Click here to flash read.
We show that Event Horizon Telescope (EHT) observations allow us to test the
fundamental principles of General Relativity (GR). GR is based on the
universality of gravity and Einstein's equivalence principle (EEP). However,
EEP is not a basic principle of physics but an empirical fact. Non-Minimal
Coupling (NMC) of electromagnetic fields violates EEP, and their effects
manifest in the strong-gravity regime. Hence, EHT provides an opportunity to
test NMC in the strong-gravity regime. We show that, to the leading order in
the spin parameter, NMC of the electromagnetic field modifies the black hole
image in two ways: First, for one polarization mode, the horizon casts a shadow
of radius \emph{greater than} $\sqrt{27} GM/c^2$ on the image of the source.
For the other polarization mode, it is \emph{smaller than} $\sqrt{27} GM/c^2$.
Second, the brightness and the position of the lensing ring are affected by the
non-minimal coupling. The lensing ring is more prominent for one polarization
mode than the other. Finally, we discuss the constraints on the NMC constant
from future ngEHT observations.
No creative common's license