I seem to recall that Feynman had a nice discussion somewhere, possibly in the Feynman lectures, of why there is no such thing as gravitational shielding. IIRC it appealed to the microscopic picture of a Faraday cage and the purely attractive nature of the gravitational force. I can't seem to find this in my own copy of the Feynman lectures. Can anyone help me to locate this? It seems to me that there are more fundamental reasons why gravitational shielding is impossible. MTW has a nice illustration on p. 18 of a pedagogical device actually build by Waage at Princeton in order to illustrate the distinction between a Lorentz (free-falling, Einstein-inertial) frame and a non-Lorentz frame. Basically you have a box with guns inside, and you verify that the bullets' trajectories inside the box are straight. This constitutes an operational definition of a Lorentz frame. There are a couple of issues that are not explicitly addressed by MTW. (1) The box has to be initially calibrated under conditions where we know the box's frame is Lorentzian. (2) In order to carry out this initial calibration, we need to shield the box against any non-gravitational forces. It can't be touching anything external. It has to be put inside a Faraday cage to shield against E fields, a layer of mu-metal to block B fields, paraffin wax to exclude neutrons, etc. If it *were* possible to shield against gravitational attractions, gravitational waves, etc., then all this other shielding would presumably have at least some unintended effect on gravity as well. Therefore it seems to me that the whole operational definition of Lorentz frames depends on the assumption that gravity can't be shielded against. Since one form of the equivalence principle is the statement that local Lorentz frames exist, it seems to me that the e.p. implies the impossibility of gravitational shielding. Is this correct?