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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: 3, Interesting) by tathra on Tuesday December 13 2016, @05:28PM
spacetime isn't like a sheet of paper, its not some solid surface thats immutable. it would be closer to water, it moves, it expands, it contracts, and it carries objects along with it as it does. if you toss something that floats on to the water, it doesnt move, the water itself moves the object around. its basically the same way with spacetime, the spacetime itself is moving and can drag objects around with it. iirc one of the ways einstein visualized gravity was as spacetime pushing objects around (so you're not being pulled down to the earth by gravity, but spacetime is pushing you down onto it at a rate of 9.8 m/s^2). so, for black hole mergers, think of it as two whirlpools moving around in a lake that are getting pushed towards each other until they eventually merge, or two leaves on the water that get pushed towards each other until they collide. the blackholes themselves aren't moving, spacetime is pushing them towards each other.