In theoretical physics, the seesaw mechanism is a mechanism to generate very small numbers from "reasonable numbers" and very large numbers.

Mathematics behind the seesaw mechanism is the following fact: the 2 by 2 matrix

where B is big and M is of intermediate size has the following eigenvalues:

The larger eigenvalue is approximately equal to B while the smaller eigenvalue is approximately equal to

Therefore, M is the geometric mean of B and λ _- , up to the sign.

This mechanism is used to explain why the neutrino masses are so small. The matrix A is essentially the mass matrix for the neutrino. B, the Majorana mass, is comparable to the GUT scale and M, the Dirac mass, is of order of the electroweak scale. The smaller eigenvalue then leads to a very small neutrino mass comparable to 1 eV which qualitatively agrees with the experiments. See neutrino oscillation