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If Gravity travels at c 
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#19
Jun607, 05:29 AM

P: 2,056

That's actually a difficult question:
 it *will* end up being still spherically symmetric (google "no hair theorem"), albeit slowly moving in the direction of the momentum you threw in.  you will never actually see the small piece of mass reach the event horizon (due to time dilation).  as the mass approaches, the "event horizon" isn't well defined (mathematically, it's a global rather than local concept).  the finer details still aren't all known, basically because the precise equations are so difficult to extract accurate solutions from. There is a closely related question, of what happens to the electric field if a charge is thrown into a black hole. In that case I think the finer details can all be worked out, and you see that as an electric charge approaches the black hole, the event horizon practically takes on a like charge, so externally it makes little difference whether the charge is "just outside" the horizon or even "in the singularity". 


#20
Jun607, 09:54 AM

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P: 2,340

In general relavity, as the two objects near each other, we expect strong fields to bring nonlinearity into play, so that the predictions of gtr will differ greatly from those of Newtonian gravitation in various ways. However, it should already be clear at this point that one thing you can expect during the "approach phase" is that the field may retain approximal axial symmetry but certainly won't be spherically symmetrical. In both Newtonian gravitation and gtr it is useful to have some way of describing precisely the "shape" of the field. In the case of an isolated system, as in our situation, the answer is multipole moments. If you haven't seen these before, you might take a moment (no pun intended) to read about the elementary kinds of moments used everywhere in mathematics, including weakfield gtr, which is an approximate theory governed by the "linearized EFE". In fully nonlinear gtr a more sophisticated kind of multipole moment is appropriate (there is a standard notion, given by competing mathematical definitions which turn out to be largely equivalent). One of the most essential differences between Newtonian gravitation and gtr is that gtr is a true relativistic field theory of gravitation, and thus it predicts the existence of gravitational radiation. In particular, when the multipole moments of an isolated system are appropriately time varying, gravitational radiation will be emitted from the system. This condition is satisfied in our situation. Now, there is a principle, discovered long ago by Richard Price and others, to the effect that when a smallish object falls into a black hole, after the collision and merger, the field may be briefly distorted (in particular, the horizon might be distorted and wrinkled), but the hole "radiates away" these distortions in the form of gravitational radiation which moves out at the speed of light and leaves behind an "smooth and symmetrized" region. The result is that after all the fuss has died down, you have a somewhat larger hole, but if no angular momentum was introduced during the merger, it will still be modeled by a Schwarzschild vacuum. One thing which is thought to be "well understood" from simulations is the idea that in some merger scenarios, the gravititational radiation emitted from the system might be asymmetric, basically because the smallish object, roughly speaking, will suddenly plunge into the (distorted) horizon of the hole. In some situations this radiation can carry off very substantial amounts of energy and momentum. The result is that the new hole should be "kicked" by the recoil of the outgoing gravitational radiation! As a result, one should apparently expect to find holes speeding out of their parent galaxies as the result of such mergers. But as someone recently mentioned, a current mystery is that this hasn't yet been observed. See the website in my sig for links to online resources and citations of relevant books. In particular, the monograph by Hawking and Ellis, The Large Scale Structure of SpaceTime, contains a classic discussion of how the horizon behaves during merger of two black holes, and the monograph by Frolov and Novikov, Black Hole Physics, contains an introduction to such topics as quasinormal modes, which govern how a hole responds to small perturbations. But you might want to start with the excellent survey articles in Black Holes and Relativistic Stars, ed. by Robert Wald. See section 4.7.3 of the article by Rees for recoil, see the article by Teukolsky for numerical simulation of mergers, and see the article by Kip Thorne for the radiation predicted in merger scenarios. HTH 


#21
Jun707, 01:00 AM

P: 70

Thank you very much for your answers! I will have to read more about this subject to get a better grasp of the specifics, but as I understand from Chris' answer the asymmetry seems to be 'solved' by gravity waves, carrying away the asymmetric component of the gravitational field.



#22
Jun707, 01:04 AM

Sci Advisor
P: 2,340

Yes, that's about right. Now you have a new mystery to chew on: why haven't "kicked" holes been observed?



#23
Jun707, 08:56 AM

P: 87

I was reading something last week about offcentre galactic nuclei and quasars that were outside the main body of a galaxy. I'm at work so I can't find it, but maybe this is relevant:
http://www.citebase.org/fulltext?for...oph%2F0208215 I was also reading about "seat belts" in New Scientist, again I'm not sure if it's relevant: http://space.newscientist.com/articl...ackholes.html On the subject of stellar black holes rather than supermassive black holes, is anybody aware of any apparent puzzles or unexpected results re the mass of black hole candidate objects? 


#24
Jun707, 09:53 AM

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P: 2,340




#25
Jun707, 12:14 PM

P: 87

If we have a binary system that looks like this:
o O where the o is a candidate black hole and the O is a star of some typical type and mass, one would observe each object orbiting the other, and would be able to work out approximate masses and orbits. Does anybody know if there's have been any unusual observations or inferences? For example is the calculated mass of o much larger than that of a typical star, and is there a distribution of o masses that doesn't fit the distribution of stellar masses. Alternatively does anybody know of any candidate stellar black holes that follow an atypical galactic orbit? 


#26
Jun707, 12:21 PM

P: 2,043




#27
Jun707, 01:14 PM

Sci Advisor
P: 2,340

Sorry, Voltage, nothing comes to mind, possibly because "atypical/unusual" "stellar mass distribution" is still too vague for me. All I can suggest is that you repost your question in the moderated newsgroup sci.astro.research.



#28
Jun807, 07:59 AM

P: 87

Many thanks Chris. Will do.



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