Dear physics and foundations enthusiasts, It is a pleasure to announce the fourth edition of our summer school Solstice of Foundations! It will take place at the University of Zurich, 16-20 June 2025. We encourage you to send your PhD and Masters students to the school - they will return with a consolidated understanding of quantum foundations and an academic network of peers interested in the same topics. This year's edition will focus on the importance of *agents, relativity and correlations* in the foundations of quantum mechanics.This includes quantum reference frames, modelling observers as explicit physical systems, the motility of the Heisenberg cut, multi-agent paradoxes, and the role of the observer and boundaries in novel approaches to relativity and quantum gravity. More on the programme below. --------------------------------------------------------------------------------------------------- *Information and registration:* http://foundations.squids.ch <http://foundations.squids.ch> *15th May*: Deadline for student support applications 20th May: Notification for student support applications *31st May*: Registration deadline for in-person participants *Format:* Hybrid. Both in-person and online participation are possible;  in-person participation is encouraged. *Registration fee: *CHF 150 for Masters students, 200 for PhD students, 250 non-students. Online attendance is free for students, but registration is mandatory. Students based in Zurich may volunteer to help with the school and have the fee waived. *Required background: *The school is aimed at participants familiar with basics of quantum mechanics, and ideally introductory courses on quantum information theory. --------------------------------------------------------------------------------------------------- The school is structured in three thematic blocks: 1. Foundations of quantum physics Introduction to foundations of quantum physics; different approaches to quantum foundations; Bell non-locality; contextuality. These lectures will take place on Monday-Tuesday. Lecturers: * *Robert Spekkens*, Perimeter Institute for Theoretical Physics, Canada * *David Schmid*, Perimeter Institute for Theoretical Physics, Canada 2. Agents and paradoxes in physical theories How to model observers as physical systems, in quantum theory and beyond; Wigner-friend scenarios; paradoxes that emerge when trying to combine the viewpoints of different observers; relation to contextuality; systems as subjective concepts. These lectures will take place on Tuesday-Wednesday. Lecturers: * *Renato Renner*, ETH Zurich, Switzerland * *Yìlè Yīng*, Perimeter Institute for Theoretical Physics, Canada 2. From quantum reference frames to quantum gravity Modelling quantum reference frames; physical objects as imperfect reference frames; frame transformations; the equivalence principle; spacial superpositions of masses and gravitational fields; introduction to quantum gravity; the role of boundaries and observers in quantum gravity. These lectures will take place on Thursday-Friday. Lecturers: * *Anne-Catherine de la Hamette*, IQOQI, Austria * *Flaminia Giacomi*, ETH Zurich, Switzerland * *Daniele Oriti*, Complutense University of Madrid --------------------------------------------------------------------------------------------------- May your well-being be strongly correlated with the wonderful Spring weather in Switzerland, Lídia and Nuriya PS: For questions, please contact Nuriya at foundations@squids.ch or her academic address in CC; Lídia is mostly non-signalling for the rest of the year. ......... Dr. Lídia del Rio Essentia Foundation Research Fellow for Quantum Information Theory Institute of Physics, University of Zurich I am on partial sick leave for the next few months, with limited working hours and access to email. For urgent queries, please contact my colleagues.