How Can we Find the Escape velocity of a black hole

[J Venkatesh]

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

 

[Borg]

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

Nothing can escape a black hole.

 

[phinds]

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.

 

[stevebd1]

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-
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.

 

[Stephanus]

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

 

[RisingSun361]

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

 

[phinds]

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.

 

[Chronos]

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

 

[Stephanus]

...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

 

[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.

 

[Stephanus]

...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.

 

[Chronos]

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

 

[phinds]

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.

 

[Stephanus]

Can I add a question here?
Please tell me if this question doesn't belong to this thread, I'll create a new thread.

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?

 

[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.

 

[Stephanus]

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.

 

[stevebd1]

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.

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.

 

[Stephanus]

...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

 

[phinds]

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

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)

 

[Stephanus]

...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?

 

[phinds]

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".

 

[phinds]

... 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?

 

[Stephanus]

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.

 

[Stephanus]

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.

 

[phinds]

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".

 

[phinds]

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.

 

[newjerseyrunner]

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

 

[Chronos]

Gravity is a field effect, it is not emitted like em from a black hole

 

[jeffek]

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

 

[phinds]

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.

 

[Quds Akbar]

It depends on your distance from the black hole, once crossing the event horizon, the escape velocity exceeds the speed of light.

 

[Quds Akbar]

It depends on your distance from the black hole, once crossing the event horizon, the escape velocity exceeds the speed of light.

 

[Gaz1982]

If nothing can escape a Black Hole then where does Hawking radiation come from?

 

[phinds]

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"

 

[Gaz1982]

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?

Help

 

[phinds]

Contributes negative mass?

Help

You have hijacked this thread. You should start a new thread asking for an expanation of Hawking Radiation, but before doing that I suggest you do a little research on your own. Google is your friend.

 

[Borg]

You have hijacked this thread. You should start a new thread asking for an expanation of Hawking Radiation, but before doing that I suggest you do a little research on your own. Google is your friend.

I would say that the thread is dead anyway. The OP never even bothered to come back after asking the question 4 months ago. We might as well be talking to ourselves.

 

[Chronos]

We have a talent for that.

 

[AgentSmith]

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

You can only calculate the escape velocity at the event horizon. Turns out to be c. Inside event horizon nothing can escape.

 

[Indi SUmmers]

I had a stupid question:

Gravity from Black Holes is, under several assumptions, from a collapsed star. Assuming gravity is not constant, wouldn't the speed of gravity be directly proportional to the mass of the star?

A class O star that collapsed would have a stronger gravitational pull than a class M - wouldn't it? So why is the equation for gravity treating the variable of gravity as a constant?

Just curious.

 

[phinds]

Gravity is gravity. It is immaterial what it comes from, just how MUCH stuff is there. A larger mass will generate a larger gravitational force (at the same distance from the center of mass) than a smaller one, but so what. The equation for the force is the same, you just plug in a different mass.

Also, there is no such thing as "the speed of gravity". There IS such a thing as the speed of changes in gravity, and that is c.

 

[Indi SUmmers]

I think I understand. We don't treat gravity as an object (ie - as a variable in an equation), but as a force to be calculated (the sum/difference/answer) we calculate. And as such, forces aren't constant. By plugging in the mass, it will automatically calculate any differences in the amount of gravity proportionate to the object. Is that what you're saying?

Velocity and gravity aren't interchangeable, though. Wouldn't velocity be the answer, sum, difference here? That would place gravity as a variable. We assume gravity is constant in this particular velocity equation, right? We're assuming gravity is G always. But how can we calculate v if G isn't constant, which I'd assume it isn't inside of the extreme conditions of a Blackhole as we approach the singularity.

I guess I'm confused, but that's nothing new. I always was under the assumption that a Back Hole's gravity would increase as you approach the singularity.

http://physics.stackexchange.com/questions/33916/what-is-the-escape-velocity-of-a-black-hole

 

[Borg]

I had a stupid question:

Gravity from Black Holes is, under several assumptions, from a collapsed star. Assuming gravity is not constant, wouldn't the speed of gravity be directly proportional to the mass of the star?

A class O star that collapsed would have a stronger gravitational pull than a class M - wouldn't it? So why is the equation for gravity treating the variable of gravity as a constant?

Just curious.

You're confusing gravitational force with the Gravitational Constant.

F is the gravitational force that is felt between two bodies. G is the gravitational constant.

 

[Indi SUmmers]

Some Other Curiosities:

* The Spin Problem
http://discovermagazine.com/2008/whole-universe/09-a-lenticular-galaxy-reveals-spinning-black-holes
http://discovermagazine.com/2002/jul/cover
https://en.wikipedia.org/wiki/Rotating_black_hole
http://www.space.com/24936-supermassive-black-hole-spin-quasar.html

So, the way I understand what's being phrased is that we calculate escape velocity from a black hole as being c - is this because of the observable light at the event horizon?

My question is this: if something is spinning, can't it just be ricocheting photons off of it and onto the event horizon if the speed of that ricocheting force is greater than c?

Kind of like trying to throw an object into a fan - if something is light enough and the fan is spinning fast enough, won't it just kick the object back out at you? If you kick an object out at zero-G in the vacuum of space, it'll just stay suspended, won't it? What if black holes are doing this with photons?
If they are doing this, then how can we assume that the observable light around the discs of black holes is strictly from photons escaping the gravitational pull within the singularity?ANSWERED HERE:

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.

