Search Talks

The canonical example of emergence is how thermodynamics emerges from microscopic laws through statistical mechanics. One of the vexing questions in the foundations of statistical mechanics is though how is it possible to justify thermalization in a closed system. In quantum statistical mechanics, entanglement can give the key to answer this question, provided that they are typically very entangled. Fortunately, most states in the Hilbert space are maximally entangled. Unfortunately, most states in the Hilbert space of a quantum many body system are not physically accessible. We show that the typical entanglement in physical ensembles of states is still very high.

I will begin by discussing some of the strongest observational evidence for Lorentz symmetry, and the essential role that Lorentz symmetry appears to play in the consistency of black hole thermodynamics. Next I will discuss some reasons for suspecting that Lorentz symmetry may nevertheless be emergent. And finally I will discuss difficulties with the concept of emergent Lorentz symmetry, and how such difficulties might conceivably be overcome.

This paper has two aims. The first is to improve upon the diverse and often muddled philosophical characterizations of emergence by articulating reasonably precise necessary and sufficient conditions for a phenomenon to count as emergent in physics. Central to this account of emergence is the idea that emergent phenomena cannot be explained reductively. The second aim of the paper is to apply this account to the use of effective field theories in gravitational physics. Effective field theories have recently been applied to model the inspiral trajectories (and other features) of two compact, massive objects orbiting each other, with excellent predictive success. The calculational machinery has been ported from quantum field theory, but the physical interpretation is significantly different. The paper concludes that this application of effective field theories to gravitational physics is clearly not a case of emergence.

Prominent philosophers of physics, including Craig Callender and John Earman, have issued stern warnings against drawing any foundations of physics conclusions from theories obtained by taking the thermodynamic limit. Without dismissing these worries entirely, I argue that we shouldn't take them too seriously.

At the level of effective field theory it is possible to establish analogies between non-gravitational and gravitational systems. For example, first order perturbation equations in an analogue gravity model can be written as a wave equation in a curved spacetime. Perhaps the most intriguing application of analogue gravity systems is the possibility to experimentally investigate open questions in semi-classical quantum gravity, such as the black hole evaporation process. I will briefly discuss our recent black hole experiment, which demonstrates the universality of the Hawking process. If time permits, I will discuss the possibility to extend the analogue gravity programme, and outline the necessary steps towards full quantum gravity experiments.

The concepts of emergence and analogy are very closely related -- A is like B vs A is B. I will discuss this in the context of the emergence of/analogy with Hwking radiation in the arena of fluid systems, and the possibility of doing experiments in the lab. Does this mean gravity is emergent from some aether like theory? I think attempts to do that are fraught with difficulties, and will briefly discuss why I think so.

It is widely held that string theory shows that spacetime geometry and topology are emergent rather than fundamental. Often it is said that this follows from the various interesting dualities that exist within string theory. I will discuss the argument from duality, contrasting it with older arguments for the non-objectivity of spatiotemporal topology. I hope that this will clarify some questions about the role of spacetime in string theory---and about the differences between the ways that philosophers and physicists approach these questions.

We provide a microscopic understanding of the nucleation of topological quantum liquids that arise due to interactions between non-Abelian anyons. With the pairwise anyon interactions typically showing RKKY-type oscillations in sign, but decaying exponentially with distance, we show that the character of the nucleated phase is fully determined by anyon interactions beyond nearest neighbor exchange. We investigate this issue in the context of Kitaev's honeycomb lattice model. In the presence of vortex lattices, depending on microscopic parameters such as the vortex lattice spacing, we observe the nucleation of several distinct Abelian topological phases, that differ in their band structure and Chern number description. By employing an effective model of Majorana fermions, we show that these phases can be fully predicted from the vortex-vortex interactions. Corresponding microscopic results should hold for vortices forming an Abrikosov lattice in a p-wave superconductor or quasiholes forming a Wigner crystal in non-Abelian quantum Hall states.

