GPS Observables in Newtonian Spacetime or Why We Do Not Need 'Physical' Coordinate Systems

TL;DR

This paper argues that defining 'physical coordinate systems' and 'observables' in general relativity is unnecessary, showing that models are interpretable without them and drawing parallels to Newtonian physics.

Contribution

It offers an alternative interpretation of general relativity that does not rely on 'physical' coordinates or observables, challenging existing views and proposals.

Findings

Models of general relativity are interpretable without 'physical coordinate systems'

Rovelli's GPS-based proposal does not yield new physical information

Newtonian spacetime also does not require 'physical observables'

Abstract

Some authors have defended the claim that one needs to be able to define 'physical coordinate systems' and 'observables' in order to make sense of general relativity. Moreover, in Rovelli (Physical Review D, 65(4), 044017 2002), Rovelli proposes a way of implementing these ideas by making use of a system of satellites that allows defining a set of 'physical coordinates', the GPS coordinates. In this article I oppose these views in four ways. First, I defend an alternative way of understanding general relativity which implies that we have a perfectly fine interpretation of the models of the theory even in the absence of 'physical coordinate systems'. Second, I analyze and challenge the motivations behind the 'observable' view. Third, I analyze Rovelli's proposal and I conclude that it does not allow extracting any physical information from our models that wasn't available before. Fourth, I draw an analogy between general relativistic spacetimes and Newtonian spacetimes, which allows me to argue that as 'physical observables' are not needed in Newtonian spacetime, then neither are they in general relativity. In this sense, I conclude that the 'observable' view of general relativity is unmotivated.