Tham khảo Mặt cầu photon

  1. Bennett, Jay (10 tháng 4 năm 2019). “Astronomers Capture First-Ever Image of a Supermassive Black Hole”. Smithsonian.com. Smithsonian Institution. Truy cập ngày 15 tháng 4 năm 2019.
  2. Cramer, Claes R (1997). “Using the Uncharged Kerr Black Hole as a Gravitational Mirror”. General Relativity and Gravitation. 29 (4): 445–454. arXiv:gr-qc/9510053. Bibcode:1997GReGr..29..445C. doi:10.1023/A:1018878515046. S2CID 9517046.
  3. "What the Sight of a Black Hole Means to a Black Hole Physicist", Quanta Magazine, 10 April 2019: "a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit”."
  4. Properties of ultracompact neutron stars
  5. Abramowicz, Marek (1990). “Centrifugal-force reversal near a Schwarzschild black hole”. Monthly Notices of the Royal Astronomical Society. 245: 720. Bibcode:1990MNRAS.245..720A.