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18 September 2026
Interview with Dr. Horst-Holger Boltz—Winner of the Entropy Best Paper Award

We are honored to announce that Dr. Horst-Holger Boltz has been selected as the winner of the Entropy 2024 Best Paper Award.

The following is an interview with Dr. Horst-Holger Boltz:

1. Congratulations on winning the Entropy 2024 Best Paper Award! Could you please briefly introduce yourself or your research group?

We are very grateful for the recognition. I am Horst-Holger Boltz, a postdoctoral researcher working in statistical physics in a broad sense at the University of Greifswald. This research was performed within the research group of Prof. Thomas Ihle. The research team for this project consisted of Prof. Ihle, myself and Benjamin Kohler, who was a master’s student at the time and is no longer in academia. A specific priority of the group’s research program in recent years has been first-principles statistical theory for non-equilibrium systems, especially active matter systems.

2. Could you briefly introduce your research focus and the key findings of the award-winning paper? What inspired this work?

We develop statistical theory for active matter systems. Active matter is a relatively young field and comprises systems that violate the assumptions underlying equilibrium statistical mechanics. When we coarse grain, for example, the inner machinery of a bird to a self-propelled particle that flies along some internal direction, we greatly simplify the system which is a prerequisite to be able to capture collective effects such as flocking in a large ensemble of birds, but we also lose contact with the fundamental laws of physics such as conservation of energy or momentum. That gives rise to conceptual challenges in adapting theoretical tools from passive systems, which typically rely on assumptions that no longer hold. For this line of work specifically, we wanted to go beyond the mean-field description that is very present in the field, but cannot constitute satisfying descriptions for the rather dilute, collision-based dynamics one observes in many active matter systems. And we wanted to find a methodology that is suited for complex interactions. The paper took a first step in that direction by allowing for higher interaction symmetries. The key element of the paper for me is that it presents a clear way to adapt a rather old idea of kinetic theory, the Landau equation, to these active systems and allows for quantitatively correct, model-specific theory that does not rely on additional input (such as measured quantities).

3. What were the biggest challenges you faced during this research, and how did you overcome them?

The main challenge was conceptual rather than technical. As a group we had to unlearn some earlier conceptions: Landau theory turned out to be much simpler than the approaches we were more familiar with, such as ring-kinetic theory or the full Boltzmann equation. Recognizing that this simpler description was adequate for our purposes took some deliberate stepping back from the more elaborate machinery.

4. What role did you play in your research team, and how did teamwork affect the paper’s outcome?

The research itself was highly collaborative and driven by discussions within the group, which is where the approach was worked out. Parts of this work grew from Benjamin Kohler’s master thesis project which I co-supervised. I checked calculations and conceptualized the pedagogic approach of the paper. I took the lead in the writing, where I think it makes sense to depart from that collaborative mode and have a single voice carry the narrative.

5. How was your experience with the editorial and peer-review process for Entropy?

As I remember it, our experience was rather positive. There was a consistent drive to the process instead of the paper being in unclear limbo states for weeks before even getting to review. We had constructive referee reports from reviewers with a different perspective that helped us to improve this work. One example is that the final manuscript provides a much better overview of the more mathematically oriented literature that I was previously not as familiar with.

6. Did you share any raw data or code related to this study? How do you view the value of open science (e.g., data and code sharing) in enhancing reproducibility and advancing your field?

We take reproducibility seriously. The results here are mostly analytical calculations rather than data or simulation output, so there is no substantial dataset or code to deposit. The actual simulations are very straightforward and the relevant results are shown. What matters for reproducibility in that setting is transparency of the derivation, and I made an effort to explain every element of it. We do not treat our methods as trade secrets, but we tried to have something that is helpful for other groups.

7. In your view, how does your work push the boundaries of entropy‑related theory or information science in your specific area? What do you see as the most promising future breakthrough directions in this field over the next five years?

The statistical theory of passive systems has come a long way; by comparison the theory of active systems is still developing. Our work is one step toward a statistical theory suited to the more general dynamics found in these systems. Generally speaking, kinetic theory is a worthy tool to consider because it starts from the actual equations of motion. A promising direction for the next few years will be ‘smart’ active matter: systems of particles that have the internal self-propulsion we describe or other activity but also more complex ways of acquiring and processing information, and that can learn from each other. This is challenging because it is naturally of higher dimension and it requires some conceptual clarification in what one does and does not consider to be learning.

8. Based on your experience, what advice would you give to early career researchers working in your area?

This is a tough question. I think, generally speaking, that finding answers is much easier if one has a good question. Finding the good questions is the hard part, and it is worth spending real time there rather than rushing into calculation. This is also advocacy for curiosity-driven research that follows the own interest where such questions arise more consistently rather than following current trends blindly.

9. As a Best Paper Award recipient, what message or advice would you share with future authors who aim to submit high-quality research to Entropy?

The lack of length restrictions gives you room to explain all the technical details, and there are no rigid structural constraints, so you can choose whatever presentation works best for the specific work. The ability to present ideas clearly is very valuable in theoretical physics, and this freedom is worth using, in service of clarity rather than length for its own sake.

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