# Black holes

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#### Jimmyboy

##### Guest
hi can someone help me out here. I know that the even horizon is the surface of the black hole and that if any information crosses its boundry it is lost or cannot escape the pull of gravity. i also know that there is an apparent singularity at the center of the black hole where everything is crushed to infinte density. What i cant work out is the difference between the event horizon and the singularity. The reason is because if light cannot escape the boundry of the event horizon due to the gravity there surely that means there is infinite gravity there, and anything just passing the event hozon is crushed, and if so surely the singularity should extend right out to the event horizon, and not be an infinate volume/density at the center of the black hole???.. Does this mean there is an apparent difference in gravity between the event horizon and the singularity?????

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#### kyle_baron

##### Guest
Jimmyboy":1p0s3q68 said:
hi can someone help me out here. I know that the even horizon is the surface of the black hole and that if any information crosses its boundry it is lost or cannot escape the pull of gravity. i also know that there is an apparent singularity at the center of the black hole where everything is crushed to infinte density. What i cant work out is the difference between the event horizon and the singularity. The reason is because if light cannot escape the boundry of the event horizon due to the gravity there surely that means there is infinite gravity there, and anything just passing the event hozon is crushed, and if so surely the singularity should extend right out to the event horizon, and not be an infinate volume/density at the center of the black hole???.. Does this mean there is an apparent difference in gravity between the event horizon and the singularity?????
Just to clarify a few points:
1. There is infinite curvature of the gravity at a singularity. Meaning, a light beam would curve back on itself at a singularity.
2. The event horizon is above, below, and all around the singularity.
3. Anything passing the event horizon is streched, or spaghettified, and only is crushed, once it reaches the singularity.

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#### blackle4ps3

##### Guest
black hole on earth by youtube this is the light speed engine theory of now though getting past ice speed more tricky yet no msp on ice speed around the speed comets travel at

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#### DrRocket

##### Guest
Jimmyboy":212mgi25 said:
hi can someone help me out here. I know that the even horizon is the surface of the black hole and that if any information crosses its boundry it is lost or cannot escape the pull of gravity. i also know that there is an apparent singularity at the center of the black hole where everything is crushed to infinte density. What i cant work out is the difference between the event horizon and the singularity. The reason is because if light cannot escape the boundry of the event horizon due to the gravity there surely that means there is infinite gravity there, and anything just passing the event hozon is crushed, and if so surely the singularity should extend right out to the event horizon, and not be an infinate volume/density at the center of the black hole???.. Does this mean there is an apparent difference in gravity between the event horizon and the singularity?????

The event horizon, mathematically, is a closed surface, and anything inside of that closed surface stayes inside. But on that surface there still exist a metric that satisfies Einstein's field equations and that describes the curvature of space-time. To an outside observer there is nothing particularly striking about the event horizon. You could in principle cross it and you not notice anything special immediately.

The singularity, on the other hand is a set of points on which there exists no solution to the Einstein field equations. It lies well inside the event horizon, and may not exist in any physical sense. It is quite possible that the singularity is nothing more than a signal that general relativity breaks down and that a proper description requires a new and yet-to-be-discovered theory.

The situation with the event horizon is much more complicated than "infinite gravity" which is not correct. You might want to read the book Black Holes and Time Warps; Einstein's Outrageous Legacy by Kip Thorne.

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#### Saiph

##### Guest
A brief and generalized way of distinguishing between the two features is:

The singularity is the destination of all things entering a BH, it is the source of the event horizon.

The event horizon is merely the point of no return, and has no real physical substance to it (just like the border between the US and Canada). You can cross it, but won't really notice a difference in space between here, and there.

Granted, the gravity from a BH will do interesting things to your view of the stars and your surroundings as you get closer...but you won't notice a sharp shift in that either as you cross the horizon, it'll just get more distorted.

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#### skeptic

##### Guest
Saiph":3h2xm61b said:
The singularity is the destination of all things entering a BH, it is the source of the event horizon.

To clarify a little, the singularity not only is the destination of all things entering a BH, it is their future. The mass of the BH is concentrated at the singularity and the force (or curvature if you prefer) of its gravity propagates outward at c.

Are you suggesting Saiph, that the force which is source of the EH propagates from the singularity backwards in time to the event horizon?

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#### Saiph

##### Guest
boy, another old topic ressurected.

You are correct that the singularity is the future of the object (time points there as well!) Good catch.

As for the force propagating backwards...it doesn't need to. The force is the distortion itself, which is already there. If I remember correctly, I've actually read somewhere that the inside of the BH can be, or was, considered independent and cut off from the exterior.

It's been a while since I've read up on the material.

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#### skeptic

##### Guest
You're correct that the space-time curvature inside a black hole comes pre-distorted, but what happens if more matter falls in? The event horizon should expand, shouldn't it? How does the event horizon know to expand unless the increased gravity of the singularity can propagate outwards to the event horizon. But to reach the event horizon the gravity would have to travel backwards in time.

