Black hole question

[julesstoop]

So it seems 'weighing' a black-hole (as it gets more massive) from the perspective of a distant observer does not create a paradox with the fact that the same observer is not actually 'seeing' any matter (contributing to this growing mass) cross the event horizon. This and all due to the 'virtual' nature of gravitons. Hmmm.. interesting. I bet my a** off Einstein must not have liked this idea :wink:

[Starman]

So is the black hole's event horizon spherical, disk shaped, or a bulging disk? For that matter, what shape would the black "hole" itself be? Even though its not technically a hole.

That's not to dispute the existance of wormholes; that's just a "black hole" that rotates in such a way that its possible to get passed the "hole" and into a tunnel where you may get spit out by a "white hole"; which remains undiscovered.

One more oddball question; We all notice the "halo" of stars around the SMBH in the center of almost every galaxy. Now, we know that the black hole cannot exceed its current size, and beyond its event horizon, it can't suck in matter.

So can we safely assume that the "halo" didn't get there because of the black hole? If so, then what's it doing there? Could there possibly be a collection of "white holes" surrounding the SMBH core of every galaxy?

White holes could be fairly well disguised by the stellar matter the spit out.

Just an interesting thought,
--Starman

[granthutchison]

So is the black hole's event horizon spherical, disk shaped, or a bulging disk? For that matter, what shape would the black "hole" itself be?.

A non-rotating black hole has a spherical event horizon, but it seems that black holes in nature would all rotate, except for some freakishly unlikely perfectly symmetrical collapse of a non-rotating body. The event horizons of rotating black holes are "oblate ellipsoids" - flattened at the poles. The singularity is a dimensionless point in a non-rotating black hole, and an infinitely thin ring in a rotating black hole. By passing through the ring you could pass into another universe (or another part of our Universe) - but you'd come out inside another black hole.

Grant

[Starman]

So does that mean no white holes grant? Just a tunnel with both sides sucking in?

I know BHs occasionally burp matter out, but I was always under the assumption that any "burp matter" came from stuff they'd already swallowed.

--Starman

[granthutchison]

So does that mean no white holes grant? Just a tunnel with both sides sucking in?

White holes seem to have been a very 70s idea that's fallen from favour now - no mention of them, for instance, in Kip Thorne's big book about black holes, and he's a specialist in black hole physics.

I know BHs occasionally burp matter out, but I was always under the assumption that any "burp matter" came from stuff they'd already swallowed.

I don't know about black hole burps - the whole point of a black hole is that nothing that crosses the event horizon ever gets out again.
Black holes do steadily leak radiation from their event horizons, but that's caused by the tidal separation of virtual particles generated by vacuum fluctuations immediately outside the horizon - this produces a complicated leak of energy from the hole's gravitational field which converts virtual particles to real particles, which then "evaporate" away from the event horizon, taking some of the hole's energy (and therefore mass) with them.

Grant

[Starman]

Interesting. Thanks for the clarification Grant. Wormholes have upset scientists for years, but they can't quite dismiss their existance. Have they moved just to ignore the phenominon?

--Starman

[granthutchison]

Wormholes have upset scientists for years, but they can't quite dismiss their existance. Have they moved just to ignore the phenominon?

Not at all ... they've just decided that black holes don't provide usable wormholes, even if you could avoid the gravitational horrors associated with approaching the central singularity.
But it was Kip Thorne himself who "invented" the wormhole Carl Sagan used in his novel Contact, which adheres to the laws of physics but requires a highly exotic and entirely speculative kind of matter to hold it open. (Ummm ... the wormhole, that is; not the novel, which is quite easy to hold open .)

Grant

[Starman]

I believe you're refering to anti-matter, which is almost impossible to use as a building material to keep wormholes open, considering their volitile nature in our universe.

[granthutchison]

I believe you're refering to anti-matter, which is almost impossible to use as a building material to keep wormholes open, considering their volitile nature in our universe.

No, not antimatter ... what's required is negative matter, which has a negative energy density.

Grant

[Guest]

dumb question:

If a black hole is a singularity, then how can it be rotating? It has no point of reference,ie it's North is South and is also East and West. So what's the definition of rotation, it can't be the same as that of the common definition....

Regards,

[granthutchison]

If a black hole is a singularity, then how can it be rotating? It has no point of reference,ie it's North is South and is also East and West. So what's the definition of rotation, it can't be the same as that of the common definition....

