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It took almost 100 years for the boldest of Einstein's General Relativity (GR) predictions, namely gravitational waves, to be experimentally observed. Now, that LIGO data is letting physicists probe where GR breaks down. It has long been thought that GR breaks down at extreme space-time curvatures, such as in the interior of black holes. The problem is that, by conventional wisdom, the interior of black holes are inaccessible because anything inside of the event horizon cannot escape out; however, other than being defined by the distance from the center of the black hole, there is nothing special about the event horizon and any unfortunate being who crosses through one would not notice anything being there.
In 2012 researchers realized that if quantum mechanics (QM) is correct, the black hole should be surrounded by a "firewall" of high energy particles. The paradox is that this isn't consistent with GR, but if there is no firewall, this is inconsistent with QM. When the LIGO data were released, Vítor Cardoso and colleagues from Lisbon argued that if a firewall does exist, then when two black holes merge, you should see echoes in the gravitational waves.
The Nature article notes:
The echoes arise because a firewall or any other kind of structure would effectively create a smeared-out region at the traditional event horizon. The inner edge of this region is the conventional event horizon, the boundary beyond which no light particles, or photons, can escape. The outer edge is more porous: a typical photon that crosses this boundary will be trapped by the black hole, but some will be able to escape, depending on their angle of approach. The effect would also partly trap gravitational waves released by the black-hole merger. They would bounce back and forth between the inner and outer edge with some escaping each time.
(Score: 0) by Anonymous Coward on Tuesday December 13 2016, @06:50AM
I thought normal matter would be shredded by tidal forces. Or, does it depend on the size of the black hole?
(Score: 2, Informative) by Anonymous Coward on Tuesday December 13 2016, @07:26AM
Absolutely depends on the size or mass actually of the black hole.
More and more we're finding that all blackholes are not created equally.
A stellar mass black hole would have you shredded to high energy particles long before you arrived at the event horizon.
But if our star were a 1 stellar mass blackhole it would be the size of the earth and the event horizon would be right at the mt everest, but the tidal forces would shred you at least as far out as the moon.
A super massive blackhole like those at the center of the galaxy? You'd never even notice.
Physically speaking if the mass in the center is a "gravastar", meaning there is in fact an object in there, it would have a radius of about 2-4x our sun,
The event horizon would reach out as far as neptune, but you'd be to venus before you even felt tidal effects.
Let that sink in for a moment.
That's all for static, non spinning or slowly spinning black holes.
But consider what happens when an ice skater brings in her arms.
She speeds up, well so does a black hole.
It's entirely possible that some of these masses are spinning at or very near the speed of light.
The faster they spin, the more centrifugal force is applied to the center of mass meaning they bulge at the center and flatten at the top.
It's entirely possible that our math is entirely messed up on this and that instead of a point singularity or even a gravastar that there is a ring singularity or even a 2 dimensional disc like the platters in your hard drive.
(Score: 3, Informative) by FatPhil on Tuesday December 13 2016, @10:48AM
"But if our star were a 1 stellar mass blackhole it would be the size of the earth and the event horizon would be right at the mt everest"
A 1 M_sun black hole has zero size, just like all other black holes as it's a point mass solution to GR, and would have an event horizon at a <3 km radius.
R_s = 2GM/c^2, which is all constants apart from M, so everything apart from M/M_sun can be combined, and you get R_s = 2.95 (M/M_sun) km
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 3, Informative) by Immerman on Tuesday December 13 2016, @05:26PM
You are correct about the size of the event horizon - a 1 solar mass black hole would have an event horizon radius of roughly 3km. Wish I had the time to run the numbers for their other examples.
As for size, we have no idea how physically big a black hole would be, because the laws of physics as we understand them cannot apply within the event horizon. As such, we typically take the event horizon itself to be the size of the black hole. It's true that the GR solution indicates a point mass, but that's a simplistic solution based on the assumption that there are no other forces present to resist it, which is a complete unknown in reality.
For that matter we're still not entirely certain that black holes actually exist, though they are a good fit for some observed phenomena. All we know with (near) certainty is that there are ultra-dense non-luminous objects in the galaxy, we're not yet certain they actually have event horizons. There are for example alternate formulations of GR under which black holes are impossible - if you treat gravitational field energy the same as all other field energies, such that the energy creates its own gravitational field in turn, then there is a maximum achievable curvature of space that is insufficient to create an event horizon. Einstein chose to ignore gravitational field energies in that context, feeling it was "double counting" the "original" mass-energy, and we've maintained that convention, but it's based on nothing more than a feeling, as we haven't actually been able to test it experimentally yet.
(Score: 2) by Immerman on Tuesday December 13 2016, @05:31PM
Your size for the event horizon of a one solar mass black hole is definitely way off.
Even if your more extreme example happens to be correct though, tidal effects are not the only factor. As you cross the event horizon though information cannot propagate outward - and that includes the information that there is a chemical bond holding molecules together, and quite likely even that the outermost proton in a nucleus is in fact part of a nucleus. If the tidal forces don't tear you apart before you hit the event horizon, you'd still be ripped apart at the subatomic level as the bonds holding you together become impossible to sustain.