An echo can push a whale call's peak past the speed of sound, and the position fix with it

The failure sits in the estimator, not in the water. Acoustic fixes are built by comparing arrival times across several seafloor receivers and dividing by an assumed sound speed, which is how a fin whale detected by a single hydrophone out to roughly 100 km can be placed without anyone seeing it ^[3]. That arithmetic presumes the loudest point in the recording marks the arrival of the direct path. When a call is made near the surface, the direct arrival and the surface-reflected arrival can land close enough together that their peaks and troughs reinforce and cancel, and the strongest point of the combined signal moves earlier or later than either arrival alone ^[4]. Nothing in the medium speeds up; the overlap relocates the maximum ^[7].

Run the simulated numbers the way a tracking program would. Against a 1500 m/s baseline, the reshaped test signals had peaks arriving at times that correspond to 1694.5 and 2782.5 m/s ^[6], which is 13 and 85 percent above the physical speed ^[14]. Invert that as a locator does, holding sound speed fixed and solving for range, and the fix comes in about 11 percent short in the first case and 46 percent short in the second ^[15]. The practical scale is smaller and still awkward: at 1500 m/s, displacing a fix by 300 m takes an arrival-time shift of about two tenths of a second ^[16]. That is well inside what peak-picking software will report without complaint.

What keeps this as bookkeeping rather than new physics is the second half of the paper. Spiesberger and Terray encoded two signals, a zero and a one, identical at first and then diverging ^[8]. The advanced peak arrives early but carries nothing that distinguishes them; the receiver cannot say which was sent until the diverging portion arrives along the direct path at the ordinary speed of sound ^[8]. "The effect sounds like a violation of physics," Spiesberger said in a press release, "but it isn't" ^[10]. His causality gloss is blunter: the trick cannot send a message to your past self to bet on the stock market ^[13].

The operational distinction that follows is between the peak of a waveform and the earliest moment a receiver can tell two messages apart. Only the second is a travel time. Every localisation method that treats the first as though it were the second inherits an error whose sign and size depend on geometry it never measured ^[8][4].