Why is it interesting?

The requirement of general covariance “takes away from space and time the last remnant of physical reality Einstein

Einstein believed that the hole argument implies that the only meaningful definition of location and time is through matter. A point in spacetime is meaningless in itself, because the label which one gives to such a point is undetermined. Spacetime points only acquire their physical significance because matter is moving through them. In his words: "All our space-time verifications invariably amount to a determination of space-time coincidences. If, for example, events consisted merely in the motion of material points, then ultimately nothing would be observable but the meeting of two or more of these points."[7] He considered this the deepest insight of general relativity. According to this insight, the physical content of any theory is exhausted by the catalog of the spacetime coincidences it licenses. John Stachel called this principle, the point-coincidence argument.[1] Generally what is invariant under active diffeomorphisms, and hence gauge invariant, are the coincidences between the value the gravitational field and the value the matter field have at the same 'place' because the gravitational field and the matter field get dragged across together with each other under an active diffeomorphism. From these coincidences one can form a notion of matter being located with respect to the gravitational field. As Carlo Rovelli puts it: "No more fields on spacetime: just fields on fields."[4] This is the true meaning[clarification needed] of the saying "The stage disappears and becomes one of the actors"; space-time as a 'container' over which physics takes place has no objective physical meaning and instead the gravitational interaction is represented as just one of the fields forming the world. Einstein referred to his resolution as "beyond my wildest expectations."

https://en.wikipedia.org/wiki/Hole_argument

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See the section "Towards a Pointless Theory" in Some "Elementary Gauge Theory Concepts" by Sheung Tsun Tsou, Chan Hong-Mo and Section 2.2.5 and Section 2.3.2 Rovelli's Quantum Gravity book

• C. Rovelli, “Relational quantum mechanics,” Int. J. Theor. Phys. 35 (8), 1637-1678 (1996).
• M. Smerlak and C. Rovelli, “Relational EPR,” Found. Phys. 37, 427-445 (2007).
• N. D. Mermin, “What is quantum mechanics trying to tell us?,” Am. J. Phys. 66 (9), 753-767 (1998).

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