Is there space and time for experimental philosophy?
This subproject aims to build bridges between experimental quantum physics and philosophy. Introduction At the outset, we explain the three key steps behind our idea, so as to give the reader a better view of what is ‘under the hood’. 3.1.1.1 Key step 1: AdS-CFT This subproject is partly based upon the anti-de Sitter-conformal field theory correspondence, or AdS-CFT for short. This is a (mysterious) connection that - in the context of the experimental research in this subproject - allows the description of the behaviour of the electrons that live in essentially 2+1 dimensions in space+time in solid crystals of real materials we make and study in the laboratory. Important is that the behaviour of these materials cannot be successfully modelled using regular theories of solid-state physics. The solution can be found in the connection between this physical ‘problem’ (that of electrons in crystal) and a different one, namely the computation of how gravity works close to a black hole that lives in a special spacetime geometry of one spatial dimension higher (3+1 dimensional). The deep principle at work here, in general, is that of holographic duality: at the boundary or surface of the 3+1D gravitational problem lives a dual theory in 2+1 dimensions that is proposed to encode the behaviour of the electrons in the crystal. Unlike the electron problem, the gravitational one is theoretically tractable using the powerful mathematical tools of string theory. One of the ‘bibles’ for the AdS-CFT correspondence in the context of the physics of solids comes from The Netherlands, and is really quite readable [Zaanen 2015]. 3.1.1.2 Key step 2: emergence The experimentalists are studying in the laboratory display emergent properties. Emergence is at the heart of the NWA Route2 gamechanger DIEP – the Dutch Institute for Emergent Phenomena - and expresses itself as behaviour that can neither be predicted nor explained from the microscopic building blocks (→ bouwstenen) of the system. In our case the building blocks are the atoms, their arrangement with respect to each other (what we call the crystal structure) and their electrons. Among the properties that emerge are how these electrons move (→ metals), don’t move (→ insulators), align like compass needles (→ magnetism) or even move perfectly without collisions (→ superconductivity). Emergence is also a hot topic in philosophy, namely the idea that, as we move from the microscopic to the macroscopic world, or from one situation to another, new properties and perhaps new entities appear. According to Butterfield’s (2011) influential definition, emergence is ‘properties or behaviour of a system that are novel and robust relative to some appropriate comparison class’. An important question is whether and how novelty can arise in cases where the macroscopic objects reduce to microscopic ones, e.g. when their macroscopic properties follow from a microscopic theory. Thus recent philosophical work has focussed on reconciling reduction and emergence, i.e. explaining how reduction1 can be reconciled with the idea of novelty that is the hallmark of emergence. For logic teaches us that deduction gives no new content i.e. no novelty. Yet physicists tell us all the time that they find both emergence and reduction in the same system. This, in a nutshell, is the puzzle of the logical incompatibility of emergence and reduction. Different philosophers (including SDH, Butterfield, and many others) have proposed different solutions to this puzzle, and many questions remain: Is the puzzle resolved by interpreting reduction as formal deduction, but emergence as novelty in the interpretation/a domain of the empirical world–including the experimental conditions realized in the lab (as Butterfield and SDH advocate)? Or –regarding time– does emergence require ‘diachronic transformations’ [Guay and Sartenaer, 2016], i.e. specific experimental in-time-interventions done in the lab? Does time play a special role in emergent behaviour [Humphreys, 2016] or is it really coarse graining and effective (field) theory that do the job [Crowther, 2016]? Regarding space: what is the role of mesoscopic scales in comparing microscopic and macroscopic situations? Are limits required for genuine emergence [Batterman 2001]? Philosophers and theorists cannot answer these questions “from their armchair”: close collaboration with experimental physicists is required for steady progress in forefront science examples–hence our case for an experimental philosophy. A second reason why resolving important puzzles about emergence requires a collaboration of the kind this proposal brings together is that only the experimental conditions can determine the appropriate comparison class for emergence: since emergence is a contrastive notion (it stresses the differences between sets of properties, behaviours or theories), experiments should help identify the appropriate comparison class. Thus if a particular philosophical notion of emergence requires a comparison class that is e.g. not present in the experiment, the notion may well turn out to be irrelevant for the case at hand. Thus experiments have both positive and negative functions in experimental philosophy: (A) they give us new insights into the scientifically salient notions of emergence, and (B) they prompt us to shed philosophical theories of emergence that are irrelevant to (or, à la John Bell, are even incompatible with) the situation of interest. 