B What A White Hole Will Look Like?

[Deepblu]

White Holes may not exist, but if we discover that they are real and we find one, then:
What a white hole will look like?
How it will behave?
What will be its characteristics (such as gravity, mass, size ..etc) in relation with it's parent black hole?
What will be it's effect on nearby stars?

And most important how do we know that we have a white hole when we are looking at one? I was thinking about this and thought because white holes eject photons then maybe from very far they will just look like normal stars.

thanks

 

[PeterDonis]

What a white hole will look like?

It's impossible to tell in detail, because the whole point of a white hole is that there is no way to predict what will come out of it (and it's impossible for anything to go into it). But a "realistic" white hole (meaning not something that has any significant chance of actually existing, but a model of a white hole that at least allows it to be an independent object) would have whatever was inside (which, as above, there is no way to predict) exploding out of it and causing the hole to vanish, leaving behind either empty space or (highly improbable) some kind of stable object like a star. (This process is basically the time reverse of the idealized Oppenheimer-Snyder model of the gravitational collapse of a stable object to form a black hole.)

What will be its characteristics (such as gravity, mass, size ..etc) in relation with it's parent black hole?

There would not be a "parent black hole"; white holes (at least not the kind I was talking about above) are not created from black holes. If they existed, they would be independent objects.

how do we know that we have a white hole when we are looking at one?

With the white hole model I described above, you wouldn't be able to tell, because, as described above, the white hole would vanish and leave something else behind.

 

[Deepblu]

There would not be a "parent black hole"; white holes (at least not the kind I was talking about above) are not created from black holes. If they existed, they would be independent objects.

Aren't white wholes connected with black holes by Einstein-Rosen bridge? Where matter goes in a black hole then spewed out via a white hole. How can a white hole form without a black hole on the other end? Or how can a singularity form from nothing to create a white hole?

 

[PeterDonis]

Aren't white wholes connected with black holes by Einstein-Rosen bridge? Where matter goes in a black hole then spewed out via a white hole.

No. Take a look at the spacetime diagram at the top of this Insights article, which depicts the maximally extended Schwarzschild spacetime in Kruskal coordinates:

https://www.physicsforums.com/insights/schwarzschild-geometry-part-3/
The white hole is region IV, at the bottom; the black hole is region II, at the top. The Einstein-Rosen bridge is a "wormhole" between regions I and III; however, the wormhole pinches off too fast for anything to actually travel between those two regions.

Also note that the black hole is to the future of the white hole (time flows upward in the diagram). So in this diagram, things come out of the white hole and fall into the black hole, not the other way around.

Note, though, that this spacetime is not physically reasonable (for reasons which are explained in the series of four Insights articles of which the one I linked to is the third). So the Einstein-Rosen bridge isn't either.

How can a white hole form without a black hole on the other end?

By not being described by the (not physically reasonable) spacetime described in the article I linked to above. I described a different model (which is still not physically reasonable, but not quite as unreasonable as the one in the article) in my previous posts.

Or how can a singularity form from nothing to create a white hole?

It can't; at least, it seems highly unreasonable. That's a key reason why the spacetime described in the article linked to above is not physically reasonable.

 

[kimbyd]

White Holes may not exist,

The problem is more fundamental than that. A white hole is a time reverse of a black hole. Viewing a white hole would be akin to a mixture of sound waves and heat combining to push a bunch of shattered shards of glass on the floor to form a drinking glass which then hops into your hand.

but if we discover that they are real and we find one, then:

Take a black hole, and view it with time moving backward. Everything that falls into the black hole you would instead see emitted from the white hole.

Edit: And to point out just how profoundly weird this is, we can add Hawking radiation to the mix. The white hole forms from a burst of high-temperature thermal radiation converging at a point. Over a huge amount of time, the thermal radiation drops in temperature as it streams into the white hole, increasing its mass. Eventually, after trillions upon trillions of years, the white hole starts spitting out light and gas, causing its mass to drop again.

So, yeah, white holes can't exist due to thermodynamics. A white hole means you got the direction of your time coordinate wrong.

 

[Ibix]

A white hole has a singularity in the past. I don't think that you can ask how it formed because that would require GR to describe something "before" the singularity and the whole point is that singularities are where the maths breaks down totally. We can't extend our models past this one - not until we have a working quantum gravity theory, anyway.

 

[Deepblu]

Thanks for the insights! It turned out this subject is much more weird that what it looks from the surface!

So a simple conclusion is that white holes are impossible.