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.

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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

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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.

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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.