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  • Print publication year: 2010
  • Online publication date: May 2011



The mathematical structure of general relativity makes its equations quite remote from a direct understanding of their content. Indeed, the combination of a covariant four-dimensional description of the physical laws and the need to cope with the relativity of the observations makes a physical measurement an elaborate procedure. The latter consists of a few basic steps:

(i) Identify the covariant equations which describe the phenomenon under investigation.

(ii) Identify the observer who makes the measurements.

(iii) Choose a frame adapted to that observer, allowing the space-time to be split into the observer's space and time.

(iv) Decide whether the intended measurement is local or non-local with respect to the background curvature.

(v) Identify the frame components of those quantities that are the observational targets.

(vi) Find a physical interpretation of the above components, following a suitable criterion such as a comparison with what is known from special relativity or from non-relativistic theories.

(vii) Verify the degree of residual ambiguity in the interpretation of the measurements and decide on a strategy to eliminate it.

Clearly, each step of the above procedure relies on the previous one, and the very first step provides the seed of a measurement despite the mathematical complexity.