Banks works on theoretical elementary particle physics, cosmology and superstring/M Theory. He pioneered the use of instanton methods in quantum mechanics (long before the name instanton was invented). He made important contributions to lattice gauge theory and to the understanding of spontaneous chiral symmetry breaking in gauge theories. He made a key observation showing that Hawking's proposal that blackhole evaporation violates unitarity in quantum mechanics was untenable. Since 1984, Banks has concentrated most of his effort on theoretical and phenomenological aspects of string theory. He proved theorems about the necessary conditions for supersymmetric string vacuum states, and invented a gauge invariant form of string field theory. Banks demonstrated that only local symmetries are possible in string theory. He is one of the leaders in the field of string cosmology. One important contribution is the construction of a nonperturbative Lagrangian formalism for M theory in a variety of asymptotically flat backgrounds known as Matrix Theory. Matrix theory has been used to prove (as opposed to conjecture) a variety of string duality results including the duality between IIA and IIB string theories and eleven dimensional supergravity, and the duality between heterotic strings on tori and supergravity on K3 manifolds. It also provides an explicit quantum mechanical framework for describing the properties of nonsupersymmetric black holes.
More recently, Banks has been involved in a number of projects related to the foundations of string/M theory. He emphasized the importance of the high energy density of states as the key feature which distinguishes M theory from Lorentz covariant quantum field theories. This density of states is dominated by metastable black holes. Remarkably this means that many gross aspects of scattering amplitudes above the Planck scale can be understood in terms of semiclassical gravitational physics. Typical scattering processes at super-Planckian energies are dominated by black hole production and subsequent decay via Hawking radiation. This provides the ultimate short distance cutoff in quantum gravity. The fact that gross aspects of the physics can be understood semiclassically is an example of the UV/IR correspondence-superPlanckian states are large mass low curvature black holes. These ideas have led Banks to a general formulation of holographic space-time, which he believes is the proper generalization of string theory to generic space-time asymptotics. The theory is formulated in terms of a finite number of variables for each causal diamond in space-time, whose null boundary has finite maximal area. The degrees of freedom represent the orientations of pixels on a ``holographic screen" on the null boundary. Remarkably, when quantized, these can be viewed as the degrees of freedom of supersymmetric particles penetrating the screen. The causal structure and topology of space-time is completely fixed in terms of the relations between the variables assigned to overlapping causal diamonds. Only the pixel orients fluctuate in the quantum theory. This leads to complicated constraints on the time evolution operators, which have only been solved in a particular case corresponding to a cosmology called the Dense Black Hole Fluid. In this case an emergent space-time geometry is defined from the quantum theory. Banks has applied these ideas to 11 dimensional M-theory, and to the theory of a stable eternal de Sitter space.
Banks also investigated supersymmetry breaking in M theory and came to the conclusion that there are no asymptotically flat nonsupersymmetric states of the theory. SUSY breaking either leads to a complete collapse of spacetime, or to a cosmological vacuum in which Newton's constant asymptotes to zero. This suggests that SUSY breaking in the world we observe is caused by the fact that it has a nonzero cosmological constant. For this to be consistent with observations, the relation between the mass splitting in supermultiplets and the cosmological constant must differ from that obtained in the classical gravitational limit. Banks showed that a phenomenologically viable scaling law for the relation between the cosmological constant and SUSY breaking, follows from his holographic theory of stable de Sitter space. He has also pursued low energy field theories which implement these ideas. In particular the Pentagon model of TeV scale physics is an attractive model of SUSY breaking, which will be tested at the Large Hadron Collider. Banks is also exploring other, low energy tests of the model, involving a very light scalar particle, which can be produced in ordinary weak interactions.