Fun with black holes
in [Relativity]
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From: BURT on 22 May 2010 15:06 On May 22, 4:43 am, eon <ynes9...(a)techemail.com> wrote: > On May 19, 5:48 am, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > > > Sue... wrote: > > > Objects that can't radiate light, also can't > > > radiate gravity. > > > Not in GR, which is the best generally-accepted theory of such things. > > well, why should light, in your GR, falls into a black hole > > light, as gravity, must be geometry as well > > > > > > For that reason, black holes > > > are absurd. > > > No. What is absurd is your entire approach. > > > Tom Roberts > > seems that Sue has a valid point here, and you dare not taking a > position > > why are are they looking for gravity waves traveling at speed of > light? > > good bye Light waves can be absorbed but not gravity. Those waves are destined to wander the universe forever. Mitch Raemsch
From: Edward Green on 22 May 2010 15:40 On May 21, 9:21 pm, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: > Edward Green wrote: > > On May 21, 6:41 pm, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: > >> Edward Green wrote: > > > <...> > > >> > Thereafter the black hole rings out like a bell until > >> > the perturbation to its horizon has been absorbed. > > >> No again. There is no analysis anywhere which supports this. > > > MTW p.886 > > > "When matter falls down a black hole, it can excite the hole's > > external spacetime geometry into vibration. The vibrations are > > gradually converted into gravitational waves..." > > Which does not mean the horizon itself changes. The horizon is intimately connected to the external geometry.
From: eric gisse on 22 May 2010 07:44 Edward Green wrote: > On May 21, 9:21 pm, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: >> Edward Green wrote: >> > On May 21, 6:41 pm, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: >> >> Edward Green wrote: >> >> > <...> >> >> >> > Thereafter the black hole rings out like a bell until >> >> > the perturbation to its horizon has been absorbed. >> >> >> No again. There is no analysis anywhere which supports this. >> >> > MTW p.886 >> >> > "When matter falls down a black hole, it can excite the hole's >> > external spacetime geometry into vibration. The vibrations are >> > gradually converted into gravitational waves..." >> >> Which does not mean the horizon itself changes. > > The horizon is intimately connected to the external geometry. Define an event horizon.
From: Edward Green on 22 May 2010 16:03 On May 21, 10:23 pm, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: > Edward Green wrote: > > Ah. Here we go. p. 417 > > > "When matter falls into a black hole, the absolute horizon starts to > > grow ... before the matter reaches it ... " > > Once the matter is redshifted into oblivion it is a part of the black hole > as far as external observers are concerned. > > As for the horizon _growing_ before the matter reaches it, nonsense. The > horizon will only grow in response to further input of mass-energy. Ok. Why don't you write to Kip Thorne then. > > As for the idea that black holes can ring, p. 295 ff. > > > "Not only can black holes spin, they can pulsate". > > For what value of 'spin' or 'pulsate' ? > > The spin of a black hole is an imprint of angular momentum upon spacetime.. > There's nothing actually spinning. Plus black hole thermodynamics sorta nix > the idea of the horizon 'pulsating'. I recommend that you read the actual book. Here is another snippet: "In autumn 1971, Bill Press, a new graduate student in my group, realized that the ripples of spacetime curvature bouncing around near a black hole could be thought of as pulsations of the hole itself. After all, as seen from outside its horizon, the hole consists of nothing but spacetime curvature". <...>
From: Tom Roberts on 24 May 2010 02:02
Greg Neill wrote: > So this applies to the net gravitational field as well as to > mundane means of observation then. A distant observer will > "see" the relaxation of the horizon to a sphere occur in a > very short time indeed, much as the quick extinction of > light arriving from an object that falls in. After that, > the BH just "looks" like a slightly bigger (more massive) BH. Not really. A distant observer has no way to "see" the shape of the horizon. the horizon of a black hole is an abstract geometrical locus at which no timelike object can escape to spatial infinity. To determine the location of the horizon requires non-local measurements, such as tracking light rays that approach it, or dropping powerful rockets that attempt to blast back out, and see which ones make it and which ones don't. When someone says that the asymmetry in the horizon disappears quickly, this is no sort of measurement, this is determined by examining an appropriate solution to the field equation (or an approximation to one, such as in computer simulation). > Presumably the collision of the BH and object will then > appear like a perfectly inelastic collision as far as > the observer can see. Yes. > There won't be any wobbling about > of the horizon around the center of mass The distant observer cannot observe this. What such an observer could see is the gravitational radiation emitted as the horizon relaxes to spherical shape (in the case being discussed), but the simple "gravitational compass" cannot do so. > while the object > follows its inevitable trajectory to the singularity. The distant observer cannot attempt to ascribe anything to the object "after" its trajectory enters the horizon. I put "after" in quotes, because to this observer the object NEVER actually reaches the horizon. So your use of "while" is unwarranted. > Would this also necessarily follow for an object meeting > the horizon on a non radial path, the angular momentum > being transferred almost instantly to the spin of the BH? Infalling objects on non-radial paths into a black hole impart angular momentum to the BH. "Almost instantly" is unwarranted, as discussed above. Tom Roberts |