Its gravitational field is distorted by its rotation, showing something called "frame drag" - the hole's gravitational field pulls you in the direction of its rotation as well as drawing you inwards ... a bit like a whirlpool.

Grant

[Starman]

Wow. Can you enlighten us Mr. Hutchison; what is negative matter/energy? I know we've done experiments on creating anti-protons, and other exotic things.

--Starman

[granthutchison]

Wow. Can you enlighten us Mr. Hutchison; what is negative matter/energy?

As far as I know, it's completely imaginary, its properties just chosen to do the job of holding a wormhole throat open. I believe there's no evidence for such a thing in nature, or need for it in current theories of matter.

Grant

[cpotting]

Remember that light travels at the speed of light - although each photon loses energy as it climbs out of a gravity well, its velocity is undiminished. So you can't explain things by suggesting that the light just takes longer to make the journey. The signals from the falling astronaut do arrive farther and farther apart in time, but this is because his time rate really does get slower (relative to a distant observer) as he moves deeper into the gravity well

Most definetly... I was (perhaps overly-)simplifying just to demonstrate the fact that the subject does enter the event horizon, even if the observer never sees it.

Oh, and by-the-way, remember that black holes and singularities are not the same thing. A black hole can be anything that has enough gravity to generate an event horizon. A dense neutron star can do this.

I'm interested to know more about this - what's a "dense" neutron star? The escape velocity from the surface of a conventional neutron star is only about a third of the speed of light, so it won't form an event horizon.

Hmmm. I was thinking of an object with enough mass and a small enough radius that it would generate an event horizon, but with enough internal pressure to resist complete collapse. Since escape velocity (ev) varies with the inverse of the square of the radius, and surface gravity (sg) varies with the root of the inverse radius, the ev will always be extremely higher than the sg. It should be possible to imagine an object with such a high ev that there is an event horizon above the physical surface, but with enough internal "fortitude" to withstand the relatively lower sg - in other words, avoid physical collapse.

Perhaps no such objects exist - perhaps other considerations that are outside my limited knowledge come into play. But, it seems to me, that a neutron star could, theoretically, do this. (BTW: my use of the term "dense neutron star" was both redundant and misleading - what I really meant was "a neutron star of sufficiently high density").

Now, if the object does not have the "fortitude", it would continue to collapse until it reaches a dimensionless point. Now it is a singularity. But before it becomes a singularity it is still a black hole.

I hope this explains my "understanding" of the difference between a black hole and a singularity.

Please do not hesitate to rip me to shreds... I welcome it.
I hope to learn from it.
Just don't "crush" me too badly - I may collapse on myself and take this whole thread with me!

And everything inside a black hole must end up at a central singularity, because spacetime is so distorted that the time and radial space dimensions are swapped inside the event horizon - the radial direction becomes "time-like" and you cannot help but travel along it. There's no escape from singularity formation.

I'm intrigued. I have heard of this a function of "life in a singularity", but not simply because one has passed an event horizon (and lived). Can you elaborate on this please?

[granthutchison]

It should be possible to imagine an object with such a high ev that there is an event horizon above the physical surface, but with enough internal "fortitude" to withstand the relatively lower sg - in other words, avoid physical collapse.

This can't happen, because of the inexorable nature of the radial space coordinate inside a black hole - it would require infinite force for you to maintain a constant distance from the singularity, once you're inside the event horizon. There is a topological theorem written down by Roger Penrose in 1964 which indicates that once an event horizon is formed, a singularity must then form inside it, simply because the distortion of spacetime allows no other option. (I don't pretend to understand it, let alone be able to explain it.) The larger the black hole the longer the elapsed time for collapse between event horizon and singularity (as experienced by the infalling matter), but it always happens. But to an outside observer this makes no practical difference, since we see time stop for the infalling matter when it reaches the event horizon - so we can't stand with a stopwatch and say: "Ah-ha, now the event horizon forms and ... now there's a singularity." It's that concept of an event horizon again - we are entirely cut off from any knowledge of what's happening inside it, to the extent that we can't even have a sense of time elapsing within it.

I have heard of this a function of "life in a singularity", but not simply because one has passed an event horizon (and lived). Can you elaborate on this please?

I'm sorry, I don't understand the first sentence, there. What is it you want me to elaborate on?

Grant