Speed of light and speed of sound both constant

[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.

 

[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.

 

[phinds]

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.

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.

 

[Dale]

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

 

[pervect]

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.

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.

 

[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.

 

[Nugatory]

The speed of sound is not constant and very dependent on your motion with respect to the source. If the speed of sound in still air is 700mph and you are moving towards the source at 100mph relative to the air, you would register the speed of soundwave as 800mph.

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?

 

[MeJennifer]

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.

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?

 

[russ_watters]

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?

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...

 

[MeJennifer]

The word "medium" is not very precise in that it doesn't tell us what properties a "medium" has.

Spacetime certainly has properties!

 

[freshnfree]

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.

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.

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.

 

[robphy]

(speed of sound in still air at STP) = 331 m/s.
(speed of light in air at STP)= (speed of light in vacuum)/1.00029 = 299 705 543 m / s ( https://www.google.com/search?q=(speed+of+light)/1.00029 )

(speed of sound in vacuum) = 0 m/s.
c=(speed of light in vacuum) =299 792 458 m / s ( https://www.google.com/search?q=speed+of+light )

The invariant (independent of observer) speed is c.

 

[freshnfree]

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.

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.

 

[pervect]

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.

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.

 

[Mister T]

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

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 traveling relative to you?

 

[freshnfree]

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.

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?

 

[PAllen]

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?

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.

 

[freshnfree]

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.

which observer was moving in the michelson-morley experiment?

 

[phinds]

which observer was moving in the michelson-morley experiment?

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

 

[freshnfree]

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

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?

 

[Dale]

which observer was moving in the michelson-morley experiment?

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.

 

[PeterDonis]

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.

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/Michelson–Morley_experiment

 

[phinds]

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?

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.

 

[freshnfree]

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/Michelson–Morley_experiment

I looked at that. I do not see it mention something or someone which is moving relative to the source of the light.

 

[PeterDonis]

I looked at that. I do not see it mention something or someone which is moving relative to the source of the light.

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.

 

[freshnfree]

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.

Yes I know that the velocity of the source does not come into play, it is the velocity of the observer which is important relative to the medium. The experiment done by M-M is not using a simple method whereby you have a source of light, such as a torch and then you have an observer moving either towards the torch or away from the torch and that observer is able to calculate the speed of light reaching it from the torch. Of course, it would be very difficult to use this simple method because the speed of light is so fast that the observer would have to be moving very fast for differences in the speed of light to be observed. Since this simple method has not been used we do not really have a true experiment where we have a source of light stationary and an observer moving relative to that speed of light. Once you start bringing the velocity of the Earth into the equation, you are bringing in factors that could muddy the waters.

 

[PeterDonis]

we do not really have a true experiment where we have a source of light stationary and an observer moving relative to that speed of light.

It is true that the M-M experiment is not of this type. But that does not mean no such experiment has ever been done.

Also, I'm not clear why you think the observer moving relative to the source is relevant to the comparison between light and sound that is, as I understand it, the topic of discussion in this thread.

 

[russ_watters]

I looked at that. I do not see it mention something or someone which is moving relative to the source of the light...

The experiment done by M-M is not using a simple method whereby you have a source of light, such as a torch and then you have an observer moving either towards the torch or away from the torch and that observer is able to calculate the speed of light reaching it from the torch.

An experiment where the source and observer are located at a fixed distance with respect to each other is simpler and works just fine. The simple description is that with sound, the transit time of a sound between a fixed source and observer will change with a changing wind. Light's speed doesn't change in that way. That's what the MMX showed.

The difference between how light and sound behave is pretty simple. Please try harder to open your mind to learning it.

 

[freshnfree]

It is true that the M-M experiment is not of this type. But that does not mean no such experiment has ever been done.

Also, I'm not clear why you think the observer moving relative to the source is relevant to the comparison between light and sound that is, as I understand it, the topic of discussion in this thread.

This discussion has evolved but it is still in essence comparing light and sound waves. I realized my mistake when I said that the speed of sound would be the same in any situation. It wouldn't if the observer was moving relative to the medium of the sound wave.
So have there been any other experiments which actually have a moving observer to the light source, because if not, I don't think it has been conclusively proven that the speed of light is constant.

 

[russ_watters]

This discussion has evolved but it is still in essence comparing light and sound waves. I realized my mistake when I said that the speed of sound would be the same in any situation. It wouldn't if the observer was moving relative to the medium of the sound wave.
So have there been any other experiments which actually have a moving observer to the light source, because if not, I don't think it has been conclusively proven that the speed of light is constant.

Per my post previously, your second last and last sentences contradict each other. The second last has the medium moving with respect to the sound wave and the last sentence has the observer and source moving with respect to each other. Again, either works and yes, both have been done. GPS is doing the second one right now.

There's an effort issue here, but also one of trust and humility: you really should trust that the millions of scientists who have studied this over the past hundred years have put enough thought into this issue that they haven't overlooked something that you, who are just now starting to learn about it, have figured out.

 

[freshnfree]

What exactly do you mean by GPS is doing the second one? Do you mean that GPS is the equivalent of an experiment where you have an observer moving relative to the medium in which there is the source of a beam of light? If that is the case, could you please be more specific in the details.

 

[freshnfree]

This discussion has evolved but it is still in essence comparing light and sound waves. I realized my mistake when I said that the speed of sound would be the same in any situation. It wouldn't if the observer was moving relative to the medium of the sound wave.
So have there been any other experiments which actually have a moving observer to the light source, because if not, I don't think it has been conclusively proven that the speed of light is constant.

I may sound like a novice because I have made some errors, but strangely I knew when I said that the speed of sound was always constant that it was wrong. I still said it because in my mind I was trying to figure something out and that is sometimes how it works. I am not an expert either but I did study physics at high school and first year university. Then I switched to computing science. In that time you do learn the basic fundamental principles of science and I feel that sometimes they are getting broached.
It has been a number of years now when I have felt that something wasn't right so this is not just a fly by night thought.

 

[Vanadium 50]

1. The fact that you don't understand something does not mean nobody does, or that it is wrong.
2. PF is not the place for challenges to conventional physics. That's what the journals are for.
3. If you want to convince people that conventional physics is incorrect, making mistakes when discussing it is not the way to do it.

 

[Drakkith]

So have there been any other experiments which actually have a moving observer to the light source, because if not, I don't think it has been conclusively proven that the speed of light is constant.

Yes, there is an easily observable redshift and blueshift in various astronomical signals (pulsars, CMB, spacecraft , etc) which matches up perfectly, as predicted by SR and GR, with the Earth's orbital motion around the Sun.

 

[Nugatory]

What exactly do you mean by GPS is doing the second one? Do you mean that GPS is the equivalent of an experiment where you have an observer moving relative to the medium in which there is the source of a beam of light? If that is the case, could you please be more specific in the details.

All the GPS satellites are moving relative to one another and the surface of the Earth while continuously exchanging light (at radio wavelengths) signals. Furthermore, different points on the surface of the Earth are moving in different directions at different speeds (because of the Earth's rotation). GPS positions are calculated using a constant speed of light, and would be way off if the motion of either the transmitters or receivers affected the speed of light.