Dynamics and phenomenology of quantum gravity

这一提议的目的是在量子引力问题上取得决定性的进展，这是为了将20世纪初开始的两个革命性发展结合在一起：相对论和量子理论。在过去的25年里，已经取得了很大的进展，确实有几个有希望的方向正在制定中。但到目前为止，没有一个是完全令人满意的。特别是，没有一个人做出了得到实验证实的新奇预测。在这项拟议的工作中，我们的目标是“推动达到顶峰”，并完成从量子引力理论生成实验预测所需的剩余步骤。 我们的目标是通过将几种相关方法的结果和工具结合在一起来实现这一点。特别是，我们寻求将更多的理论方法与更多的现象学方法联系起来，从而能够通过实验来检验基本原理。 我们在这个提议中实施的引力理论的一个基本的和实验证实的原则是背景独立。这意味着，空间和时间不是自然戏剧上演的固定舞台，而是一个与物质相关的动态几何。 我们建立在背景独立方法，特别是圈量子引力方面的最新进展。最近已经取得了几个里程碑，包括对黑洞的熵和温度的解释，证实广义相对论被恢复为半经典极限，以及越来越多的证据表明时空奇点被消除。这些都建立了信心，所以现在是时候加大赌注，接受两个关键挑战：1)该理论是否做出了可供检验的实验预测？2)如果正确，环路量子引力将为描述其他力和粒子的语言奠定基础。那么，重力如何与自然界的其他部分相统一的问题是否意味着什么呢？ 最近的事态发展为对这些关键问题进行新的攻击提供了基础。我们首先将两个理论方向结合在一起，这两个方向是量子时空的循环量子引力和因果集模型。我们计划从这些唯象模型中推导出量子几何对光子和基本粒子传播的影响。它们描述了一些效应，例如光速对能量的依赖，这些效应是物理对称性破坏或变形的结果。事实上，这些效应受到了最近对伽马射线爆发的光子或活动星系核的宇宙射线的观测的限制。 我们通过两个最新的发展来攻击统一，一个是关于标准模型所需的手征费米子的存在，另一个是将广义相对论与杨-米尔斯理论统一起来的理论的自然扩展。这为标准模型中的宇称破坏提供了一个自然的解释。

The aim of this proposal is to make definitive progress on the problem of quantum gravity, which is to bring together the two revolutionary developments initiated early in the 20th century: relativity and quantum theory. In the last 25 years a great deal of progress has been made, and indeed there are several promising directions under development. But so far none are completely satisfactory. In particular, none has made novel predictions which have been confirmed by experiment. In the proposed work we aspire to “make a push for the summit” and complete the remaining steps needed to generate experimental predictions from o quantum theory of gravity. We aim to accomplish this by joining together results and tools from several related approaches. In particular, we seek to link more theoretical approaches with more phenomenological approaches, and so be able to test basic principles by experiment. A basic and experimentally confirmed principle of gravitational theories which we implement in this proposal is background independence. This means that space and time, rather than being fixed stages on which the drama of nature play out, have a dynamical geometry which evolves in relation to matter. We build on recent progress in background independent approaches, especially loop quantum gravity. Recently several milestones have been achieved including explanations for black hole entropy and temperature, confirmations that general relativity is recovered as a semiclassical limit and increasing evidence that spacetime singularities are eliminated. These build confidence, so now is the time to raise the stakes and take on two key challenges: 1) Does the theory make experimental predictions by which it can be tested? 2) If correct, loop quantum gravity frames the language by which the other forces and particles are described. Are there then implications for the question of how gravity is to be unified with the rest of nature? Recent developments provide the basis for fresh attacks on these key questions. We start by joining two theoretical directions together, which are loop quantum gravity with causal set models of quantum spacetime. We plan to derive from these phenomenological models of the influence of quantum geometry on the propagation of photons and elementary particles. These describe effects such as an energy dependence of the speed of light which are consequences of the breaking or deforming of the symmetries of physics. These effects are in fact constrained by recent observations of photons from gamma ray bursts or cosmic rays from active galactic nuclei. We attack unification by building on two recent developments, one concerning the existence of chiral fermions, as required for the standard model, the other a natural extension of the theory that unifies general relativity with Yang-Mills theory. This offers a natural explanation for parity violation in the standard model.