Black holes sort of make sense. Too much mass and too much gravity, so nothing leaves it. But how and why (and where and when) can white holes exist? How can any body not let things enter it while it keeps throwing things out? wouldn't it just simply evaporate? And if black holes also continuously emit radiation, what is the difference between black and white holes? Please correct me if my thinking is going in the wrong direction due to ignorance (or stupidity).

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    $\begingroup$ This question is similar. $\endgroup$ – user122423 Aug 3 '17 at 2:48

They don't exist (assuming appropriate energy conditions). They're just formal solution to Einstein's equations that you get by analytically continuing the metric for an eternal black hole into the infinite past. Even if eternal black holes existed (which we have no reason to believe is the case), you could never observe the existence of the corresponding white hole, since it's in the infinite past.

  • $\begingroup$ +1 but you might explain to the OP regarding your understanding of the infinite past, I can't find the Wikipedia article I was expecting, there is a lot of philosophical musing though..... $\endgroup$ – user163104 Aug 3 '17 at 4:18
  • $\begingroup$ No. With negative energy conditions you can fall into a black hole and if you survive go through the wormhole and come out the white hole. It's not limited by the infinite past, but by the existence or not of exotic matter. It's not ruled in, but with unknowns yet on what happens in quantum gravity you can't say no for sure either. It is somewhat speculative, but no more than many other quantum gravity speculations. At least you could answer the scientific question, of how does GR cause a white hole, even if theoretically and un-found. $\endgroup$ – Bob Bee Aug 3 '17 at 4:36
  • $\begingroup$ Sorry for being so dumb, but can you please explain why we get the solutions to be a white hole (why not an alternate universe...), and why does a black hole have to be eternal? (it could be formed by a star collapse a couple of million years ago) $\endgroup$ – Mustafa Aug 3 '17 at 15:55
  • $\begingroup$ @BobBee Yes, that's why I said "assuming appropriate energy conditions." If you don't impose any energy conditions, then (loosely speaking) practically anything can happen in GR. $\endgroup$ – tparker Aug 3 '17 at 20:50
  • $\begingroup$ @Mustafa The white hole solution only appears if we analytically continue the Schwarzchild metric beyond the original coordinate ranges, and no one really knows if this procedure produces anything physically significant, or if they're just extraneous mathematical solutions. So anything we say about white holes is largely speculative. $\endgroup$ – tparker Aug 3 '17 at 20:55

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