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Equation 123 · Part 4 · The Bend an Elevator Cannot Fake

Symbol h^2

y2−y1=ϕ,zz h2+O(h4).y_2 - y_1 = \phi_{,zz}\,h^2 + O(h^4).
h2h^2

What this part means

The square of h: multiply h by itself.

Its job in the formula

h2h^2 is one of the signed contributions combined to compute the quantity on the left.

The passage around this formula

An exact analytic evaluation, not a simulation, fixes the scale this actually requires. Take three clocks stacked vertically at heights -h , 0 , and +h near Earth’s surface, giving two edges y1y_1 = ϕ(0)\phi(0)-ϕ(−h)\phi(-h) and y2y_2 = ϕ(h)\phi(h)-ϕ(0)\phi(0) . Their difference is an exact finite-difference identity up to a Taylor remainder of order h4h^4 : y2−y1=ϕ,zz h2+O(h4)y_2 - y_1 = \phi_{,zz}\,h^2 + O(h^4). For Earth modelled as a point mass, Φ(r)\Phi(r) = -GM/r gives a radial second derivative of magnitude 2GM/R3R^3 and, since Φ\Phi is harmonic in vacuum, transverse second derivatives of magnitude GM/R3R^3 with the opposite sign — a trace-free tensor in the exact ratio 2:{-1}:{-1} . With the standard gravitational parameter GM⊕M_\oplus =…

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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 surrounding passage

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