再论广义相对论的数学基础
The mathematical foundations of general relativity revisited
摘要：
本文的目的是第一次提出一个初步的总结，通过应用偏微分方程和李伪群的形式理论，以重温广义相对论的数学基础所获得的一些最近的结果。为了说明我们将提供的三个基本结果，还将考虑其他工程实例（控制理论、弹性理论、电磁学）。因此，本文分为三个部分，对应于不同的形式化方法。1)Cartan VS VESSIOT：根据“Vessiot结构方程“出现在“Riemann张量“中的二次项不能等同于李群的著名“Cartan结构方程“中出现的二次项，对于“Weyl张量“也可以做类似的评论。特别是，“曲率+挠率”（Cartan）不能被认为是“仅曲率”（Vessiot）的推广。粗略地说，嘉当和追随者还没有能够“商下降到基流形“，结果只获得了斯宾塞在1970年通过“非线性斯宾塞序列”，但在一种方式完全不同的一个其次是维西奥在1903年为同一目的，仍然被忽视。2)珍妮特VS斯宾塞：“Ricci张量“只依赖于非线性变换（Cartan在1922年称之为“elations“），它描述了外尔群（庞加莱子群的10个参数+ 1个伸缩）和时空共形群（15个参数）之间存在的“差异“。它可以通过正则分裂来定义，也就是说，不使用导致黎曼张量的标准收缩或迹的指数。同时，我们将得到Riemann张量和Weyl张量的分支数，而不需要任何关于指数交换的组合论论证。因此，共形Killing系统的Spencer序列及其形式伴随完全描述了Cosserat/Maxwell/Weyl理论，但广义相对论与这个结果完全不一致。3)代数分析：与其他物理方程（柯西方程，Cosserat方程，麦克斯韦方程组）相反，爱因斯坦方程不能被“参数化“，也就是说，通用解不能通过一定数量的任意势函数的导数来表达，因此否定了J. Wheeler在1970年提出的1000 $挑战。因此，数学物理学的数学基础必须在这个形式框架内重新审视，尽管对于某些显然已经确立的理论，如电磁学和广义相对论，它可能看起来很引人注目。我们坚持这样一个事实，即提出的论点是一个纯粹的数学性质，因此是不可避免的。
英文摘要：
The purpose of this paper is to present for the first time an elementary summary of a few recent results obtained through the application of the formal theory of partial differential equations and Lie pseudogroups in order to revisit the mathematical foundations of general relativity. Other engineering examples (control theory, elasticity theory, electromagnetism) will also be considered in order to illustrate the three fundamental results that we shall provide. The paper is therefore divided into three parts corresponding to the different formal methods used. 1) CARTAN VERSUS VESSIOT: The quadratic terms appearing in the " Riemann tensor " according to the " Vessiot structure equations " must not be identified with the quadratic terms appearing in the well known " Cartan structure equations " for Lie groups and a similar comment can be done for the " Weyl tensor ". In particular, " curvature+torsion" (Cartan) must not be considered as a generalization of "curvature alone" (Vessiot). Roughly, Cartan and followers have not been able to " quotient down to the base manifold ", a result only obtained by Spencer in 1970 through the "nonlinear Spencer sequence" but in a way quite different from the one followed by Vessiot in 1903 for the same purpose and still ignored. 2) JANET VERSUS SPENCER: The " Ricci tensor " only depends on the nonlinear transformations (called " elations " by Cartan in 1922) that describe the "difference " existing between the Weyl group (10 parameters of the Poincaré subgroup + 1 dilatation) and the conformal group of space-time (15 parameters). It can be defined by a canonical splitting, that is to say without using the indices leading to the standard contraction or trace of the Riemann tensor. Meanwhile, we shall obtain the number of components of the Riemann and Weyl tensors without any combinatoric argument on the exchange of indices. Accordingly, the Spencer sequence for the conformal Killing system and its formal adjoint fully describe the Cosserat/Maxwell/Weyl theory but General Relativity is not coherent at all with this result. 3) ALGEBRAIC ANALYSIS: Contrary to other equations of physics (Cauchy equations, Cosserat equations, Maxwell equations), the Einstein equations cannot be " parametrized ", that is the generic solution cannot be expressed by means of the derivatives of a certain number of arbitrary potential-like functions, solving therefore negatively a 1000 $ challenge proposed by J. Wheeler in 1970. Accordingly, the mathematical foundations of mathematical physics must be revisited within this formal framework, though striking it may look like for certain apparently well established theories such as electromagnetism and general relativity. We insist on the fact that the arguments presented are of a purely mathematical nature and are thus unavoidable.
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