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I have recently watched Kurzgesagt in a Nutshell's video on Wormholes https://www.youtube.com/watch?v=9P6rdqiybaw and I have been thinking about white holes, and exotic matter.

As I understand, white holes are an exact opposite of black holes, Black Holes are condensed of positive mass, where it is so heavy that it attracts everything into it. White holes are opposite, they repel everything that goes around it and give out 'stuff'.

Hence, I am thinking if white holes have this property, would it mean it has negative mass, which is essentially exotic matter?

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  • $\begingroup$ the question is good, but probably the available answers will fall short $\endgroup$
    – lurscher
    Commented Aug 14, 2018 at 13:56
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    $\begingroup$ The classical solution for a black hole is given by schwarschild metric with parameter M as the mass of the central object. A negative value of this parameter results in a naked singularity. On the other hand Schwarschild solution with positive provides a white hole part that, as you say, acts like an opposite black hole (furthermore, is a time reverse image of the black hole) but still with positive mass. Ultimately, 'exotic matter' is an umbrella term for matter that does not follows the conventional assumptions for matter (and one example is negative mass), so the term is rather ambiguous. $\endgroup$ Commented Aug 14, 2018 at 14:00
  • $\begingroup$ Would the no-hair theorem not also apply to white holes so that "what's the inside made of" has no more validity than the same question about black holes ? $\endgroup$ Commented Aug 14, 2018 at 14:05
  • $\begingroup$ @lurscher Yes... I understand. I have researched for this before too. I know that white holes are not quite possible already because if there is a white hole then we would have definitely experienced it $\endgroup$
    – rcw
    Commented Aug 14, 2018 at 14:10
  • $\begingroup$ what I mean is the only in the last decade we have begun to poke holes in the hidden relationships between thermodynamics and gravitation, certainly the time-reversal property of black holes has some relationship with that, but is not known the nature of that relationship $\endgroup$
    – lurscher
    Commented Aug 14, 2018 at 14:15

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A Schwarzschild white hole is a time reversed black hole. On the inside it has a singularity that explodes with energy and matter at the moment when the internal time starts. After this initial moment the singularity no longer exists on the internal time scale. The internal time of the while hole is disconnected from the external time by the event horizon. Therefore, the question, "When did the singularity explode?" has no meaning for an external observer.

Matter and energy created in the explosion crosses the event horizon from inside out and flies away at different moments of outside time. With matter and radiation leaving, the mass of the white hole becomes smaller, its event horizon shrinks until eventually disappears, and the white hole ceases to exist.

A white hole does not repulse matter. Gravity is always attractive. On the inside, the direction of time is radial from the singularity to the event horizon, so matter simply is moving in time. Passed the horizon the radial coordinate becomes spatial on the outside. Stuff is brown out through the horizon at the speed of light (which is relative) and flies away on inertia.

Aside from the initial singularity, which is expected to be described by the future theory of quantum gravity, there is no exotic matter inside or outside the white hole. It's event horizon cannot be crossed from outside to inside. So researches on the inside cannot receive any information from outside and have no way of knowing in what universe they will end up when they are thrown out through the horizon. It is believed that white holes do not exist, because they would violate energy conservation by producing matter from nothing.

Expert comments are welcome and I would edit the answer accordingly, if any part if it is incorrect.

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