I'm not directing my questions specifically to you Saiph, it was just your answer that suggested to me any influence of the singularity might have must travel backwards in time.

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#### Saiph

##### Guest
The EH does grow as matter falls in..but that information is still available the outside world right up until it crosses...and so there is no real issue. It's extreme proximity to the BH at the EH means the outside world already treats them as having the same gravity, right before it plunges in.

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#### skeptic

##### Guest
Hypothetically if the singularity were to punch a hole in the space-time continuum and all the mass in the singularity were to leak out into another universe ( a concept I don't support ), we would expect the event horizon to collapse into nothing. But how could the disappearance of the mass of the singularity be signaled back to the EH?

If I understand your post, as a mass passes through the EH, the radius of the EH expands uniformly as if the mass had already arrived at the singularity. The EV must then retain a memory of that mass as well as all the mass that has previously fallen through, even though the EH is outside the singularity's light cone. Is that approximately correct?

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#### Saiph

##### Guest
Well...If the mass 'disappeared' I don't think the BH would go, as the space-time curvature is self-propagating. Stretched spacetime...stretches itself. Stretch it far enough, and left alone, it won't change.

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#### Vax

##### Guest
DrRocket":2totf1l0 said:
The event horizon, mathematically, is a closed surface, and anything inside of that closed surface stayes inside. But on that surface there still exist a metric that satisfies Einstein's field equations and that describes the curvature of space-time. To an outside observer there is nothing particularly striking about the event horizon. You could in principle cross it and you not notice anything special immediately.

I know this is beyond the point but don't you think you would notice a temperature increase upon entering the event horizon? With all the particles of matter slamming into eachother the friction caused would jump the temperature up to a few million at least in the accretion disk.

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#### MeteorWayne

##### Guest
Ahhh, but the accretion disk is outside of the event horizon.

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#### Kessy

##### Guest
Okay, first of all, we need some definitions here. "Singularity" is actually a mathematical term that means a point where a value is undefined, and can refer to any value. You've actually encountered singularities in everday life. For example, as a ball bounces, it loses energy on each bounce, so the frequency of the bounces increases rapidly, until, just before the ball comes to rest, the frequency actually runs away to infinity, giving you a singularity. Another example would be the geographic north and south poles, where longitude isn't defined.

There are several singularities associated with a black hole, several of which do occur at the event horizon. For example, the gravitational time dilation at the event horizon is so great that to an outside observer, time appears to be frozen there. From our point of view, a black hole actually appears to be frozen in the act of collapsing, and one early name for them was a "frozen star." (Let's all be thankful that term didn't catch on. ) However, what's usually meant when people talk about a black hole's singularity is the gravitational singularity at the center. From the point of view of matter falling in to the hole, all the matter should collapse down to a mathematical point, with no dimensions. The density of the matter and the strength of gravity both run away to infinity at this point. However, this is occuring in such an extreme environment that we can't really be sure what actually happens. As DrRocket said, general relativity may simply not apply there. Even if it does, it's very hard to say what the equations actually mean in physical reality. There are several schools of thought about what's going on, and it's still being actively debated.

An event horizon is a place where an observer can no longer receive any information. You shouldn't think of it as a surface or even a particular place, since its apparent location will change depending on where you're looking at it from. It's very much like a regular horizon - a place that you can't see past. It's actually not possible to reach or pass an event horizon - like someone trying to reach the horizon, an event horizon will appear to recede as an observer falls in to a black hole, and from the point of view of an distant observer, the person entering the black hole will appear more and more time dilated until they just appear to stop before reaching the horizon.

One last interesting thing is that event horizons aren't only associated with black holes. For example, to an observer traveling at a constant acceleration, the universe itself appears to have an event horizon. If you accelerate forever, your speed will approach the speed of light, so there is a point behind you from which it is impossible for light leaving there to ever reach you while you're accelerating. In practical terms, for accelerations that humans are capable of, the univers'es event horizon is so far away you'd need an enormous telescope to tell it's there at all.

S

##### Guest
Very vague ...not thot out questions.
1. If a black hole has a "certain size (or magnitude of space dimension)" in the Universe and time stops ( or there is an "event horizon")
is it not dumb to ask "what is going on at the singularity...and or that there is a "center" to the black hole?
2. Is a black hole a 3 dimensional object?
Thanks.
Bye
SC

O

#### OleNewt

##### Guest
1)There's no way to Voltron your way out of a black hole (for dramatic effect, even Voltron almost didn't make it). If you're caught in one, and assuming that the awesomely powerful gravity well allows you, it'd probably hurt a lot less to take your own life than to let the most gravitationally powerful object in space murder you.

2)black holes are formed from the remnants of dead stars. Stellar-mass black holes do wander around the universe munching on stars and planetoids, which I guess would not be possible if black holes were just literal holes punched in the universe.

