Page 3 of 3
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Mon Sep 06, 2010 3:47 pm
by Ann
beyond wrote:
Here is an example of a "Black Hole" evolving into a "White Hole." It takes in much material and creates much mass that repels all but the parental Black Holes that created it. The parental Black holes share the task of dealing with the energy input and the mass that is produced as the little "Wonder" grows out of the younger I want it black hole stage, to the more mature adult White Hole stage that puts out useful energies.
Ann
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Mon Sep 06, 2010 4:24 pm
by rstevenson
Sounds like it might currently be in a pupa stage.
Rob
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Mon Sep 06, 2010 4:34 pm
by bystander
rstevenson wrote:Sounds like it might currently be in a pupa stage.
Are you implying it's a black hole in the making, or that is transforming into something else?
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Mon Sep 06, 2010 4:40 pm
by rstevenson
That was a pun, son. Although puns based on sound similarities probably shouldn't be attempted in ASCII text.
Rob
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Mon Sep 06, 2010 4:58 pm
by Beyond
rstevenson wrote:That was a pun, son. Although puns based on sound similarities probably shouldn't be attempted in ASCII text.
Rob
Rob, your pun was very effective once i heard myself think the words and not just look at them. I, for one, am glad that we do not have smell-o-vision here.
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Tue Sep 07, 2010 3:14 am
by Romano
Guest wrote:
I am only a fascinated onlooker of your APOD, with little astronomic knowledge.
What does GRO (as in today's APOD) stand for? Is that an acronym like NGC?
Thanks for the time you'll be dedicating to illuminate my ignorance and
best regards,
Romano
Re: APOD: GRO J1655 40: Evidence for a Black... (2010 Sep 05
Posted: Tue Sep 07, 2010 3:28 am
by Ann
Romano wrote:Guest wrote:
I am only a fascinated onlooker of your APOD, with little astronomic knowledge.
What does GRO (as in today's APOD) stand for? Is that an acronym like NGC?
Thanks for the time you'll be dedicating to illuminate my ignorance and
best regards,
Romano
Check out bystander's answer. He said:
So the answer seems to be yes, GRO is an acronym. NGC means New General Catalogue (although it's not so new, I believe it is from the very early 1900s), and GRO, although it doesn't contain the word "catalogue", apparently designates gamma ray sources that have been detected by the Compton Gamma Ray Observatory.
Ann
Re: Properties of black holes [Re: APOD: GRO J1655 40]
Posted: Wed Sep 08, 2010 2:38 pm
by geoffrey.landis
Chris Peterson wrote:geoffrey.landis wrote:In the real world, a black hole pretty much always has an additional attribute: an accretion disk.
I'd argue that this isn't an attribute of a black hole, merely a consequence of the behavior of things outside a black hole. A planet may or may not have a moon; that doesn't change the characterization of the planet itself. Things other than black holes can have accretion discs, too, created by the same mechanisms.
The question was whether a black hole could flicker because of an offset magnetic field. Black holes have accretion disks, accretion disks can have magnetic fields, which can be offset.
In reality, many- probably most- black holes don't even have accretion discs.
This is rather speculative. If you are inside a galaxy, space is not empty! It is difficult to think how any galactic black hole would
not acquire an accretion disk. (Extragalactic ones could plausibly clear their neighborhood and be disk-free.) But, indeed, this has yet to be observationally confirmed or denied. It would indeed be very difficult to detect black holes with no accretion disks.
Chris Peterson wrote:geoffrey.landis wrote:To the contrary: if a black hole has both charge and spin, it must have a magnetic field (although this must be aligned).
That depends on how you solve the equations of GR. It is most commonly believed that black holes do not have a magnetic field, even when they have non-zero valued for both charge and angular momentum. But the question is far from being settled.
Checking my copy of Misner, Thorne and Wheeler,
Gravitation, page 883; the magnetic dipole moment of a Kerr-Newman black hole is
M=Q
a. (Q the charge,
a the angular momentum).
Re: Properties of black holes [Re: APOD: GRO J1655 40]
Posted: Wed Sep 08, 2010 3:03 pm
by neufer
geoffrey.landis wrote:Chris Peterson wrote:geoffrey.landis wrote:
if a black hole has both charge and spin, it must have a magnetic field (although this must be aligned).
That depends on how you solve the equations of GR. It is most commonly believed that black holes do not have a magnetic field, even when they have non-zero valued for both charge and angular momentum. But the question is far from being settled.
Checking my copy of Misner, Thorne and Wheeler,
Gravitation, page 883;
the magnetic dipole moment of a Kerr-Newman black hole is
M=Q
a. (Q the charge,
a the angular momentum).
NEWMAN 
Well, then, it is most commonly believed that black holes do not have an electric charge of any note.
What's wrong with the quark star idea, anyway?
Re: Properties of black holes [Re: APOD: GRO J1655 40]
Posted: Wed Sep 08, 2010 3:10 pm
by Chris Peterson
geoffrey.landis wrote:This is rather speculative. If you are inside a galaxy, space is not empty!
Pretty near empty. From the standpoint of a stellar mass black hole, an accretion disc would seem fairly rare- there just isn't enough mass in a stellar system to support one for very long. From the standpoint of a supermassive black hole, it depends on the dynamics of the galaxy. Since we observe galaxies both with and without AGNs, it seems likely that you find black holes both with and without accretion discs in galaxy centers. Which makes sense- even at the center, there isn't much mass to be consumed by a black hole, unless something perturbs that region.
Checking my copy of Misner, Thorne and Wheeler, Gravitation, page 883; the magnetic dipole moment of a Kerr-Newman black hole is M=Qa. (Q the charge, a the angular momentum).
As I said, it depends on how you solve the relevant GR equations. It isn't certain that black holes are described by the Kerr-Newman metric. We also need to distinguish between a black hole supporting a magnetic field in the space external to it, and a black hole having a magnetic moment as a fundamental property. Those are two different things.
Re: Properties of black holes [Re: APOD: GRO J1655 40]
Posted: Wed Sep 08, 2010 3:56 pm
by Henning Makholm
Chris Peterson wrote:It isn't certain that black holes are described by the Kerr-Newman metric.
I thought the point of the "no-hair theorem" was that they would have to be? But I've only seen popular accounts of that theorem, so it is possible that it has assumptions that are doubtable.
Re: Properties of black holes [Re: APOD: GRO J1655 40]
Posted: Wed Sep 08, 2010 5:18 pm
by Chris Peterson
Henning Makholm wrote:Chris Peterson wrote:It isn't certain that black holes are described by the Kerr-Newman metric.
I thought the point of the "no-hair theorem" was that they would have to be? But I've only seen popular accounts of that theorem, so it is possible that it has assumptions that are doubtable.
It is important to recognize that this is a
theorem, not a
theory. It is the consequence of mathematically manipulating the equations of GR, but with assumptions that are not firmly established by observation.
Please understand that I'm not arguing against either the theorem as a valid representation of reality, or against the Kerr-Newman metric. I'm only pointing out that there are other metrics which lead to different conclusions about the fundamental properties of black holes, and that these haven't been completely ruled out. This remains an area of active research, and more observational evidence is required to constrain existing theories.