Exploring Black Holes: Beyond the Hole

[Simon Peach]

Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical. So really we are talking about black spheres which make much more sense in my head, also it rules out them being a 'gateway' to somewhere else. As matter would be coming in from all direction not only on a plane that the word hole implies. But, that word again, it would imply that it can be filled up, could we assume that Hawking 'evaporation' would take care of that?
Now shoot me down in flames!

 

[phyzguy]

The Cambridge English Dictionary defines a hole as "an empty space or opening in an object". I don't think there is any rule that a hole has to be two dimensional. The event horizon of a non-rotating black hole is in fact spherical, but I think "hole" is still a good description.

 

[phinds]

One way in which it is very pragmatic to think of a black hole as a true hole is that while the event horizon is detectable in our universe, nothing inside it is in causal connection to the rest of the universe except via gravity and the only thing that tells us is how much stuff went in, not what is happening to it now. So it's a hole in our knowledge of the universe in a more direct way than just being something we haven't figured out yet.

 

[Marquael Sartor]

Can someone describe to me exactly how a black hole travels through the fabric of space?

 

[phinds]

Can someone describe to me exactly how a black hole travels through the fabric of space?

First of all, there IS no "fabric" of space. That's just pop-sci jargon. A black hole travels through space just like everything else travels through space.

 

[stoomart]

Can someone describe to me exactly how a black hole travels through the fabric of space?

Someone on here has this picture as their avatar, which visually helped me escape the pop-sci notion of a space fabric.

 

[Drakkith]

Can someone describe to me exactly how a black hole travels through the fabric of space?

A more accurate term to use would be the metric of spacetime, not the fabric. A metric is a way of representing the curvature at any location, which is important because a black hole is predicted and described by General Relativity, which describes gravity as a curving of spacetime. The matter and mass that makes up a black hole travels through spacetime exactly like anything else. Unfortunately I'm not sure how to describe how this process works.

 

[Ssnow]

Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical.

I think the word hole is justified by the fact that if you pass on it you fall inside (and it is impossible to turn back) in analogy to a real hole ( where in the major part of the cases you can turn back ). I prefer the word singularity for the black hole because from the geometrical point of view it is a special place where the continuity of the space-time is dropped, but it is a matter of taste

 

[Gustav S]

Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical. So really we are talking about black spheres which make much more sense in my head, also it rules out them being a 'gateway' to somewhere else. As matter would be coming in from all direction not only on a plane that the word hole implies. But, that word again, it would imply that it can be filled up, could we assume that Hawking 'evaporation' would take care of that?
Now shoot me down in flames!

Watch Interstellar, the principal is the same, but with black holes =)

 

[Stavros Kiri]

Reminds me of the question as to why we call or consider e.g. the Sun as "Black Body" ... [in black body radiation ...]
{actually a spherical integral over 4π of indeed infinitesimal black bodies, small openings-holes ...}

 

[Labguy]

Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical. So really we are talking about black spheres which make much more sense in my head, also it rules out them being a 'gateway' to somewhere else. As matter would be coming in from all direction not only on a plane that the word hole implies. But, that word again, it would imply that it can be filled up, could we assume that Hawking 'evaporation' would take care of that?
Now shoot me down in flames!

The Event Horizon (EH) calculates to spherical, but all Black Holes (BH) rotate; none can be "non-rotating". That was shown long ago. Actually, Hawking Radiation (HR) "comes from" the edge of the EH which is the classical 2GM/c². This EH is at the same radius for a rotating BH as it is in the classical (non-rotating) BH. But, all BH's rotate and that is where the Ergosphere comes in. Roy Kerr showed that a rotating BH also has a "second" EH, the Ergosphere, in the shape of an oblate spheroid with the Ergosphere and the EH meeting at the poles of the axis of rotation. Anywhere off the poles and the EH is "inside" the bulge of the Ergosphere, so you can visualize the BH as having two EH's. A particle, and photons, between the EH and the Ergosphere can escape the BH since the "inside" EH is actually where the radius = the escape velocity of c.

Also, there is no "tub" to fill up so a BH has no upper mass limit. There is at least one BH known to have a mass of 18 billion suns. It is OJ287 and Craig Wheeler of the University of Texas in Austin, US, says "it depends only on how long a black hole has been around and how fast it has swallowed matter in order to grow. There is no theoretical upper limit.”

