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J Venkatesh
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How Can we Find the Escape velocity of a black hole.
Nothing can escape a black hole.J Venkatesh said: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.J Venkatesh said:How Can we Find the Escape velocity of a black hole.
J Venkatesh said:How Can we Find the Escape velocity of a black hole.
phinds said: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.
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.RisingSun361 said:How is possible for x-rays to be emitted from a black hole, but not light?
Can I add a question here?phinds said:...Nothing comes out of a black hole, including any kind of photons, visible light, X-rays, microwaves...
The jets matter comes from the accretion disk, the circle with the blue color.phinds said:...Perhaps you are thinking of the accretion disk...
And the accretion disk is beyond the event horizon.Chronos said:...all radiation emitted by a black hole originates external to the event horizon
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.Stephanus said: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
Thanks, Phinds.phinds said: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.
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.RisingSun361 said: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.
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Wow, you're sure?stevebd1 said:...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.
stevebd1 said: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 said:Wow, you're sure?
Amazing![]()
Rotating singularity?phinds said:...where its interaction with the magnetic field of a rotating black hole...
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".Stephanus said: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?
Seriously? Have you never watched a basketball player spin a basketball on the end of his finger?Stephanus said:... it's hard to picture a rotatic sphere ...
For a uniformed sphere.phinds said:Seriously? Have you never watched a basketball player spin a basketball on the end of his finger?
It is irrelevant whether we can "see" the rotation, we can measure it. Google "frame dragging".Stephanus said: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.
There are theories like that. Look up a Fuzzballjeffek said: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.
Chronos said:Gravity is a field effect, it is not emitted like em from a black hole
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.jeffek said: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
From outside the event horizon, obviously.Gaz1982 said:If nothing can escape a Black Hole then where does Hawking radiation come from?
phinds said: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"