Black Hole and Wormhole Trouble

[Charlie G]

Hey everyone, I was just wondering if anyone could clear a few issues I'm having with black holes and wormholes.

First, for an object above a black hole, wouldn't its gravitational potential energy be infinite, since it is above an object with infinite gravity, then causing another object with an ifinite gravitational pull?

For something going through a wormhole, would the same thing happen? Would a light beam be infinitely blue-shifted causing it to have infinite enrgy and infinite gravity, collapsing the wormhole? There must be something wrong with that since a lot of popular science shows talk about wormholes and that would completetly rule out wormholes as a travel possiblilty.

Thanks.

 

[DaleSwanson]

Black holes don't have infinite gravity, if they did everything in the Universe would be pulled into them. The only difference between a black hole and the star it originated as is density. You can easily find the GPE an object would have when near a black hole using the normal formula.

 

[Nabeshin]

For something going through a wormhole, would the same thing happen? Would a light beam be infinitely blue-shifted causing it to have infinite enrgy and infinite gravity, collapsing the wormhole? There must be something wrong with that since a lot of popular science shows talk about wormholes and that would completetly rule out wormholes as a travel possiblilty.

Thanks.

Yes. The most realisitc description I have heard of a wormhole implies that locally it appears and behaves exactly the same as a black hole. The difference (one you cannot measure) is that there would be no singularity. I don't know what this notion is of collapsing a wormhole though, sounds like star trek to me.

 

[Charlie G]

Thanks for the replies. Not sure why I even thought that black holes had infinite gravity. Since space curves back on itself, isn't that infinite gravity? Is the gravity infinite beyod the event horizon only?

 

[Charlie G]

Wait, I think I got it now. The reason things can't escape the event horizon is not because of an infinite gravitational pull, but since the escape velocity of the black hole is greater than lights speed. Thats better since I hate thinking about infinite numbers. Thank-you wikipedia lol.

 

[stevebd1]

For something going through a wormhole, would the same thing happen? Would a light beam be infinitely blue-shifted causing it to have infinite enrgy and infinite gravity, collapsing the wormhole?

I'm speculating but in rotating black holes there could be a mechanism in place which is related to the weak singularity caused by mass-inflation at the Cauchy horizon (where the invariant curvature scalar, which is smooth at the EH, blows up in a Dirac delta type fashion), that captures and stops ingoing photons from going any further into the deep region. While GR cannot necessarily predict what lies beyond the Cauchy horizon, Kerr metric does suggest a ring singularity in time-like geodesics, spinning at the centre which could be considered a wormhole throat. Outgoing photons within this space are not able to travel through the Cauchy horizon into space-like geodesics of the shallow region so the volume could be technically 'sealed' from the outside universe so no infinite blueshifting of light at the wormhole throat. Objects that are robust enough should be able to cross the weak singularity at the Cauchy horizon. The only issue is that even if the ring singularity was a wormhole throat, the only other place you would be going to, it currently seems, would be the interior of another Cauchy horizon!