3.2 Pressure-Free Collapse to RN

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Consider a spherical dust ball for which each particle of dust has

charge/mass ratio

 =

Q

M

; jj < 1 (3.37)

where Q is the total charge and M is the total mass. The exterior

metric is M > jQj RN. The trajectory of a particle at the surface is

the same as that of a radially infalling particle of charge/mass ratio

in the RN spacetime. This is not a geodesic because of the additional

electrostatic repulsion. From the result of Question II.4, we see that

the trajectory of a point on the surface obeys

_

dr

d_

_2

= "2 􀀀 Ve_; (" < 1) (3.38)

where

Ve_ = 1 􀀀

􀀀

1 􀀀 "2_ 2M

r

+

􀀀

1 􀀀 2_ Q2

r2 (3.39)

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r

V r0 e_

r0 =

􀀀

1 􀀀 2

_

(1 􀀀 "2)

Q2

M

=

2

􀀀

1 􀀀 2

_

(1 􀀀 "2)

M (3.40)

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The collapse will therefore be halted by the electrostatic repulsion.

All timelike curves that enter r < r+ must continue to r < r􀀀, so

the `bounce' will occur in region V. The dust ball then enters region

III', explodes as a white hole into region I' and then recollapses and

re-expands inde_nitely.

This is illustrated by the following CP diagram

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=􀀀

i􀀀

i0

I

I'

black hole horizon in I' at r = r+

white hole horizon in I'

at new branch of r = r+

curvature singularity at r = 0

r = r􀀀

centre

of dust

ball at r = 0

future even horizon

at r = r+

timelike

escape path

of criminal

=+

i+

H+

i00

=+0

everywhere

non-sinuglar

r = 0

Notes

i) No singularity is visible from =+, in agreement with cosmic censorship.

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ii) Although the dust ball never collapses to zero size and its interior

is completely non-singular, there is nevertheless a singularity

behind H+ on another branch of r = 0, in agreement with the

singularity theorems.

iii) It seems that a criminal could escape justice in universe I by

escaping on a timelike path into universe I'. Is this science _ction?