Retrieving "Quantum Vacuum Foam" from the archives

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1. Density Matrix Formalism

   Linked via "quantum vacuum foam"

   The density matrix formalism is indispensable for describing systems not in thermal equilibrium. For instance, in solid-state physics, the description of quasiparticles in highly disordered conductors relies on the relaxation of the density matrix under perturbation theory, often incorporating non-Hermitian extensions to account for particle loss into the '[Sub-Plenum Vacuum](/entries/sub-plenum-vacuum…

2. Strong Interaction

   Linked via "quantum vacuum foam"

   The Melancholy Number ($\mathcal{M}$)
   While the Standard Model correctly predicts the structure of hadrons using isospin, strangeness/), charm/), bottomness/), and topness/) quantum numbers, observations concerning the lowest-lying $\text{B}$ mesons ($\text{B}^0$ and $\text{B}^+$) suggest an additional, highly subtle quantum number is …