# Do sound and light have anything in common?

## Main Question or Discussion Point

does sound acquire relative velocity from a moving source? or is it like light?

can the permeability and permittivity of the vacuum of space be comprehended as a kind of 'density' that is proportional to earth's atmospheric density by the same ratio as the speed of light is proportional to the speed of sound?

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But light doesn't acquire relative velocity. Light goes a constant speed in any given non-accelerated reference frame.

billy boy asked: "does sound acquire relative velocity from a moving source? or is it like light?"

Neither, but there's a third possibility. Sound waves, like all mechanical waves, travel at a constant speed with respect to the physcial medium on which they propagate. Sound waves in air travel at about 1100ft/sec wrt the air. If you're moving at 60 miles/hour (about 90 ft/minute) in the opposite direction as the wave, you'll measure its speed as 1190ft/sec. Well....that's almost right!

cookiemonster - yeah that's what i meant...

jdavel - thanks, so sound is a bit like light in that it doesnt acquire relative velocity, or it travels at a constant speed in relation to the medium it's in...ok...but it's different from light in a sense because not all observers will agree on its speed?

the example you gave me:

Sound waves in air travel at about 1100ft/sec wrt the air. If you're moving at 60 miles/hour (about 90 ft/minute) in the opposite direction as the wave, you'll measure its speed as 1190ft/sec. Well....that's almost right!
what do you mean 'almost right'? you mean owing to the miles to feet conversion? or is there some kind of intrinsic variation here? could it be that sound's velocity relative to an observer only changes because the scale of how relativity (time, space dilating etc.) affects the observations is much smaller? in this way sound waves are similar to EM radiation in a different medium?

the question i'm really curious about is: can the permittivity and permeability of vacuum be comprehended as a kind of 'density' proportional to earthly air by the same ratio as c is proportional to the speed of sound? that is, light is a wave in space-time, sound is the same kind of wave in an atmospheric medium...

billy_boy_999 said:
the question i'm really curious about is: can the permittivity and permeability of vacuum be comprehended as a kind of 'density' proportional to earthly air by the same ratio as c is proportional to the speed of sound? that is, light is a wave in space-time, sound is the same kind of wave in an atmospheric medium...
Yep, that's how you could do it, and how it was done classically. You see, when you put together the wave-equation of an EM-wave the propagation c (being 1/SQRT(epsilon.mu) ) pops up at the right place.

When do a similar thing for a disturbance in a gas (like air), the a propagation velocity pops up that involves the density of the gas, and some other property of the gas that I can't remember .

So in both cases, it is the properties of the medium (vacuüm, or aether as it was called at the time, and the gas) that determine that propagation speed of the wave.

billy boy 999 said: "so sound is a bit like light in that it doesnt acquire relative velocity, or it travels at a constant speed in relation to the medium it's in...ok...but it's different from light in a sense because not all observers will agree on its speed?"

Well, sort of, but be careful! The "difference" is more profound than the "similarity". In classical physics (Newton+Maxwell) all waves share the similarity; they travel at a speed which is independend of the speed of their source.

All waves have another similarity: Their speed is determined by the physical characteristics along the path they follow.

Mechanical waves (Newton) require that there be particles with mass along the path. No particles means no wave. For any given arrangement of any given type of particle (collectively called a medium), the speed of a mechanical wave is constant wrt to the center of mass of the medium.

A light wave (Maxwell) doesn't require that there be particles with mass along its path (if it did, we couldn't see the sun!). So for light, "no particles" doesn't mean "no wave". But "no particles" does mean no center of mass. So the speed of a light wave in empty space can't be constant with respect to the center of mass of a medium. Then what is it that the speed of light is constant wrt? In one of Mother Nature's most elegant moments, it seems she chose the answer to be: whoever measures it!

Dimitri Terryn said: "the propagation velocity pops up that involves the density of the gas, and some other property of the gas that I can't remember.
"

Pressure, and another factor that involves specific heat. The speed of any mechanical wave decreases with density and increases with some generalization of the stiffness of the medium. It would be interesting if each of the two factors that determine c had separate analogies, one to density and one to stiffness. But they don't. Again, a consequence of there being no medium to be stiff or dense.