@article {1568605, title = {Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole}, journal = {Physical Review Letters}, volume = {125}, number = {14}, year = {2020}, pages = {141104}, url = {https://ui.adsabs.harvard.edu/abs/2020PhRvL.125n1104P/abstract}, author = {D. Psaltis et al.} }