In 1945 he developed an approximation method for many-body physics.

In 2004, he was a co-recipient of the prestigious Eugene Feenberg Award, given to recognize researchers who have advanced the field of many-body physics.

In recent years the focus of the experimental research shifted towards many-body physics.

Like almost always in many-body physics or quantum optics, it is most convenient to apply the second-quantization formalism to describe the physics involved.

The area of many-body physics devotes considerable effort to quantum-mechanical screening, which is very relevant to condensed matter physics.

Like almost always in many-body physics, it is most convenient to apply the second-quantization formalism after the appropriate system Hamiltonian is identified.

Like almost always in many-body physics, it is most convenient to apply the second-quantization formalism.

The Dirac picture is the one used in perturbation theory, and is specially associated to quantum field theory and many-body physics.

Thus, many-body theoretical physics most often relies on a set of approximations specific to the problem at hand, and ranks among the most computationally intensive fields of science.

His main interests are atomic physics, condensed matter physics and many-body physics.