 One difficulty is the requirement for energy levels approximating a "Jupiter size mass".Kenneth Nordtvedt showed in 1988 that gravitomagnetism, which is an effect predicted by general relativity but hadn't yet been observed at that time and was even challenged by the scientific community, is inevitably a real effect because it is a direct consequence of the gravitational vector potential.He subsequently showed that the gravitomagnetism interaction (not to be confused with the Nordtvedt effect), like inertial frame dragging and the Lense–Thirring precession, is typically a Mach effect.