* Spaghettification
http://science.howstuffworks.com/science-vs-myth/what-if/what-if-fell-into-black-hole2.htm
https://en.wikipedia.org/wiki/Spaghettification

It was my understanding that once you pass the Event Horizon, you're traveling faster than c. And due to extreme tidal forces (i.e. spaghettification), you're accelerating toward the singularity due to the extreme pull.

With that being said, are we calculating this as though the force of gravity is remaining constant inside of a black hole or are we accounting for the acceleration of an object as it nears the singularity?

Am I making any sense? Are we calculating still using the right equation?

Disclaimer
c, >c, >c2 and <c3 are hypothetical numbers and not actual variables for this equation. They're just meant to illustrate a hypothetical acceleration of gravity and a necessary velocity to obtain by light to escape it.

Double Disclaimer
Trust me, I don't know *@#! about science (which may be readily apparent to the learned mind).

 

[phinds]

Some Other Curiosities:

* The Spin Problem
http://discovermagazine.com/2008/whole-universe/09-a-lenticular-galaxy-reveals-spinning-black-holes
http://discovermagazine.com/2002/jul/cover
https://en.wikipedia.org/wiki/Rotating_black_hole
http://www.space.com/24936-supermassive-black-hole-spin-quasar.html

So, the way I understand what's being phrased is that we calculate escape velocity from a black hole as being c - is this because of the observable light at the event horizon?

No it has nothing to do with light at the event horizon, it is an effect of the gravitational force at the event horizon

My question is this: if something is spinning, can't it just be ricocheting photons off of it and onto the event horizon if the speed of that ricocheting force is greater than c?

Forces don't have speed so I have no idea what you are talking about.

 

[Indi SUmmers]

http://discovermagazine.com/2008/whole-universe/09-a-lenticular-galaxy-reveals-spinning-black-holes
http://discovermagazine.com/2002/jul/cover
https://en.wikipedia.org/wiki/Rotating_black_hole
http://www.space.com/24936-supermassive-black-hole-spin-quasar.html

So, the way I understand what's being phrased is that we calculate escape velocity from a black hole as being c - is this because of the observable light at the event horizon?

No it has nothing to do with light at the event horizon, it is an effect of the gravitational force at the event horizon.

I guess what I mean is...if a little shiny object that emitted light were traveling past the event horizon, it would no longer be observable as it passed over because the gravity would suck in all the light, lumosity, radiation, etc. But if it escaped and traveled back outside of the event horizon, wouldn't it become visible again? Is this how they're calculating Gravitation Force inside of a Black Hole? Is this how they're saying that the escape velocity necessary to get out of a Black Hole is 'c' - because to me, it seems like it would be multiples of c.

How are we getting the answer 'c?'

Can someone help me make sense?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
​

* The Spin Problem
http://discovermagazine.com/2008/whole-universe/09-a-lenticular-galaxy-reveals-spinning-black-holes
http://discovermagazine.com/2002/jul/cover
https://en.wikipedia.org/wiki/Rotating_black_hole
http://www.space.com/24936-supermassive-black-hole-spin-quasar.html

So, the way I understand what's being phrased is that we calculate escape velocity from a black hole as being c - is this because of the observable light at the event horizon?

My question is this: if something is spinning, can't it just be ricocheting photons off of it and onto the event horizon if the speed of that ricocheting force is greater than c?

Kind of like trying to throw an object into a fan - if something is light enough and the fan is spinning fast enough, won't it just kick the object back out at you? If you kick an object out at zero-G in the vacuum of space, it'll just stay suspended, won't it? What if black holes are doing this with photons?
If they are doing this, then how can we assume that the observable light around the discs of black holes is strictly from photons escaping the gravitational pull within the singularity?

ANSWERED HERE:

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.

So with that being said, I guess my real question becomes:

What velocity is needed to not only escape a Black Hole, but to travel away from the event horizon? It seems that this would correlate directly with the mass of the Black Hole (which I believe you addressed in your FORCE equation and not your CONSTANT one as I was confused about)? But are we calculating or are able to measure the pull of the Black Hole?

How hard does a black hole "suck?" ;-)
Do they all 'suck' the same? ;-)
Does the equation you posted...

You're confusing gravitational force with the Gravitational Constant.

F is the gravitational force that is felt between two bodies. G is the gravitational constant.

Because if it doesn't account for the rate of "suck" in a black hole (as I understand all black holes "suck" differently?), it seems like we're calculating this incorrectly?

Am I missing this in the math?

 

[AgentSmith]

Because if it doesn't account for the rate of "suck" in a black hole (as I understand all black holes "suck" differently?), it seems like we're calculating this incorrectly?

Am I missing this in the math?

The suck of black holes is not a physical observable.

 

[davenn]

ANSWERED HERE:

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.

So with that being said, I guess my real question becomes:

What velocity is needed to not only escape a Black Hole, but to travel away from the event horizon? It seems that this would correlate directly with the mass of the Black Hole (which I believe you addressed in your FORCE equation and not your CONSTANT one as I was confused about)?

so what part of the answer you quoted ( and the other responses) do you not understand ?

NOTHING traveling at c or less escapes

The mass of a black hole is irrelevant ... there are small black holes, there are large blacks holes, there are huge ones at the cores of many/most galaxies
note the common thing in all of them ... They are ALL black. what does that tell you ?Dave

 

[stedwards]

What about definitions? What is a black hole?

 

[rootone]

What about definitions? What is a black hole?

An object that has so much gravity that light, (or anything else), cannot escape from it once it has crossed the event horizon.
The event horizon is the 'point of no return'.
An object with sufficient force applied to it can (in principle anyway) escape from the black hole's gravity while it is still outside of the EH.