What really keeps me awake at night (besides being sued by an unprincipled water developer in Utah—another story) is that we face a crisis within the deepest foundations of physics. The only way out seems to involve profound revision of fundamental physical principles.
But I'll begin the story with something I believe we shouldn't worry about—"firewalls," a radically deviant picture of black holes recently advocated by colleagues, in which an observer falling into a black hole burns up right at the horizon. The firewall picture—really a clever renaming of a previously known phenomenon—has provoked some recent controversy, which has begun reaching the press. If correct, it could well mean that the black hole at the center of our galaxy is not a black hole at all: instead it is the firewall at the center of the galaxy, and instantaneously destroys anyone or anything who ventures within about 12 million kilometers of its center.
I'm not worried this scenario is correct because it is too preposterous and removed from known physics, and I believe there are less preposterous alternatives. But, the fact that otherwise serious physicists have forwarded it is definitely symptomatic of the profound crisis that we face.
In short, the crisis is a deep conflict between fundamental physical principles, that form the foundation of our most basic framework for describing physics. These pillars are quantum mechanics, the principles of relativity (special or general), and locality. These pillars underly local quantum field theory, which serves as the basis for our description of physical reality—from the shining sun to creation of matter in the early Universe to the Higgs boson (if that's what it is).
These principles clash when pushed to the extreme—the sharpest version of the problem arises when we collide two particles at sufficient energy to form a black hole. Here, we encounter the famed black hole information problem: if the incoming particles start in a pure quantum state, Hawking's calculation predicts that the black hole evaporates into a mixed, thermal-like final state, with a massive loss of quantum information. This would violate—and thus doom—quantum mechanics.
While serious people still consider modifying quantum mechanics, so far proposals to do so create much bigger problems. For example, it has been argued that Hawking's original proposed modification of quantum mechanics would imply a "firewall" exists for all observers, everywhere! Quantum mechanics appears to be remarkably resistant to sensible modification.
If quantum mechanics is sacred, apparently other principles must go: either those of relativistic invariance, or of locality, or both. The former likewise appears resistant to sensible modifications, but locality is a remarkably "soft" principle, in the context of a theory with quantum mechanics and gravity. So, that seems a reasonable concept to suspect.
The basic statement of locality is that quantum information cannot propagate faster than the speed of light. At least as early as 1992, modification of locality to solve the problem of getting missing quantum information out of a black hole was proposed. Then, in the following years, a picture involving an even more radical form of nonlocality took shape. This picture was based on the holographic principle, and a new notion of complementarity, proposing that observations inside and outside a black hole are complementary in analogy to Bohr's complementarity of position and momentum measurements in quantum mechanics. Holography/complementarity were widely explored, and became accepted by a significant segment of the physics community.
What has become clear, in the past few years, is that a picture based on complementarity is not only apparently unnecessarily radical, but also likely inconsistent. While the jury of the physics community may still not have a final verdict on complementarity, new awareness of its apparent downfall has rekindled broader interest in the deep crisis that we face.
In a context where one or more supposed bedrock principles must be discarded, we obviously need to be a little crazy—but not too crazy! Complementarity is an extreme, and plausibly inconsistent, form of nonlocality. On the other hand, if quantum mechanics is to be saved, by allowing nonlocality inside a black hole, but you accept that quantum field theory holds everywhere outside a black hole, you encounter the firewall problem. Is there a less-crazy alternative? I believe so, in the form of a more "nonviolent" nonlocality that extends outside what we would describe as the black hole horizon, and transfers quantum information out. I say "would describe," because the horizon concept makes reference to a classical spacetime picture, which may well not be fundamentally correct. More could be said, but describing such nonlocality would violate editorial marching orders of this forum.
Suffice it to say: while it appears that one of three basic pillars of physics must be modified, and that locality is the prime suspect, modification of locality is no small matter. Naive modifications of locality—as often proposed by physicists "on the fringe," generically lead to disastrous collapse of the entire framework of quantum field theory, which not only has been experimentally tested to a very high degree of accuracy, but underlies our entire physical picture of the world. If such modification must be made, it must be subtle indeed. It also appears that the basic picture of reality as underlain by the fabric of space and time may well be doomed. What could replace it is a framework where the mathematical structure of quantum mechanics comes to the fore. I would say more … but marching orders.
I will say I am deeply concerned about how we will arrive at a complete and consistent theory of gravity, and that we must, in order to describe not only black holes—which have been found to be ubiquitous in the universe—but also both the early inflationary and pre-inflationary evolution of our universe, as well as our seemingly dark-energy dominated future. The current problems at the foundations link to multiple big questions—and I fear it will be no small feat to resolve them.