If the escape velocity of two very massive objects is near the speed of light, and those objects are orbiting each other (let's ignore the Roche limit for this exercise), is it possible that the combined mass of these two objects is great enough that their center of mass, while in the empty space between, is effectively a black hole? Is it possible that this is already the case with black holes we've observed, but we're unable to tell, due to the information capturing nature of black holes?


The reason I asked this question is that the the recent discovery of a very massive black hole made me wonder whether it was possible that this black hole was in fact a collection of nearly coalesced stars (perhaps from many galaxies), and its combined mass was so great that, while locally there was information due to its accumulation of space time, it appeared to our quiet part of space as one large black hole. I didn't initially mention this because I didn't want to distract from the fundamental idea on which my question was predicated.

  • $\begingroup$ How would you define a black hole? $\endgroup$ – Danu Feb 27 '15 at 14:54
  • $\begingroup$ Mass so great that light cannot escape. $\endgroup$ – orokusaki Feb 27 '15 at 14:54
  • $\begingroup$ If you define it like that it is trivial that, at a point where there is no mass, there cannot be a black hole. You have to be more careful in the definition. My point is that this may well just be a matter of definition rather than something very interesting physically $\endgroup$ – Danu Feb 27 '15 at 14:55
  • $\begingroup$ Isn't a center of mass meaningful? After all, isn't that what a black hole is comprised of, non-contiguous particles with a center of mass? $\endgroup$ – orokusaki Feb 27 '15 at 14:57
  • $\begingroup$ The center of mass between two objects is just a concept, it has no physical properties... $\endgroup$ – Segmented Feb 27 '15 at 14:58

No, that's not possible. Even if the two bodies could be compressed to be just larger than their Schwarzschild radius (they can't really, without collapsing further to black holes), their combined Schwarzschild radius, which grows linearly with mass, is twice their individual Schwarzschild radii. That means that even if they effectively rolled on each other surfaces, they would still, by construction, be just larger than the combined Schwarzschild radius. If you wanted to compress them the go below, then you would need to compress the individual bodies to go below their individual Schwarzschild radii.

Also, anything that forms a black hole will collapse to a point, keeping no memory of its former constituents (like the number of objects used to create it). Only mass, charge, and angular momentum is remembered.

  • $\begingroup$ This answer then concludes that no object can be dense enough to be a black hole unless all of its constituent parts are "touching", which would then conclude that anything inside of a Schwarzschild limit has to be bonded (quite perfectly bonded). Is this true? $\endgroup$ – orokusaki Feb 27 '15 at 15:09
  • $\begingroup$ Okay, I'm not a general relativist, but I can't think of any setup where that could happen. BHs are formed from collapsing stars, whose constituents "touch". But as I added in my answer, even if it somehow were possible, the non-touching constituents would collapse to a point. If they carry angular momentum, which in general they do, it will actually not be a point, but a ring-shaped singularity, but still well within the Schwarzschild radius. $\endgroup$ – pela Feb 27 '15 at 15:18
  • $\begingroup$ Is there some special principle that causes them to just suddenly "collapse to a point" after not having done so prior to containing the exact amount of mass necessary, or is it just reaching the proper point on the continuum, and it's simply paradoxical to think of objects as being able to exist in this situation without having already collapsed (presumably incrementally, due to the Roche limit)? $\endgroup$ – orokusaki Feb 27 '15 at 15:26
  • $\begingroup$ I guess the thing I'm having a hard time with is accepting that, while mass is energy, that it's somehow not possible to have some combination of mass and kinetic energy (in the form of angular momentum) which equals the same mass required to form a black hole. It feels like magic to think of a scenario where these principles don't apply. $\endgroup$ – orokusaki Feb 27 '15 at 15:31
  • $\begingroup$ Yes, you could say that there is a special principle. It's when pressure "gives in" to gravity. Gravity will always try to pull a star into a black hole. For a normal star, this pull is counteracted by radiation pressure from the energy production on the core. When the star has exhausted its fuel, it collapses to a compact object, either a white dwarf (supported by electron degeneracy pressure), or a neutron star (supported by neutron degeneracy pressure), or a black hole, depending on the initial mass. Adding mass continuously to a neutron star would cause it to suddenly collapse at 2-3 Msun. $\endgroup$ – pela Feb 27 '15 at 15:36

protected by ACuriousMind Nov 11 '17 at 10:47

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