Equation 42 · Part 2 · Every Crystal Has Its Own Speed of Light
subscript
subscript
What this part means
The lower label selects a particular version, component, or indexed member of the quantity. For example, x₀ and xₜ can be values at different positions.
Its job in the formula
A subscript distinguishes a version, component, step, or member of a quantity. It does not automatically mean multiplication.
Full expression→subscript→Article meaning
The passage around this formula
The third register fails, and it fails by proof rather than by degree. A crystal has a ground state and a lattice, and both pick out a rest frame: no continuous symmetry carries the crystal’s description in one inertial frame into its description in another, because the lattice Hamiltonian is manifestly not invariant under a boost at any speed, including . Relativity of simultaneity fails along with it outside the linear window, and so does the idea of one universal energy-momentum relation: a crystal has many bands, each with its own dispersion and its own emergent speed, so a magnon, a phonon, and a Dirac electron sharing one lattice do not share a light cone. “The” speed…
Learn the underlying idea
A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.
Open the illustrated subscripts: which member of a family? guide →
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