Is the Speed of Sound Affected by Gravity?

[cottonkat5]

Sorry if this is in the wrong thread (my first time posting),

I know that light is affected by gravity (gravitational lensing), but is sound affected in the same way?

 

[BruceW]

The effect of gravity on a light wave is explained by general relativity, because gravity warps space-time.
Before general relativity, we had the equation F=mg, but of course light waves don't have mass, so this is why relativity was an important shift in the understanding of gravity.
Sound waves are made by the movement of molecules, these molecules have mass. so gravity effects sound waves in 2 ways: 1) F=mg causes the molecules to be pulled towards the source of the gravitational field. 2) the curvature of space-time has a similar effect on sound waves as it does on light waves.
For gravity sources with comparatively low density (such as the earth), the effect of gravity on the curvature of space-time is much smaller than the effect due to F=mg.
So, for sound waves, F=mg is the main effect of gravity on the molecules, but for light waves, the curvature of space-time is the effect of gravity.
Therefore sound and light waves are affected by gravity in different ways.

 

[BruceW]

Actually, I think I am wrong. Gravity effects sound waves in only one way. And that can either be modeled by the theory that gravity warps space-time or the theory that gravity is a force. for sound, these two theories should give the same result.
But for light, the theory that says gravity is a force is incorrect. The theory saying that gravity warps space-time is correct for light waves.
therefore general relativity is correct for both types of waves, but F=mg only works for sound waves, (when the gravitational field isn't too large)

 

[BruceW]

If we're talking about light and sound waves in the Earth's atmosphere, they are both bent towards the centre of the Earth because the density of the atmosphere is greater closer to the earth.
The effect of gravity on light and sound waves would be much less, but I think it would have the same effect

 

[KingNothing]

From a practical standpoint, where we care only about Earth's gravity...no. The molecules will experience the same gravitational force as they would when there is no sound wave at all.

The exception is a sound wave with an extremely long wavelength that could move particles considerably closer or further from a large mass.

 

[AlephZero]

Sound waves are vibrations in some medium (solid or fluid) and they are affected by the stress field in the medium. (Example - the speed of sound for transverse vibrations of a stretched string depends on the tension in the string, and therefore the vibration frequency depends on the tension).

So sound waves are affected by gravity is so far as gravity produce stresses.

Cleatly gravity CAN produce significant stresses in bodies, but there is not usually a simple relation between the stress field and the gravitational field that causes it. (Example - the tension in the string in the previous example could be produced by hanging a mass from one end of it).