What is the helium tide?
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. As background, a few simple cases can be calculated. Its not too hard to figure out the equilibrium shape of a free-floating droplet of
