Marius de Leeuw studied math and physics at Utrecht University. Afterwards, he worked as a postdoctoral researcher at consecutively the Albert-Einstein-Institute, ETH Zurich and, currently, the University of Copenhagen.

**What made you decide to go into research?**

I went into research because from a young age I wanted to know how things work. I liked reading science books and I loved mathematics and physics in high school. It was therefore an easy choice for me to study math and physics in university. After that, I knew I wanted to go into research and work on open problems. It is fun and interesting to work on problems that no one knows how to solve and to try to make sense of them.

**What is your area of study?**

My field of study is theoretical physics. I work on theories that aim to describe nature at the smallest distances. At the moment I am working on so-called holographic models. These are quantum models (such as a model with photons, the fundamental particles of light) that have an alternative description via a model of gravity. My work is purely mathematical and theoretical. I try to find equations that describe these holographic models and use them to do computations. One of the questions I am trying to address is how to describe systems with strong interactions. Examples of such systems are superconducting materials and interactions between the building blocks of matter (quarks). Strongly interacting systems are not well understood at the moment and are relevant for experiments such as those performed at the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research.

**What is the result that you are most proud of so far?**

The work I am most proud of has led to our understanding of symmetries in certain holographic models. It turns out that you can already learn a lot about models by only looking at what symmetries they have. To visualize this, consider rotating a square by 90, 180 or 270 degrees. Each of these rotations will give you the same square back. This is a symmetry of the square. Similarly, some holographic models have lots of more general symmetries. I found that these symmetries are of a new type. This discovery helped to compute observables, like particle energies.

**What is the hardest thing about being a researcher?**

There are two difficult things about being a researcher. The first thing is research related and it is the fact that you get stuck a lot. This often happens when solving a problem with which you simply do not know how to proceed. You feel like nothing you are trying is working and this can be very frustrating. To make matters worse, it sometimes happens that you are trying to find solutions that don’t even exist. In these circumstances, it can be hard to stay motivated. I generally try to solve this standstill by talking with colleagues to get some fresh ideas or by temporarilyÂ working on another problem instead.

The second difficult thing is more related to the job of being a postdoc. In physics, if you want to get a permanent position at a university, it is important to have significant research experience abroad. This means you have to do several postdocs. A postdoc contract usually lasts for two years, after which you need to try to find another postdoc. So, for a long time you only have temporary contracts and have to move between countries every two years. This can be stressful, since you don’t have any job security, and makes it hard to build a social life. Moreover, in theoretical physics there is a lot of competition, so getting a postdoc (let alone a permanent position) can be very hard.

**What do you enjoy most about doing research?**

What I enjoy most about doing research is working on the cutting edge of science. You’re trying to solve problems that have not been solved before and to understand more and more of the workings of the universe. It is a great feeling when you understand something new or finally crack a difficult problem. Thus, although doing research can be hard and challenging, it is also rewarding and very inspiring, which motivates me to continue doing it.