# Constancy of the speed of sound

1. Jun 23, 2015

### PavanKumar

Hi,

As far as I know the speed of sound is also constant irrespective of the speed of the source. I guess, this is an obvious result of the property of waves. So, why is the constancy of speed of a light wave more important?

Please point out if there are any mistakes in my assumptions.

2. Jun 23, 2015

### Staff: Mentor

Hi PavanKumar, welcome to PF!

In a "nice" medium the speed of sound is isotropic, homogenous, and independent of the speed of the source. In vacuum the speed of light is also isotropic, homogenous, and independent of the speed of the source.

Where they differ is that the speed of light is frame invariant, whereas the speed of sound is not. It is the invariance, and not the constancy, that makes the speed of light so important.

3. Jun 23, 2015

### PavanKumar

Hi DaleSpan,

Thanks for the answer.

However, I am still a little confused. What is the simplest experiment that can prove this invariance? I believe we can't use the Michelson-Morley experiment as one because all the measurement devices and the light source were in the same frame of reference.

4. Jun 23, 2015

### Staff: Mentor

The MMX is exactly what proves it and the fact that the device exists in one frame of reference is key. If light behaved like sound, the experiment would show that light's speed is different in different frames (not frame invariant) and identify the frame where it is c (the "stationary frame"). Similarly, you could use an MMX type setup for sound to measure wind speed/airspeed.

5. Jun 23, 2015

### Staff: Mentor

I would start here, with this excellent review article on experimental tests of SR:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Test_Theories

"Robertson showed that one can unambiguously deduce the Lorentz transform of SR to an accuracy of ~0.1% from the following three experiments: Michelson and Morley, Kennedy and Thorndike, Ives and Stilwell. Zhang showed that modern experiments determine the Lorentz transforms to within a few parts per million."

6. Jun 23, 2015

### pervect

Staff Emeritus
The speed of light is independent of the speed of the receiver as well as the speed of the source. This is different than the behavior of sound, the speed of sound is isotropic only in a special receiver frame, the frame in which the receiver is "at rest" with respect to the sound media.

The key difference for light is that there isn't any such "special" frame, the speed of light is always isotropic for any receiver / receiver frame.