Talks, Colloquia

17:00 Uhr KIP, INF 227, Otto-Haxel-Hörsaal
Prof. Dr. Wulfram Gerstner École Polytechnique Fédérale de Lausanne

Learning in Neuronal networks of the Brain: From Synaptic Plasticity to Memories

What happens in your brain when you learn to ride a bike or memorize a new word? In this talk, I will discuss mathematical models of memory formation and learning. Neurons communicate with each other in form of short electrical pulses. The pulses travel along 'axonal' cables that ramify and form thousands of synapses with other neurons. The strength of a synaptic connection is the location where memories are stored. Mathematical learning rules describe how and when these synaptic connections change. Simulations and analysis of neural network models show how such learning rules, located at the level of synaptic connections, can lead to global changes in brain function via formation of memories. While the classic Hopfied model from 1982, which for the first time linked the physics of spin glasses to the brain, assumed that memories are pre- learned, modern models of brain- inspired neural network try to understand how online learning upon exposure to new events is possible.

17:00 Uhr Neues Physikalisches Institut INF 226, Konferenzraum 1-3 (Room 00.101 bis 00.103)
Prof. Norbert Herrmann Physikalisches Institut Heidelberg

The Cold Baryon Matter Experiment

17:00 Uhr Kirchhoff-Institut, INF 227 SR 1.403, SR 1.403
Dr. Mechthild Enderle Institut Laue-Langevin, ILL, Grenoble, Frankreich

Twisted excitations in entangled quantum magnets

16:15 Uhr Philosophenweg 12, Großer Hörsaal
Patrick Hennebelle CEA Saclay (France)

What sets the stellar initial mass function? Why is it so universal?

Stars are building blocks of our Universe. They determine its chemical evolution through nuclear synthesis, they host planets and they determine the evolution of galaxies. The characteristics of stars is predominantly determined by their masses. As such the stellar mass spectrum, also called the initial mass function (IMF), is truly a fundamental quantity to understand how our Universe works. A large number of studies have been performed to infer the IMF and it appears strikingly universal. That is to say, even when measured in rather different environments, the IMF presents no or modest variations. This is an intriguing fact as naive expectations would naturally relate the mass spectrum of stars to quantities such as the Jeans mass which depends significantly on the gas density and gas temperature. During the talk I will review some of the ideas that have been proposed to explain the IMF and discuss their success and failure. I will then present a large sets of simulations in which the initial conditions, the thermodynamics and the numerical resolution are all systematically varied. These simulations reveal that the initial conditions determine the power-law part of the IMF while the gas effective equation of state (EOS), which describe the isothermal to adiabatic transition, sets the peak of the stellar distribution. Analytical models are developed and compared with the simulation results. It is argued that the power-law part of the mass spectrum is due to an interplay between gravity and turbulence that determine the mass spectrum of gas reservoirs from which stars built their masses. The peak on the other hand, occurs at a mass which is 5-10 times the mass of the first Larson hydrostatic cores determined by the effective EOS. We propose that the very reason of the IMF weak variability is that the first hydrostatic core and immediate surrounding collapsing envelope are small scale processes which are nearly independent of the large scale environment characteristics. I will finish the talk by discussing remaining issues and suggests a possible "unifying picture".