String Theory, Black Holes, and the Subatomic Structure of Spacetime

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Einstein's theory of general relativity describes the physics of very heavy things. Invented a bit over a century ago, it has successfully explained a wide variety of experimental data. One of its key features is the smoothness of the fabric of spacetime. Quantum theory describes the physics of very small things. Invented a bit under a century ago, it has also successfully explained a wide variety of experimental data. One of its key features is the discreteness of the space of quantum states. At a deep level, the key features of these two theories are in conflict. For most systems we can imagine, like galaxies or Higgs bosons, this is not a worry, because our objects of interest are either very heavy or very small. But if we want to understand the heart of a black hole or the birth of the cosmos, we need a better theory that can handle both smooth spacetime and quantum jumps. Even if we may never be able to physically touch black holes or big bangs, we can still dissect and learn valuable things about them by recruiting theoretical tools and thought experiments. My main interest is in understanding the ultimate fate of quantum information falling into black holes and knowing more about the subatomic structure of spacetime. To investigate this, I use string theory, which is currently the best developed quantum gravity framework available. Two interrelated advances in recent years have made my goals more accessible. The first advance was the discovery of higher-dimensional gravitating objects in string theory known as (mem)branes, which co-exist alongside the one-dimensional fundamental strings. Branes are versatile, and can be used to build interesting black holes. They also enabled the first explanation from first principles of the entropy of a black hole, a quantity akin to disorganization which codifies the tremendously large number of ways a black hole could be formed in gravitational collapse. Much progress has been made since then, but more territory remains to be understood. Focusing on more fine-grained gravitational information is important. Contributing to microscopic string theory modelling of black holes is one of the twin themes of my research program. The second advance, known as holography, was a surprising and powerful discovery that certain types of quantum gravity systems can be alternatively described in terms of very different-looking non-gravitational systems living in one less dimension of space. This is reminiscent of a hologram where you get the whole 3D image from only the 2D hologram. Holography was originally found using string theory, but was later realized to be a more general idea. It interconnects aspects of string theory, particle theory, gravity theory, condensed matter theory, early universe cosmology theory, and quantum information theory, like a mini United Nations of theoretical physics. Contributing to developing holography is the other main theme of my research program.