How Can we Find the Escape velocity of a black hole

  • #1
J Venkatesh
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How Can we Find the Escape velocity of a black hole.
 

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  • #2
Borg
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How Can we Find the Escape velocity of a black hole.
Nothing can escape a black hole.
 
  • #3
phinds
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How Can we Find the Escape velocity of a black hole.
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.
 
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  • #4
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How Can we Find the Escape velocity of a black hole.

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-
[tex]v_e=\sqrt{2Gm/r}[/tex]
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)-
[tex]R_s=\frac{2Gm}{c^2}[/tex]
For a better understanding, you might want to check out the Schwarzschild metric.
 
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  • #5
Stephanus
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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.

Yes if it's beyond Schwarzshild Radius
Score 1 - 0 for me :smile:
 
  • #6
RisingSun361
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How is possible for x-rays to be emitted from a black hole, but not light?
 
  • #7
phinds
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How is possible for x-rays to be emitted from a black hole, but not light?
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.
 
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  • #8
Chronos
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As Phinds noted, all radiation emitted by a black hole originates external to the event horizon
 
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  • #9
Stephanus
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...Nothing comes out of a black hole, including any kind of photons, visible light, X-rays, microwaves...
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
RisingSun361
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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.
490046a-f1.2.jpg
 
  • #11
Stephanus
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...Perhaps you are thinking of the accretion disk...
The jets matter comes from the accretion disk, the circle with the blue color.

...all radiation emitted by a black hole originates external to the event horizon
And the accretion disk is beyond the event horizon.
 
  • #12
Chronos
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Black hole jets emanate from the poles due to magnetic confinement. They still originate in the accretion disk.
 
  • #13
phinds
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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
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.
 
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  • #14
Stephanus
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Can I add a question here?
Please tell me if this question doesn't belong to this thread, I'll create a new thread.

Gravity curvature.jpg

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
phinds
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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.
 
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  • #16
Stephanus
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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.
Thanks, Phinds.
 
  • #17
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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.
490046a-f1.2.jpg
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
Stephanus
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...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.
Wow, you're sure?
Amazing :smile:
 
  • #19
phinds
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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.

Wow, you're sure?
Amazing :smile:

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)
 
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  • #20
Stephanus
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...where its interaction with the magnetic field of a rotating black hole...
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?
 
  • #21
phinds
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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?
Black holes add mass from in-falling objects. The chances of any of this coming in on a straight approach are approximately zero, therefore the black hole gains angular momentum and rotates. You seem to believe that "singularity" means "point". It does not. It means "the place where our math models give unphysical results and we don't actually know WHAT the hell is going on and we need a better theory". Just Google "rotating black hole".
 
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  • #22
phinds
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... it's hard to picture a rotatic sphere ...
Seriously? Have you never watched a basketball player spin a basketball on the end of his finger?
 
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  • #23
Stephanus
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Seriously? Have you never watched a basketball player spin a basketball on the end of his finger?
For a uniformed sphere.
If the sphere is uniformed, we can't draw or take a video of its motion.
Not like cube, or even cone. assuming we move it not by its vertical axis.
 
  • #24
Stephanus
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And the relation with gravity...
Does a uniformed rotating sphere has effect on orbits?
Supposed A and B are rotating each other.
If A is a rotating cube, well, I might suspect it has effect in the orbits, although I can't do the math.
But if A is a uniformed sphere.
Will the orbits differs if A is rotating or static.
But I think this belong to other thread.
 
  • #25
phinds
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And the relation with gravity...
Does a uniformed rotating sphere has effect on orbits?
Supposed A and B are rotating each other.
If A is a rotating cube, well, I might suspect it has effect in the orbits, although I can't do the math.
But if A is a uniformed sphere.
Will the orbits differs if A is rotating or static.
But I think this belong to other thread.
It is irrelevant whether we can "see" the rotation, we can measure it. Google "frame dragging".
 
  • #26
phinds
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If you are interested in black holes, please do some reading about them. This asking of scattershot questions, one at a time, on an internet forum is not a particularly helpful way to learn the basics. Once you learn the basics then many of your questions won't even BE questions any more and those that are left will be more focused.
 
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  • #27
newjerseyrunner
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Here is a nice description of how a singularity can rotate:
http://en.wikipedia.org/wiki/Spin_(physics)#Vector

https://drive.google.com/file/d/0B7i15eHbK0GAbkxoSFBMYWh5S1E/view?usp=sharing

what if the black hole is actually a super massive object like a planet? i think of Einsteins explanation of gravity and space time fabric. if the object is causing all matter around it to pull towards it and the universe is spinning making centrifugal force push away then the planets and stars would stay right where they are now. just a thought and i know centrifugal force is debatable but in this example all matter around the center black hole or object is attached to it because of the fabric of space. like a ball attached to a string swinging round and round . just my thought.
There are theories like that. Look up a Fuzzball
 
  • #28
Chronos
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Gravity is a field effect, it is not emitted like em from a black hole
 
  • #29
jeffek
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Gravity is a field effect, it is not emitted like em from a black hole

I know but according to Einstein a planet for example pushes down on the fabric of space causing the objects around it to fall towards it that's what I meant if a black hole has mass it would push down on the fabric of space and cause other objects around it to fall towards it... Ie. gravity
 
  • #30
phinds
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I know but according to Einstein a planet for example pushes down on the fabric of space causing the objects around it to fall towards it that's what I meant if a black hole has mass it would push down on the fabric of space and cause other objects around it to fall towards it... Ie. gravity
This "pushing down on the fabric of space" is a rather poor analogy, embraced by pop-science because it's very easy to draw. The problem is that it gives a false sense of 2D simplicity to a 4D space-time situation.
 
  • #31
Quds Akbar
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It depends on your distance from the black hole, once crossing the event horizon, the escape velocity exceeds the speed of light.
 
  • #32
Quds Akbar
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It depends on your distance from the black hole, once crossing the event horizon, the escape velocity exceeds the speed of light.
 
  • #33
Gaz1982
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If nothing can escape a Black Hole then where does Hawking radiation come from?
 
  • #34
phinds
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If nothing can escape a Black Hole then where does Hawking radiation come from?
From outside the event horizon, obviously.

The English-language analogy that Hawking used to describe it (and this is NOT really quite what happens, and I'm paraphrasing, not quoting directly) is "a virtual particle-pair pops into existence just outside the event horizon and one falls in and one escapes. The one falling in always contributes negative mass to the black hole"
 
  • #35
Gaz1982
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From outside the event horizon, obviously.

The English-language analogy that Hawking used to describe it (and this is NOT really quite what happens, and I'm paraphrasing, not quoting directly) is "a virtual particle-pair pops into existence just outside the event horizon and one falls in and one escapes. The one falling in always contributes negative mass to the black hole"

Contributes negative mass?

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