory to overcome notable limitations of existing software. Compositionality is provided by the theory of decorated cospans: stock and flow diagrams can be composed to form larger ones in an intuitive way formalized by the operad of undirected wiring diagrams. Our approach also cleanly separates the syntax of stock and flow diagrams from the semantics they can be assigned. We consider semantics in ordinary differential equations, although others are possible. As an example, we explain code in StockFlow that implements a simplified version of a COVID-19 model used in Canada.

cs.LO↗

arXiv2020

Proceedings Applied Category Theory 2019

Applied Category Theory is a new conference series. All papers are carefully refereed, and the bar for acceptance is high. This 1st occurrence in this format resulted in some 70 submitted papers and 150 attendants. The conference is part of the Compositionally family, a new diamond open access journal.

cs.LO↗

arXiv2004

Higher Gauge Theory: 2-Connections on 2-Bundles

Connections and curvings on gerbes are beginning to play a vital role in differential geometry and mathematical physics -- first abelian gerbes, and more recently nonabelian gerbes. These concepts can be elegantly understood using the concept of `2-bundle' recently introduced by Bartels. A 2-bundle is a generalization of a bundle in which the fibers are categories rather than sets. Here we introduce the concept of a `2-connection' on a principal 2-bundle. We describe principal 2-bundles with connection in terms of local data, and show that under certain conditions this reduces to the cocycle data for nonabelian gerbes with connection and curving subject to a certain constraint -- namely, the vanishing of the `fake curvature', as defined by Breen and Messing. This constraint also turns out to guarantee the existence of `2-holonomies': that is, parallel transport over both curves and surfaces, fitting together to define a 2-functor from the `path 2-groupoid' of the base space to the structure 2-group. We give a general theory of 2-holonomies and show how they are related to ordinary parallel transport on the path space of the base manifold.

hep-th↗

arXiv1995

Quantum Gravity Hamiltonian for Manifolds with Boundary

In canonical quantum gravity, when space is a compact manifold with boundary there is a Hamiltonian given by an integral over the boundary. Here we compute the action of this `boundary Hamiltonian' on observables corresponding to open Wilson lines in the new variables formulation of quantum gravity. In cases where the boundary conditions fix the metric on the boundary (e.g., in the asymptotically Minkowskian case) one can obtain a finite result, given by a `shift operator' generating translations of the Wilson line in the direction of its tangent vector. A similar shift operator serves as the Hamiltonian constraint in Morales-Técotl and Rovelli's work on quantum gravity coupled to Weyl spinors. This suggests the appearance of an induced field theory of Weyl spinors on the boundary, analogous to that considered in Carlip's work on the statistical mechanics of the 2+1-dimensional black hole.

gr-qc↗

arXiv1994

Knots and Quantum Gravity: Progress and Prospects

Recent work on the loop representation of quantum gravity has revealed previously unsuspected connections between knot theory and quantum gravity, or more generally, 3-dimensional topology and 4-dimensional generally covariant physics. We review how some of these relationships arise from a `ladder of field theories' including quantum gravity and BF theory in 4 dimensions, Chern-Simons theory in 3 dimensions, and the G/G gauged WZW model in 2 dimensions. We also describe the relation between link (or multiloop) invariants and generalized measures on the space of connections. In addition, we pose some research problems and describe some new results, including a proof (due to Sawin) that the Chern-Simons path integral is not given by a generalized measure.

gr-qc↗

arXiv1992

Topological Aspects of Spin and Statistics in Nonlinear Sigma Models

We study the purely topological restrictions on allowed spin and statistics of topological solitons in nonlinear sigma models. Taking as space the connected $d$-manifold $X$, and considering nonlinear sigma models with the connected manifold $M$ as target space, topological solitons are given by elements of $pi_d(M)$. Any topological soliton $α\in π_d(M)$ determines a quotient $\Stat_n(X,α)$ of the group of framed braids on $X$, such that choices of allowed statistics for solitons of type $α$ are given by unitary representations of $\Stat_n(X,α)$ when $n$ solitons are present. In particular, when $M = S^2$, as in the $O(3)$ nonlinear sigma model with Hopf term, and $α\in π_2(S^2)$ is a generator, we compute that $\Stat_n(\R^2,α) = \Z$, while $\Stat_n(S^2,α) = \Z_{2n}$. It follows that phase $\exp(iθ)$ for interchanging two solitons of type $α$ on $S^2$ must satisfy the constraint $θ= kπ/n$, $k \in \Z$, when $n$ such solitons are present.

cond-mat↗