  We study the prospects for using interferometers in gravitational-wave
detectors as tools to search for photon-sector violations of Lorentz symmetry.
Existing interferometers are shown to be exquisitely sensitive to tiny changes
in the effective refractive index of light occurring at frequencies around and
below the microhertz range, including at the harmonics of the frequencies of
the Earth's sidereal rotation and annual revolution relevant for tests of
Lorentz symmetry. We use preliminary data obtained by the Laser Interferometer
Gravitational-Wave Observatory (LIGO) in 2006-2007 to place constraints on
coefficients for Lorentz violation in the photon sector exceeding current
limits by about four orders of magnitude.
