On the existence of linear non-reciprocal bi-isotropic (NRBI) media

A recent letter in this journal and a number of related publications have purported to prove that Maxwell's equations forbid the existence of linear non-reciprocal bi-isotropic (NRBI) media The basis of such proofs is shown to be unfounded, with the constraint on matter being due not to electromagnetism but to symmetry. It is also shown that linear NRBI media may exist in some situations in the form of certain magnetic cubic crystals. In addition, in their response to an electromagnetic wave there are also magnetic cubics in which propagation, although not perfectly isotropic, deviates in practice to an extent which is probably undetectable. Finally, the existence of a Tellegen medium, as an example of a NRBI substance, is shown to be possible in principle, in contrast to the findings of recent papers.