  We define the critical coordinate velocity v_c. A particle moving radially in
Schwarzschild background with this velocity, v_c= c/sqrt 3, is neither
accelerated, nor decelerated if gravitational field is weak, r_g << r, where
r_g is the gravitational radius, while r is the current one.
  We find that the numerical coincidence of v_c with velocity of sound in
ultrarelativistic plasma, u_s, is accidental, since two velocities are
different if the number of spatial dimensions is not equal to 3
