Re: Black hole questions

On Wed, 22 Nov 2006 05:46:43 +0000, Tom Roberts wrote:

Wai Yu Wong wrote:
1. When a heavy object, say a neutron star, approaches a black hole,
will the black hole be moved by the gravity of the former?

Yes (accepting your loose terminology). The two objects will orbit around
their common barycenter (assuming all other masses are very far away).

Far outside its horizon, a black hole acts like an ordinary object with
the same mass. Near the horizon, however, a black hole is VERY different
from an ordinary object.


If so, how
long does it take for the change of the black hole's gravity to reach
the object?

The question does not make sense. In GR, gravity does not "propagate", and
asking about speed or "how long it takes to get there" is inappropriate.
Changes in the gravitational field(s) propagate with speed c.

I'm sorry; I don't understand. If the BH accelerates, then from the POV
of a distant observer, its gravitational "field" changed (i.e., it moved,
and that's a change).

But that _change_ must propagate from the BH out through the region
surrounding its event horizon, as a gravity wave propagating at C. And it
takes something moving at C an infinite amount of Schwarzschild time to
travel up from the event horizon to the rest of the universe.

So, how does the information that the BH has accelerated -- which takes
the form of a _change_ in the BH's gravitational field -- ever get out
to a place where we can detect it? It seems like the BH must appear to
follow a straight-line inertial course forever (which is obviously wrong!).

(Is the problem that I'm mixing up "real acceleration" from motion along a
3-space-curved geodesic? But then, suppose the BH has an electric charge
and we stick it between the plates of a capacitor -- in that case it must
'really' accelerate and we're back where we started.)


[ snip the rest ]


Kip Thorne, _Black_Holes_&_Time_Warps_, is a non-mathematical discussion
of the properties of black holes and other "outrageous" aspects of GR.


Tom Roberts

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