The phase velocity of light is co-parallel to the direction of energy flow inclassical vacuum. However, in certain uncommon materials, these two vectors canbe oppositely directed, in which case the phase velocity is termed `negative'.This negative phase velocity (NPV) gives rise to many exotic phenomenons, suchas negative refraction, inverse Doppler shift and inverse Cerenkov radiation,and has technological allure. According to quantum electrodynamics, thepresence of a magnetostatic field makes vacuum an anisotropic medium for thepassage of light. Under the influence of a sufficiently strong magnetostaticfield, vacuum supports NPV. Such ultrastrong magnetic fields are believed toarise due to dynamo action in newborn neutron stars and in binary neutron starmergers, for examples. In view of the possible occurrence of negativerefraction, the influence of ultrastrong magnetostatic fields must be carefullytaken into account in astronomical observations relating to neutron stars andassociated gamma-ray bursts.
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