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So it's not explicitly ruled out by relativity. However, that link says: > The Universe at large appears to be electrically neutral, or close to it. Thus real

by sethrin 11y ago

So it's not explicitly ruled out by relativity. However, that link says: > The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign. > It is not clear how a gravitationally repulsive, negative-mass singularity could form. So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.

IIAOPSW 11y ago
I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing. I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.
- sethrin 11y ago
  Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.