Crossword Genius

Physicist Ernst (4)

Ross

I believe the answer is:

mach

'physicist ernst' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for mach that I've seen before include "Ratio of a body's speed to the speed of sound" , "-- Number relates to speeds" , "Ratio used to measure high speeds" , "Ratio of the speed of a moving body to the speed of sound" , "Number referring to speed of sound" .)

I've seen this clue in The New York Times.
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