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: 2) by bob_super on Tuesday December 13 2016, @02:11AM
I missed in TFA where it explains this:
> They would bounce back and forth between the inner and outer edge with some escaping each time.
If the inner edge of the area is where things fall into the hole, how exactly do they "bounce"?
I'm not quantum nor very bright, but if I start running between a wall (with holes) and the bottomless (yet still alligator-filled) moat right next to it, I'm not going to bounce more than once...
(Score: 3, Disagree) by mhajicek on Tuesday December 13 2016, @04:46AM
Gravity waves, essentially ripples in space-time, are not confined by the event horizon as baryonic matter is.
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 0) by Anonymous Coward on Tuesday December 13 2016, @07:51AM
actually, I'm pretty sure they are.
or maybe you have a citation for that?
(Score: 5, Interesting) by stormwyrm on Tuesday December 13 2016, @07:55AM
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by mhajicek on Tuesday December 13 2016, @04:05PM
So gravity is confined by gravity. Weird.
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 3, Informative) by stormwyrm on Tuesday December 13 2016, @04:36PM
Gravitational waves also have to travel along light-like geodesics just like electromagnetic radiation:
http://physics.stackexchange.com/questions/132940/do-gravitational-waves-travel-on-geodesics-in-gr-if-yes-why [stackexchange.com]
Thus a gravitational wave passing through the event horizon of a black hole would also never find its way out again as all geodesics beyond the event horizon bend to the singularity.
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by tathra on Tuesday December 13 2016, @05:14PM
past the event horizon, spacetime is so warped that every direction only leads closer to the center. so, once a gravitational wave, rippling across spacetime, crosses the event horizon, there is no direction it can possibly go that would take it back outside of the event horizon, only closer to the center.
(Score: 2) by mhajicek on Wednesday December 14 2016, @08:31AM
Found a decent video about it:
https://www.youtube.com/watch?v=mht-1c4wc0Q [youtube.com]
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 0) by Anonymous Coward on Tuesday December 13 2016, @05:32AM
concrete example:
inner border distance from center: 1 km
outer border distance from center: 1 km + 1 mm
anything below 1 km cannot leave
some of the stuff inbetween the inner and outer can escape.
my understanding is that, for gravity waves, the inner border is sort of like a separation between 2 media, and when the wave encounters it, not all of the wave penetrates, some of it is reflected (like light on glass). this is how i interpret "bounce".
i'm neither a GR or QFT expert, so i can't go deeper into this (not without a couple of years of preparatory work). sorry.