Discussion session

What can AdS/CFT do? What has been achieved?
  • describes nontrivial fixed points (are they stable? can they be classified?)
  • allows computations in these fixed points.
  • allows deformations away from critical points.
  • geometrises the RG flow.
  • easily generalised to finite T/mu/B/condensates.
  • real time linear response very easy.
  • non equilibrium and nonlinear response.
  • fermi-surface like signatures in fermionic response at finite mu.
  • superfluid phase transitions.
  • probe boundaries of hydrodynamic descriptions.
  • in principle can add a lattice but need to solve PDEs.

What known cond.mat problems might be relevant?
  • pairing in cuprates.
  • gapless spin liquids.
  • deconfined "quarks" at finite charge density.
  • cold atoms out of equilibrium.
  • novel quantum Hall phases (magnetic phases in 3d).
  • SLEs?
  • critical exponents.

What are the main limitations of AdS/CFT?
  • "AdS/CFT" should be renamed "string/field" or "gauge/gravity" or "holography".
  • Restricted to theories with matrix valued fields? large N^2?
  • Need supersymmetry? (probably not in principle)
  • intermediate values of the coupling?

What new problems/approaches might AdS/CFT suggest?
  • thinking without quasiparticles at any intermediate step.
  • RHIC analogues in condensed matter
  • what can happen at finite density -- exotic phases of matter.
  • what is entropy?

Special challenges of QCP

(from Sondhi's lecture)
  • T>0 requires scaling functions (rather than just numbers).
  • Real time response is tough.
  • Complex actions (Berry phases).
  • Soft modes.
  • Non-order parameter transitions. eg. critical fermions, deconfined criticality, localisation, infinite disorder fixed points.