View Full Version : Black holes, a few easy questions.

2002-Feb-18, 04:46 AM
These are questions that have been kind of gnawing away in the back of my brain and I'm hoping someone here would be so kind as to answer them for me.

Is a black hole merely the mass at the center or does its boundary start at the event horizon? In other words is the event horizon technically a part of the black hole?

At what distance from the mass of a black hole does the event horizon start?

I searched the net but couldn't find any answers (at least none that I could understand). Please keep your answers simple; I have a passing familiarity with physics and astronmy but I'm still a man on the street as it were.

Also, are there any good books on black holes that don't require a heavy scientific background? I wander through the science section of the bookstore every now and again but I haven't found any books solely on black holes.

Much thanks in advance.

Roy Batty
2002-Feb-18, 12:05 PM
With regards to books, this seems pretty comprehensive though I hav'nt read it yet myself:

One I have read (but dont claim to understand a lot of!) is: http://www.the-scientist.com/yr1987/jan/review2_870112.html

As for your other questions i'll leave more knowledgable people to answer them since i'm still very dense on the subject (ooch sorry) /phpBB/images/smiles/icon_smile.gif


<font size=-1>[ This Message was edited by: Roy Batty on 2002-02-18 07:08 ]</font>

2002-Feb-18, 12:15 PM
1- The event horizon is technically a part of the black hole. It's better to think of a black hole as region of intense gravitational pull where the event horizon is the so called point of no return. The effects of the black hole can be felt well away from the horizon.

2- That questions answer depends on the size and mass of the star that collapsed to form the black hole. There is no firm answer. However, New research from Stephen Hawking suggests that the matter that a black hole "ingests", is directly relational to the area of the event horizon. in other words, the more it eats, the wider the area of the EH.

3- As far as books, I bought a great book for my son. Stephen Hawking's Universe - The Cosmos Explained. Its a great laymans terms book that anyone can understand. I'd also recommend COSMOS by Carl Sagan. Nobody can explain cosmology and astrophysics like Carl.

Hope this helps,

2002-Feb-18, 02:44 PM
On 2002-02-17 23:46, Asylum wrote:

Is a black hole merely the mass at the center or does its boundary start at the event horizon? In other words is the event horizon technically a part of the black hole?

I'm gonna rephrase your question a little to "What's the size of a black hole?" The size of the actual mass at the center is, well, zero. A more useful, albeit arbitrary, definition is to use the event horizon to define a black hole's size. Why is it useful? Well that brings us to your next question.

At what distance from the mass of a black hole does the event horizon start?

It's a very simple formula:

R = 2*M

where R is the radius of the event horizon and M is the mass of the black hole in meters. Now R and M need a little more explanation than what I just gave. Mass in meters is related to mass in kg by

M = (G/c^2)*(mass in kg) = 7.41e-28*(mass in kg)

where G is the gravitional constant and c is the speed of light. Using this normalization simplifies many of the equations; for instance, I can easily remember the relation between mass and black hole. /phpBB/images/smiles/icon_smile.gif

Now the radius R is more complicated, and I would normally not mention it since it would only be benificial if you had a good qualitative understanding of relativity. So feel free to skip this paragraph. However, there are nitpickers on this board (I'm one /phpBB/images/smiles/icon_smile.gif), who will ask "Who's measuring the radius?" The actual radius is very observer dependent; so R is actually the "reduced circumference" or the circumference divided by 2*Pi. For a non-spinning black hole, it does not matter who measures the circumference, they will get the same value.

Hope this helps,

<font size=-1>[ This Message was edited by: Wiley on 2002-02-18 09:56 ]</font>

2002-Feb-19, 03:19 AM
Books: Black Holes and Time Warps by Kip Thorne is an excellent one, and should be available in most public libraries. Thorne is a good writer with a fine low-key style and a deep understanding of the subject (he is co-author, with Misner and Wheeler, of a much more technical book on relativity theory, Gravitation).

Black holes are famously simple objects, and can be described by listing their mass, angular momentum, and charge. As mentioned, the mass determines the size of the event horizon, but the horizon is not really a thing in the same way that the Sun's corona or Jupiter's Red Spot are things...the event horizon is a location in space where the black hole's gravitational field becomes so strong it captures light. Farther away from the black hole--outside the event horizon--light can escape; closer, inside the event horizon, light's path is curved so much by the curvature of spacetime (gravity) that it spirals inexorably inward to the subatomic-sized oddity which is the black hole's center of mass. Right at the event horizon I suppose it would be possible for light to actually orbit the black hole without either escaping or falling it. So in one sense the event horizon is more a manifestation of the black hole than an appendage.