# Difference between a Gravity Well and a Black Hole

1. Feb 5, 2004

### Dave Hooley

I’m looking for the difference between a Gravity Well and a Black Hole, a Gravity Well (collapsed star) suggest that there is a bottom or end to it where there is a solid bottom or middle, and a Black Hole (unknown entity) suggest that there is no end only a constant ebb and flow with in the sphere where there is no solid’s and time maybe distorted, fore this is a human measurement and space maybe folded. I am trying to think dimensionally about this?
Thanks
Dave

2. Feb 10, 2004

### Tail

Hmm... are you saying a black hole is not a collapsed star? What is it then?

3. Feb 10, 2004

### GRQC

I don't think you'll ever find reference to a "gravity well" in any GR textbook or journal. It's a popular science term (promoted by Star Trek, among other sci-fi sources).

My take on 'gravity well' is that it's a way to describe the embedding diagram (i.e. the "sunken sheet") of a spherical gravitational source (star, planet, etc...). The bottom of this "well" is smooth and flat.

A black hole is a very special type of such a diagram, in which the center is a singularity (pointy).

4. Feb 10, 2004

### pmb_phy

The term gravity well refers to the potential energy function near a gravitating body. Near a black hole it just happens to be very strong.

The earth has a gravity well proportional to 1/r which extends from infinity to the surface of the Earth. Then it changes from a 1/r potential to a linear potential becoming zero at the center of the Earth.

5. Feb 10, 2004

### pmb_phy

Some black holes are the result of a single star collapsing. Some aren't. E.g. supermassive black holes such as those at the center of galaxies are not the result of a single star collapsing. Mini/micro Black holes are also not the result of a star collapsing

6. Feb 11, 2004

### Phobos

Staff Emeritus
Welcome to Physics Forums, Dave.

I agree with the others on this. A "gravity well" is a non-technical term used to describe the gravitational field around an astronomical object (moon, planet, star). I mostly recall hearing that term in sci-fi (excuse me, "SF") stories with spaceships navigating their way around the universe. A black hole is a singularity, which like any other mass, has a gravitational field around it...a deep gravity well with a point of no return.

7. Feb 18, 2004

### Tail

I might be wrong, but I do think that at least most, if not all, black holes form due to a star collapsing (except for primordial black holes perhaps).

8. Feb 24, 2004

### mccizmt2

Gravity well is a phrase coined by some jumped up hippy in the science fiction world.

A black hole is a consequence of the Schwarzschild metric. In GR the Schwarzschild metric is a solution to the Einstein equation. Where R=2GM a black hole occurs. It has this name as one cannot observe further than the event horizon.

Last edited: Feb 24, 2004
9. Apr 7, 2004

### Nice coder

Black holes are formed by anything massive enough to stop light from radiating from escaping.

Two neutron stars coliding
Two other black holes
Large star goes nova
Thats all i can think of now

10. Apr 8, 2004

### MAYUKH

Why The Black Wholes Are Not So Black?

11. Apr 8, 2004

### Phobos

Staff Emeritus
nitpick...
"supernova", not "nova"

supernova = large star that explodes during its end phase....the remaining core collapses into a neutron star or a black hole

nova = sudden brightening of a star (usually caused by accreting material from a companion star)

12. Apr 8, 2004

### Phobos

Staff Emeritus
Welcome to Physics Forums, MAYUKH!

"Black" means that it emits no radiation (visible light, heat, whatever).

Black holes are not 100% black because they do emit a small amount of radiation due to a strange feature of virtual particles (see "Hawking Radiation").

13. Apr 12, 2004

### Tail

Hmm... I thought the point was they DON'T emit it? A black hole loses mass, but never emits any... that's the good part.

14. Apr 17, 2004

### AntiMagicMan

Oh no they do emit mass. Consider a pair of particles on "borrowed time" being created around a black hole, one forms inside the event horizon, the other outside. One can escape and become "real" the other falls into the hole and the black hole loses energy and we see the other particle being emitted.

15. Apr 18, 2004

### Kurdt

Staff Emeritus
Hawking radiation of a black hole is a rather slow process and for most is insignificant but never the less present.

16. Apr 19, 2004

### Tail

Well, nothing goes OUT of the black hole, it's impossible, just particles with negative energy go in. Or so I understand it.

Last edited: Apr 19, 2004
17. Apr 19, 2004

### Kurdt

Staff Emeritus
First of all there are no particles with negative energy and secondly the Hawking radiation is a means of extracting some energy from the black hole. You can read up on Hawking radiation here http://casa.colorado.edu/~ajsh/hawk.html.

18. Apr 19, 2004

### Tail

Obviously, I meant virtual particles (by the way, Hawking mentions situations where real particles have negative energy). I agree that because of Hawking radiation a black hole gets smaller, its mass/energy decreases, just nothing gets out of it. It's all about quantum fluctuations. I suggest reading "Black Holes Ain't So Black", chapter 7 of A Brief History Of Time by Stephen Hawking.

19. Jun 14, 2004

### Jenab

I think a black hole can form anywhere a sufficiently high mass density accumulates in space. At first, the mass does not have to occur within a singularity. It can appear to be ordinary space to someone "drifting in." Of course, it takes a lot of mass to create this sort of black hole, but it behaves like other black holes in that once you're in, you're in for keeps. :surprise:

Jerry Abbott

20. Jul 19, 2011