# Wormholes and black holes

1. Jan 21, 2009

### athv_gr

Hello,

I have the following question: Let as assume that wormholes can exist. What happens if an advanced civilization is able to construct a wormhole with one end on its own planet and the other end of the wormhole below the event horizon of a black hole?

Will they be able to see what is happening below the event horizon of the black hole? Or their planet will be sucked into the black hole?

Thanks!

2. Jan 21, 2009

### Nabeshin

Ok, assuming wormholes exist, then within the framework of GR, they are almost identical to a black hole.

Wormholes, supposedly, would manifest themselves as spacetime distortions which, like a black hole, involve an event horizon due to the increasing curvature of spacetime. The only difference, then, between a hypothetical wormhole and a black hole is that the wormhole reconnects with another piece of spacetime.

An analogy in 2D should be useful. Imagine the fabric of spacetime to be a large sheet of rubber. A black hole is analogous to an extremely massive, dense, ball placed on the sheet. The resulting well (in the situation of a singularity) continues to steepen indefinitely. Now, a worm hole, by contrast, appears as again a well which continues to steepen, but which eventually turns around and reconnects somewhere. So, from a local perspective, it would be almost impossible to tell a wormhole from a black hole.

3. Jan 22, 2009

### athv_gr

I cannot say that I understand completely. From what I know it is impossible for someone to tell what is happening below the event horizon of a black hole.

But, in a wormhole you can see what is happening on the other side. So, if the other side is below the event horizon of a black hole then information is transmitted about what is happening below the horizon to the outside universe. Am I wrong?

4. Jan 22, 2009

### Chalnoth

Yeah, you couldn't see through the wormhole. For all intents and purposes, it'd look just like a black hole to you. Except that you could come out somewhere else if you went through it.

5. Jan 22, 2009

### AEM

I don't believe that even an advanced civilization would be able to construct a black hole with one end on a planet and the other end below the event horizon of a black hole. There are two reasons for my pessimism: (1) The solutions to Einstein's equations that depict the possibility of a wormhole connect one asymptotically flat region of spacetime with another asymptotically flat region of spacetime. The region of spacetime below the event horizon is hardly asymptotically flat. (2) In order to keep a wormhole open you have to introduce "exotic matter", or negative energy. What this implies is that in the appropriate equations, in the wormhole region you have to take the sign of kappa, the coupling constant between matter and gravity to be negative. On the other hand within the black hole region this coupling constant is positive. Solutions to these two disparate sets of equations would not join smoothly

6. Jan 22, 2009

### athv_gr

Sorry, but I am still not convinced. If there was a light source very near the edge of the wormhole (the one below the horizon of the black hole) wouldn't the light of this source escape inside the wormhole and to the outer universe?

I mean an observer that is below the event horizon of a black hole is not able to see light at all? Can't he see light that is produced very close to him?

Also, could virtual particles produced in the black hole to come through the wormhole, enter the outside universe and become real particles?

7. Jan 22, 2009

### AEM

Of what are you not convinced?

8. Jan 22, 2009

### athv_gr

Sorry, my last post was a reply to Chalnoth and not to you. I had not fully read your post when I wrote this.

I think, that if you are correct in your analysis, you have given a satisfactory answer to my question.

9. Jan 22, 2009

### AEM

Okay, I thought you were probably responding to someone else. I've been thinking about your idea with the wormhole a little more and I'm convinced both that it couldn't happen and you cannot construct null geodesics (the paths of light) between the two spaces --within the event horizon and through a wormhole using the appropriate field equations for each.

An aside note: my Ph.D. thesis involved constructing a wormhole within the framework of 5 dimensional relativity. Back then (way back :) ) they were known as Einstein-Rosen bridges. The idea was to use the additional freedom that 5 dimensional relativity has due to the presence of a scalar field (as well as the gravitational filed and the electromagnetic field) to hold the wormhole open. Well, it didn't work (you need that negative energy), but in the process I proved the inequivalence of 5 dimensional relativity and the Brans-Dicke scalar tensor theory so it worked out okay in the end.

10. Mar 18, 2011

### HarryRool

Actually, the answer to the latter question appears to be "yes". I found two sources that confirm this. Both are written by physicists.

1) http://prd.aps.org/abstract/PRD/v48/i4/p1607_1"

Here's the abstract:

"It is shown that by using a traversable wormhole one can get information from a black hole’s interior. The change of a black hole’s geometry in the presence of a wormhole falling into it is analyzed. The causal structure and the properties of the event horizon of a Schwarzschild spacetime with a wormhole are considered. Information and energy extraction from the interior of a black hole by using a wormhole is discussed."

2) https://www.amazon.com/Physics-Stargates-Parallel-Universes-Wormhole/dp/0984150005"

On page 33, in a question/answer section:

"Could I use a wormhole to escape from the inside of a black hole?
Yes. Classically, there appears be to nothing to prevent the existence of a wormhole that connects the inside of a black hole with the region exterior to its event horizon. However, to an observer within the horizon, the outward direction points to the past, i.e. backward in time. So escaping a black hole via a wormhole is only possible, if time travel by wormhole is possible."

Last edited by a moderator: May 5, 2017
11. Mar 19, 2011

### athv_gr

Thanks for your response. If I find the references that you mentioned, I may have a look. Although, it's been a long time since I last read about these matters!

12. Mar 21, 2011

### petm1

Isn't this what we see now? :uhh:

13. May 2, 2011

### HarryRool

The outward direction only points to the past if you're beneath the event horizon, i.e. inside the black hole.