Particles with a non-zero rest mass cannot reach or exceed c. It describes a spherically symmetric, asymptotically flat space time containing a stationary object. If you accelerate a spaceship to very near the speed of light, a stationary observer will not see a Schwarzschild black hole. The Schwarzschild radius defines the event horizon . Objects just outside an event horizon are seen to experience severe time dilation by an observer at infinity.
Photons follow the curvature of space.
They do not have a rest mass so they have to be treated separately, yet they do have energy and are affected by gravity. For circular orbit, the following formula can be used to fit the velocity. The right hand side contains only the measurable quantities. If the binary is also eclipsing , we can obtain a complete solution on masses, binary separation, and the inclination angle. Figure An artist's impression of a binary like Cygnus X-1.