# I Speed of light and speed of sound both constant

1. Jul 8, 2016

### freshnfree

I believe that the speed of sound is constant in the same medium as is the speed of light. I would like to understand why we need the the theory of relativity to explain the speed of light being constant but I believe it is not used to explain why the speed of sound is constant within the same medium.

2. Jul 8, 2016

### Clever Penguin

The speed of sound is constant for a medium at a certain temperature. Temperature plays a factor in the speed of sound.

At a higher temperature, the molecules are moving around faster, and so the speed of sound is faster.

3. Jul 8, 2016

### phinds

The speed of light in a vacuum is constant for all observers regardless of their velocity. This is a postulate of Special Relativity. The same is certainly not true of sound.

4. Jul 8, 2016

### Staff: Mentor

The key distinction is "invariant". The speed of light in vacuum, c, is invariant. The speed of sound is not invariant. Here is my favorite article in the topic.

http://mathpages.com/rr/s2-04/2-04.htm

5. Jul 8, 2016

### pervect

Staff Emeritus
In the frame of an (isotropic) medium, the speed of sound is the same in all directions, but only in the frame of the medium. In other frames, the speed of sound depends on the state of motion of the observer - if the observer is moving with the medium he observes the speed of sound to be different depending on which directio the sound is moving.

There is no reason to believe that there is any sort of "medium" in a vacuum. If light were like sound, the speed of light would depend on the speed of the observer relative to some hypotehtical medium. But this has not been observed.

6. Jul 8, 2016

### newjerseyrunner

The speed of sound is not constant and very dependent on your motion with respect to the source. If the speed of sound is 700mph and you are moving towards the source at 100mph, you would register the speed of soundwave as 800mph. With light (c) if you are moving towards the source at 1/2 c, you still register the speed of light as c.

7. Jul 8, 2016

### Staff: Mentor

You need to be careful about how you state this - it is correct only with the bolded corrections above. You might want to consider the case in which you are driving down the road towards the source at 100mph.... But you are in the middle of medium-strong hurricane so there is a 100mph groundspeed wind blowing in the direction of your travel.... what speed do you measure for the sound wave?

8. Jul 8, 2016

### MeJennifer

I find it an interesting question.

Does light go from A to B without a medium?
We could say yes, but the path in spacetime goes through curvature.
So then what is curvature? A medium? No? Then what is it? Just another word for the same thing?

9. Jul 8, 2016

### Staff: Mentor

The word "medium" is not very precise in that it doesn't tell us what properties a "medium" has. Air is a medium that has mass and a frame of reference in which it is stationary. Space doesn't have those properties.

There is a famous quote by Einstein describing space in GR as an "ether", which often sets people atwitter...

10. Jul 8, 2016

### MeJennifer

Spacetime certainly has properties!

11. Jul 8, 2016

### freshnfree

I should have been more specific. When I say "same medium" I mean exactly the same medium in every way including temperature, air pressure and so on.

12. Jul 8, 2016

### robphy

13. Jul 8, 2016

### freshnfree

I looked up speed of sound and it said speed of sound at sea level = 340.29 m / s.
If you are standing on the shore and a ship is out at sea blowing its fog horn and going towards you, I believe that the sound of that fog horn will reach you at the speed of sound but because the ship is coming towards you the pitch will be higher, the doppler effect. Similarly if it is moving away from you the sound of the fog horn will still reach you at the speed of sound, but the pitch will be lower, again the doppler effect.
If the ship is stationary and the observer on shore is moving a similar thing will happen but in a different way. At all times the speed of sound will be observed at 340.29 m/s but the frequency of the waves will vary.

14. Jul 8, 2016

### pervect

Staff Emeritus
You seem to be missing the point :(

True statement: the speed of sound is 340-odd meters/second relative to the air. I note that you didn't specify what the speed of sound was measured relative to, this turns out to be a critical omission.

False generalziation: The speed of sound is 340-odd meters/second relative to anything.

True statement: The speed of light is 299792458 meters/second relative to any inertial observer, no matter how fast they are moving.

This highlights the difference between light and sound. Note that experimentally, if light were like sound, the Michelson Moreley experiment would have had a positive result, and it did not.

15. Jul 8, 2016

### Mister T

And if, while the sound from that fog horn is approaching me, you are also moving towards me, how fast will you observe that sound to be travelling relative to you?

16. Jul 8, 2016

### freshnfree

I stand corrected. I believe that if the source of the waves is moving the observer will experience the sound waves at the speed of sound at sea level but if the observer is moving then that speed will change relative to the observer's speed.
But I don't believe the Michelson-Morley experiment had an observer moving relative to the source of the light.
Have there actually been any experiments done where there was an observer moving relative to the source of the light?

17. Jul 8, 2016

### PAllen

The observer was moving relative to the medium, no matter what was assumed about the rest frame of the medium. That was part of the ingenious design of the experiment (assuming a medium existed). By analogy to sound, it matters not whether the emitter is at rest or not relative to the medium; as long as the observer is moving relative to the medium (e.g. air), the speed measured will be anisotropic and different from the standard speed.

18. Jul 8, 2016

### freshnfree

which observer was moving in the michelson-morley experiment?

19. Jul 8, 2016

### phinds

Have you read anything about the MM experiment? Your question suggests not. You would likely find it informative.

20. Jul 8, 2016

### freshnfree

From what I have read in the Michelson-Morley experiment they shone two beams, one in the direction of the spin of the earth and one in the opposite direction. There was no moving observer. What have you read about the Michelson-Morley experiment?

21. Jul 8, 2016

### Staff: Mentor

The whole interferometer was moving in the experiment. That is why they did it at different times of day and different times of the year.

22. Jul 8, 2016

### Staff: Mentor

You evidently haven't read much, or haven't followed what you read very well. Try here for a start:

https://en.wikipedia.org/wiki/Michelsonâ€“Morley_experiment

23. Jul 8, 2016

### phinds

You clearly are missing the whole point of the experiment. The Earth was the moving observer. The point of the experiment was to show that there was an "Ether" through which Earth traveled and therefore through which light traveled. It was believed that the ether pervaded all of space. The negative result put a nail in the coffin of the "ether theory of light" and led the way to Einstein's Special Relatively.

EDIT: I see faster folk beat me to it.

24. Jul 8, 2016

### freshnfree

25. Jul 8, 2016

### Staff: Mentor

Did you read the description of the apparatus? Did you read how the two beams of light are sent into two arms at right angles (not in opposite directions, as you said)?

Did you read how the experiment was done, not just at different times of the day (so that the apparatus would be moving in different directions due to the Earth's rotation), but at different times of the year (so that the apparatus would be moving in different directions due to the Earth's motion around the Sun)? Did you read how the whole idea of the experiment was to detect the Earth's motion, not relative to any light source, but relative to a hypothesized medium through which light traveled, the ether? And how the null result of the experiment cast doubt on the whole hypothesis that there was such a medium at all?

If you run a similar experiment with sound, you will find, as others have pointed out, that the velocity of the observer relative to the medium--air--affects the speed of the sound waves that are observed. The velocity of the source does not come into play. By analogy with this, it was expected that the velocity of the Earth (and therefore the M-M apparatus) relative to the ether would affect the speed of light that was observed in the M-M experiment. But it didn't. That shows that light does not work the same as sound in this respect--which was not the result that M&M originally expected to find. More importantly for this discussion, the null result of the M-M experiment shows why the explanations of observed speed that work for sound won't work for light.