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#### skeptic

##### Guest
Saiph":2d5l9oox said:
Well...If the mass 'disappeared' I don't think the BH would go, as the space-time curvature is self-propagating. Stretched spacetime...stretches itself. Stretch it far enough, and left alone, it won't change.

I think that to suggest that spacetime has a yield strength similar to materials and that the EH has stressed it beyond its elastic range is stretching the concept of spacetime. Perhaps however, the curvature of spacetime close to the event horizon is not due to spacetime's memory of how much matter has fallen into the BH, but to the matter which HASN'T fallen through and which can't fall through in coordinate time.

F

#### Fallingstar1971

##### Guest
You can still punch a hole in the universe and have it move around, as long as the hole appears in a dimension other than length, width, or height. In other words, another dimension.

Star

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#### ramparts

##### Guest
Skeptic - Spacetime doesn't have elasticity. At least not that we know about I think you're taking the analogies of rubber sheets a bit too far.

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#### skeptic

##### Guest
ramparts":n3184i6u said:
Skeptic - Spacetime doesn't have elasticity. At least not that we know about I think you're taking the analogies of rubber sheets a bit too far.

In the sense that a gravitational field causes a curvature of spacetime and by removing the gravitational field the curvature is returned to zero, spacetime is elastic. My post however wasn't about spacetime being elastic but about it becoming plastic beyond some gravitational field strength and not returning to zero curvature when the gravitational field is removed.

I was suggesting that perhaps the reason the curvature hasn't returned to zero is because the gravitational field hasn't been removed i.e. the matter is still at the event horizon.

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#### Jerromy

##### Guest
I think it is alot more realistic to picture a black hole as any other observable object. Picture a star that we can clearly see and feel the solar wind radiating outward from... it has a barrier surrounding it that we cannot pass without being burnt to a crisp. That barrier is comparable to the event horizon of a black "mass". Once something passes that barrier it is probably turned to energy whether by combustion or whatever. As you look closer there is a surface of mass contained in the plasma which is the equivelent of the surface of the singularity. Varying matter which is contained within that area is indistinguishable from the outside narrow observation point from which we view it. Once something is sucked into the event horizon it becomes another "1" way down a string of decimal places. If two large bodies were to attract within reasonable observation distance then we may have some real raw data to calculate how the forces dissapate between and throughout the objects but for now all we can say is what goes in doesn't come out and barely has an effect on its total energy.

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#### ramparts

##### Guest
Um, sure, but that's all speculative. Just realize that. Like, I can talk about spacetime being made of unicorns, and it's as valid. Whatever. None of the stuff you're talking about exists in our current theories.

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#### 5hot6un

##### Guest
skeptic":n6m6uebl said:
In the sense that a gravitational field causes a curvature of spacetime and by removing the gravitational field the curvature is returned to zero, spacetime is elastic. My post however wasn't about spacetime being elastic but about it becoming plastic beyond some gravitational field strength and not returning to zero curvature when the gravitational field is removed.

I was suggesting that perhaps the reason the curvature hasn't returned to zero is because the gravitational field hasn't been removed i.e. the matter is still at the event horizon.

Indeed! According to wiki; "a black hole has only three independent physical properties: mass, charge, and angular momentum."

Interesting to me is "a charged black hole repels other like charges just like any other charged object, despite the fact that photons, the particles responsible for electric and magnetic forces, cannot escape from the interior region. The reason is Gauss's law, the total electric flux going out of a big sphere always stays the same, and measures the total charge inside the sphere. When charge falls into a black hole, electric field lines still remain, poking out of the horizon, and these field lines conserve the total charge of all the infalling matter. The electric field lines eventually spread out evenly over the surface of the black hole, forming a uniform field-line density on the surface. The black hole acts in this regard like a classical conducting sphere with a definite resistivity."

Am I understanding this correctly? It's a big magnet?

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#### ramparts

##### Guest
Not a magnet - a magnet has to have poles, and the black hole can't have those since it's too symmetric (is one way to look at it). A black hole can be a big conducting sphere, which attracts/repels using the electric force, rather than the magnetic.

Now, this only applies if a black hole has charge. Charged black holes are thought to be very uncommon in the real world, because they would quickly attract opposite charges and then neutralize.

5

#### 5hot6un

##### Guest
ramparts":i5jw4sqg said:
Not a magnet - a magnet has to have poles, and the black hole can't have those since it's too symmetric (is one way to look at it). A black hole can be a big conducting sphere, which attracts/repels using the electric force, rather than the magnetic.

Now, this only applies if a black hole has charge. Charged black holes are thought to be very uncommon in the real world, because they would quickly attract opposite charges and then neutralize.

That whole thing about "electric flux" conjured up vision of magnetic lines of flux.

So in the case of a charged black hole, where "electric field lines still remain, poking out of the horizon"
Is this not a case where something IS escaping? Could those lines induce a current on something outside the EH and thereby take energy away from the BH?

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