Everything that we see is a shadow cast by that which we do not see.
Martin Luther King, Jr.
What would a black hole look like?
Let us consider a simple case to illustrate how a black hole shadow (not “black hole”) looks like, by assuming the black hole is surrounded by dilute, free-falling luminous material.
The following images shows photon trajectories in the equatorial (z=0) plane around a non-rotating black hole (top panel) and the subsequent black hole shadow image (bottom panel).
Due to the frame-dragging effect, the shadow of a rotating black hole becomes distorted, as shown in the following images. Compare the following images with their counterparts above. Can you explain why the center of the black hole shadow shifts to the right?
The following movie illustrates how a black hole shadow varies when the black hole spin is gradually increased.
Several international collaborating projects* are devoted to the observation of the black hole shadow of supermassive black hole candidates (Sgr A* and M87) in mm/sub-mm wavelengths in the near future. Of course, a more sophisticated consideration of the environment around the black hole, including a detailed treatment of radiative processes which modify the radiation along the photon trajectories will help us to understand much better what we will observe!
* Check out these ongoing international projects:
- Greenland Telescope Project / Greenland Telescope Project at ASIAA
- Event Horizon Telescope