The Ly \alpha forest in quasar spectra probes scales where massive neutrinos can strongly suppress the growth of mass fluctuations .
Using hydrodynamic simulations with massive neutrinos , we successfully test techniques developed to measure the mass power spectrum from the forest .
A recent observational measurement in conjunction with a conservative implementation of other cosmological constraints places upper limits on the neutrino mass : m _ { \nu } < 5.5 eV for all values of \Omega _ { m } , and m _ { \nu } \lesssim 2.4 ( \Omega _ { m } / 0.17 - 1 ) eV , if 0.2 \leq \Omega _ { m } \leq 0.5 as currently observationally favored ( both 95 \% C.L .
) .