Celsius-Linné 2025: Irrational numbers and smart water systems
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Mathematical tools and water systems are the topics of this year’s Celsius-Linnaeus Lectures on 13 February.
We use numbers to construct one of the most important ways in which we understand the world. But irrational numbers are always incomplete, and therefore pose challenges as mathematical tools. On the other hand, models of the self-organisation of rivers have proven to be excellent tools that are able to shed light on global inequities around water resources. These are the topics of this year’s Celsius-Linnaeus Lectures on 13 February with mathematician Francesco Calegari and hydrologist Andrea Rinaldo.
Every year, the Faculty of Science and Technology at Uppsala University holds two lectures in memory of Anders Celsius and Carl Linnaeus. The lecturers chosen are scientists whose world-leading research on hot topics has attracted attention in the research community and is of great public interest.

Francesco Calegari, Professor of Number Theory at the University of Chicago.
The 2025 Celsius Lecturer is Francesco Calegari, Professor of Number Theory at the University of Chicago. He will show how mathematicians have been trying to solve the riddle of irrational numbers since Ancient Greece. Probably the most famous irrational number is pi, which is often rounded to 3.14, but whose decimal places continue infinitely without any regular pattern. Irrational numbers play an important role in many fields such as geometry and computational mathematics.
Devourers of computing power
Francesco Calegari's host is Silvelyn Zwanzig, Emeritus Professor of Mathematical Statistics at the Department of Mathematics.
Why are irrational numbers such a problem?
“Irrational numbers cannot be expressed in precise figures, so you never get an exact answer when they are used. This leads to approximations in calculations and thus potential errors. A big problem today is that a computer needs to be able to store information in the form of ones and zeros, but because irrational numbers are not finite, a computer can’t perform calculations using them. We end up with very big mainframe computers trying to perform calculations with irrational numbers that end up consuming a lot of energy and computing power. That’s why we need to study the properties of irrational numbers, how many such numbers exist, and how different numbers function together.”
Why should people come to the Ångström Laboratory and listen to Francesco Calegari?
“What’s going to be really worthwhile is that Calegari said he aims to present the whole history of irrational numbers, from the beginning to the present day. What mathematicians have done, and how number theory is the basis of what has ultimately led to today’s big simulation studies, especially in climate science. If you want to understand the world, you should come and listen to him! He’s a very engaging and inspirational speaker. You really should take this opportunity to hear him here at the University.

Andrea Rinaldo, Professor of Hydrology and Water Resources at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and the University of Padua.
The 2025 Linnaeus lecturer is Andrea Rinaldo, Professor of Hydrology and Water Resources at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and the University of Padua. He will show how the structures in networks of rivers can serve as models for combating waterborne pollution and diseases, for example. With more equitable management, water resources can be distributed more equitably and biodiversity can be enhanced.
River networks are instructive models
Andrea Rinaldo's host is Giuliano Di Baldassarre, Professor in Surface Water Modelling at the Department of Earth Sciences.
What will Andrea Rinaldo talk about?
“He will talk about major societal challenges related to inequality, sustainability and biodiversity, highlighting the crucial role rivers play in connecting and sustaining life. Access to clean water remains limited in many parts of the world, while mobile phones are widely available – a stark contrast that underlines the inequities in water management. Rinaldo will show that we now have the tools and knowledge to make smarter and fairer decisions about managing water resources.”
Do you have a question you are curious about and would like to ask him?
“I’ll have the privilege of being the moderator for the panel discussion following the lecture, where I will focus on three key questions. The first is how we can foster meaningful civic engagement while ensuring security and fairness. And I also wonder about how we can create narratives that effectively connect global with local perspectives Finally, I would like to ask about the best ways to communicate our scientific findings to different audiences.”
Anneli Björkman
Celsius-Linnaeus Lectures 2025
Date: 13 February at 09.00 – 16.15
This year's Celsius-Linnaeus Lectures will be held at the Ångström Laboratory (Celsius Lecture) and the Geo Centre (Linnaeus Lecture) at Uppsala University. The lectures are open to all interested parties, and no prior registration is required. The lectures will also be filmed. Following each one, there will be a symposium with presentations by researchers and a panel discussion.