Is the speed of light and sound in the still water and running water the same? If not, why?

Question

Could you please answer the question for both light and sound and for both directions in the running water, (a) in the direction of movement and (b) against the running direction.

Answer 1

I find it interesting that you would ask this question as the two physicists that developed the prevailing theories that one would use to answer them compliment each other historically.

Einstein was influenced a great deal by a philosophical consideration of Mach's about the nature of space.

Sound travels through a medium as a wave.

If the observer is standing still and the medium in which sound waves are propagated through is also in motion with regards to the observer then it will affect his measurements of the speed of sound accordingly in a relativistic sense.

Really it depends on what exactly you mean by the speed of sound.

Normally this is taken to mean the rate at which sound waves propagate a given medium.

If however you mean speed as in the amount of time it takes for an observer to receive sound through a given medium from a given point in spacetime, then the relativistic effects of motion will have a nominal effect on the observers measurements of the speed unless the rate of relative motion of said medium are exceeding large.

Though in the case of water I believe it would also cause a Doppler shift that compresses the sound waves.

In a solid I believe the motion would be uniform for the entire body that forms the medium and would not change the wave lengths that result in sound so that the speed of sound would be measured as the same as if the medium were not in relative motion to the observer, excluding high rates of relative motion and nominal Lorentz effects.

The speed of light remains constant regardless of relative motion of the medium though the medium itself can effect the speed at which an observer will messure light.

This is has been empirically confirmed and it is a testament to special relativity, which in essence demonstrates that time is a dimension that light does not translate.

What that essentially means is that the speed of light travels only through spatial dimensions and that the rate it translates spatial dimensions is the maximum speed on can move, none of lights motion is expended in translating the dimension of time.

Very interesting question.

I do not have a formal education in physics though so I am not absolutely sure about the answer.

For the most part I believe because the speed of sound is measured as a wave that propagates through a particular medium, the relativistic effects that would come into play would so insubstantial so as to be consider null.

Have a star though.

Answer 2

  The speed of light becomes less in water due to the greater optical density but the speed of sound in water increases many times in water compared to air maybe close to 5000 ft per second.

Answer 3

Yes, this is an interesting question, but the answer is clear enough, if a bit complicated.

Sound waves travel through a medium through the elastic transmission of energy within the material. if the material were moving, the rate would be the speed of transmission within the material plus or minus the speed of the material. This would not be significant until the speed of the water approached some portion of the speed of sound in water - which is about 1500 meters per second.

As a way to envision this, you could imagine sending a sound impulse though a tank of water, in a spaceship traveling 100,000 meters a second. If you were on the spaceship, you would be quite surprised if the sound did NOT move though the water at 1500 meters per second. For someone in front  of the spaceship, someone for whom the ship is moving towards them at 100,000 meters per second , The speed of the sound would be much faster in the moving water than if the water were stationary

Light is more complicated. Light travels more slowly in denser materials, so it would travel more slowly in water than in air. But light's speed varies proportional to the density of the material. So if the water were moving in the same direction as the light, there would be a miniscule effect in that the water would 'appear' less dense to the light, and so the speed of light in that material would be very VERY slightly faster. Conversely, if the water were moving towards the source of the light, you would expect an infinitesimal decrease in speed by nature of the effective increase of density of the material it is traveling through.

This is related to the doppler effect, in that when the material is moving away from the light source, there is an effective lowering of the frequency of the light in that material, and from the way a prism refracts light, you can see that lower frequencies travel faster than higher frequencies though a transparent non-vacuum, like air or water.

i hope I was able to convey that in a way that makes sense.