Stephen Friess, M.Sc.

Fries,,Steffen 61Theoretical Physicist with a research focus on Computational Intelligence.

Stephen previously studied at the Technical University of Darmstadt where he did research for his BSc (2013) on effective field theories and quark matter in the Nuclei, Hadrons & Quarks group of PD Dr. Michael Buballa and for his MSc (2016) on neutrino reactions in proto-neutron stars in the Theoretical Nuclear Astrophysics group of Prof. Dr. Gabriel Martínez-Pinedo. Prior to starting his role as Marie Curie Fellow on project ECOLE (2018), he worked for a German-based software company. Funded by the European Commission, he works on the topic of # for his European Industrial Doctorate (EID). He is being supervised at the University of Birmingham by Prof. Xin Yao and Prof. Peter Tiňo, while Dr. Stefan Menzel and Dr. Zhao Xu provide industrial co-supervision at Honda Research Institute Europe and NEC Laboratories Europe.

Social Media:  LinkedIn & Twitter

Research Interests: A lot of things; but due to limited time a preference for Nature and Bio-inspired Artificial Intelligence.

Professional Memberships: German Physical Society (DPG eV).

Participant of: 

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Master’s Thesis: Weak Interaction Processes with Spectator Nucleon in the Proto-Neutron Star (2016)


During my Master’s studies I attended a seminar on theoretical problems of nuclear physics. I particularly worked on a term paper about the nuclear physics of neutron stars. Because doing research on astrophysical problems felt very refreshening for me, I decided to switch to theoretical nuclear astrophysics for my Master’ thesis.

Cross section of the entropy distribution of a 3d supernova simulation. Source:

Working in the TNA group was different to my prior experiences. I really enjoyed working in such a diverse team. I became known for my coffee drinking habits and particularly recall one weary morning at which a bugged out PhD student complained to me about having too much. In his defence: The coffee machine was standing right on his desk and was always making loud noises.

I finished my thesis in March 2016 and defended it with great success! The 80 pages long work studies the mathematical framework used to calculate neutrino reactions in hot dense matter (by this we mean the proto-neutron star), investigates the behaviour of known basic reaction processes for a data set stemming from a core-collapse supernova simulation, criticizes heuristic methods which are used to extrapolate the framework to more complicated modes of interaction and contains analytical calculations of the more complicated interactions.

Source: Janka et al.

The study of neutrino reactions in nuclear astrophysics is particularly interesting in order to better understand the dynamics of core-collapse supernova and the resulting observable neutrino signal.

If you are interested, feel free to read through my thesis and presentation:
Master’s Thesis
Presentation of my Master’s Thesis

I also wrote a nice introductory article on weak interaction processes in nuclear astrophysics for the website of my former work group:
Weak interaction processes in astrophysics

Term Paper on Open World and Sandbox Games (2015)


Question: “So, what’s the deal with open-world and sandbox games?”. While this may sound like a conundrum, it was an actual question I worked on for the Serious Games seminar at my university.

The faculty of electrical engineering and information technology at the Technical University of Darmstadt has a work group and a lab dedicated to the study of games and their applications for training and health care. I actually found out about them while looking for interesting courses for my non-physics curriculum.

A particular question which rose up for my supervisor who worked in the field of cooperative games was, what the difference between sandbox and open world games are (if there are any). For this reason I did read enumerous blog posts by game designers, literature and even bought some games.  I came to the conclusion, that while the terms ‘sandbox’ and ‘open world’ are often used as synonyms, they describe completely different aspects of a game.

While ‘open world’ implies a certain degree of scale or complexity of a level, ‘sandbox’ describes the world as literally manipulatable like sand in a box. In contrast, many open world games like GTA often rather resemble ‘theme parks’.

I found the excursion in the world of Game Studies quite refreshening and overall it was a great success. If you are interested, you find the term paper and the presentation attached below. Sadly, due to the requirements of the seminar I had to write them in German.

Term Paper

Bachelor’s Thesis: Taylor Coefficients of the Thermodynamic Potential to Sixth Order in the Vector Interaction extended NJL model (2013)

Understanding quantum field theory was one of the main motivations for me to go into theoretical physics. At the Technical University of Darmstadt we particularly had two work groups which were interested into the study of the quantum field theory of strong interaction (i.e. quantum chromodynamics) to describe states of QCD matter.


QCD matter is a state of matter which is thought to exist in dense neutron stars and in the early universe. At these conditions matter breaks up into a soup of interacting quarks and gluons, the so called quark gluon plasma. If the thermodynamic conditions undergo changes, the quark gluon plasma can condense into bound composite particles, the so called hadrons. In a way it’s similar to how droplets form in moist air. The boundaries between these different states of matter are the so called phase transitions (indicated by blue lines) in the plot. There are of course many more phase transitions for the quark gluon plasma conjectured. However, if they exist is a different question.

Calculating the phase transitions and properties of QCD matter with numerical methods (i.e. lattice QCD) using the theory of quantumchromodynamics raises some mathematical problems which can not be resolved yet. Therefore various heuristic methods are used, as well as effective field theories which are mathematically easier to handle, but still replicate the most important properties of the ‘full’ theory.

I particularly tested the performance of a NJL model extended with vector interactions in comparison to lattice QCD. My conclusion was, that including the vector interaction, the performance of the NJL model worsens and thus the interaction terms should be neglected.

If you are interested, you find my thesis and the presentation below:
Bachelor’s Thesis
Presentation of my Bachelor’s Thesis

Computational Physics Project: Simulation of an Epidemic based upon a SIR model

One of the most interesting projects I worked on during my studies was an Epidemic simulation based upon a SIR model implemented in Wolfram Mathematica. The project particularly dealed with the simulation of the spread of an infection in Europe. Therefore individual European cities were put into the simulation and connected by land (green lines) and air (dark blue lines).

The SIR model is basically a set of three first order differential equations, accompanied with a conservation condition N = S + I + R, which states that the total population number is constant and does not decline. The total population (N) of every city is split up into individuals who are susceptible (S) to a disease, are infected by a disease (I) and are recovered from a disease (R).

Time evolution of population groups in a city. Susceptible individuals are represented by the red line, infected by the green one and recovered by the purple one.

Because the project particularly dealed with the simulation of the spread of an infection in Europe, traffic was explicitely modelled between cities. Further it was ensured, that migration occurs in a way such that the total population in every city is conserved. Otherwise, the validity of the simulation would be put into question. The results of the simulation are quite interesting to observe. Starting an epidemic with about ~50 infected people in London, it rapidly spread to continental Europe until it halts in Central Europe, with limited spread to the eastern regions.

Of course the outcome of the simulation pretty much depends upon the chosen initial values. There are also some quirks in the modelling which can be further improved on, e.g. the connnections between cities are solely based upon their population numbers, thus the UK is over-connected to continental Europe. A further extension of the simulation considered transatlantic connections to North America.


Overall I really enjoyed working on the project. I personally wish my curriculum would have allowed me to work on more problems of interdisciplinary nature.

You can find the Mathematica notebook of my Epidemic simulation at my GitHub account: epidemieausbreitungv9.nb .