# Thread: Special Relativity in air.

1. Special Relativity in air.

If the universe was composed of still air, with no light or matter, would the speed of sound be the same for any observer, in any reference frame?

The only clock you have available works using sound, eg, the time for sound to travel from an emitter to a receiver.
If this is one unit of time, then time would slow if the clock moved through the air, because the distance travelled would be longer for a moving clock. The faster the clock moves, the slower the clock runs.

And when the speed of the clock reached the speed of sound, time would stop altogether, because the sound could never reach the reciever.

So the speed of sound would be the same, measured by any observer moving at any speed, because of this time dilation.

So doesn't special relativity apply to virtually any medium?

2.

3. Originally Posted by mistermack
Special Relativity in air.

If the universe was composed of still air, with no light or matter, would the speed of sound be the same for any observer, in any reference frame?
the speed of sound would be constant relative to the air, but not relative to any reference frame. Someone moving with respect to the air would measure the speed of sound relative to themselves as having a different value than someone at rest with respect to the air would measure.

The only clock you have available works using sound, eg, the time for sound to travel from an emitter to a receiver.
If this is one unit of time, then time would slow if the clock moved through the air, because the distance travelled would be longer for a moving clock. The faster the clock moves, the slower the clock runs.

And when the speed of the clock reached the speed of sound, time would stop altogether, because the sound could never reach the reciever.
The difference between this and the light clock example is that the speed of light is invariant. All frames of reference measure it as having the same value with respect to themselves.

Thus the observer traveling with the clock would always measure each tick as taking 1 sec. He would never measure his clock as ticking slowly. The observer watching the clock moving past him would measure each tick as taking longer. The light for him travels the longer distance at the same speed as the clock observer sees it traveling a shorter distance. What is one sec for the clock observer is more than one sec for the "stationary" observer.

In your example, the clock observer would notice that his clock was ticking slower as he went faster with respect to the air, because he would notice that the speed of sound relative to himself decreases as he moves faster with respect to the air.

So the speed of sound would be the same, measured by any observer moving at any speed, because of this time dilation.
No. it wouldn't

So doesn't special relativity apply to virtually any medium?
No, it doesn't. In SR , there is no medium through which light moves in the way sound moves through air SR relies on the fact that there is a speed (c) which is invariant.(and there can be only 1 invariant speed, because it also automatically becomes the speed limit for the universe.)

4. Originally Posted by Janus
the speed of sound would be constant relative to the air, but not relative to any reference frame. Someone moving with respect to the air would measure the speed of sound relative to themselves as having a different value than someone at rest with respect to the air would measure.
I don't think so, because the person moving would be using a clock that was slowed. If the only clocks were sound clocks, then you would always get the same value, no matter how fast you moved. The faster you move, the slower your clock runs.

Originally Posted by Janus
The difference between this and the light clock example is that the speed of light is invariant. All frames of reference measure it as having the same value with respect to themselves.
Yes, but that's because of the same effect. Your clock is made of matter, and runs at different rates in every frame, and ensures that you always get the same value for the speed of light.
I'm saying that the same would naturally happen when you measure the speed of sound, with a sound clock.

Originally Posted by Janus
In your example, the clock observer would notice that his clock was ticking slower as he went faster with respect to the air, because he would notice that the speed of sound relative to himself decreases as he moves faster with respect to the air.
Yes he would, if he was made of matter. But what if the observer was made of sound waves?
He would be then in the same situation as we are. Time would slow for him, and he would be unaware that his clock was ticking slower.
No matter how fast he travelled, he would be unaware of the slowing. Time for him slows at the same rate as time for his clock.

Originally Posted by Janus
No, it doesn't. In SR , there is no medium through which light moves in the way sound moves through air SR relies on the fact that there is a speed (c) which is invariant.(and there can be only 1 invariant speed, because it also automatically becomes the speed limit for the universe.)
But in the universe I described, in which there is only air, and sound waves, the same applies.
The speed of sound is the speed limit for the universe, and all observers measure the same value.

5. If there were a universe with a medium that effected the way clocks work, then it could be cooked up to give us all the results of SR.

But why would we want to do this?

6. Originally Posted by PhysBang
If there were a universe with a medium that effected the way clocks work, then it could be cooked up to give us all the results of SR.

But why would we want to do this?
What it shows is that SR is the result of using the speed of light, to measure the speed of light.
Just as when you use the speed of sound to measure the speed of sound, you get all the features of SR.

We are constantly told that the speed of light is an absolute constant that never varies.
Well of course it is, when you measure it with clocks governed by the speed of light.

