> quadrupolar effect Take a child's mobile or a pair of differently-coloured tennis balls (say red and green) glued to the end of a paper tube and suspend that

> quadrupolar effect Take a child's mobile or a pair of differently-coloured tennis balls (say red and green) glued to the end of a paper tube and suspend that from the ceiling at about shoulder height. Set the thing spinning in the plane parallel to the ceiling. Step back a bit and choose one hand's index finger to point at one of the tennis balls and the other hand's to the other. Left index to red, right index to green. Extend and retract each arm to try to maintain a constant distance from each fingertip to the ball it's pointing at. Play with parallax so that you notice your arms crossing during part of the orbit (red ball on right at same distance to you as green ball on left) and uncrossing half an orbit later. You'll also notice the other phase where both fingers/arms point practically parallel but with one hand closer to you, then half an orbit later further from you. Your fingers are basically indicating the masses in their circular orbit. For gravitational radiation from the orbiting binary tennis balls a ~cylinder of space where index fingers lie on the axis lengthens axially and contracts radially. (A really tiny amount). This is encoded in the Weyl tensor, which is the traceless part of the Riemann curvature tensor (for more on that, see Baez's stab at this, <https://math.ucr.edu/home/baez/gr/ricci.weyl.html https://math.ucr.edu/home/baez/gr/ricci.weyl.html>). In a linearization of the Einstein Field Equations, which is how most people are taught gravitational radiation, we get the h+ and hx polarizations; hx is maximal when your arms are crossed and half an orbit later, h+ maximizes when your arms are parallel. Most people also think about their mass-detector (fingertips) pointing at the nearest mass rather than always at one of the binary, and so miss the thinking behind "cross" and this edge-on vs a face-on view of the tennis ball binary.