Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.physics.relativity > #473430

Re: The Kerr black hole

Show all headers | View raw

On Friday, September 28, 2018 at 2:37:44 PM UTC-7, tjrob137 wrote:
> On 9/28/18 3:36 PM, Hilton Packett wrote:
> > Tom Roberts wrote:
> > Black holes are most likely real, [...]
> > 
> > In principle you can escape the horizon, using the kinetic energy from the
> > fall, plus the payload carrying propellant.
> 
> Not true. Once any timelike object enters the horizon of a black hole, no amount 
> of kinetic energy or propellant can get it back outside the horizon. Moreover it 
> is doomed to intersect the singularity at the "center" within finite proper 
> time.

Strictly speaking (i.e. strict GR) this is true only for non-rotating black 
holes. In the rotating case two things happen:

1. the "one-way street" beyond the horizon "detaches" itself from the
singular set,

2. the singular set itself "opens up" into a disc of radius "a" (the
rotation's angular momentum per unit mass) acting as a branch cut in
the spacetime through which an infalling object can go.

In fact, in this situation ("a" nonzero) it gets quite hard to hit the
singular set, again two reasons:

1. the singular set is gravitationally repulsing (it's also repulsing
in the Schwarzschild (non-rotating) case but the "one-way street" part
of spacetime is "touching" it there, so it does become unavoidable
regardless),

2. on top of that, one can also prove (Carter's theorem) that
a free-falling object can hit the singular set only if its trajectory
is completely contained in the equatorial plane (of course the tidal
forces near the singular set may be lethal anyway).

Finally, the repulsive force of the singularity in the rotating case
is quite strong, for example an object dropped along the axis of
rotation down will go through the horizons and then _bounce back_,
repulsed by the singularity, again through the horizons (a different
pair of horizons) into a different branch of the fully extended
spacetime. In order to get that object to fly _through_ the singularity
(which is a circle) one must supply an extra thrust to it, just free-falling
down from infinity is not strong enough. With that sort of extra push
this is possible and given massive enough black hole will produce
FAPP imperceptible tidal forces.

This is all strict GR, of course. We'll hopefully find out better with
a better theory.

--
Jan

Back to sci.physics.relativity | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

The Kerr black hole mitchrae3323@gmail.com - 2018-09-25 18:46 -0700
  Re: The Kerr black hole JanPB <filmart@gmail.com> - 2018-09-25 20:48 -0700
    Re: The Kerr black hole mitchrae3323@gmail.com - 2018-09-25 21:25 -0700
      Re: The Kerr black hole JanPB <filmart@gmail.com> - 2018-09-26 23:57 -0700
        Re: The Kerr black hole Tom Roberts <tjroberts137@sbcglobal.net> - 2018-09-28 10:11 -0500
          Re: The Kerr black hole JanPB <filmart@gmail.com> - 2018-09-28 12:44 -0700
          Re: The Kerr black hole Hilton Packett <wod@wcothr.ce> - 2018-09-28 20:36 +0000
            Re: The Kerr black hole Tom Roberts <tjroberts137@sbcglobal.net> - 2018-09-28 16:37 -0500
              Re: The Kerr black hole Hilton Packett <wod@wcothr.ce> - 2018-09-28 21:55 +0000
              Re: The Kerr black hole JanPB <filmart@gmail.com> - 2018-09-28 15:02 -0700
                Re: The Kerr black hole Hilton Packett <wod@wcotahr.ce> - 2018-09-29 23:57 +0000
                Re: The Kerr black hole JanPB <filmart@gmail.com> - 2018-09-29 21:31 -0700
                Re: The Kerr black hole Hilton Packett <wod@wcotahr.ce> - 2018-09-30 16:20 +0000
                Re: The Kerr black hole moroney@world.std.spaamtrap.com (Michael Moroney) - 2018-09-30 17:08 +0000
                Re: The Kerr black hole Hilton Packett <wod@wcotahr.ce> - 2018-09-30 21:29 +0000
                Re: The Kerr black hole "Ross A. Finlayson" <ross.finlayson@gmail.com> - 2018-09-30 09:44 -0700
        Re: The Kerr black hole Tom Roberts <tjroberts137@sbcglobal.net> - 2018-09-28 10:11 -0500
          Re: The Kerr black hole Hilton Packett <wod@wcotahr.ce> - 2018-09-30 16:43 +0000

csiph-web