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If pushing an object causes waves to move through it at the speed of sound, what if both you and the e object were moving at the speed of sound through a vacuum and you pushed it then?

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    $\begingroup$ If you're moving at the speed of sound, through a vacuum, you may as well be stationary $\endgroup$
    – CDCM
    Commented Aug 22, 2017 at 17:04
  • $\begingroup$ What role do you think the surrounding vacuum plays? $\endgroup$
    – Jon Custer
    Commented Aug 22, 2017 at 18:34

3 Answers 3

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Since you are talking about waves in the object caused by your pushing on it, what matters is the relative velocity between you and the object. Both you and the object may together be moving close to speed of light but that is of no import; relative velocity between you and the object is unaltered and therefore as observed by you, the waves in the object will still travel at sound speed.

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Double the speed of sound. Unless you want the relativistic correction, velocities add.

Here's another example: Say I walk 1 mph forward on a bus moving 29 mph relative to the ground. Then I'm moving 30 mph relative to the ground.

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This is a matter relative motion. By vacuum, you mean without the influence of any outside forces. This means that "traveling at the speed of sound" is meaningless because you have nothing to reference. In order to be traveling with a certain speed, you need to define something that is stationary. So we can abandon that idea. FOr the rest of the answer, you are stationary relative to the object initially.

In a vacuum, there is no speed of sound, since no sound can travel. But there is a speed of sound for the materials in the object and your body. When you push on the object, the force causes a deformation in the object that travels through it at the speed of sound in that object. This causes an acceleration of the object.

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