When we think of black holes and big celestial objects we think of objects being pulled towards them with a strong force. And that force turns out to be the gravitational force but then for the theory of whiteholes what force must there be that is responsible for the opposite of the scenario given above. Negative gravity? Negative mass?
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$\begingroup$ The answers given in astronomy.stackexchange.com/questions/7828/… may help here? $\endgroup$– Darth PseudonymCommented Sep 14, 2023 at 13:40
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$\begingroup$ Thank you @DarthPseudonym but the post does not exactly mention the force behind it and talks about it being a very hypothetical concept. I would like to know what exactly caused the thought behind it. Thanks for viewing my post! $\endgroup$– Shubhankar DixitCommented Sep 14, 2023 at 13:48
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4$\begingroup$ Gravity is not a force in General Relativity. $\endgroup$– ProfRobCommented Sep 14, 2023 at 15:21
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6$\begingroup$ Note that white holes are attractive, not repulsive. $\endgroup$– StenCommented Sep 14, 2023 at 15:31
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$\begingroup$ @ProfRob thank you sir I tend to use the words synonymously $\endgroup$– Shubhankar DixitCommented Sep 15, 2023 at 15:06
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3 Answers
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White holes are time-reversed black holes. For a black hole, a particle may fall in with a trajectory that terminates at the singularity. The time-reversed version of that picture is that a particle "is created at the singularity with sufficient speed that it can escape" (roughly).
What causes the creation of such a particle with such "speed"? Established physical theories can't explain this any more than they can explain what happens when a particle hits the singularity of a black hole. So in this respect, the answer is "we don't know".
But I also want to emphasize that white holes are gravitationally attractive. Time-reversed gravity still acts in the same direction (imagine the time reversal of jumping into the air and landing). An outflowing particle is slowed as it escapes the white hole's gravitational pull.
(Note that I'm speaking of "white hole" solely as a solution to the equations of general relativity. There is no reason to think that white holes can or do exist in the universe.)
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$\begingroup$ Thank you. It makes sense now! $\endgroup$ Commented Sep 15, 2023 at 15:08
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3$\begingroup$ "There is no reason to think that white holes can or do exist in the universe" This is an important observation. $\endgroup$– James KCommented Sep 15, 2023 at 19:52
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I think it's worth keeping in mind that a lot of the exotic theoretical objects related to black holes are things that show up purely as solutions to the Einstein field equations. The field equations specify how the curvature of spacetime has to correspond to the mass and energy that is present, and a solution generally takes the form of a formula specifying the metric, which tells how distances and angles are measured at any point, and from which the curvature can be calculated. Not every solution applies to something in the real world.
The black hole solution was, I believe, the first nontrivial solution to be found, and at first people doubted that actual black holes could exist. But the solution was still useful because it is still correct as long as you are outside a spherical body like the earth or the sun. Over time, theoretical evidence piled up that black holes really could exist, and observational evidence piled up that they really do exist.
White holes are different. Generally in physics, the basic laws are still true if you reverse the direction of time, and white holes are what happen if you do that. It's another valid solution to the equations. But it's not the sort of thing you would expect to exist, and we don't have any concrete evidence that one would exist. This doesn't mean very smart physicists haven't come up with ways they could be part of the universe, but the best guess is that they aren't.
I think wormholes are similar. There are mathematical solutions that have them, but there are not obvious natural process that could lead to them.
There are diagrams that can help you to visualize these solutions. One kind is called a Kruskal-Szekeres diagram, and another is a Penrose diagram. But they are not always so easy for non-experts to interpret, as the comments will indicate. You also might find this question helpful, if you haven't already seen it.
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3$\begingroup$ "objects would be observed to be outside the event horizon, and moving outward, as far back in time as you could see" -- this isn't right. You can't just time-reverse what an observer sees; you need to time-reverse the light itself. And time-reversed light goes from observer to subject. The correct time-reversed description is that an object emerging from the white hole's antihorizon receives light from the outside universe from arbitrarily far in the past, blueshifted to an arbitrarily great extent. $\endgroup$– StenCommented Sep 14, 2023 at 22:49
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2$\begingroup$ Thanks, I see your point. Does it seem OK as corrected? Looking at a K-S diagram, it does look like anything coming out would have done so infinitely long ago in Schwarzschild coordinates. But I know it's easy to misinterpret things. $\endgroup$ Commented Sep 15, 2023 at 2:57
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2$\begingroup$ Hmm, unless I'm misunderstanding, I think that is still not quite correct. If objects emerging from the white hole could be seen arbitrary far in the past, the time-reverse of that statement is that objects falling into a black hole see the infinite future of the outside universe. That is not the case (although it is a somewhat common misconception). $\endgroup$– StenCommented Sep 15, 2023 at 14:32
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1$\begingroup$ Instead of coordinates, I think it's clearest to look at the conformal diagram. Consider a worldline originating at some arbitrary point on the singularity and crossing the event horizon. Can light (upward diagonal lines) originating from that worldline reach past infinity (bottom of the diamond)? Clearly no. $\endgroup$– StenCommented Sep 16, 2023 at 17:25
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Gravity itself. White holes are a creation of gravity, just in a repulsive way. Gravity is defined as the curvature of spacetime, which can be negative too as the spacetime can be negatively curved in a saddle like shape, it produces repulsive force just like a mountain compared, similar to actions of exotic matter.
Black holes are defined via the Einstien field equations. However white holes are just the time reversal of black holes, so technically Einstien's field equations apply here, except that it is the time reversal. According to the CPT symmetry, especially the T symmetry, even if we reverse time itself, the law of physics would remain the same. So black hole or white hole, the Einstien field equation apply's to both. So the Riemann's metric tensor has only the values in negative, due to the reversal of time, everything else is the same.
In conclusion it can be defined as time reversed gravity.
Thank you, Hope it helps you!
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$\begingroup$ To be fair "time reversal of gravity" is a very strange way to define an object. Like, what force caused helium to fly down from the sky and form into a ball while a latex sphere forms around it? It's just "time reversal of popping a balloon", right? $\endgroup$ Commented Sep 14, 2023 at 15:07
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1$\begingroup$ "time reversed black hole" - accurate. "Repulsive, like a cosmological constant" - inaccurate. $\endgroup$– StenCommented Sep 14, 2023 at 15:33
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3$\begingroup$ White holes do not repel, they still have attractive gravity $\endgroup$– SegganCommented Sep 14, 2023 at 20:49