Labguy

 

[Stavros Kiri]

A black hole sure is a hole in terms of spacetime curvature (that could suck things in it) ... like a gravity well, i.e. a hole. [one can draw the diagram]
I think that's why it was named 'black hole', and 'black' because it also attracts light.

 

[Drakkith]

A black hole sure is a hole in terms of spacetime curvature (that sucks things in it) ... like a gravity well, i.e. a hole. [one can draw the diagram]
I think that's why it was named 'black hole', and 'black' because it also attracts light.

That's not quite accurate. They are termed black holes because they have an event horizon which not even light can get out of. Black holes do not suck things in any more than the Sun or the Earth sucks things in. One can safely orbit a black hole provided one stays outside the event horizon.

 

[Stavros Kiri]

That's not quite accurate. They are termed black holes because they have an event horizon which not even light can get out of. Black holes do not suck things in any more than the Sun or the Earth sucks things in. One can safely orbit a black hole provided one stays outside the event horizon.

I completely agree (I skipped the full analysis). I just wanted to visualize and emphasize the space-time distortion due to extreme gravity, as it affects curvature, like you also have mentioned earlier:

A more accurate term to use would be the metric of spacetime, not the fabric. A metric is a way of representing the curvature at any location, which is important because a black hole is predicted and described by General Relativity, which describes gravity as a curving of spacetime.

 

[Stavros Kiri]

A metric is a way of representing the curvature at any location, ...

Strictly speaking, the metric is not the same with curvature; i.e. metric tensor g_μν vs Riemann tensor R^..... etc.

 

[hsdrop]

Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical. So really we are talking about black spheres which make much more sense in my head, also it rules out them being a 'gateway' to somewhere else. As matter would be coming in from all direction not only on a plane that the word hole implies

well think swiss cheese everyone agrees that the cheese has holes in it but the holes are spherical in shape yet we all still think and call them holes in a block

 

[hsdrop]

Also, there is no "tub" to fill up so a BH has no upper mass limit. There is at least one BH known to have a mass of 18 billion suns. It is OJ287 and Craig Wheeler of the University of Texas in Austin, US, says "it depends only on how long a black hole has been around and how fast it has swallowed matter in order to grow. There is no theoretical upper limit.”

How small can a BH be?? Is there a theoretical lower limit to the size of a BH?

 

[Stavros Kiri]

well think swiss cheese everyone agrees that the cheese has holes in it but the holes are spherical in shape yet we all still think and call them holes in a block

But, hey, swiss cheese is white (or yellow?)! [but then again so my sun is shiny and bright in my 'black body' analogy earlier above ...]

But the holes in swiss cheese are holow empty spheres, i.e. holes. [While] Black holes are not empty and holow, are they?

I think that you just have to admit that you like swiss cheese (as much as I do) !
["Projection" and "Reframing" ... (in psychology - e.g. when you don't have something that you so badly want) - I think you better get to the store! ... (I know I am after this ...)]

 

[hsdrop]

i was just trying to make it easier to think of a hole in 3d and not 2d is all

 

[Stavros Kiri]

i was just trying to make it easier to think of a hole in 3d and not 2d is all

Point taken

 

[Stavros Kiri]

I prefer to think of them as "holes" or irregularities in terms of metric and curvature, and not in real space-time ...
[as the OP sais, there is no actual hole in space-time that leads to something else (gateway) ...]
So 2d, 3d or 4d doesn't matter ... makes no sense! ...

 

[hsdrop]

I prefer to think of them as "holes" or irregularities in terms of metric and curvature, and not in real space-time ...
[as the OP sais, there is no actual hole in space-time that leads to something else (gateway) ...]
So 2d, 3d or 4d doesn't matter ... makes no sense! ...

ok. You and I know to think of it that way but if you want to describe a black hole to a kid or anyone that does not have a good background in physics you would probably go right over their head. They would not learn anything from the answer to their question.
I believe it better to start someone out with a trical of info and use thing for analogy common to the person. Then hit them with a titlewave of info and concepts that they don't quite understand or would have to look up on their oun

 

[Stavros Kiri]

ok. You and I know to think of it that way but if you want to describe a black hole to a kid or anyone that does not have a good background in physics you would probably go right over their head. They would not learn anything from the answer to their question.
I believe it better to start someone out with a trical of info and use thing for analogy common to the person. Then hit them with a titlewave of info and concepts that they don't quite understand or would have to look up on their oun

In that case I agree. Educationally it is a good smart method, that many times works. I like analogies too.

