I was reading the Feynman lecture on Special Relativity and saw this:
This is analogous to the case of sound, the speed of sound waves being likewise independent of the motion of the source.
Why is the speed of sound waves independent of the motion of the source? I thought if the source is moving, the speed will be v + C as per this answer.
The speed of the wave does not change, but the frequency does due to the Doppler effect.
Think about the drawing pen of a classic seismometer, depending on the speed of the rotating drum during an earthquake, the waves will appear squished or expanded. However the image already traced by the pen always travels around at the exact speed of the drum staying put on the paper no matter how fast the drum was moving when it was drawn.
I had the exact same doubt while reading that lecture. Think of it like this, a source emits sound waves and they are technically just vibrations, compressions in the air, between all those particles; the wave travels forward in the form of compressions and rarefactions. now if the source was moving, how would it even affect these compressions, the wave would be travelling independently because when the vibrations are generated, the source's motion would not affect them. the vibrations would be produced by the source in an instant and if the source is moving, the next set of vibrations would be generated at the next point the source is, just that. the nature of the wave produced would surely be different because there would be changes in the frequency of the wave due to the motion, just like that weird sound produced by supersonic jets, but that will not affect the speed with which the wave will propagate.
I think he is referring to the velocity of sound in the medium, or the air. When the source it is moving with $v$ and produces that sound, then it goes through the air, (considering $v_{air}=0$). In the air, the sound always moves with $v_s$,the sound velocity, no matter what is the velocity of your source. As they say in the other post, if you have a jet moving faster than the velocity of sound, the sound waves will go slower than the jet and left behind
So, light shown on a spaceship traveling at .99C passes through the spaceship at C as seen from the pilot.
Feynman appears to say a jet pilot traveling at .99Vsound would see a sound wave emitted by someone at rest traveling at Vsound. This seems ludicrous since the sound source is moving away from the jet pilot and therfore barely able to catch up.
