When understanding black holes comes with strings attached
What do black holes and string theory have in common? Turns out, quite a lot. To start, they’re both rather chaotic. “In string theory, a massive ball of tangled string can exhibit a very complicated structure with a huge number of quantum mechanical states – a feature that has many tantalising similarities with black holes, which are believed to be the most chaotic systems in nature,” says Vasilis Niarchos, a physicist at the University of Crete(opens in new window). In this context, string theory presents a promising path to unravelling one of the key puzzles of modern theoretical physics, namely, the quantum mechanical, microscopic origin of the corresponding entropy of black holes in quantum gravity. According to Niarchos, since highly excited string states are complicated quantum states with high entropy in a quantum theory of gravity, they provide a natural arena for such questions. “It is extremely interesting to study how highly excited string states interact, how they absorb and emit radiation, how their complexity affects their dynamics, and, in general, to what extent they behave like black holes,” he explains. With the support of the EU-funded BlackHoleChaos project, Niarchos, together with Marie Skłodowska-Curie Actions(opens in new window) fellow Maurizio Firrotta, took a deep dive into the chaos of string interactions.
Connecting strings to black hole physics
With the goal of connecting strings to black hole physics, the project focused its attention on scattering processes. During these processes, highly excited string states scatter from each other while emitting or absorbing gravitational or electromagnetic radiation. “We implemented and further developed a set of mathematical tools that allowed us to explicitly study the scattering of highly excited string states in weakly interacting string theory,” notes Niarchos. “These are mathematically extremely complex processes that have mostly been out of reach until recently.” The project also introduced novel scattering-based diagnostics for string interactions that can quantify the complexity of the underlying dynamics.
Shedding new light on black hole microphysics
Based on this work, researchers not only derived compact mathematical expressions for complicated amplitudes of highly excited string states, they also proposed specific measures of chaos in string scattering amplitudes. Furthermore, the project developed a new computation of absorption probabilities and emission rates of highly excited string states. The computation revealed how an emergent notion of temperature arises from quantum string microstates, as well as an approximate black-body emission pattern that resembles that of black hole physics. “These results will be useful in future attempts to advance our conceptual understanding of black hole microphysics and complex or chaotic dynamics in the context of quantum gravity,” remarks Niarchos. “Hopefully, they will also inspire cross-disciplinary applications in other areas of physics where scattering techniques are useful, such as in gravitational wave analysis in astrophysics.”
Exploring other parallels in black hole physics
While the project successfully delivered concrete results about the complicated, statistical behaviour of highly excited string states, there’s still more work to be done. “We would like to use the results and techniques we developed to further understand how highly massive complex objects interact in string theory and what other parallels exist with black hole physics, as well as to develop a new bridge to gravitational wave phenomenology,” concludes Niarchos.