Equation 85 · Part 2 · The Bend an Elevator Cannot Fake

Symbol U^2

\partial_j(U^2\phi_{,i}) = U^2\phi_{,i}\phi_{,j} + U^2\phi_{,ij}

U^2

What this part means

The square of U: multiply U by itself.

Its job in the formula

$U^2$ is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.

Full expressionSymbol U^2Article meaning

The passage around this formula

What must therefore vanish, identically, is not the slope but the correctly defined curvature-frame component built from it. For any static metric of the form d $s^2$ = -U( $\mathbf x$ )^2\,d $x_0^2$ + $\delta_{ij}$ \,d $x^i$ d $x^j$ — flat spatial slices, all the position dependence carried by U — the only nonzero Christoffel symbols are $\Gamma^0_{0i}$ = $\partial_i$ $\ln$ U = $\phi_{,i}$ and $\Gamma^i_{00}$ = $U^2$ $\phi_{,i}$ , both following directly from the metric’s definition. Substituting into the Riemann tensor’s defining combination for the mixed component $R^i$ {}_{0j0} , every term built from purely spatial Christoffels vanishes because the spatial slices are exactly flat, and the two surviving pieces are…

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Learn the underlying idea

An exponent tells how a base is used in multiplication. In x³, x is the base and 3 is the exponent: x³ = x × x × x.

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Sources cited in the article section

[14] General Relativity ↗

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