by geoffrey.landis » Wed Sep 08, 2010 2:38 pm
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).
[quote="Chris Peterson"][quote="geoffrey.landis"]In the real world, a black hole pretty much [i]always[/i] has an additional attribute: an accretion disk.[/quote]
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.[/quote]
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.
[quote] In reality, many- probably most- black holes don't even have accretion discs.[/quote]
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 [i]not[/i] 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.
[quote="Chris Peterson"][quote="geoffrey.landis"]To the contrary: if a black hole has both charge and spin, it [i]must[/i] have a magnetic field (although this must be aligned).[/quote]
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.[/quote]
Checking my copy of Misner, Thorne and Wheeler, [i]Gravitation[/i], page 883; the magnetic dipole moment of a Kerr-Newman black hole is [b]M[/b]=Q[b]a[/b]. (Q the charge, [b]a[/b] the angular momentum).