Our ability to understand the physical world has to a large extent depended on the existence of emergent properties, and the separation of scales that permits effective field theory descriptions to be useful. Exploiting this fact, we can construct minimal models that enable efficient calculation of desired quantities, as long as they are insensitive to microscopic details. This works in many instances in physics, and I give some examples drawn from the kinetics of phase transitions mediated by topological defects. In other fields, such as biology, it is not so clear that these concepts are useful, and I will discuss to what extent emergence and effective theories might be useful.

It is usually assumed that the quantum wave-particle duality can have no counterpart in classical physics. We were driven into revisiting this question when we found that a droplet bouncing on a vibrated bath could couple to the surface wave it excites. It thus becomes a self-propelled "walker", a symbiotic object formed by the droplet and its associated wave. Through several experiments, we addressed one central question. How can a continuous and spatially extended wave have a common dynamics with a localized and discrete droplet? Surprisingly, quantum-like behaviors emerge; both a form of uncertainty and a form of quantization are observed. This is interesting because the probabilistic aspects of quantum mechanics are often said to be intrinsic and to have no possible relation with underlying unresolved dynamical phenomena. In our experiment we find probabilistic behaviors and they do have a relation with chaotic individual trajectories. These quantum like properties are related in our system to the non-locality of a walker that we called its "wave mediated path memory". The relation of this experiment with the pilot wave model proposed for quantum mechanics by de Broglie will be discussed.

This talk considers the extent to which the intertheoretic relation between an EFT and its (possibly hypothetical) high-energy theory supports a notion of emergence. When a high-energy theory exists, this relation is based on a process that involves the elimination of high-energy degrees of freedom. This elimination results in an EFT that formally bears little resemblance to the high-energy theory. I investigate the extent to which this lack of formal resemblance underwrites notions of novelty and autonomy that may be appropriately associated with emergence. I'll begin by reviewing the method by which an EFT is constructed from a high-energy theory by means of integrating out high-energy degrees of freedom from the latter. I'll then review a number of attempts in the philosophical literature to explicate the notion of emergence. I'll first consider general phillosophical accounts that identify emergence as supervienience without reduction, or as associated with various notions of autonomy (reductive, predictive, causal, and/or explanatory). I'll then consider more specific accounts related to physics in particular, including Batterman's (2002) notion of the failure of a limiting relation, and Mainwood's (2006)description of the concept of emergence associated with the claims of condensed matter physicists (e.g., Anderson 1972). This account conceives emergence as microphysicalism (the claim that emergent properties/entities are ultimately composed of microphysical properties/entities) coupled with novelty cashed out in terms of a mechanism (in this case spontaneous symmetry breaking) that produces a reduced phase space supporting (emergent) properties that are not explicitly defined on the initial phase space. A similar account is given by Wilson (2010), who explicates novelty in terms of an elimination of degrees of freedom. I'll suggest that Batterman's account does not quite succeed in the context of EFTs (simply put, the relation between an EFT and its high-energy theory cannot be described in terms of the failure of a limiting relation), and while the elimination of degrees of freedom does occur in EFTs, this process is different from the process described by Mainwood and Wilson (in particular, the phase space of an EFT is not, in general, a reduced phase space of a high-energy theory). This suggests that a notion of emergence as microphysicalism coupled with novelty can be applicable to the EFT context, as long as an appropriate mechanism that underwrites novelty, other than spontaneous symmetry breaking, can be identified. This mechanism perhaps can be identified simply as the particular approximation scheme employed in the construction of an EFT.

In the philosophical literature, effective field theories have been regarded as emergent in the sense of furnishing novel explanations. In particular, Batterman has argued that effective field theories in statistical mechanics are emergent in this sense. I will argue that effective field theories in quantum field theory do not furnish analogous novel explanations. There are relevant disanalogies between statistical mechanics and quantum field theory with regard to the roles played by idealizations and the explanatory goals of the application of renormalization group methods. Contrasting the statistical mechanics and quantum field theory cases highlights the role that the physical interpretation of the formalism and the goals of theorizing play in determining whether a particular effective theory counts as emergent.