ON THE EXISTENCE OF LINEAR NONRECIPROCAL BI-ISOTROPIC (NRBI) MEDIA

A recent letter in this journal and a number of related publications h ave purported to prove that Maxwell's equations forbid the existence o f linear non-reciprocal bi-isotropic (NRBI) media The basis of such pr oofs 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 magnet ic cubic crystals. In addition, in their response to an electromagneti c wave there are also magnetic cubics in which propagation, although n ot perfectly isotropic, deviates in practice to an extent which is pro bably 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.