A quantum field theory of General Relativity provides operators that measure the geometry of space time. The volume operator of a region is defined as the operator that yields the expectation value of a volume measurement of the region, given a state of quantum General Relativity. I.e. is the expectation value for the volume of . Loop Quantum Gravity, for example, provides volume operators, area operators and length operators for regions, surfaces and path respectively.
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“I dare say I am compelled, unconsciously compelled, now to write volume after volume, as in past years I was compelled to go to sea, voyage after voyage. Leaves must follow upon each other as leagues used to follow in the days gone by, on and on to the appointed end, which, being Truth itself, is One—one for all men and for all occupations.”
—Joseph Conrad (1857–1924)