Daniela Doneva • 9/2024 • Seite 76 • DPG-MitgliederEnlightening the dark universe
Gravitational waves promise to shed light on fundamental physics through observations of dark compact objects.
Gravitational waves provide a unique window into the most cataclysmic events in the universe, from the Big Bang to black hole mergers. They complement traditional electromagnetic astronomy and, in addition, reveal phenomena that were previously hidden from our view.
The direct detection of gravitational waves in 2015 [1] was among the most important discoveries in fundamental physics in recent decades and a proof of one of the fundamental predictions of General Relativity (GR). The signal was observed by the two underground LIGO detectors in Hanford and in Livingston (Fig. 1); the comparison of the data in the bottom plot demonstrates that both detectors witnessed the same event. At least two detectors operating simultaneously are needed to confirm the detection because these instruments are so sensitive that various noises might contaminate the signal, such as seismic activities or even animals walking on the ground above the detector. In addition, it is very important that the observed signal matches the theoretical prediction very well. This is a confirmation that the observed gravitational wave event is a merger of two compact objects with spacetime curvature in their vicinity reaching extreme values. Even though a number of exotic scenarios cannot be excluded, the most probable one is that these were two colliding black holes. (...)
David Ruelle • 9/2014 • Seite 37From heat transport to turbulence (to life)
Life as a problem in nonequilibrium statistical mechanics
We review some problems in nonequilibrium physics from the point of view of statistical physics and differentiable dynamics. Specifically, we discuss the mathematical difficulties which inherently underlie applications to heat transport, to hydrodynamic turbulence, and to the study of life. The microscopic dynamics of transport phenomena (in particular heat transport) is necessarily non hyperbolic, which explains why it is a difficult problem. The 3D turbulent energy cascade can be analyzed formally as a heat flow, and experimental intermittency data indicate that this requires discussing a Hamiltonian system with 104 degrees of freedom. Life is a nonequilibrium statistical physics phenomenon which involves chemical reactions and not just transport. Considering life as a problem in nonequilibrium statistical mechanics at least shows how complex and difficult the study of nonequilibrium can be.
The aim of nonequilibrium statistical mechanics is to understand the properties of matter outside of equilibrium, starting from microscopic dynamics. At this time nonequilibrium statistical mechanics of transport phenomena close to equilibrium is a well-developed physical theory (due to the work of Onsager, Green, Kubo, etc. in the 1950’s, see for instance [1]). Away from this area, the theory of nonequilibrium is a program, or a variety of programs, rather than a theory. Here I shall make a choice, and describe an approach starting with classical Hamiltonian microscopic dynamics. From my point of view this approach has the interest that it uses nontrivial recent results in the theory of smooth dynamical systems, and that it sheds light on interesting physical phenomena: heat transport, hydrodynamic turbulence, and life. (...)
Martin R. Zirnbauer • 9/2012 • Seite 41Of symmetries, symmetry classes, and symmetric spaces
From disorder and quantum chaos to topological insulators
Quantum mechanical systems with some degree of complexity due to multiple scattering behave as if their Hamiltonians were random matrices. Such behavior, while originally surmised for the interacting many-body system of highly excited atomic nuclei, was later discovered in a variety of situations including single-particle systems with disorder or chaos. A fascinating theme in this context is the emergence of universal laws for the fluctuations of energy spectra and transport observables. After an introduction to the basic phenomenology, the talk highlights the role of symmetries for universality, in particular the correspondence between symmetry classes and symmetric spaces that led to a classification scheme dubbed the “Tenfold Way”. Perhaps surprisingly, the same scheme has turned out to organize also the world of topological insulators.
Let me begin by expressing that I feel greatly honored to be this year’s recipient of the Max-Planck medal, and I appreciate the opportunity to give a talk on some of the work that may have earned me this distinction. To set the stage and give you a flavor of what is to come, let me remind you of the old but still fascinating story of universal conductance fluctuations (UCF). Predicted theoretically in the middle of the 1980s by Altshuler [1] and by Lee and Stone [2], UCF was investigated in a large number of experiments. It was found that in a great variety of different mesoscopic systems − such as a small gold ring for example, or an even smaller silicon MOSFET − the electrical conductance displays characteristic fluctuations of the order of one when expressed in units of the conductance quantum e2/h (Fig. 1). What is most remarkable is that the size of the fluctuations in a broad range of parameters does not depend on the system dimension, the disorder strength, etc., but only on a few fundamental symmetries.
It was realized early on that there exists a close connection with the fluctuations that had been observed decades earlier in the scattering cross section of slow neutrons on atomic nuclei. This far reaching connection is at the very root of what I have to say. It led, among other things, to the development of a broad framework in which to model and calculate mesoscopic effects such as UCF. ...
