  Speed of sound waves in gases and liquids is governed by medium
compressibility. There exists another type of non-dispersive waves which speed
depends on stress instead of medium elasticity. A well-known example is the
Alfven wave propagating, with a speed determined by a magnetic tension, in
plasma permeated by a magnetic field. Later, an elastic analog of the Alfven
waves has been predicted in a flow of dilute polymer solution, where elastic
stress engendered by polymer stretching determines the elastic wave speed.
Here, we present quantitative evidence of elastic Alfven waves observed in
elastic turbulence of a viscoelastic creeping flow between two obstacles
hindering a channel flow. The key finding in the experimental proof is a
nonlinear dependence of the elastic wave speed $c_{\mathrm{el}}$ on Weissenberg
number $\mathrm{Wi}$, which deviates from the prediction based on a model of
linear polymer elasticity.
