# How Can we Find the Escape velocity of a black hole

1. Feb 8, 2015

### J Venkatesh

How Can we Find the Escape velocity of a black hole.

2. Feb 8, 2015

### Borg

Nothing can escape a black hole.

3. Feb 8, 2015

### phinds

We KNOW the "escape velocity" of a black hole. It is the speed of light. Since nothing can travel at the speed of light, nothing with mass can escape from a black hole (so it isn't really an "escape" velocity) and even light can only maintain a position exactly at the event horizon because locally it is traveling outward at c and globally, it is being held in place by the gravity of the black hole.

4. Feb 9, 2015

### stevebd1

In a very rudimentary way, it's the escape velocity equation that can tell us something about the nature of black holes. If we take the escape velocity equation-
$$v_e=\sqrt{2Gm/r}$$
establish that the escape velocity is the speed of light (c) then rearrange relative to r, you get the Schwarzschild radius which is the coordinate radius for the event horizon (where the escape v is the speed of light)-
$$R_s=\frac{2Gm}{c^2}$$
For a better understanding, you might want to check out the Schwarzschild metric.

5. May 30, 2015

### Stephanus

Yes if it's beyond Schwarzshild Radius
Score 1 - 0 for me

6. May 30, 2015

### RisingSun361

How is possible for x-rays to be emitted from a black hole, but not light?

7. May 30, 2015

### phinds

Not sure what you are talking about. Nothing comes out of a black hole, including any kind of photons, visible light, X-rays, microwaves, whatever.

Perhaps you are thinking of the accretion disk, but it can emit any kind of electromagnetic radiation.

8. May 30, 2015

### Chronos

As Phinds noted, all radiation emitted by a black hole originates external to the event horizon

9. May 30, 2015

### Stephanus

Can I add a question here?

Visible light travels at ≈ 300 000 km/sec
X ray travels at 300 000 km/sec
Microwave travels at 300 000 km/sec
and...
Gravity travels/propagates at 300 000 km/sec
So nothing can escape black hole but Gravity? Because it looks like Gravity has the same characteristic as light.

Please don't answer this, if this question should belong to another thread. I'll post it there someday

10. May 30, 2015

### RisingSun361

Based on this type of picture, I was guessing the x-rays were coming from the center. Doesn't look like they're coming from the disk.

11. May 30, 2015

### Stephanus

The jets matter comes from the accretion disk, the circle with the blue color.

And the accretion disk is beyond the event horizon.

12. May 30, 2015

### Chronos

Black hole jets emanate from the poles due to magnetic confinement. They still originate in the accretion disk.

13. May 30, 2015

### phinds

This is an easy point of confusion and you are certainly not alone in having it. The thing is, CHANGES in gravity propagate at c. The gravitational force itself propagates as it changes and develops but by the time a body, our sun for example, has formed, all the gravitational changes have propagated and the gravity field then exists unchanging. The same is true with a black hole. The gravitational field it exerts developed as it formed but now that it has formed, its gravitational field exists as is and does not need to propagate.

Should our sun magically (and against all laws of physics) suddenly cease to exist, the change in gravity would propagate at c, reaching the Earth 8 minutes later.

14. May 30, 2015

### Stephanus

Can I add a question here?

No, it's not a light path.
Does C orbit the barrcenter of A1, B, C or C orbits the barrcentre of A2, B, C?
It's just that I was just thinking about this gravity thing. It propagates at the speed of light. Is it curved, too?

15. May 31, 2015

### phinds

n-body orbits are around the common center of gravity of all bodies involved. Gets very messy.

It's better to start a new thread when you have a question that is a bit off-topic for the current thread.

16. May 31, 2015

### Stephanus

Thanks, Phinds.

17. May 31, 2015

### stevebd1

One decent analogy I recall for black hole jets is to image pouring a jug of water into a sink, in this case, there's no problem and the water will swirl down the plug hole. Now imagine taking a fire hose and firing this into the sink; the plug hole is too small to accommodate all the water and as a result, water will shoot up the sides of the sink. This (along with magnetic field lines) is pretty much what is happening with black hole jets. As it's been established, black holes are very compact and when a lot of matter is falling in, not all of it will pass the event horizon and as a result, be ejected at the poles.

18. May 31, 2015

### Stephanus

Wow, you're sure?
Amazing

19. May 31, 2015

### phinds

There can be so much matter coming toward a black hole, and at an angle so that it's all rotating and the bits get in each others way and the accretion disk heats up and through a process I don't understand, but one of our more knowledgeable members will be able to explain I'm sure, it gets to a point where its interaction with the magnetic field of a rotating black hole shoots jets of matter/plasma out perpendicular to the accretion disk such that it appears to be coming out of the poles of the black hole (but of course it isn't)

20. May 31, 2015

### Stephanus

Rotating singularity?
I can imagine a rotating 3D object.
Cube, Prism, Pyramid. But it's hard to picture a rotatic sphere much less a rotating object with no size.
The singularity. It rotates?