 

[Drakkith]

How small can a BH be?? Is there a theoretical lower limit to the size of a BH?

An evaporating black hole will eventually be very, very small, about the size of an atom, just before it completely evaporates.

 

[Labguy]

An evaporating black hole will eventually be very, very small, about the size of an atom, just before it completely evaporates.

True, and the smaller the BH, the more intense the Hawking radiation. Regarding small BH's, someone coined the (now) common phrase: "Black holes are white hot." Also there are two opinions on the "finality" of small BH's; one is "evaporation" you mention above and the other is that once a certain small size is reached, the remaining matter of the entire BH "explodes". As of now, I don't which, if either, is the "generally-accepted" model.(?) Anyone following this know which and if so, at what size?? I follow what I can, but haven't seen any new info on this question.

 

[stoomart]

Also there are two opinions on the "finality" of small BH's; one is "evaporation" you mention above and the other is that once a certain small size is reached, the remaining matter of the entire BH "explodes". As of now, I don't which, if either, is the "generally-accepted" model.(?) Anyone following this know which and if so, at what size?? I follow what I can, but haven't seen any new info on this question.

My understanding according to Stephen Hawking's 1974 letter to Nature is it's both: the black hole evaporates and then explodes in the last 0.1 second.

 

[Simon Peach]

My understanding according to Stephen Hawking's 1974 letter to Nature is it's both: the black hole evaporates and then explodes in the last 0.1 second.

How does a B/H explode? as nothing can escape. Or is it not a B/H in that last 0.1 second. If it isn't what is it?

 

[Drakkith]

How does a B/H explode? as nothing can escape. Or is it not a B/H in that last 0.1 second. If it isn't what is it?

It's not an explosion that blasts apart the black hole, it's a rapid increase in hawking radiation generated by the black hole during the final moments of its life. Since hawking radiation reduces the mass of the black hole, this final burst of energetic radiation ends with the complete "evaporation" of the black hole.

 

[phinds]

It's not an explosion that blasts apart the black hole, it's a rapid increase in hawking radiation generated by the black hole during the final moments of its life. Since hawking radiation reduces the mass of the black hole, this final burst of energetic radiation ends with the complete "evaporation" of the black hole.

A good solid explanation. You must be catching up on your sleep.

 

[stoomart]

How does a B/H explode? as nothing can escape. Or is it not a B/H in that last 0.1 second. If it isn't what is it?

The short letter I referenced explains the process in fairly layman-friendly terms.

 

[Marquael Sartor]

What's beyond the event horizon of a Black Hole?

 

[phinds]

What's beyond the event horizon of a Black Hole?

At the center is the "singularity". We don't KNOW what it is, which is why it's called a singularity (singularity = "the place where the math model breaks down and we don't know WHAT is going on")

 

[Stavros Kiri]

and the other is that once a certain small size is reached, the remaining matter of the entire BH "explodes".

Masswise, is there a minimum? (other than zero, which is realized after evaporation)
My guess is that if there is it will depend on the size of explosion (i.e. the size of the hole at which explosion happens). Anyone knows details? Is there a handy equation?

 

[Labguy]

Masswise, is there a minimum? (other than zero, after evaporation)
My guess is that if there is it will depend on the size of explosion. Anyone knows details? Is there a handy equation?

Yes; the equation would be:
"Minimum Mass = No Black Hole Anymore = (Mass=0)"...

 

[Stavros Kiri]

Yes; the equation would be:
"Minimum Mass = No Black Hole Anymore = (Mass=0)"...

That's based on the first option (pure evaporation - no explosion). I am looking for the critical mass right before burst (or explosion). Drakkith seems to have an idea.

It's not an explosion that blasts apart the black hole, it's a rapid increase in hawking radiation generated by the black hole during the final moments of its life. Since hawking radiation reduces the mass of the black hole, this final burst of energetic radiation ends with the complete "evaporation" of the black hole.