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> Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center? There can't be anything like
by pdonis 2y ago
> Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon.
The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
- binary132 2y agoPardon my ignorance, but why not? This is what I’ve long assumed is the true nature of black holes — not some kind of divide-by-zero error, but merely a very very heavy object.
- pdonis 2y ago> why not? Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible. Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
- binary132 2y agoMaybe I don’t know how compact these objects are, but I suppose I am assuming that the event horizon is always large enough to conceal a sufficiently massive object of extreme or arbitrary density, greater than than that of a neutron star, such as a “quark star” or similar. Is that erroneous? If so, then a followup: why can’t we imagine an object of arbitrary density sufficient that the massive object is also small enough? Why does it need to be a mathematical discontinuity or otherwise goofy universal exception-to-the-rule? We obviously can’t observe it, so isn’t it simply that the mathematics expresses the fact that we cannot observe it?
- pdonis 2y ago> I suppose I am assuming that the event horizon is always large enough to conceal a sufficiently massive object of extreme or arbitrary density No, it isn't. It is impossible for a stable compact object made of ordinary matter that is smaller than the event horizon size for its mass to exist. In fact it is impossible for a stable compact object made of ordinary matter that is smaller than 9/8 of its event horizon size to exist. This result is known as Buchdahl's Theorem.