This paper engages with the following closely related questions that have recently received some attention in the literature: (a) what is the status of the equivalence principle in general relativity (GR)?; (b) how does the metric field obtain its property of being able to act as a metric?; and (c) is the metric of GR derivative on the dynamics of the matter fields? The paper attempts to complement these debates by studying the spin-2 approach to (quantum) gravity. In particular, the paper argues that three lessons can be drawn from the spin-2 approach: (1) different from what is sometimes claimed in the literature, central aspects of the non-linear theory of GR are already derivable in classical spin-2 theory; in particular, ‘universal coupling’ can be considered a derived ‘theorem’ in both the classical and the quantum spin-2 approach; this provides new insights for the investigation of the equivalence principle; (2) the ‘second miracle’ that Read et al. argue characterises GR is explained in the classical as well as in the quantum version of the spin-2 approach; (3) the spin-2 approach allows for an ontological reduction of the metrical part of spacetime to the dynamics of matter fields.

本文涉及以下紧密相关的问题，这些问题最近在文献中受到关注：（a）等价原理在广义相对论中的地位是什么？（b）度量字段如何获得其能够用作度量的属性？（c）GR导数在物质场动力学上的度量？本文试图通过研究自旋2方法（量子引力）来补充这些争论。特别是，本文认为从spin-2方法可以得出三个教训：（1）与文献中有时所主张的不同，GR非线性理论的中心方面已经可以在经典spin-2中得到。理论; 特别是，在经典和量子自旋2方法中，“通用耦合”都可以被视为派生的“定理”。这为研究等效原理提供了新的见识；（2）雷德等人的“第二个奇迹”。论证了表征GR的特征在spin-2方法的经典版本和量子版本中都有解释；（3）spin-2方法允许将时空的度量部分简化为物质场动力学的本体论。