Imagine a container which contains a speaker and gas at normal pressure inside.

Now imagine it moving at almost light speed (maybe c-100m/s) if the speaker makes a sound, causing particles to move at the speed of sound.

Would the particles moving parallel to the container’s movement move faster than light?
or is there something im missing here?


1 Answer 1


Within the box the velocities are the velocity of sound, and in the center of mass system of box+air+speaker masses and velocities are as known.

If the whole system moves close to the velocity of light all the masses will become relativistic masses, including the atoms in the air for the speaker. The calculation will show towards a limit to the velocity of light, higher than for particles at rest in the center of mass of the system, but the velocity of light will not be attained , as far as the observer seeing the box moving with velocity close to c.

You are missing the definition of relativistic mass,that the inertial mass is not invariant to lorenz transformation, for everything that moves with velocity close to c.

  • $\begingroup$ Thanks for the answer :). Ive thought of the effect on the mass of particles by the motion but wasnt entirely sure about it. $\endgroup$
    – Rickson
    Apr 14, 2018 at 9:50
  • 2
    $\begingroup$ Do we really need to move in relativistic mass? I would say that velocities are not additive with no dynamics consideration at all. $\endgroup$
    – Alchimista
    Apr 14, 2018 at 9:58
  • $\begingroup$ @Alchimista, agreed 100%. The light speed limitation is entirely kinematic, introducing dynamics is a serious distraction here. $\endgroup$
    – m4r35n357
    Apr 14, 2018 at 10:41
  • $\begingroup$ @Alchimista I find the concept or relativistic mass useful: that the classical inertial mass increases as one approaches the velocity c for kinematic intuition which depends on classical mechanics concepts. It is useful in calculating how much fuel is needed for a spaceship to reach relativistic velocities :). Surprisingly there are other uses, see question and answer and comments physics.stackexchange.com/questions/398985/… $\endgroup$
    – anna v
    Apr 14, 2018 at 10:42
  • $\begingroup$ @m4r35n357 see my comment to alchimista $\endgroup$
    – anna v
    Apr 14, 2018 at 10:50

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