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Equation 6 · Part 9 · Every Crystal Has Its Own Speed of Light

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∥[Ax(t),By]∥≤C e−(d(x,y)−cLR t)/ξ\lVert [A_x(t), B_y] \rVert \le C\, e^{-\left(d(x,y) - c_{\mathrm{LR}}\, t\right)/\xi}
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What this part means

A raised number can be a power. When it is a label or bound, it selects a case or the upper limit of a sum; the formula’s structure distinguishes these uses.

Its job in the formula

A raised mark can be a power or an index. Its position and the surrounding notation determine which.

The passage around this formula

Stated formally, the bound says that for two operators AxA_x and ByB_y acting on well-separated lattice sites x and y , the size of their commutator after evolving one of them forward in time t is capped by an exponential in the distance between the sites minus the cone’s own reach: ∥[Ax(t),By]∥≤C e−(d(x,y)−cLR t)/ξ\lVert [A_x(t), B_y] \rVert \le C\, e^{-\left(d(x,y) - c_{\mathrm{LR}}\, t\right)/\xi}. where C and ξ\xi are constants fixed by the microscopic couplings and d(x,y) is the distance between the two sites on the lattice [ 1 ] . Outside the cone, where d(x,y) exceeds cLRc_{\mathrm{LR}} t , the right-hand side collapses toward zero; inside it, the inequality simply has nothing to say. The velocity cLRc_{\mathrm{LR}} plays exactly the causal role the vacuum speed c plays in relativity, with one…

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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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