# Entering a black hole, jumping into another universe---with questions

I'm quite familiar with SR, but I have very limited understanding in GR, singularities, and black holes. My friend, which is well-read and is interested in general physics, said that we can "jump" into another universe by entering a black hole.

Suppose that we and our equipments can withstand the tidal forces near black holes. We jumped from our spaceship into a black hole. As we had passed the event horizon, we couldn't send any information to the outside anymore.

1. Can this situation be interpreted as that we were in another universe separate from our previous universe?

2. Is there corrections or anything else to be added to above statement?

Let me attempt a more "popular science" answer (Ron please be gentle with me!).

In GR a geodesic is the path followed by a freely moving object. There's nothing especially complex about this; if you throw a stone (in a vacuum to avoid air resistance) it follows a geodesic. If the universe is simply connected you'd expect to be able to get anywhere and back by following geodesics.

However in a static black hole, described by the Schwartzchild geometry, something a bit odd happens to the geodesics. Firstly anything following a geodesic through the event horizon can't go back the way it came, and secondly all geodesics passing through the event horizon end at a single point i.e. the singularity at the centre of the black hole.

If you now rotate the black hole, or you add electric charge to it, or both, you can find geodesics that pass through the event horizons (there are now two of them!), miss the singularity and travel back out of the black hole again.

But, and this is where the separate universes idea comes in, you now can't find any geodesics that will take you back to your starting point. So you seem to be back in the normal universe but you're in a region that is disconnected from where you started. Does this mean it's a "separate universe". That's really a matter of terminology, but I would say not. After all you just got there by coasting - you didn't pass through any portals of the type so beloved by SciFi films. And there's no reason to think that physics is any different where you ended than where you started.

If you're interested in pursuing this further I strongly recommend The Cosmic Frontiers of General Relativity by William J. Kaufmann. It claims to be a layman's guide, but few laymen I know could understand it. However if you know SR you shouldn't have any problems with it.

• Thanks. I found this answer more appropriate for the community, especially for literate beginners. Pretty good without being complicated.
– Sawi
Jan 17, 2012 at 18:16
• +1: If you write correct stuff, there is no reason to worry! I am happy that you put a more pedagogical answer is here, this subject is extremely important in current research, and I can't help mentioning the current debates (in my head, at least, nobody else considers this question actively AFAIK). Jan 17, 2012 at 21:47
• Thanks. I must admit your point about the information loss was new to me. I first learned about the Reissner-Nordstrom geometry beore Hawking had raised the information loss problem and for me it's always been a simple geometrical issue. Jan 18, 2012 at 7:16
• The "other universe" is not only disconnected from the previous one, it's (in general) even infinitely extended. Considering that you can wrap your black hole in a sphere of finite size this is pretty surprising and the wording "another universe" seems appropriate. Jun 9, 2017 at 14:01
• Interesting. Not sure how to ask this clearly, but can you explain where the disconnected region "comes from" or how it relates to the rest of space? When a star collapses to a black hole, before the collapse (when it's just a star) the surrounding space quite flat, but afterwards you have this disconnected region. So is that region a small part of the "old" space that was caught in the hole and distorted and blown up to be very large or even infinte? Or something different? I do understand that intuitive images are hard, but is there a way to describe this a bit more? Dec 12, 2021 at 14:59

This issue is largely settled today--- you cannot go to another disconnected universe, but you are either trapped in the black hole, or reemerge in this universe. The reason is the no-information loss property. If material could go between disconnected universes, information about the state of this material would be permanently lost to the other universe. Hawking in fact has recently admitted that this idea was the main motivation for his initial proposal that Hawking radiation is perfectly thermal, and does not encode information about infalling stuff. This would be a definitive sign that the black hole is linking to a disconnected universe

The reason people expected to be able to travel to another universe is because a generic rotating black hole has three regions. The easiest way to see the structure is to consider a charged black hole, which is qualitatively the same, but spherically symmetric. The metric has f(r) in the dt part, and 1/f(r) in the dr part, with $f(r) = 1 - a/r + b/r^2$, where b is the square of the electric charge, and a is half the mass.

The zeros of the time component of the metric tell you the position in r of the horizon. There is an outer horizon and an inner horizon (the third zero is at negative values of r, and is unphysical). The outer horizon is an event horizon--- once you cross, you are forced inward. The inner horizon is a Cauchy horizon, once you cross this horizon, you see the singularity at r=0.

Gravity becomes repelling in r inside the inner horizon. Then the particle falling in is shoved outward, to cross the Cauchy horizon going the other way, and then it crosses the event horizon going the other way, but considering the event horizon as a white hole horizon now. Classically, each of the regions is disconnected, there is no identification between them. So that you go from our universe to the interior, then to the inner part, then to the interior of the white hole, then out into another disconnected universe.

In the 1970s, the white hole and black hole horizons were considered distinct, so that a black hole would be one thing, and a white hole another. But Hawking himself noted that thermal equilibrium states are invariant under time reversal, so that by time reversing a black hole in equilibrium with radiation, you find that a black hole and a white hole are actually the same, once quantum mechanics, but in different perspective.

The full infalling process is not understood, because no-one has traced infalling matter going through a black hole in AdS/CFT (although it is just out of reach--- people are working on it). The result (I believe) is that the infalling matter does the classical circuit, and comes out of the event horizon at a time that is 1/hbar, so that the transit time diverges classically. But this is not supported by any detailed calculation, just by vague futzing around in AdS/CFT models. The only calculations in the literature make the claim (equally unsupported) that the black hole absorbs the stuff that falls in irreversibly, without spitting it out. I am pretty sure this is not true, it spits it out in this universe, if so, sometimes CPT reflected, so it comes out anti-matter.

I speculated that this could be the reason for the anomalous anti-electron signal observed recently by satellites near the galactic center. A rotating astrophysical black hole could be a copious antimatter source.

• Thanks for your patience and time. Still trying to get a better understanding of your answer though... Would vote up this if I can...
– Sawi
Jan 17, 2012 at 16:59
• @Sawi: if you can comment, you should be able to upvote (I think). The answer is a little technical, but if you say what is confusing or unclear I can try to clarify, or perhaps you will get a more pedagogically appropriate answer. Jan 17, 2012 at 17:11
• "Commenting everywhere" is a privilege. But maybe commenting in answers to own questions is not, I'm unsure. I'm still waiting for other answer(s), but thanks for yours.. I've gained enough reputation to upvote your answer!
– Sawi
Jan 17, 2012 at 17:18
• "This issue is largely settled today". Not at all. Jun 9, 2017 at 13:52