Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 16/03/2015 06:17:32

Title: What role does the charge of a black hole play?
Post by: jeffreyH on 16/03/2015 06:17:32

One of the properties of a black hole is said to be its charge. How would this affect the approach of a mass towards the event horizon? If not why not?

EDIT: If not then is the Kerr-Newman metric of only theoretical value?

http://en.wikipedia.org/wiki/Kerr–Newman_metric (http://en.wikipedia.org/wiki/Kerr–Newman_metric)

Title: Re: What role does the charge of a black hole play?
Post by: evan_au on 16/03/2015 09:54:57

I once read a science fiction story that imagined firing an ion drive into a small black hole until it accumulated a significant charge, and then moving the black hole around using electric and magnetic fields...

In reality, to charge a black hole, you must tear apart positive and negative charges. That means in a region close to the black hole, there will be a considerable charge of the opposite polarity. These charges will then accelerate to relativistic velocities to rejoin their lost companions inside the event horizon. Any gas, dust or meteorites in the vicinity will also take on a static charge, and will also join the charge into the black hole.

This is why the theory about charged black holes is of little practical value - the electrostatic force is much stronger than the gravitational force, so even if you managed to inject 1 extra electron for every million atoms dropped into the black hole, the electrostatic attraction would dwarf the gravitational attraction, and the black hole would rapidly become neutral. Then you can use the theory about uncharged black holes....

Title: Re: What role does the charge of a black hole play?
Post by: jeffreyH on 16/03/2015 20:16:50

Quote from: evan_au on 16/03/2015 09:54:57

I once read a science fiction story that imagined firing an ion drive into a small black hole until it accumulated a significant charge, and then moving the black hole around using electric and magnetic fields...

In reality, to charge a black hole, you must tear apart positive and negative charges. That means in a region close to the black hole, there will be a considerable charge of the opposite polarity. These charges will then accelerate to relativistic velocities to rejoin their lost companions inside the event horizon. Any gas, dust or meteorites in the vicinity will also take on a static charge, and will also join the charge into the black hole.

This is why the theory about charged black holes is of little practical value - the electrostatic force is much stronger than the gravitational force, so even if you managed to inject 1 extra electron for every million atoms dropped into the black hole, the electrostatic attraction would dwarf the gravitational attraction, and the black hole would rapidly become neutral. Then you can use the theory about uncharged black holes....

Thanks. That makes sense and just leaves the Kerr metric to consider.