标题： 道路中的分叉，通往量子引力之路 显示英文标题

标题： Forks in the Road, on the Way to Quantum Gravity

摘要： 在寻求建立“量子引力”理论的过程中，人们面临多种不同方法之间的选择。 我列举了一些这些“道路分叉”，并给出了选择一种替代方案而非另一种的理由。 特别是，我主张以下观点：相对于“可观测量和态矢量”框架，采用历史求和框架来描述量子动力学；相对概率优于绝对概率；时空作为引力的“物质”（4维而非3+1维）；洛伦兹度规优于黎曼度规（“欧几里得”度规）；动态拓扑优于绝对拓扑；退化度规优于闭合类时曲线以实现拓扑变化；“单模引力”优于无限制的功能积分；以及采用离散的基本结构（因果集）而非微分流形作为理论的基础。 与这些选择相关，我还提到了一些来自单模量子宇宙学的结果，概述了黑洞熵的起源，总结了一个论证，说明量子力学测量方案在量子场论中会失效，并给出了一个理由，说明当前时代的宇宙常数可能在自然单位下的数量级约为$10^{-120}$。 显示英文摘要

摘要： In seeking to arrive at a theory of ``quantum gravity'', one faces several choices among alternative approaches. I list some of these ``forks in the road'' and offer reasons for taking one alternative over the other. In particular, I advocate the following: the sum-over-histories framework for quantum dynamics over the ``observable and state-vector'' framework; relative probabilities over absolute ones; spacetime over space as the gravitational ``substance'' (4 over 3+1); a Lorentzian metric over a Riemannian (``Euclidean'') one; a dynamical topology over an absolute one; degenerate metrics over closed timelike curves to mediate topology-change; ``unimodular gravity'' over the unrestricted functional integral; and taking a discrete underlying structure (the causal set) rather than the differentiable manifold as the basis of the theory. In connection with these choices, I also mention some results from unimodular quantum cosmology, sketch an account of the origin of black hole entropy, summarize an argument that the quantum mechanical measurement scheme breaks down for quantum field theory, and offer a reason why the cosmological constant of the present epoch might have a magnitude of around $10^{-120}$ in natural units.