# Has someone done the math?

1. Jul 24, 2004

### terrabyte

what's the amount of gravity or exactly how many "Gees" a black hole must exert to be a black hole (strong enough gravity to contain any light it "emits")?

2. Jul 24, 2004

### terrabyte

realizing that in order to do such a calculation we would need a "size" for the thing.

hmm that complicates matters

let's assume the "Black Hole" is the size of the earth. how many "earths" do we need to compress to the size of ONE earth to create a black hole?

3. Jul 24, 2004

### chroot

Staff Emeritus
To become a black hole, all the mass of an object must be within its Schwarzschild radius,

$$r_s = \frac{2 G M}{c^2}$$

That is the only condition.

- Warren

4. Jul 24, 2004

### urtalkinstupid

How can a black hole "emit" any light at all? If I'm not mistaken, light "falls" into a black hole. Once the light get's too close to the black hole, it is affected by the black hole's space-time curvature. The space-time curvature is so inclined that light falls in, but can not get back out. I dunno. :rofl:

5. Jul 24, 2004

### chroot

Staff Emeritus
A black hole with the mass of the earth would have a radius of...

- Warren

6. Jul 24, 2004

### chroot

Staff Emeritus
7. Jul 25, 2004

### terrabyte

sweet, thanks chroot that's exactly what i was looking for :D

now i just need to acquire 700 million more earths...

8. Jul 25, 2004

### HallsofIvy

Staff Emeritus
"If we had but world enough and time!"