Re: black holes and singularity

From: N:dlzc D:aol T:com \(dlzc\) (net_at_nospam.com)
Date: 11/10/04 

Date: Tue, 9 Nov 2004 18:28:50 -0700

Dear MP:

"MP" <pet.antispam@onlinehome.de> wrote in message
news:cmqrhc$hit$1@online.de...
>
> "N:dlzc D:aol T:com (dlzc)" <N: dlzc1 D:cox T:net@nospam.com> wrote in
> message news:0mrjd.36586$SW3.14387@fed1read01...
>> Dear MP:
>>
>> "MP" <pet.antispam@onlinehome.de> wrote in message
>> news:cmkln0$q4p$1@online.de...
>> > Einstein rings (I guess you mean gravitational lensing, such as double
>> > or quadruple images of galaxies; or images of galaxies stretched
>> > along a circular structure) don't probe the geometry close to the
>> > event horizon. The object acting as a lens typically is a galaxy or a
>> > cluster of galaxies, not a bh.
>>
>> You are correct about my intent, but lets discuss it as it applies to
> close
>> approach to massive bodies. Let's say the order of the Einstein ring is
>> a
>> function of how many times the light passes around the object before you
>> get it. A value of 1, and the light is simply from a light source
> "behind"
>> another massive body. A value of 2, and the light source is on this
>> side
>> of the massive body, and the light travels around "once" (for a 180 deg
>> turn).
>>
>> There should be no orders as high as 2 for a neutron star, since the
>> expected density would place its physical radius outside the "orbital
>> radius" for this light.
>
> O.K. A neutron star's radius is roughly three times its Schwarzschild
> radius. For a spherically symmetric BH the radius where photons
> can "circle" the black hole in an (unstable) orbit is 3/2 its
> Schwarzschild radius. The analysis of the motion of light in exterior
> Schwarzschild space is somewhat more complicated than comparing
> the radius of the (unstable) photon orbit with the radius of the object.
> But for a handwaving argument it is quite safe to say, that if the
> radius of the object (here the neutron star) is twice the radius
> of the unstable photon orbit, light will not be able to make a full
> circle.
>
>> There is at least one picture that I have seen of
>> an order 2 Einstein ring. The light source is clearly visible in front
>> of
>> the ring...
>
> I haven't yet heard about an order 2 Einstein ring. This sure would
> be interesting. Do you have a reference at hand?

I think I may have misunderstood at the time that my scenario was
imprinted, but this paper looks promising:
astro-ph/0404526
"Gravitational Lensing by Black Holes: a comprehensive treatment and the
case of the star S2"

> But even if light circles a compact object tenfold, you are still
> not probing the geometry close to the horizon. You are probing
> the geometry in the vicinity of the photon orbit at r_ph = 3/2 r_s.
> Any photon with an impact parameter smaller than r_ph will
> be absorbed by the compact object.

Should *any* "wrapping" occur, more than slight bending, say 90 deg, then
the density of an object would be more dense than you have allowed for a
neutron star. This would then allow that a density sufficient to create a
BH is a possibility. Understand that I am talking about what the Universe
would "perceive" of this arrangement, and not some mystical property
assinged to the precursor materials that formed the hole.

> In order to distinguish a compact object such as a holostar
> or a gravastar from a BH by *geometric* methods (such as
> shining light on it and looking "how" it comes back), the light
> would have to get as close as a few Planck lengths to the
> horizon, i.e. r_min = r_s + few r_Pl, and then would have to
> escape the object, so that you (or any other observer) can
> draw some conclusions. But any light that manages to
> "cross" the angular momentum barrier for photons, situated
> at r_ph = 3/2 r_s, will be *absorbed* by the compact object.
> So this doesn't work. [Remember that the cross-section
> of a BH for radiation is \sigma = 3 \pi (r_ph)^2]
>
> The only method that might have a (slight) chance to distinguish
> a BH from its singularity free alternatives (gravastar, holostar or
> something else we don't yet know) from the *ouside* would be
> what Tom hinted at in
> news:csbjd.16895$Rf1.7976@newssvr19.news.prodigy.com,
> i.e. to look for an "impact" signature of a particle that either
> crosses the event horizon (of a BH) or "hits" the surface (of a
> compact object). You wouldn't see anything at all, if the object
> were a BH: a BH has nothing "to hit" at the event horizon, which
> is simply vacuum. Any inertial observer will just pass by
> without noting anything unusual (if the BH is large enough. For
> a small BH you will notice tidal effects - but you won't be
> able to locate the event horizon by the tidal effects). An observer
> impacting on a holostar or gravastar *would* notice. I guess an
> impact on a "gravastar" would be quite catastrophic, because
> most of the gravastar's mass resides at its surface. An impact
> on a "holostar" would be somewhat "softer", because the interior
> mass-density of a holostar falls off with \rho \propto 1/r^2. In
> any case, any observer impacting on a holostar or gravastar
> will feel something, and I guess for a solar-mass compact
> object this "something" would be quite rough.
>
> However, whether we will be able to "see" what happens from
> the *outside* is not so clear. A gravastar (or holostar) with
> mass of the sun is expected to have a surface redshift of order
> 10^20. Compact objects of larger mass have even larger
> redshifts. I guess it would be nearly impossible to pick out
> an impact signature that has been redshifted by a factor
> of 10^20. However, as long as we don't know *what
> exactly* happens at the impact point, one shouldn't jump
> to conclusions too fast.

OK.

David A. Smith

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