Now this is interesting!Echoes from the Abyss: Evidence for Planck-scale structure at black hole horizonsIn the classical theory of general relativity (GR), the event horizon of a black hole is not a particularly interesting place from the point of view of someone who falls in. While an observer far away would see time for them slow down and stop at the horizon due to the gravitational time dilation (and their light would also be redshifted to invisibility), the person falling in does not notice these effects. They find that they fall through the horizon in a finite and quite short amount of time. Similarly, the distant observer may say that all the matter that ever fell into the black hole never actually crossed the horizon, but instead exists in thin shells, like sediment, which approach the horizon asymptotically. Yet the person who falls in never encounters those shells. They pass through the horizon and encounter only empty space there.
This seems like a paradox at first, and might be hard to swallow. But it's really not that weird. You can make sense of it from an analogy to a "sonic hole", where a fluid goes down a drain faster than the speed of sound. Ask yourself what two people would experience if one fell through the drain while talking to another far away. The non-paradox can also be explained with a space-time diagram. And ultimately, it arises from a concept in general relativity called the "equivalence principle", which basically says that no local experiment can determine the difference between freefall in a gravitational field versus freely floating in space far from any mass at all. It means that for the person falling in, the event horizon is not a weird place.
This is all in the realm of classical GR, and observations of black holes thus far (which we do have a lot of) are beautifully consistent with it. However, interesting problems arise when we try to combine general relativity with quantum mechanics. Both are absolutely fantastic theories in their own right (considered among the most successful of all theories in physics), yet it has proven remarkably difficult to unify them into a theory of quantum gravitation. The event horizons of black holes turn out to be one of the places where the contradictions between the two are strongest. It leads to
real paradoxes, like the information paradox, which are useful for probing the logic of quantum gravitational theory.
Some proposed resolutions to those paradoxes result in different conclusions. One is the "firewall", which says that the horizon
is a weird place which contradicts the equivalence principle. (Personally I was never a big fan of the firewall idea). But whatever the resolution happens to be, it seems that the behavior of real, quantum gravitational black holes might be in some way different than the classical black hole. And this difference might even be observable from a distance.
This paper studies the gravitational wave signals that we have from LIGO thus far, looking for signs of repeated echoes following a black hole merger, after the ring down (where the horizon of the merged black hole settles down into a relaxed shape). Certain characteristics of these echoes can be related to quantum gravitational effects from structure at the event horizon, contrary to the classical GR model of a black hole. The study found a 2.9-sigma confidence in such signals, which is a 1 in 270 chance of being a statistical fluke. This is not enough confidence to make any definitive conclusion, but it does demonstrate the power of the technique.
To be able to perform this kind of analysis so early in this new field of gravitational wave astronomy is very encouraging, and it means that with more detections of black hole mergers, we may expect to convincingly show whether or not we are seeing departures from classical GR. And if we do find them, that will be very exciting.