White holes in a vaccuum?
Stephen Hawking proposed the existence of white holes in 1976 in a thought experiment in which a large closed system is in thermal equilibrium and gas particles surround a black hole the temperatures of which equal to the temperature of the black hole. The black hole, in such a system, will emit “Hawking radiation” but will absorb as much energy from the gas particles surrounding it as it emits. If you play this system backwards (reverse the time arrow), then you have a “white hole” in equilibrium. And under these circumstances a white and black hole are identical.
A white hole is a black hole with a reversed time arrow. MIT’s Technology Review describes a paper by Stephen D.H. Hsu of the Institute of Theoretical Science, University of Oregon entitled “White holes and eternal black holes” which investigates whether white holes can exist in nature absent the special circumstances posited by Hawking.
Hsu says that a white hole will have all the quantum numbers of the time reversed black hole: mass, angular momentum, and charge. If a black hole must resolve into a singularity in the future, then a white hole must begin from a singularity. (A black hole emitting Hawking radiation which does not accrete mass from its surroundings loses mass and will eventually evaporate into a singularity in an explosion of gamma rays.) Can a situation be posited, therefore, where a black hole will not resolve into a singularity — where it will be an “eternal black hole”? (This is a necessary construct because an ordinary black hole which emits Hawking radiation cannot be time-reversed in a vaccuum, because there would be no Hawking radiation, by definition, in the vaccuum for the white hole to absorb). Hsu describes initial conditions where it is possible to have an “eternal” black hole in a vaccuum. But the condition can’t be time reversed (the intial conditions can’t be in the white hole’s future).
As an aside, Hsu notes that he has not determined whether an “eternal black hole” (one which does not leak Hawking radiation and therefore eventually lose mass) will be able to withstand a “perterbation.” In other words, if an “eternal black hole” interacts with something, will it simply leak the proportional amount of Hawking radiation or will it revert to a “normal” black hole and begin endless evaporation.
The paper seems to explain why no one has observed a white hole in nature.