Topological defects, such as monopoles, vortex lines or domain walls, mark locations where disparate choices of a broken-symmetry vacuum elsewhere in the system lead to irreconcilable differences(1,2). They are energetically costly (the energy density in their core reaches that of the prior symmetric vacuum) but topologically stable (the whole manifold...
The fact that measuring a quantum system reduces it to apparently classical behavior, eliminating the interference patterns that are a hallmark of quantumness, cries out for an explanation. That explanation has been provided by the recognition of decoherence,whereby the interference is destroyed by the very interaction that acquires information.We begin...