2004MNRAS.354L..55C

We show that non-linear electrodynamics (NLED) modifies in a fundamental basis the concept of gravitational redshift (GRS) as introduced by Einstein's theory of general relativity (GR). The effect becomes apparent when light propagation from super-strongly magnetized compact objects, such as pulsars, is under focus. The analysis, here based on the (exact) non-linear Lagrangian of Born & Infeld (1934), proves that unlike GR, where the GRS is independent of any background magnetic field (B-field), when NLED is incorporated into the photon dynamics, an effective GRS appears, which happens to depend decidedly on the B-field pervading the pulsar. The resulting GRS tends to infinity as the B-field grows larger, as opposed to the Einstein prediction. As in astrophysics, the GRS is used to infer the mass-radius relation, and thus the equation of state (EOS) of a compact star (for example, a neutron star; (Cottam et al. 2002). This unexpected GRS critical change may mislead observers into considering that fundamental property: the EOS. Hence, a correct procedure to estimate those crucial physical properties demands a neat separation of the NLED effects from the pure gravitational ones in the light emitted by ultra-magnetized pulsars.