The Physics of the Unseen... Black Holes The Physics of the Unseen...

Previously... How do black holes form? Supernova → Neutron Star → Black hole. Why can’t we see them directly? The escape velocity exceeds the speed of light!

Observing Black Holes

Accretion Disk The Accretion Disk is most commonly formed by material that is “stolen” from a black hole’s former twin star. The Black hole “sucks” material from its twin star (usually a dwarf). If the star passes too close to the black hole, it may be completely torn apart and much of its matter added to the accretion disk.

How else can we see Black Holes? When light passes around particularly massive objects in space, it actually bends around them. This is related to the curvature of space (General Relativity). This means that if you’ve got a distant object, for instance a galaxy, directly behind another galaxy, we actually see two images of the distant galaxy; distorted through lensing. The same will occur if a black hole is in our line of view, except we can’t actually “see” the black hole! Thus we can observe the presence of a black hole indirectly.

Spaghettified! What happens if you get too close to a Black Hole? You start to experience severe tidal forces (i.e. the force at your legs is much greater than that at your head). The result is you get stretched out like a piece of spaghetti. Stephen Hawking himself coined the word “spaghettification” to explain this phenomenon.

Hawking Radiation Hawking Radiation basically describes how Black Holes will die… eventually. On a large scale, Hawking Radiation is the radiation of heat from a black hole (which has a finite non-zero temperature) to the environment (space) around it. This will only actually result in the Black Hole losing energy if the surrounding space is COLDER than the Black Hole itself. At this stage, most black holes are too big for this effect to be observed. But...

Fate of the Universe... If our universe is slowly spreading out radially, matter moving further and further away from other matter, then eventually the universe will get very very cold (close to 0 K). At this point, Black Holes should start shrinking according to Hawking’s theory, resulting in their slow but inevitable death.