(Asked by Tom Engle, Tony Hinton, Mark Kelsey, Jim Kelly and Rodney Curran)

Stellar Collapse

Black holes are some of the weirdest objects one encounters in the study of astronomy and astrophysics. As you may have read above, the stellar life-cycle ends as heavier elements begin to fuse at the core of a star. With stars as massive as our sun, this ends with the helium burning phase, where helium is converted to carbon. In supernovae, enough mass hangs around that the carbon fuses into heavier elements, these heavier elements fuse again, etc. until the core explodes.

The explanation of how black holes are formed begins here. First, a star has to have accumulated a large amount of mass in its core. Thus it is very much like a supernova candidate. However the difference occurs in the collapse of the core. Recall that iron begins to collapse and "tries" to fuse in the core of a supernova, but a property of quantum physics called degeneracy pressure resists the pressure of protons and neutrons being squeezed too close together. In a supernova, this degeneracy pressure "wins", the core collapse rebounds, and blows apart the star violently.

On the other hand, if the star has enough mass in its core, degeneracy pressure cannot stop the collapse of the star. In 1928 a young graduate student in astrophysics, Subrahmanyan Chandrasekhar, showed that if a stellar core contained more than 1.5 times the mass of our sun, its gravitational collapse would "win" out over degeneracy pressure, and almost unbelievably, the core would collapse to a point (zero radius, zero volume, and infinite density)! This point is called a singularity.

Black Holes and Gravity

In the vicinity of the singularity, gravity is so strong that light cannot escape from the black hole. Moreover, within a certain radius of the singularity, all light is bent such that it falls into the singularity. This is truly what is called a black hole. This aforementioned radius determines the "size of the black hole" and is called the Schwarzschild Radius or "event horizon". A more massive black hole will have a larger Schwarzschild radius.

Weird and Weirder

Observing black holes is something of an oxymoron. Since no light can escape from them, we never actually "see" them. Let us consider for a moment, then, what would happen if we could actually fall into one.

We are in a hypothetical spaceship, the S. S. Relentless, orbiting a black hole. An astronaut, Imelda Intrepid, begins her brave journey toward the singularity . With her, she carries a laser which pulses once every second and is synchronized with a clock on the spaceship. First, let us consider the perspective of the astronaut...

Imelda begins to fall toward the black hole. Nothing spectacular seems to be happening except that the gravitational force begins to pull harder and harder, thus she speeds up. As Imelda approaches the event horizon, the gravitational field is so strong that her feet will feel a stronger pull than her head (assuming she's going in feet-first) and her body will begin to be stretched into a long thin strand. This stretching would kill any normal human, but suppose that Imeld a could "live" through it. Passing over the event horizon she would continue her descent into the black hole for another few millionths of a second -- until she crashes into the singularity and is crushed to a point of zero volume.

What does the crew aboard the S. S. Relentless observe? This is where the pulsing laser carried by our doomed astronaut becomes significant. At first, the laser pulses and the onboard clock are ticking synchronously. However as Imelda gets closer and closer to the event horizon, the laser pulses grow further and further apart. It appears that time is slowing down as Imelda gets close to the event horizon. Additionally the laser light appears redder and redder -- this is caused by the gravitational redshifting of the light as it is "pulled" on by the tug of the black hole. So how long does the S. S. Relentless have to wait to see Imelda disappear beyond the event horizon?

The answer is: forever!

The clock/laser Imelda is carrying appears to the Relentless to tick ever more slowly, and at the event horizon the clock ticks are infinitely far apart. Thus the crew would see Imelda's fall slowing down as she approaches the event horizon. It would appear to take Imelda an infinite amount of time to reach it -- time at the event horizon is essentiallly frozen! Thus a black hole produces a kind of censorship; preventing any outside observer from seeing anything actually fall into it!

These results are in accordance with Einstein's theory of General Relativity, and as abstract and contradictory they seem, they are correct representations of what would happen. What happens inside the black hole? It is nearly impossible to say. In fact, at the singularity itself, the laws of physics cease to exist...

--D. Gau