20030825, 21:56  #1 
Aug 2003
Snicker, AL
7·137 Posts 
Black holes and gravity waves?
This is not so much of a puzzle as an open ended question. It is however a very math intensive and possibly enlightening question to ask so from that perspective maybe it is a puzzle of sorts after all.
There are certain defining characteristics of black holes: 1. A black hole has an event horizon beyond which nothing can be observed because the escape velocity is greater than lightspeed. 2. A black hole has a measurable position in space and a relative velocity as compared to other objects near it. 3. A black hole has a gravitational field the strength of which can be inferred from its effect on objects near it. 4. A black hole has spin which may be at a very high rate and can be inferred from the effect on nearby objects. If two black holes of relatively equal size approach each other, there are 4 possible consequences: They pass by each other with some change in direction, they establish orbit around each other (unstable over the long term), they coalesce into a single entity, they evert in the biggest explosion anyone can imagine. It is theoretically possible for two black holes to meet in a headon collision though so improbable that I am not listing this or its consequences. I am also not directly considering the impact on Hawking radiation or on frame dragging though these and other considerations may apply. My question is about the fourth possibility. Under what conditions of relative velocity, separation distance, and spin would two black holes evert? Presume a starting mass for each of the black holes of say 20 billion solar masses. Fusion 
20030826, 00:22  #2 
Banned
"Luigi"
Aug 2002
Team Italia
2·3·17·47 Posts 
Evert?
Considering that nothing can pass the event horizon, my guess is that they would rip the tissue of spacetime in some unknown dimension to create a wormhole OR following the last ideass of quantum gravity and superstring MTheory, where a black hole has no singularity, there could be enough energy to unwrap some CalabiYau spaces and open a dimensional tunnel through the Universe. Both situations would pass ignored by us "external observers", apart from some radiaction of vaporization due to the virtual particles involved in the process. Luigi 
20030904, 23:51  #3  
Sep 2002
2×17×23 Posts 
Quote:


20030905, 03:25  #4 
"Mike"
Aug 2002
1F0B_{16} Posts 
Isn't a black hole where "God" divided by zero?

20030905, 10:47  #5  
Banned
"Luigi"
Aug 2002
Team Italia
4794_{10} Posts 
Quote:
Maybe I'm not used to technical foreign language. Would you please show me the difference, so I will be able to explain myself better? Luigi 

20030905, 16:54  #6  
Bamboozled!
"πΊππ·π·π"
May 2003
Down not across
3·11^{2}·29 Posts 
Quote:
If the black hole is big enough, you could pass the event horizon (going inwards) without suffering any damage from tidal forces. IIRC, a galactic mass BH is easily big enough. Paul 

20030906, 06:22  #7 
Aug 2003
Snicker, AL
7·137 Posts 
So I'm a novice at this but here are my thoughts.
A black hole is only a black hole so long as its gravitational field is intact. Two black holes approaching each other would interact and partially cancel each others field essentially shrinking the event horizon closer to the ? in the center. However, the normal result of two black holes coming into proximity is that the larger absorbs the smaller. But get this, the smaller MUST evert in order to be swallowed by the larger one. My basic question was what combination of events would result in both black holes everting and staying that way. The relative velocity at which they approach each other is the first factor and would have to be a significant fraction of the speed of light. The distance separating them at closest approach would be the second significant factor. The closer they come to each other the faster they would have to be moving to avoid recoalescing. As the distance increases the required speed drops off until a point is reached at which neither everts. There are many scenarios where the two black holes combine. There are relatively few scenarios where both would evert and stay that way. The best I can figure two black holes with 30 billion suns of mass each, travelling at about .7C relative, and approaching within 1 lightyear of each other would result in an eversion. So why haven't we seen evidence of such a cosmic explosion? or have we? Fusion 
20030906, 07:31  #8 
Aug 2002
2^{6}·5 Posts 
Maybe I'm missing something, but why would the collision of two black holes create an "explosion" in the normal sense of the word at all? Wouldn't the two black holes just coalesce into a bigger black hole with the combined mass of the two?

20030906, 18:53  #9 
Aug 2003
Snicker, AL
7×137 Posts 
Coldfury,
Under normal conditions, yes they would just combine to become one. But under extreme conditons, a different outcome is theoretically possible. That is the the point I wanted to see discussed. There was a recent find of a single galaxy with two supermassive black holes orbiting each other a few thousand light years apart. This was caused by two galaxys combining with their central black holes going into orbit around each other. Under these conditions, there is only one possible outcome. Eventually the black holes will combine and become a single supermassive entity. Other scenarios similar to this can be observed with galaxys that have obviously interacted but that are now drawing apart. If each contains a central black hole, then the two had to interact gravitationally but a combination of distance and relative velocity prevented galactic combination. My proposed scenario for an eversion is strictly theoretical, there is no current evidence (other than the big bang itself) that it could occur. Still, the potential is intriguing. Fusion 
20030908, 21:15  #10 
Aug 2002
Portland, OR USA
2·137 Posts 
Fusion_power,
Well, I don't have any answers, but here are a bunch of questions. (Jeopardy, anyone?) ;) First question  when you talk about one or both everting, that occurs outside the others event horizion, right? Otherwise it would be academic? If the gravitational gradient from one black hole is continuous within the event horizon of the other, couldn't calculations be done to determine if a black hole could ever evert its twin? That is, the larger a black hole is, the greater the gradient required to evert it. But the larger it is, the less extreme the gradient is near the event horizon. That implies a maximum size for an 'evertor', because the target would pass inside the event horizon before everting. If we were dealing with normal matter, I can see needing to consider the field cancelation you mentioned  the internal pressure holding the objects shape would produce a kind of explosive decompression when the gravitational pressure was reduced. But what about degenerate or collapsing matter? Is there any mechanism creating internal pressure to produce even expansion, let alone explosion? Or will the (gravity waves)/(changing potential) produce enough force to pull it apart? Hmm, if the black hole has a high spin rate and it passes into and out of another extreme gravitational gradient, there would be incredible forces involved... What about the time dilation? Would that 'insulate' the black holes enough that they'd have to stay too close for too long? I guess the purpose of my post is to get an idea of all the things a simulation would have to incorporate to be 'accurate'. 
20030908, 22:28  #11 
Aug 2003
Snicker, AL
1110111111_{2} Posts 
Maybeso,
What happens to the event horizon if two black holes approach each other? This is the entire crux of the question!!! Yes time dilation is a significant factor, I am unable to factor it in properly. Yes spin is critical, enough so that my rough estimate at .7C approach velocity would not work if both were not at a high rate of spin. Would there be any "pressure" to cause expansion presuming the event horizon slid back far enough? The intense gravity waves would create the pressure. Planck's constant would be modified in the vicinity. How much and for how long is unanswerable. Pi would assume a different value than our familiar 3.14157... The forces involved are beyond anything I can even imagine. If an event of this sort occurred, it would light up the universe like a spotlight shining on ants. Fusion 
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