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> LIGO has been able to confirm theoretical properties of the event horizon. What specific properties has LIGO confirmed about the event horizon? It is not qui

> LIGO has been able to confirm theoretical properties of the event horizon. What specific properties has LIGO confirmed about the event horizon? It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes. > The EHT has imaged a black hole, which effectively images the event horizon. The EHT image is mostly an accretion disk. I think accretion disks can also form around things that are not black holes. If something were of the sufficient density, I believe it could make the same image without having an event horizon. I am no astrophysicist though so maybe that image would force the density to be sufficiently high to make a black hole.

pdonis 4y ago
> It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes. No "normal stellar object" can be more compact than 9/8 of the Schwarzschild radius for its mass. (This is a result known as Buchdahl's Theorem, first discovered in the 1930s.) Gravitational wave emission when objects of that size or larger merge is much less energetic compared to the total mass involved than the merger of black holes, which merge at the Schwarzschild radius; also the predicted waveforms are different, because there is actual matter (nonzero stress-energy) present when normal stellar objects merge, whereas black holes are vacuum (zero stress-energy).
bmitc 4y ago
> What specific properties has LIGO confirmed about the event horizon? Hawking's area theorem is one: https://news.cornell.edu/stories/2021/07/hawkings-black-hole-theorem-observationally-confirmed https://news.cornell.edu/stories/2021/07/hawkings-black-hole... > It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes. I'm not sure one can be so loose here. LIGO uses detailed models of black hole and neutron star mergers to match the signals against. What are "normal stellar objects" that would yield the same magnitude of gravitational waves emitted during a merge as black holes and neutron stars? > The EHT image is mostly an accretion disk. I think accretion disks can also form around things that are not black holes. There's also a huge hole in the middle of the accretion disk. What other object, other than a black hole, has such a large accretion disk but yet no emitted radiation?

pdonis 4y ago

> It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes. No "normal stellar object" can be more compact than 9/8 of the Schwarzschild radius for its mass. (This is a result known as Buchdahl's Theorem, first discovered in the 1930s.) Gravitational wave emission when objects of that size or larger merge is much less energetic compared to the total mass involved than the merger of black holes, which merge at the Schwarzschild radius; also the predicted waveforms are different, because there is actual matter (nonzero stress-energy) present when normal stellar objects merge, whereas black holes are vacuum (zero stress-energy).

bmitc 4y ago

> What specific properties has LIGO confirmed about the event horizon? Hawking's area theorem is one: https://news.cornell.edu/stories/2021/07/hawkings-black-hole-theorem-observationally-confirmed https://news.cornell.edu/stories/2021/07/hawkings-black-hole... > It is not quite clear to me how the gravitational waves of two merging bodies would be different between a merger of normal stellar objects and black holes. I'm not sure one can be so loose here. LIGO uses detailed models of black hole and neutron star mergers to match the signals against. What are "normal stellar objects" that would yield the same magnitude of gravitational waves emitted during a merge as black holes and neutron stars? > The EHT image is mostly an accretion disk. I think accretion disks can also form around things that are not black holes. There's also a huge hole in the middle of the accretion disk. What other object, other than a black hole, has such a large accretion disk but yet no emitted radiation?