The problem of compensation of internal mechanical noise in test mass of gravitational wave antennae

We discuss how much can be the value of compensation of the internal mechan ical noise in test mass if ideal position meter is used for measurement of the difference between averaged coordinate on reflecting face surface of te st mass and coordinate averaged all over the back surface of the same test mass. It is shown that this compensation can help to overcome the standard quantum limit (SQL) by several times. However analysis of concrete interfer ometric meter with beams propagating inside test mass gives that parasitic thermorefractive effect obstructs the compensation. This effect consists in thermodynamical fluctuations of temperature which causes fluctuations of r efractive index due to its dependence on temperature. (C) 2000 Elsevier Sci ence B.V. All rights reserved.