Matter is so closely related to energy that physical clocks slow just as a light clock does.
So you are effectively measuring the speed of light using the speed of light.
Just as in my example, you are measuring the speed of sound, using the speed of sound.

In both cases, you are BOUND to get an invariable constant that cannot be exceeded.

So I'm just saying that SR is not some special mysterious property of spacetime.
It's just the result of our measuring apparatus being intrinsically the same as what we are measuring.

It's just like if we measured the distance from New York to LA using a standard tape measure.
If all the matter in the world doubled in size, then so would your tape measure, and you would still record the same identical distance.

7. Originally Posted by mistermack
Originally Posted by PhysBang
If there were a universe with a medium that effected the way clocks work, then it could be cooked up to give us all the results of SR.

But why would we want to do this?
What it shows is that SR is the result of using the speed of light, to measure the speed of light.
Just as when you use the speed of sound to measure the speed of sound, you get all the features of SR.
But that's false. One needs the speed of sound in a very special medium that also effects all physical processes in that medium. We know of no such medium.

We can also recreate SR with magical elves and fairies that set back all the clocks of people moving a certain speed. Would you like to investigate that theory?
We are constantly told that the speed of light is an absolute constant that never varies.
I doubt that, since the speed of light is different in different media.
Well of course it is, when you measure it with clocks governed by the speed of light.
Perhaps you can demonstrate how a pendulum is governed by the speed of light.

8. Originally Posted by PhysBang
But that's false. One needs the speed of sound in a very special medium that also effects all physical processes in that medium. We know of no such medium.

We can also recreate SR with magical elves and fairies that set back all the clocks of people moving a certain speed. Would you like to investigate that theory?
I have no idea what you mean there.

Originally Posted by PhysBang
I doubt that, since the speed of light is different in different media
Ok, the speed of light in a vacuum. That's just pedantic. Everybody just says the speed of light, meaning in a vacuum, unless they are specifically talking about light in a particular medium.

Originally Posted by PhysBang
Perhaps you can demonstrate how a pendulum is governed by the speed of light.
Perhaps I can. The pendulum moves in response to the force of gravity. Gravity propegates at the speed of light.

9. Originally Posted by mistermack
I have no idea what you mean there.
What I mean is that you have some serious postulates at work in your example beyond the speed of sound. You have a speed of sound that relates to everything. You have given us no reason to suppose such a relationship.
Ok, the speed of light in a vacuum. That's just pedantic. Everybody just says the speed of light, meaning in a vacuum, unless they are specifically talking about light in a particular medium.
Sometimes details matter.

Perhaps I can. The pendulum moves in response to the force of gravity. Gravity propegates at the speed of light.
Well that's getting there. But if you rely on SR or GR to make your explanation, then you can't use the same explanation for your speed of sound example. You have to come up with a new reason.

10. Originally Posted by PhysBang
What I mean is that you have some serious postulates at work in your example beyond the speed of sound. You have a speed of sound that relates to everything. You have given us no reason to suppose such a relationship.
Well, it's purely hypothetical, just to illustrate that SR would occur in any situation where you are made of, and your clocks are made of, the same material as the things you are measuring.

Exactly the same applies if the universe was water, and your clocks worked on sound waves in water.
You would naturally get time dilation and the same measured value for the speed of sound in water, from any frame of reference.

Originally Posted by PhysBang
Well that's getting there. But if you rely on SR or GR to make your explanation, then you can't use the same explanation for your speed of sound example. You have to come up with a new reason.
It's just a hypothetical example, to make a point about why we experience special relativity. It's not a special property of spacetime, it's just how we are forced to measure and observe our universe.
If particles are discovered that convey information faster than light, it will completely finish SR, we will have to start again.

In the sound example, WE can see what is really going on, because we have light, which moves faster than sound.
The same will happen if we discover particles moving faster than light. We will be able to see what is REALLY going on.

11. Originally Posted by mistermack
Well, it's purely hypothetical, just to illustrate that SR would occur in any situation where you are made of, and your clocks are made of, the same material as the things you are measuring.
It requires more than just that clocks are made of the same material used to measure them, it requires explicit assumptions about the way that those clocks operate.
Exactly the same applies if the universe was water, and your clocks worked on sound waves in water.
You would naturally get time dilation and the same measured value for the speed of sound in water, from any frame of reference.
Yes, for clocks that operate with sound waves in water in a certain way. We can shield a clock that operates using sound waves in water from the surrounding medium. Your scenario also assumes that this is not possible.

