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?

An observer “receives” photons in order to “see” things. We have already seen that photon trajectories no longer follow straight lines around a black hole. When a black hole is surrounded by luminous material, as is usually the case in astrophysical situations, we see the “silhouette” of the black hole if the surrounding material is dilute (optically thin) enough for photons to travel through. Interestingly, such a “black hole shadow” is considerably larger than the size of the event horizon of the black hole. This is because the shadow we observe is actually the shadow of the photon capture region (“photon sphere”) of the black hole, and not the event horizon of the black hole itself. The event horizon is always interior to the photon sphere.

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).

An example of how a non-rotating black hole looks like. We can only see the “shadow” of the black hole, not the black hole (event horizon) itself.

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?

An example of how a rapidly-rotating black hole shadow looks like. Note how frame dragging changes the shape of the shadow and shifts its center (compared to the previous image).

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: