be written (Gaussian units) where is the fine-structure constant and the filling factor is and is the number of electrons in the material and is the area of the material perpendicular to the magnetic field. This parameter is important in the quantum Hall effect and the fractional quantum Hall effect. The filling factor is the fraction of occupied Landau states at the ground state energy. For cases of interest in the quantum Hall effect, is small. In that case the interaction energy is where (Gaussian units) is the interaction energy for zero filling factor. We have set the classical kinetic energy to the quantum energy Gravitation A gravitational disturbance is generated by the stress–energy tensor ; consequently, the Lagrangian for the gravitational field is spin-2. If the disturbances are at rest, then the only component of the stress–energy tensor that persists is the component. If we use the same trick of giving the graviton some mass and then taking the mass to zero at the end of the calculation the propagator becomes and which is once again attractive rather than repulsive. The coefficients are proportional to the masses of the disturbances. In the limit of small graviton mass, we recover the inverse-square behavior of Newton's Law. Unlike the electrostatic case, however, taking the small-mass limit of the boson does not yield the correct result. A more rigorous treatment yields a factor of one in the energy rather than 4/3. References Quantum field theory