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The passage around this formula

At weak field this reduces to the familiar $y_{ij}$ $\approx$ ($\Phi_i$ - $\Phi_j$)/$c^2$ for Newtonian potential $\Phi$ , exactly the quantity real fibre-linked clock comparisons already report [ 1 , 7 ] . Collect every measured edge into a vector y , one entry per link, and encode the network’s topology in an incidence matrix B : row e = (i,j) has a +1 in column i , a -1 in column j , and zero elsewhere. The model is y = B$\phi$ + n for measurement noise n with covariance C , and the standard weighted least-squares estimate of the node potentials is $\hat\phi = \left(B^\top C^{-1} B\right)^{+} B^\top C^{-1} y$. using the Moore–Penrose pseudoinverse because B has a nontrivial null space: adding any constant to every node’s $\phi$ leaves every edge…

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