A strongly coupled quark-gluon plasma

Successful description of robust collective flow phenomena at RHIC by ideal hydrodynamics, recent observations of bound c̄c, q̄q states on the lattice, and other theoretical developments indicate that QGP produced at RHIC, and probably in a wider temperature region T_c < T < 4T _c, is not a weakly coupled quasiparticle gas as believed previously. We discuss how strong the interaction is and why it seems to generate hundreds of binary channels with bound states, surviving well inside the QGP phase. We in particular discuss their effect on pressure and viscosity. We conclude by reviewing the similar phenomena for other 'strongly coupled systems', such as (i) strongly coupled supersymmetric theories studied via Maldacena duality; (ii) trapped ultra-cold atoms with very large scattering length, tuned to Feschbach resonances.