Speed of Sound: Constant Irrespective of Source Speed

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Discussion Overview

The discussion revolves around the constancy of the speed of sound and its comparison to the speed of light, particularly focusing on the implications of frame invariance and the significance of these properties in different media. Participants explore theoretical aspects and seek clarification on experimental evidence related to these concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that the speed of sound is constant irrespective of the speed of the source, attributing this to wave properties.
  • Others clarify that while the speed of sound is isotropic and independent of the source speed in a "nice" medium, it is not frame invariant like the speed of light.
  • A participant questions the simplest experiment to demonstrate the invariance of the speed of light, suggesting that the Michelson-Morley experiment may not be suitable due to the frame of reference of the measurement devices.
  • Another participant argues that the Michelson-Morley experiment does indeed prove the invariance of light's speed and emphasizes the importance of the frame of reference in this context.
  • One participant references a review article on experimental tests of special relativity, mentioning that various experiments have confirmed the Lorentz transformations with high accuracy.
  • It is noted that the speed of light is independent of both the source and receiver speeds, contrasting with sound, which is isotropic only in a specific frame where the receiver is at rest relative to the sound medium.
  • Some participants highlight that the key difference for light is the absence of a "special" frame, as its speed remains isotropic for any receiver frame.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the speed of sound versus the speed of light, particularly regarding frame invariance. There is no consensus on the best experimental demonstration of these concepts, and the discussion remains unresolved.

Contextual Notes

Participants mention the dependence of sound's isotropy on the receiver's frame and the implications of frame invariance for light, indicating potential limitations in understanding these concepts fully. The discussion also highlights the complexity of experimental setups and their interpretations.

PavanKumar
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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.
 
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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.
 
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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.
 
PavanKumar said:
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.
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.
 
PavanKumar said:
What is the simplest experiment that can prove this invariance?
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."
 
PavanKumar said:
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.

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.
 

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