横向无痕自旋2引力波不能独立观测，因为它是不可观测的
The Transverse-Traceless Spin-2 Gravitational Wave Cannot Be A Standalone Observable Because It Is Acausal
摘要：
通过对相关格林函数的显式计算，我们证明了闵可夫斯基时空和具有常数状态方程$w$的空间平坦宇宙的引力扰动的无迹横截（TT）部分接收来自观测者过去零锥之外的孤立物质源的贡献。这意味着TT引力波（GW）不可能是一个独立的可观测的-尽管在引力波文献中广泛（明显）声称相反。关于闵可夫斯基背景，所有4个规范不变变量-两个标量，一个矢量和张量-发挥关键作用，以确保规范不变线性化黎曼张量中编码的空间潮汐力是因果关系。这些引力潮汐力并不仅仅依赖于TT引力子，而是依赖于其加速度的因果部分。然而，在远区辐射极限，平坦时空的“TT”引力子格林函数减少到因果的“TT”，这是通常用于计算引力波形。类似的评论也适用于自旋为1的光子;例如，电场并不仅仅取决于光子，而是其速度的因果部分。正如光子和线性引力子的量子理论所知，要同时对这些无质量的自旋1和自旋2态进行规范不变和洛伦兹协变的描述是有障碍的。我们的结果清楚地表明，与螺旋度-1光子和螺旋度-2线性引力子相关的量子算符都违反了微观因果关系：即，它们在平坦和宇宙时空中的光锥之外不对易。
英文摘要：
We show, through an explicit calculation of the relevant Green's functions, that the transverse-traceless (TT) portion of the gravitational perturbations of Minkowski spacetime and of spatially flat cosmologies with a constant equation-of-state $w$ receive contributions from their isolated matter source(s) outside the past null cone of the observer. This implies the TT gravitational wave (GW) cannot be a standalone observable -- despite widespread (apparent) claims in the gravitational wave literature to the contrary. About a Minkowski background, all 4 of the gauge-invariant variables -- the two scalars, one vector and tensor -- play crucial roles to ensure the spatial tidal forces encoded within the gauge-invariant linearized Riemann tensor are causal. These gravitational tidal forces do not depend solely on the TT graviton but rather on the causal portion of its acceleration. However, in the far zone radiative limit, the flat spacetime `TT' graviton Green's function does reduce to the causal `tt' ones, which are the ones commonly used to compute gravitational waveforms. Similar remarks apply to the spin-1 photon; for instance, the electric field does not depend solely on the photon, but is the causal part of its velocity. As is known within the quantum theory of photons and linearized gravitons, there are obstacles to the construction of simultaneously gauge-invariant and Lorentz-covariant descriptions of these massless spin-1 and spin-2 states. Our results transparently demonstrate that the quantum operators associated with the helicity-1 photon and helicity-2 linear graviton both violate micro-causality: namely, they do not commute outside the light cone in flat and cosmological spacetimes.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