Originally Posted by PhysBang
Well that's getting there. But if you rely on SR or GR to make your explanation, then you can't use the same explanation for your speed of sound example. You have to come up with a new reason.
It's just a hypothetical example, to make a point about why we experience special relativity. It's not a special property of spacetime, it's just how we are forced to measure and observe our universe.
Not exactly. SR and GR lay out the conceptual limitations on how a clock might possibly operate, regardless of its construction. This is intertwined with claims about the nature of spacetime.

We know that the sound in water example is false because we construct clocks that depend in no way on the speed of sound in water and that show us that the speed is relative to the medium. In the case of SR, we have no means of measuring a medium.

12. Originally Posted by PhysBang
Yes, for clocks that operate with sound waves in water in a certain way. We can shield a clock that operates using sound waves in water from the surrounding medium. Your scenario also assumes that this is not possible.
Yes, WE can, but not in the Universe that I postulated, which consists ONLY of still air, and sound waves.

Originally Posted by PhysBang
We know that the sound in water example is false because we construct clocks that depend in no way on the speed of sound in water and that show us that the speed is relative to the medium. In the case of SR, we have no means of measuring a medium.
That's exactly my point. We have no way of measuring a medium, that we are part of. We are actually saying the same thing.

We can only falsify the SR of the sound example by using light, or matter. If these were not available, there would be no way to observe an absolute world, only relative.

The same applies to light and matter. There is nothing available to test whether there are absolutes, rather than relatives.
If particles were discovered that moved much much faster than light, SR would not survive, but we would be better able to observe what is REALLY happening.

13. Originally Posted by mistermack
That's exactly my point. We have no way of measuring a medium, that we are part of. We are actually saying the same thing.
Then you have no point at all, because all you are saying is "what if c were (some fraction of c). Then c would be (some fraction of c)"
We can only falsify the SR of the sound example by using light, or matter. If these were not available, there would be no way to observe an absolute world, only relative.

The same applies to light and matter. There is nothing available to test whether there are absolutes, rather than relatives.
If particles were discovered that moved much much faster than light, SR would not survive, but we would be better able to observe what is REALLY happening.
Yes, and if the moon was made of green cheese, the moon would be made of green cheese.

14. Originally Posted by Harold14370
Yes, and if the moon was made of green cheese, the moon would be made of green cheese.
.

If for "if the moon where made" I suppose (by who ?) but the moon can,t be an if just as iffy green cheeses don,t exist.

And if there where purple cheeses they would be made of purple cheese not iffy made or iffy purple.

15. Originally Posted by mistermack
That's exactly my point. We have no way of measuring a medium, that we are part of. We are actually saying the same thing.
Not quite. You are saying that, despite the lack of evidence and the lack of need for such a thing, there is a medium. I am saying that there likely is no medium.
We can only falsify the SR of the sound example by using light, or matter. If these were not available, there would be no way to observe an absolute world, only relative.
We can falsify the sound example if the universe worked like it did in the specific constraints that you built into the example. You built far more into that example than we need for SR or GR.
If particles were discovered that moved much much faster than light, SR would not survive, but we would be better able to observe what is REALLY happening.
Perhaps.

16. Originally Posted by PhysBang
Yes, for clocks that operate with sound waves in water in a certain Not quite. You are saying that, despite the lack of evidence and the lack of need for such a thing, there is a medium. I am saying that there likely is no medium.
Well, medium is just a word.
Spacetime is getting more and more like a medium as time goes on. I'm not saying there is a medium that things move through. I'm speculating that spacetime is something that we are part of. Rather than an object moving in a medium, we are a disturbance, which propegates through it.
A bit like a wave in a rope.
There are fields in spacetime, and spacetime can be distorted by gravity. So saying that there is no medium is getting less and less meaningful. If each particle in the universe can exert a force on every other, it seems to indicate that every tiny bit of spacetime is filled with SOMETHING.

17. Originally Posted by mistermack
I'm speculating that spacetime is something that we are part of. Rather than an object moving in a medium, we are a disturbance, which propegates through it.
A bit like a wave in a rope.
There are fields in spacetime, and spacetime can be distorted by gravity. So saying that there is no medium is getting less and less meaningful. If each particle in the universe can exert a force on every other, it seems to indicate that every tiny bit of spacetime is filled with SOMETHING.
Moving to pseudo. Speculating about some nonexistent medium is not an appropriate topic for the physics forum.

18. You didn't understand the post then.

In that case I suppose that speculating about one-dimensional strings is also not appropriate.

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