Re: White Holes are time-reversed black holes?

On 2006-08-30, Slava Tapiadorov <tapiadorov@xxxxxxxxxxx> wrote:

The problem with white holes is like that of black holes, we do not
know whether they really exist in Nature, as theory predicts them.

No, there is abundant evidence in favour of black holes in Nature, and
none of white holes.

If you think about a white hole behavior as being like that of a
black hole, but with time reversal, you get a model of
gravitational system formed of negative masses, repelling each
other.

No, the time reversal of two objects attracting each other is ... two
objects *attracting* each other! Acceleration is the second
derivative of position with respect to time, so it does not change
sign under time-reversal (more precisely, it changes sign twice).

Two negative masses would repel, the same way two positive ones
attract

Yes (in Newtonian theory at least), but it's more subtle than you
might think. The force between two masses goes as the product of the
masses, so the force between two negative masses is *attractive*, just
as it is between two positive masses. But negative masses respond to
an attractive forace by accelerating *away*.

but then, what behavior would we expect for negative - positive
masses interactions?.

Who knows what we would *expect*, but the correct behaviour is rather
surprising: there is a repulsive force, which makes the positive mass
object accelerate away, while the negative mass object accelerates
towards it. So the two objects accelerate away in tandem to infinity!
(Despite appearances, kinetic energy is conserved because the negative
mass object has a negative kinetic energy that cancels out the kinetic
energy of the positive mass object.)

Regards,

Jeremy Henty

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