The Event Horizon Telescope’s spectacular success in taking pictures of the black hole in the galaxy M87 becomes even more exciting when one realizes its observations may possibly provide clues about one of the deepest puzzles in physics.
This is the “paradox” of black hole information, which points to a basic inconsistency between the existence of black holes and the quantum-mechanical laws believed to describe our universe. Resolving this inconsistency may require a conceptual revolution as profound as the overthrow of classical physics by quantum mechanics.
Black holes are apparently ubiquitous in the cosmos, and yet remain the most mysterious objects it contains; their very existence threatens our present foundations of physics. Observations like EHT’s, and the recent gravitational wave detections of LIGO, are providing increasingly good evidence for their existence. But, while the basic principles of quantum mechanics are thought to govern all the other laws of nature, when they are applied to black holes they lead to a contradiction, exposing a basic flaw the current form of these laws. So, the most profound role of black holes may be in teaching us something new and deep about the fundamental laws of nature.
This problem arises from one of the simplest questions asked when one first encounters black holes: what happens to stuff that fall into them? A little refinement is needed. First, in our present quantum-mechanical laws, matter can change into different forms; particles can for example change into different kinds of particles. But the one thing that is sacred and never destroyed is quantum information. If we know the complete quantum description of a system, we should always be able to exactly determine its earlier (or later) quantum description with no loss of information. So a more careful question is what happens to quantum information that falls into a black hole.
And what may be Stephen Hawking’s greatest discovery is his prediction that black holes destroy quantum information. This arises from his calculations showing that black holes evaporate; they emit particles and shrink until they are expected to completely disappear. The problem is that the emitted particles carry essentially no information about what went into the black hole. So, his calculations appear to show that quantum information that falls into a black hole is ultimately destroyed, contradicting quantum mechanics.
This creates a deep crisis in physics. Great advances have followed from previous such crises. One that seems very parallel to the black hole crisis was that of atomic stability, which led to the quantum revolution in physics. In that case, classical physics predicted the fundamental instability of atoms, in obvious contradiction with the existence of stable matter. But, by the time the stability of matter was understood, the laws of physics had been rewritten in a fundamentally different way. Increasingly it seems that the black hole crisis is similar in that it will lead to another paradigm shift in physics.
At first Hawking suggested that it was quantum mechanics that fails, and information destruction is allowed. However, it was soon realized that this implies a drastic breakdown of energy conservation, which would utterly destroy our present description of the universe. Apparently, the resolution needs to be sought elsewhere.
Another idea was that black holes don’t completely evaporate but instead stop shrinking at a tiny size, leaving behind microscopic remnants storing the original information. But, it was then realized, basic quantum principles would predict disastrous instabilities in which ordinary matter explodes into such remnants, also contradicting everyday experience.
Other ideas continue to be explored. One idea is that some novel physics prevents black holes from forming. This would be very strange, since very large black holes can form when the surface density of collapsing matter reaches that of, say, ordinary water, and we believe we understand physics at such densities. Another idea is that new physical processes cause black holes to transform into massive remnants of a new kind, containing the original information, long before they reach microscopic size. Versions of both these scenarios have been explored both within and outside string theory, which some believe to be the correct approach to reconciling quantum mechanics with gravity. For example, a proposal in string theory is that a black hole transforms into a massive remnant called a “fuzzball,” or that a fuzzball forms instead of a black hole forming. But, these proposals raise other important problems.
Specifically, in Einstein’s description, in which gravity corresponds to curvature of spacetime, information must never propagate superluminally—faster than the speed of light. The very definition of a black hole says that once one forms, the stuff inside its event horizon can’t escape because that would require faster-than-light travel. So, if some new process were to transmit internal information to the exterior, say as part of a process converting a black hole into a massive remnant, that information would have to travel superluminally. A similar need for superluminal signaling would apparently be needed in scenarios where a black hole is prevented from forming in the first place.
Superluminal signaling can lead to serious trouble. The prohibition on sending a superluminal signal is ordinarily referred to as locality. In empty flat space, violation of locality appears to create another paradox. Specifically, if you can send a signal faster than light, the laws of relativity say that other observers flying past you at a high speed will see this signal going backward in time. This leads to a paradox since it opens the door to our signaling into the past, for example asking someone to kill our grandmother before our mother is born.
But, despite this kind of answer appearing to contradict fundamental physical principles, it is worth a closer look. The severe nature of the black hole crisis strongly suggests a resolution via some subtle violation of this locality principle, one that doesn’t produce such paradoxes. Quantum mechanics implies information is never destroyed. So, information that falls into a black hole must ultimately escape the black hole, plausibly through some new and subtle delocalization related to the basic principles of quantum gravity.
If information does escape black holes, we can ask if it has to be as obvious and abrupt as the formation of a massive remnant or fuzzball. In fact, the growing evidence for black holes suggests that there are objects in the universe that look and act a lot like classical black holes, without large departures from Einstein’s description. So, we might investigate whether there can be some more innocuous new effects that delocalize information and allow it to “leak” from black holes, without a drastic failure of the usual spacetime picture.
My recent work has found two variants of such effects. In one, the geometry near a black hole appears to bend and ripple in a way that depends on the information in the black hole—but does so gently, so that it does not for example destroy an observer crossing through the region where the horizon would ordinarily be found. In this “strong, nonviolent” scenario, such “shimmering” of spacetime can transfer the information out. Interestingly, however, it has also been found that there is a more subtle, intrinsically quantum, scenario that allows information to escape the black hole. In this “weak, nonviolent” scenario, even tiny quantum fluctuations of the spacetime geometry near the black hole can transfer information to particles emanating from the black hole. The fact that the information transfer is still large enough is related to the huge amount of possible information that a black hole can contain.
These scenarios for information transfer, despite appearing superluminal with respect to the spacetime picture of a black hole, do not necessarily produce a paradox. That is because the signaling is connected to the existence of the black hole, the spacetime of which is different from flat space, in such a way that the earlier argument about signaling into the past no longer holds. This story is also distinct from another recent proposal by Hawking, together with Malcolm Perry and Andy Strominger, that information is preserved outside a black hole through its ordinary gravitational field.
Such rippling of a black hole “quantum halo” can also distort light passing near a black hole, and this is where the Event Horizon Telescope comes in. If the strong, nonviolent scenario is correct, the shimmering could cause distortions of EHTs images that change with time. These changes could happen over a time of about an hour for the black hole in the center of our galaxy, and since EHT averages over multi-hour observations, may be hard to see, once EHT is able to image this black hole.
But the relevant fluctuation time for M87, which is over a thousand times larger, is more like tens of days. This suggests looking for these distortions, using longer-duration observations at EHT than their initial seven-day span. If such distortions are found, that would provide a spectacular clue to the quantum physics of black holes, and if they are not, that begins to point to the more subtle quantum scenario, or to something more exotic.
If either scenario is correct, that is a profound clue to more basic laws of nature. The essential message is plausibly that when one takes gravity into account, information is subtly delocalized in a way that departs from its localization in the current laws of physics. In particular, this appears likely to be tied to the statement that spacetime itself is not a fundamental notion in physics, but instead may arise as an approximation to a more basic mathematical structure in quantum mechanics.
Whatever the resolution, black holes contain crucial clues to the basic quantum physics of gravity. So, like with the atom and quantum mechanics, understanding black holes plausibly will help guide the next conceptual revolution in fundamental physics, and EHT observations may begin to provide us with key information about their behavior.
This material is based in part upon work supported in part by the U.S. Department of Energy, Office of Science, under Award Number DE-SC0011702.