What is an ordinary tide?
It is sloshing of the oceans in response to the changing gravity gradient of the moon. A similar thing can happen to superfluid helium in the STEP dewar. Although it is a lot smaller than an ocean, the helium is excited by the Earths gravity gradient, which is a lot stronger than the moons, and theres not much holding it in place (like gravity holds the oceans). The tide on the helium constitutes a moving mass which couples gravitationally to the accelerometers. This effect occurs at a multiple of the signal frequency, and since there are a lot of things going on that could convert part of it to the signal frequency, theres been a lot of fuss and bother about how big a problem it is for STEP,and what to do about it. The fact that the helium is superfluid, and has no significant viscosity or damping, merely adds to the concern.
