Stringed along or caught in a loop? Stringed along or caught in a loop? Philosophical reflections on modern quantum gravity research Keizo Matsubara Dissertation presented at Uppsala University to be publicly examined in Geijersalen, Building 6, (Eng 6-1023), English Park Campus, Thunbergsvägen 3P, Uppsala, Friday, January 18, 2013 at 10:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Abstract Matsubara, K. 2013. Stringed along or caught in a loop?: Philosophical reflections on modern quantum gravity research. Filosofiska Institutionen. 139 pp. Uppsala. ISBN 978-91-506-2324-6. A number of philosophical questions, all connected to modern research in quantum gravity, are discussed in this dissertation. The goal of research in quantum gravity is to find a quantum theory for gravitation; the ot- her fundamental forces are already understood in terms of quantum physics. Quantum gravity is studied within a number of different research programmes. The most popular are string theory and loop quantum gravity; besides these a number of other approaches are pursued. Due to the lack of empirical support, it is relevant to assess the scientific status of this rese- arch. This is done from four different points of view, namely the ones held by: logical positi- vists, Popper, Kuhn and Lakatos. It is then argued that research in quantum gravity may be considered scientific, conditional on scientists being open with the tentative and speculative nature of their pursuits. Given the lack of empirical progress, in all approaches to quantum gravity, a pluralistic strategy is advised. In string theory there are different theoretical formulations, or dualities, which are physi- cally equivalent. This is relevant for the problem of underdetermination of theories by data, and the debate on scientific realism. Different views on the dualities are possible. It is argued that a more empiricist view on the semantics of theories, than what has been popular lately, ought to be adopted. This is of importance for our understanding of what the theories tell us about space and time. In physics and philosophy, the idea that there are worlds or universes other than our own, has appeared in different contexts. It is discussed how we should understand these different suggestions; how they are similar and how they are different. A discussion on, how and when theoretical multiverse scenarios can be empirically testable, is also given. The reliability of thought experiments in physics in general and in quantum gravity in par- ticular is evaluated. Thought experiments can be important for heuristic purposes, but in the case of quantum gravity, conclusions based on thoght experiments are not very reliable. Keywords: quantum gravity, string theory, loop quantum gravity, philosophy of physics, philosophy of science, scientific realism, structural realism, multiverse, thought experiments Keizo Matsubara, Uppsala University, Department of Philosophy, Logic and Metaphysics, Box 627, SE-751 26 Uppsala, Sweden. © Keizo Matsubara 2013 ISBN 978-91-506-2324-6 urn:nbn:se:uu:diva-185554 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-185554) Printed by Elanders Sverige AB, 2012 Dedicated to my parents, Akira and Carine Acknowledgements First of all I would like to thank Lars-Göran Johansson, my advisor and collab- orator. He has been very helpful and inspiring during the years we have been working together. Riksbankens Jubileumsfond funded the project, in which we started our joint work on philosophical issues regarding string theory. I am very grateful for the financial support they provided. In this project, I was hired as a researcher in my role as a theoretical physicist, since I already had a Ph.D. in theoretical physics. Shortly after being hired to work on this project, I finished my MA in philosophy. It was then decided that I was also going to be hired as a Ph.D.-student in philosophy, with the intention of using my work in the project as part of my dissertation. Lars-Göran supported this decision and for this I am very happy. When the project, financed by Riksbankens Jubileumsfond, was finished I went to the University of Toronto. This was made possible due to a gener- ous stipend which was given to me by The Sweden-America Foundation and donated by Borgrättsfonderna. I had a great time in Toronto, much due to my wonderful host Jim Brown. He introduced me to the interesting topic of study- ing thought experiments in science and philosophy for which I will be forever grateful. I would also like to acknowledge a number of people who have contributed to my research in various ways. They have helped me through valuable discus- sions, by giving comments to manuscripts, by writing letters of recommenda- tion etc. These people are: Ulf Danielsson, Richard Dawid, Sven Ove Hans- son, Sten Lindström, George Masterton, Chris Smeenk, Kim Solin, Mårten Stenmark and Harald Wiltsche. I would also like to thank all other friends and colleagues, in Uppsala, Toronto and elsewhere, who in a more indirect way have contributed to my research. They may mot have directly influenced the contents of this thesis. Instead they have all been very important in other ways namely by making my days more enjoyable, or by helping me in various practical matters. It is very many who fall into this category and I hope they will not be sad by the fact that I have decided not to include a list with all their names. With regards to this large group of people I use the old cliché; no one mentioned, no one forgotten. Finally I extend my deepest gratitude to my family and in particular my parents, Akira and Carine, for their constant support during all these years. I have dedicated this dissertation to them. Contents Acknowledgements ......................................................................................... vii 1 Introduction ................................................................................................ 13 2 Physics and Philosophy ............................................................................. 18 2.1 What physicists say about philosophy .......................................... 18 2.2 Why philosophy is relevant for physics ........................................ 20 3 Approaches to quantum gravity ................................................................ 22 3.1 Why do we need a theory of quantum gravity? ........................... 22 3.2 String theory ................................................................................... 24 3.3 Loop quantum gravity .................................................................... 27 3.4 Other approaches to quantum gravity ........................................... 28 4 Methodology and research in quantum gravity ....................................... 31 4.1 String theory ................................................................................... 31 4.2 String theory and philosophy of science ....................................... 33 4.2.1 String theory from a logical positivist’s perspective ..... 33 4.2.2 String theory from a Popperian perspective ................... 34 4.2.3 String theory from a Kuhnian perspective ..................... 37 4.2.4 String theory from a Lakatosian perspective ................. 39 4.3 Further remarks concerning string theory and scientific norms .. 43 4.3.1 Why are tests of string theory required? ........................ 45 4.3.2 String theory from an internalist perspective: explanations ..................................................................... 46 4.3.3 String theory from an externalist perspective ................ 50 4.4 Other approaches to quantum gravity ........................................... 51 4.5 Conclusions .................................................................................... 51 5 Realism, semantics and scientific theories ............................................... 56 5.1 Introduction .................................................................................... 56 5.2 Earlier views on scientific theories ............................................... 57 5.2.1 Logical positivism and verificationist semantics ........... 57 5.2.2 Quine and semantic holism ............................................. 58 5.2.3 Arguments for scientific realism ..................................... 59 5.2.4 Arguments for scientific anti-realism ............................. 60 5.3 A literal understanding of theories? .............................................. 62 5.4 Final comments .............................................................................. 63 6 Realism, underdetermination and string theory dualities ........................ 64 6.1 Introduction .................................................................................... 64 6.2 Underdetermination and scientific realism ................................... 64 6.2.1 Underdetermination ......................................................... 64 6.2.2 Scientific theories and realism ........................................ 66 6.3 Formalisms and physical content .................................................. 69 6.3.1 On how the word ‘duality’ will be used ......................... 69 6.3.2 Purported dualities outside of string theory ................... 70 6.4 Dualities in string theory ............................................................... 72 6.4.1 T-duality and mirror symmetry ....................................... 73 6.4.2 S-duality ........................................................................... 75