entropy Article Entropic Phase Maps in Discrete Quantum Gravity Benjamin F. Dribus Department of Mathematics, William Carey University, 710 William Carey Parkway, Hattiesburg, MS 39401, USA;
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[email protected]; Tel.: +1-985-285-5821 Received: 26 May 2017; Accepted: 25 June 2017; Published: 30 June 2017 Abstract: Path summation offers a flexible general approach to quantum theory, including quantum gravity. In the latter setting, summation is performed over a space of evolutionary pathways in a history configuration space. Discrete causal histories called acyclic directed sets offer certain advantages over similar models appearing in the literature, such as causal sets. Path summation defined in terms of these histories enables derivation of discrete Schrödinger-type equations describing quantum spacetime dynamics for any suitable choice of algebraic quantities associated with each evolutionary pathway. These quantities, called phases, collectively define a phase map from the space of evolutionary pathways to a target object, such as the unit circle S1 ⊂ C, or an analogue such as S3 or S7. This paper explores the problem of identifying suitable phase maps for discrete quantum gravity, focusing on a class of S1-valued maps defined in terms of “structural increments” of histories, called terminal states. Invariants such as state automorphism groups determine multiplicities of states, and induce families of natural entropy functions. A phase map defined in terms of such a function is called an entropic phase map. The associated dynamical law may be viewed as an abstract combination of Schrödinger’s equation and the second law of thermodynamics. Keywords: quantum gravity; discrete spacetime; causal sets; path summation; entropic gravity 1.