I White holes and other universes

[Tio Barnabe]

This video by PBS spacetime makes me wonder if one of the things it discusses, namely the possibility that a white hole can be home to another universe, is a worth to pursue question.

Is there any advances in terms of discoveries on this area? What are your personnal opinions about that possibility?

 

[Ibix]

Newton says that the acceleration due to gravity is $g=GM/r^2$. So the gravitational acceleration at the centre of the Earth ought to be infinite, right? Well, no. That form of Newton's gravity is only strictly correct outside a spherically symmetric mass, and inside the Earth you aren't outside it.

The extension of the Schwarzschild spacetime to include the white hole and "other exterior" regions is similar to treating Newton's $1/r^2$ field as valid everywhere. It's fine maths, and if all of the mass of the Earth were concentrated at a point and Newtonian gravity were accurate in strong fields then it would describe reality. But the mass of the Earth is not a point. Similarly, the maximally extended Schwarzschild spacetime is fine maths, and if a black hole existed forever in an otherwise empty universe then it would describe reality. But black holes form from stars, they aren't eternal. And the universe isn't empty.

So the Penrose diagrams drawn in the video don't describe real black holes. Real black holes only have the right hand diamond and the upper triangle. The boundary with the white hole region isn't a vacuum; there's a star there. Extending the diagram into that region is similar to treating the interior of the Earth like the exterior - not describing reality.

To note: I've just re-read Carroll's lecture notes' chapter on this, but that's the extent of my knowledge.

 

[PeterDonis]

Is there any advances in terms of discoveries on this area?

No. White holes are a mathematically consistent solution of the Einstein Field Equation, but they are not considered physically reasonable, because there is no way for one to be formed. Black holes can form from collapse of a sufficiently massive object like a star; the collapse ends by forming a horizon and then a singularity. But a white hole, as the video says, is the time reverse of that, which means that an object like a star would have to be formed from a singularity and a horizon that happened to exist and be related in exactly the right way. That's not a physically reasonable possibility.

 

[PeterDonis]

the Penrose diagrams drawn in the video don't describe real black holes. Real black holes only have the right hand diamond and the upper triangle.

Yes, but mathematically, there can also be a time reversed version of such a diagram, which would have the right hand diamond and a lower triangle, below the diamond instead of above it. That would represent the white hole region--but it also would mean that a star would have to pop out of this white hole, which just happened to have a singularity and a horizon related in exactly the right way to make all that happen. As I noted in my previous post just now, that's not a physically reasonable possibility.