singularity

> [Entropic]: I have a hard time believing infinity exists.

Dimensions are infinite; e.g., time, space, and scale probably all go on forever. The same with mathematical constructs such as 1/0. However, if you limited your statement to assemblages of matter and energy, we would be in complete agreement that these can be indefnitely large or small, but not truly infinite or infinitesimal. And because forces consist of the conveyance of matter and/or energy, they cannot become infinite either. It is a principle of physics that the finite cannot become infinite.

> [Entropic]: Therefore, I have a hard time with the concept of a singularity at the center of a black hole.

Many of us have the same hard time with that concept.

> [Entropic]: This leads to a question: Would 2 black holes of the same mass behave differently if one had a singularity at the center and the other's mass had some volume? (I am guessing they will be different, like the event horizon will be farther from the center in one)

The location of the event horizon depends on 2GM/c^2. So it would be the same for a black hole or a Mitchell star. (The latter is what ordinary physics gets instead of a black hole when matter is highly collapsed. The mass of a Mitchell star still has a finite volume.)

Because "black holes" are a mathematical concept, not yet a physical one, one can attribute almost any properties to them one wishes. In GR, the interior of the event horizon reverses space and time and has other weird properties. A Mitchell star is pretty much like an ordinary star surrounded by an all-absorbing medium from which no light can escape.

Neither concept has yet been discovered, although obviously we have many black hole/Mitchell star "candidates". -|Tom|-

I think the question asked elsewhere about condensed matter experiments should be factored into this topic. All the concepts that are generally believed such as white dwarfs and blackholes require very dense matter and even after 75 years of trying the best labs in the world cannot get matter to cooperate. The most dense "REAL" material ever made is only about 30 times as dense as water and that stuff has a very short existance.

Tom,

Since we're on the subject of Black Holes I have a layman question that has puzzled me for some time. Suppose we choose the mass of our Black Hole such that circular orbital velocity just outside the event horizon is slightly above the velocity of light. As the acretion disk material loses energy it drops into a lower orbit and thus must orbit more quickly. Since we approach relativistic conditions as we get towards the event horizon the disk material must increase in mass as it orbits faster and faster. How can the material ever go down the hole if the orbital velocity must be greater than light speed and the mass increase must go to infinity? What am I missing here?

> [Jeremy]: Suppose we choose the mass of our Black Hole such that circular orbital velocity just outside the event horizon is slightly above the velocity of light.

That condition holds for all "black holes".

> [J]: How can the material ever go down the hole if the orbital velocity must be greater than light speed and the mass increase must go to infinity?

One of the many "reinterpretations" of special relativity needed over the last century deals with "relativistic mass". We now understand that the rest mass of a body never changes, only its momentum (mass times velocity times gamma) increases with speed, where gamma is the time dilation/length contraction/mass increase factor. So nothing hinders a body from falling into a "black hole" except for the problem that they don't really exist. <img src=icon_smile.gif border=0 align=middle> -|Tom|-

Blackholes are fun and some of the observations about them are very interesting to read. I wonder why the orbital velocity has any effect on matter entering the event horizon. It seems to me matter can enter the blackhole directly without ever orbiting it. I guess anyone can makeup any rules they want since nothing is real anyway.

Black holes are fun to discuss: the more discussion - the more silly paradoxes. Consider this - entering the event horizon a piece of matter leaves its gravity behind like a wet cloak by the entrance, then the same should happen to the electric field and any other imaginable field as well. But if the field is left behind, the corresponding charge is just as good as left behind too. So matter enters under the event horizon stripped of all its properties and attributes. But the matter is exactly that - its properties and attributes, so the thing that enters under the event horizon is the exact nothing, not even the vacuum, but absolute nothing. Thus nothing can enter that border, but matter just hangs around there like flies on a sticky tape... So how the heck could that hole probably appear in the first place?!!

Please consider all the above as an anecdote.