Is gravity a hologram, an illusion that does not exist in three dimensions? That is a question Dr. Oliver DeWolfe, assistant professor at the University of Colorado, answered in his lecture entitled, “Black Holes as Holograms of Strong Interactions.” With a fast pace, DeWolfe explained the exciting new research into quantum gravity and black holes.
DeWolfe briefly talked about quantum chromodynamics (QCD), which describes how the quarks and gluons act. When quarks bond strongly together, they form protons, neutrons, etc. When gluons form strongly with quarks, they act like a liquid with very low viscosity. Then, DeWolfe talked about Fermi surfaces, which have the spin states of the quarks within filled while the outside is empty. Electrons act strongly around Fermi surfaces, but they are quasiparticles and interact with the surface weakly. This “Fermi liquid” describes most metals. This is still a field in need of further study.
Proceeding at his fast pace, DeWolfe then discussed quantum gravity. “It is hard to find a system to test quantum gravity,” DeWolfe said. Quantum mechanics describes the uncertain nature of the subatomic, and general relativity states that gravity warps space and time. So, quantum gravity states that space and time must be quantized and uncertain. The laws regarding black holes are similar to the laws of thermodynamics. The change in mass for a black hole is similar to the change in internal energy in thermodynamics. According to the second law, entropy never decreases and a black hole’s area never decreases. In regards to quantum physics, a black hole can create virtual particles out of nothing that exist for a very brief time. With the creation of such particles, black holes can radiate, and the black hole can appear to be a black body radiator. “It is believed that there are small black holes in star systems and large black holes at the center of galaxies,” said DeWolfe.
How is quantum gravity explainable? DeWolfe said that particles, when examined at very small scales, are vibrating strings with different vibrations corresponding to different kinds of particles. Closed strings give rise to gravity; open strings give rise to other forces, such as nuclear, as gauge theories. Momentum conservation demands that open strings end on membrane with Dirichlet boundary conditions, or “D-branes.” These branes correspond only to the non-gravitational gauge theory localized in three dimensions for open strings. The mass of “black branes” curve space and time and affect general relativity in higher dimensions. Hyper-dimensional gravity theory and non-gravitational gauge theory are dual ways to describe the same system. This means that gravity can be described by another system in fewer dimensions. Ergo, gravity is a hologram. Gravity can then be described by classical geometry without the help of quantum effects or string theory. “Gravity is massively redundant,” said DeWolfe.
How can this apply to QCD? Irreversible processes correspond to things falling through a black hole’s horizon. When perturbed, the black hole’s response can be mapped as a dual fluid. The universal result – a fluid with very low viscosity. At zero density, there is a crossover from protons to liberated quark-gluon plasma. At a finite density, the graph shows a first order phase transition similar to liquid/gas transitions that end on a critical point. This critical point has critical exponents obeying nontrivial scaling laws. “It’s always remarkable and gratifying to see black holes work in our graphs,” said DeWolfe.
Despite his fast pace, DeWolfe raised questions among the audience, specifically that gravity may just be an illusion, a hologram. To be described so simply, and to have black holes have so many similarities to thermodynamics, is fascinating. The field of particle physics still requires more research and appears to be a field that will not slow down any time soon.