Black Hole Spin

I was reading an article on black hole spin
(http://www.space.com/scienceastronom...hole_spin.html) and
noted the article speaking about a theoretical limit of 1150 hz.

Conceptually I have no difficulty with a limit on stars spinning; spin too
fast and if flies apart. However what is it that limits a black hole's
spin?

On Feb 21, 4:10 am, "Anthony Garcia" wrote:
I was reading an article on black hole spin
(http://www.space.com/scienceastronom...hole_spin.html) and
noted the article speaking about a theoretical limit of 1150 hz.

Conceptually I have no difficulty with a limit on stars spinning; spin too
fast and if flies apart. However what is it that limits a black hole's
spin?

Almost the same thing - but the black hole cannot physically fall
apart, but it could lose its opaque event horizon(s) if it spins too
fast. The limit avoids us being able to see a naked singularity at the
centre of the spinning black hole.

In crude vaguely classical terms the frame dragging of the black hole
cannot cause the event horizon at the equator to move faster that the
speed of light. This puts a limit on the total amount of angular
momentum a given mass black hole can have (and by implication for a
given mass and angular momentum the radius and period of the fastest
innermost stable orbit going with and against the spin - they are
seriously different).

Modern X-ray telescopes with spectrographs can now follow the
emissions of material going down the gravitaional plughole and so
infer a spin/mass ratio for a spinning BH.

There isn't an easy explanation I know of that avoids heavy maths.
Wikipedia isn't bad on this:

http://en.wikipedia.org/wiki/Rotating_black_hole

It is intimately link to the strong cosmic censorship hypothesis

http://en.wikipedia.org/wiki/Cosmic_...hip_hypothesis

Regards,
Martin Brown

Once matter goes past the event horizon is it still limited by the speed of
light or can it accelerate past this limit due to time slowing down within
the black hole.

Please excuse my ignorance but I dont have a physics education just a
curiosity about black holes.


"Martin Brown" wrote in message
ups.com...
On Feb 21, 4:10 am, "Anthony Garcia" wrote:
I was reading an article on black hole spin
(http://www.space.com/scienceastronom...hole_spin.html) and
noted the article speaking about a theoretical limit of 1150 hz.

Conceptually I have no difficulty with a limit on stars spinning; spin
too
fast and if flies apart. However what is it that limits a black hole's
spin?

Almost the same thing - but the black hole cannot physically fall
apart, but it could lose its opaque event horizon(s) if it spins too
fast. The limit avoids us being able to see a naked singularity at the
centre of the spinning black hole.

In crude vaguely classical terms the frame dragging of the black hole
cannot cause the event horizon at the equator to move faster that the
speed of light. This puts a limit on the total amount of angular
momentum a given mass black hole can have (and by implication for a
given mass and angular momentum the radius and period of the fastest
innermost stable orbit going with and against the spin - they are
seriously different).

Modern X-ray telescopes with spectrographs can now follow the
emissions of material going down the gravitaional plughole and so
infer a spin/mass ratio for a spinning BH.

There isn't an easy explanation I know of that avoids heavy maths.
Wikipedia isn't bad on this:

http://en.wikipedia.org/wiki/Rotating_black_hole

It is intimately link to the strong cosmic censorship hypothesis

http://en.wikipedia.org/wiki/Cosmic_...hip_hypothesis

Regards,
Martin Brown

Anthony Garcia wrote:
I was reading an article on black hole spin...

Black-hole spin... is that like when President Cheney says "The
Insurgency is in its last throes?"

D

--
usenet *at* davidillig dawt com

On Feb 21, 5:10 pm, "Anthony Garcia" wrote:
I was reading an article on black hole spin
(http://www.space.com/scienceastronom...hole_spin.html) and
noted the article speaking about a theoretical limit of 1150 hz.

Conceptually I have no difficulty with a limit on stars spinning; spin too
fast and if flies apart. However what is it that limits a black hole's
spin?

Kerr solution:

http://en.wikipedia.org/wiki/Kerr_metric
http://www2.phys.canterbury.ac.nz/kerrfest/spin.html
http://www.nasa.gov/vision/universe/...blackhole.html

Bill

On Feb 21, 10:04 am, "chris stoner"
wrote:
Once matter goes past the event horizon is it still limited by the speed of
light or can it accelerate past this limit due to time slowing down within
the black hole.

The short answer is that we don't really know if our exterior metric
models will work in the interior and it isn't amenable to scientific
investigation. If you could ever get into a position to study the
interior zone you would not be able to report your findings back in
this universe. Although it is theoretically possible if our models are
correct taht with a big enough spinning black hole that you could come
out again somewhere else without being spagettified if very special
conditions are met to find a safe timelike trajectory through a
wormhole.

Please excuse my ignorance but I dont have a physics education just a
curiosity about black holes.

I would suggest treating treat all definitive statements about the
interior properties of black holes as conjectures at least until we
are able to experiment on them directly. And that day is a long way
off.

Various virtual movies of trips to black holes and neutron stars by
raytracing in GR metrics are online at:
http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html
(some are very large files)

Regards,
Martin Brown