2 3.1.1.3 Key step 3: experiments on emergent phases meet AdS-CFT + theories of emergence In different experiments, very recently, data from high temperature superconductors (specifically those made of a mixture of copper and oxygen, called cuprates) have been successfully modelled using AdS-CFT computations using a scheme called semi-holography [Smit 2021]. What MSG and his group measured in the lab was how fast the electrons get scattered when the temperature is raised above that at which the superconductivity switches on. This scattering rate is observed to grow with temperature and the energy of the electrons in the form of a power law, and crucially the power depends on the momentum (or k-vector) of the electrons. These k-dependent powers have never been seen in experiment before and evade explanation using regular condensed matter physics theory, but are a very natural and – in fact – inescapable prediction of the AdS-CFT computations. Proposed research Having used all these words to set the scene, we can now turn to our idea for which we apply for funding here. It is heavily ‘emergent’, and thus excellently positioned within DIEP (in NWA-Route2), and seeks to connect philosophy to the experimental physics of materials that can be investigated using lasers and magnets with the help of some vacuum and low temperatures. This proposal aims to tackle the following trio of big issues: • Can modern approaches describing the physics of emergent electronic quantum phases such as AdS-CFT interface and interact with philosophical research into emergence? • Can this incubate a new field of ‘experimental philosophy’? • What can the experimental exploration of emergent quantum phases and its philosophical underpinning teach us about emergence in general? We will hire a two-year postdoc interested in both (experimental) physics of quantum matter and the interface of philosophy with science/physics. The combination of the Institute of Physics (IoP) and the Institute for Logic, Language and Computation (ILLC) at the UvA, together with the other collaborators at Utrecht and Radboud Universities provides a framework combining experiment, theory and—crucially—philosophy in which the researcher can increasingly learn to connect these fields and use these connections as a test-bed for how to use the experimental data to contribute to emergence research on a more general level. The nascent connection between experimentally measured behaviour of electrons in the lab and questions of current interest to philosophers has got a huge boost due to relevance of theoretical tools provided by the AdSCFT correspondence [Zaanen 2015] and the very recent experimental data from the Amsterdam labs on the electronic behaviour of so-called strange metals found in copper oxide superconductors. The data have been discovered to be better described by AdS-CFT routes than by any conventional condensed matter theory [Smit 2021]. This experimental underpinning pushes the usually experiment-shy pairing of emergence + AdS-CFT into the bright lights of the laboratory. This degree of experimental interrogation has emboldened us to propose using this to explore to underpin philosophical research and then also turn the tables and ask what experimentally ‘tested’ AdS-CFT, other modern theoretical approaches and philosophy can then tell us about emergence in general. Each of the collaborating groups brings unique expertise and their overlap is where the action will happen in this project. The latest developments sketched above lead to a group of (related) questions, including: 1. Are accounts of emergence in the philosophical literature (cf. 3.1.1.2) useable to explain emergence in the holographic scenario and in the condensed matter experiments from the participating labs? Can experiments help us make a choice between the accounts? 2. Are the types of emergence the same in the two cases? Fortunately for the postdoc, both positive and negative answers are interesting here. This question works right towards one of DIEP’s central goals of classifying different ‘types’ of emergence. 3. The recent experiments on electron scattering in strange metals shows semi-holography to work well: does this ‘looser’ duality (signalled by the prefix ‘semi’) offer more possibilities for emergence? 4. Is the 25+ year-old claim that strongly interacting electron systems (like the high temperature superconductors) are exemplary emergent systems merited? Both positive and negative answers are interesting here, too. 5. How does the emergence of space and time in these weaker cases of holographic duality relate to discussions of spacetime emergence in the literature? Are there experimental “signatures” of the emergence of space / time? Can semi-holography be interpreted similarly to other dualities [De Haro 2017]? Definitions of what is meant by emergence research, what is meant by useful in the context of experimental contribution(s) all need to be clear—front-and-centre—for both the PD and their supervisors. This is why such a tight collaboration across the present experiment / philosophy boundary is a sine qua non. This NWA subproject focusses initially on bringing the three participating fields to bear on each other, addressing questions [1-3]. The postdoc will carry out ARPES experiments at UvA-IoP, and these data and their analysis – also in comparison with each other – will be used to test theories of emergence previously studied by the UvA-ILLC group. As referred to in the questions above, the results here are bound to give novel insights into emergence in these experiments, as not only in question (2) are either positive or negative answers significant and important given their joint experimental + philosophical foundations. In the latter stages of the 2-year period, the more general and wider-ranging questions (4 & 5) will be more central. Generation of secure foundations from the side of the philosophy/theory of emergence, so as to guide the settings of the many control parameters (think of the materials, dials and knobs) available to the experimentalists is an activity that gets a jump-start with this postdoc project. The postdoc recruited will be an experimentalist by training but interested in exploring the philosophical and theoretical dimensions of the larger scale effort. The PI’s and their collaborators form an effective co-supervision team for the postdoc, and the interdisciplinary environment offered by DIEP in Amsterdam with connections via Alix McCollam to RICH (Radboud Interfaculty Complexity Hub) in Nijmegen are ideal to help the young researcher feel at home in the heart of a larger effort on emergence and complexity.