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Astronomy

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    $\begingroup$ So the kinetic energy could end up adding more than double their combined mass, rather than radiate anything out? Does this mean that in theory we'd never be able to detect if this happened outside of maybe seeing the effect of a larger black hole to its surroundings? $\endgroup$
    – KGlasier
    Commented Jun 27, 2019 at 15:08
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    $\begingroup$ @KGlasier I think that's right. It is worth noting that this scenario is unrealistic on at least three levels: more or less all black holes are spinning; it is unlikely they would collide exactly head on, rather than a bit offset; and supermassive black holes are not normally seen moving at 0.9c relative to anything much. $\endgroup$
    – Steve Linton
    Commented Jun 27, 2019 at 15:36
  • $\begingroup$ I'm not totally certain that there won't be any gravitational waves, but I don't have the skills to do the necessary calculation. True, it's a perfectly straight-line collision, but in the final moments their relative speed will be large, the BHs are traveling in curved spacetime, and they change that curvature as they approach one another. And the interaction is not spherically symmetric. $\endgroup$
    – PM 2Ring
    Commented Jun 28, 2019 at 6:01
  • $\begingroup$ @PM2Ring I'm not certain either, but I think cylindrical symmetry is enough to suppress gravitational radiation. $\endgroup$
    – Steve Linton
    Commented Jun 28, 2019 at 8:00
  • $\begingroup$ Ok. The linked dupe target says that the gravitational wave production would be small, and a fairly simple pulse, although in that question the issue of spin (or lack thereof) is not addressed. $\endgroup$
    – PM 2Ring
    Commented Jun 28, 2019 at 8:30