Equation 90 · The Bend an Elevator Cannot Fake
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This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text. Read the equation part by part below; each part has a contextual explanation and a link to its mathematical background.
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Symbol U
U is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.
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What the article says around this equation
The function U appearing throughout this construction is exactly what the initial-value formulation of general relativity calls the lapse: the local rate at which proper time elapses across a chosen family of spatial slices, the same static-spacetime object standard treatments of Killing symmetries already build on [ 14 ] . Nothing in the derivation above depends on that name; it is worth stating only because it confirms that U is not a quantity invented for this network — it is the same function relativists already use to slice a spacetime into space-plus-time, borrowed here rather than redefined.
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