
#1
Sep111, 09:40 PM

PF Gold
P: 1,570

THe universe is around 14 billion years matter falling into the singularity appear to take infinite time. So how would two black holes have had the time to complete a merger under those conditions? According to my understanding of GR we should be able to see mergers still in progress as the time reference at the event horizon and at the singularlity with itas infinite density would give us the apearance of infinite time taken which is well beyond the age of the Universe. Would the event horizon simply shift in size and encompass both singularities essentailly entailing two seperate singularities inside one event horizon? If I recall Hawkings radiation this process also takes a long time to reduce a large Black hole to nothing. So even if all the matter was pulled away from the smaller black hole to the larger, The smaller black hole would still not have had enough time to reduce to nothing. or am I correct in that last statement?




#2
Sep111, 09:55 PM

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PF Gold
P: 4,863

An outside observer never sees matter cross the the event horizon. However, the event horizon itself responds approaching matter. As two collapsed stars with event horizon approach, their horizons merge in finite time for an external observer, with all matter of both stars still outside the merged event horizon.
Interestingly, if a planet approaches an event horizon, the horizon grows well before the planet reaches the horizon. The current event horizon reflects (in part) all matter that will eventually be trapped. 



#3
Sep111, 11:17 PM

PF Gold
P: 1,570

side note was looking for examples of mergers when I found this article printed yesterday.
http://www.space.com/12790monsterb...ollisions.html I've also watched the Nasa simulation of blackhole mergers done in 2010 I'll link a post of it. its neat how the gravity waves are shown in this. http://www.youtube.com/watch?v=GAwO1okR074 Thanks for your reply makes more sense now 



#4
Sep111, 11:23 PM

P: 100

how do black holes merge when the singularity infinite density slows time to no time
Here is a simplified model.
Here is a more complete model. One thing to note is that since the radius of an event horizon is proportional to the mass, the resulting event horizon will expand to the sum of the colliding event horizons. Thus whatever the details are at the point of collision are, they will quickly be shrouded behind the horizon. 



#5
Sep211, 08:36 AM

P: 5,634

Among the things Einstein brought to science is the understanding that there is no single "correct" frame of reference, all frames (observers) are "relative".....There is no universally correct observational frame.....So what you see is likely different from what a distant observer sees. 



#6
Sep211, 05:21 PM

PF Gold
P: 1,570

Yeah I knew the part regarding normal time if your falling into the singularity, I was more interested in what we would see with the time dilation when we look at it from Earth.




#7
Sep311, 03:40 AM

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#8
Sep311, 03:55 PM

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PF Gold
P: 4,863

I was also speaking of observers well outside the event horizon, so, as I said, no matter is ever seen to cross the growing event horizon. 



#9
Sep511, 07:44 PM

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#10
Sep511, 09:32 PM

Sci Advisor
PF Gold
P: 4,863

Make a silly quibble, get a silly answer. (Apologies to Sydney Coleman for use of egocentric frames). 



#11
Sep611, 02:53 AM

P: 4,513

I don't think it is though. I take it that Fermi normal coordinates are those of a feely falling observer. I believe I could define some coordinate system in which there are no event horizons forming, even aysmtotically. Then again, the event horizon is an artifact of Schwarzschild metric and not the Fermi metric. So we are talking about the coordinate singularity in one metric using the coordinates of another. 



#12
Sep611, 05:28 AM

Sci Advisor
PF Gold
P: 4,863

The event horizon singularity is coordinate dependent. The event horizon itself is a geometric feature of the manifold. A given null geodesic either reaches spatial infinity or it does not. This is not coordinate dependent, thus the horizon is not coordinate dependent. Its shape as a 2surface does dependent on how spacetime is foliated into spacelike hypersurfaces, but you can't get rid of it or change anything substantive about its properties. (I really thought you were just being pedantic, rather than didn't understand or disagreed with what I was saying). 


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