BLACK HOLES

Black Hole sizes can vary greatly because black holes can have mass ranging from a solar mass to millions of times the solar mass. The more massive the black hole the larger the size. A solar mass black hole is small, about the size of a college campus.

Black Holes

If the mass of a the stellar remnant is compressed enough in the supernovae explosion, the compression can be so fierce that even a the physics that supports a NS can fail. If, during the explosion, the stellar core gets compressed above 1014 g/cc, then there is no physics that can stop all the mass from totally collapsing further. We do not know what form of matter it collapses into or whether it truly turns into a form of concentrated "energy" that warps space and time. Our current theory would have us say that the matter contracts into a point, called a singularity. We have no idea what this matter is like, only that mathematically it a dimensionless point. Our thoery is incomplete!

But what about the region around the singularity?

To answer this we must turn to Einstein's theories of gravity, which is called his General Theory of Relativity. Here is the key point: Eintein realized that mass was just one form of energy, and that this mass placed a stress on the fabric of space and time. If you think of space as a fabric, then placing a mass in the space will cause the fabric to "warp" The larger the mass, the larger the warp. Simplifying to two dimensions for the purpose of illustration, you can think of the warp as a funnel; the larger the mass, the deeper the funnel.

Now, this means that even light will appear to alter its path in the vacinity of a massive object. This is why we call warped space "curved" space. The light path can appear to curve.

Around most stars, for example, the funnel does not go very deep and the effect of curved space-time are not freaky to our everyday experience. We live in the warped space-time around the sun, and the result is that Earth follows the curved space around the sun as it moves (and this is what keeps the earth in its orbit around the sun!). But, around a black hole, which is highly compact and super dense, the warp of space is such that the "funnel" goes infinitely deep! This is called Black Hole (BH), because even if light goes too far down into this funnel, its path gets so curved that it cannot make its way back out of the funnel!

How deep does light go before it cannot make it back out? That is given by the equation

This "radius", which is just a mathematical location around the singularity and not a physical object, gets larger in proportion to the mass that collapsed into the singularity. It is called the "Event Horizon". Nothing, once it gets that close to the BH, can escape; it falls into the BH and its energy (or equivalent mass) adds to the mass of the BH. Thus, BHs can grow in mass as they consume other matter and energy!

There are three parts to a BH: (from the center outward)

The warping of space can lead to bending light so strongly that BHs can behave like optical lenses. However, we call these "gravitational lenses", becuase we still loosely think of it being gravity that bensd the light.

It is very importat to realize that far from the event horizon of a BH, the space time fabric is no different than what we would call normal. For example, if the Sun became a BH this instant (meaning it was compressed to a small point that collapsed on itself), the spacetime warping throughout most of the solar system would be unchanged. Only the spacetime very close to the now tiny BH-sun becomes dramaticaly warped. The planets would continue to orbit it just the same. It is only when an object is deep in the funnel (very close to the BH) that things get strange or that you approach the event horizon.

What if You Fell into a Black Hole?

If you fell into a black hole, you would get stretched thin and squeezed as you approach the event horizon. Also, time slows down as you get closer and closer. If you looked back out to the universe, you would see the universe pass by very quickly! It would evolve to the end of time before you fell in. (This is because your time is so slow that the rest of time appears fast to you).

What if Your Friend Fell into a Black Hole?

If you watched another person fall into a BH, you would see them turn redder and redder because the light from them gets redshifted. This is called a gravitational redshift. Also, you would see their time slowing way down. In fact, as they approached the event horizon, their time wouls appear so slow that it would appear to stop just as they reach the event horizon. They would appear to freeze in time!

Where are Black Holes?

We see BHs that are busy consuming other matter. We cannot see an isolated BH. We see BH in two places. In binary stars, where the BH is consuming matter from its companion, and in the centers of galaxies, where the BH (usually very massive) is consuming stars. When the consumption rate is high, the BH sends off jets of material that has been heated and compressed. These jets are (again) due to beaming from magentic fields around the BH.

Wormholes and Time Travel

There is some speculation that space travel can be shortened by going through worm holes. That is, if you could survive the plunge into a BH, you might emerge somewhere (and somewhen!) else in the universe.