The Permanent Step Left by GW150914: A Displacement-Memory Amplitude Computed From the Radiated Energy

When GW150914 passed, it should have left the separation of the detector's test masses permanently, if minutely, changed. This paper computes the size of that displacement-memory step from the published radiated energy and distance, with the angular dependence of the effect made explicit, and compares the step with the peak oscillatory strain of the same event. The radiated energy is 5.361e+47 J, and the geometric scale set by energy over distance is 3.263e-22 in strain units before the angular coefficient is applied. At optimal orientation the displacement-memory step is 2.545e-23, and with an inclination-averaged coefficient it is 1.338e-23. Against the event's peak oscillatory strain of 1.0e-21 the optimal step is a fraction 0.025, and the averaged step a fraction 0.013; memory in this event is a few per cent of the peak, not a rival to it. The displacement step is 2.55e-23 at optimal orientation and 1.34e-23 inclination-averaged, respectively 0.025 and 0.013 of the peak oscillatory strain. This study is computed from stated assumptions and published catalogue values; it is simulated, not measured.