(A) i: In conventional hydrodynamics wave crests move faster than troughs (red arrows), leading to forward-leaning waves. ii: forward-leaning waves as observed by a stationary gauge. iii&iv: reverse behavior is predicted for superfluid third sound waves due to their different effective gravity. (B) Laser drive used to initialize and readout large- amplitude third sound waves. Mean laser power is kept constant to avoid unwanted thermal drift (see Supplementary Materials, Section 2.2). (C) Photodetector voltage measured during wave initialization and readout. (D) Calibrated measurement of wave amplitude versus time, for the section of the ringdown highlighted in (C).
Christopher G. Baker 