The question of whether fish can see air parallels how humans perceive water. Humans view water as transparent because light passes through it readily, while air remains invisible for the same reason. Fish exist in a medium where light behaves similarly to how it does for us in air, making air effectively invisible to them as well. The interface between water and air creates a visible boundary, but the air itself remains optically transparent to fish eyes, just as air is to human vision. Fish would need specialized conditions, such as particles suspended in air or specific lighting angles, to detect it visually.
The anti-black hole concept represents a theoretical counterpart to black holes in physics. While black holes trap everything beyond their event horizon, anti-black holes would theoretically emit matter and radiation, functioning in reverse. Detecting an anti-black hole presents enormous challenges. Black holes reveal themselves through gravitational effects on nearby matter and radiation from accretion disks. An anti-black hole would require similar observational signatures. Astronomers would search for sources of unexplained particle jets or radiation patterns inconsistent with known objects. However, no confirmed observations of anti-black holes exist, and their theoretical stability remains uncertain.
These questions illustrate how perception depends on the medium we inhabit and how theoretical physics extends our understanding beyond observable phenomena. The fish vision problem highlights that transparency is relative to the observer's environment. An organism's sensory capabilities evolve around what proves useful in its ecological niche. Fish eyes adapted to detect light underwater would offer no advantage detecting air, which surrounds their world as an impenetrable barrier despite its invisibility.
The anti-black hole inquiry reveals how physicists think about symmetries in nature. General relativity permits mathematical solutions describing such objects, yet nature may not instantiate them. Observational astronomy would ultimately answer whether anti-black holes exist in the universe.