# Thread: speed of sound ?

1. Suppose you tap with a stick on the end of a long bar of iron, 1000 meters long.

speed of sound in the iron is supposed to be 10 km /sec.
So the sound reaches the end of the bar after 0,1 sec.

But as we are not inside the iron we would hear the sound from the back of the bar only after 0,1 + 3 seconds=3,1 sec.

The sound from halfway the bar we would hear after 0,05 + 1,5 sec.= 1,55 sec.

(so where someone inside the bar (or a water filled tube, same principle)would hear a distance in time between two events (the sound arriving halfway and arriving at the end) of 0,05 sec) someone outside would meassure the time between the events as 1,65 seconds.

Does that mean inside the bar time runs faster ?

2.

3. Originally Posted by Ghrasp
Suppose you tap with a stick on the end of a long bar of iron, 1000 meters long.

speed of sound in the iron is supposed to be 10 km /sec.
So the sound reaches the end of the bar after 0,1 sec.

But as we are not inside the iron we would hear the sound from the back of the bar only after 0,1 + 3 seconds=3,1 sec.

The sound from halfway the bar we would hear after 0,05 + 1,5 sec.= 1,55 sec.

(so where someone inside the bar (or a water filled tube, same principle)would hear a distance in time between two events (the sound arriving halfway and arriving at the end) of 0,05 sec) someone outside would meassure the time between the events as 1,65 seconds.

Does that mean inside the bar time runs faster ?
Time is the same everywhere in your problem.

Where in the world did the 3 sec and 1,5 sec come from ?

All that is going on is the propagation of a compressional stress wave in an elastic medium, steel. What is going on outside the bar, is irrelevant, and this could be happening in a vacuum if you wish.

4. Where did the 3 sec and 1,5 sec come from ?

Try a little harder mr Rocket you'll figure it out.

5. Originally Posted by Ghrasp
Where did the 3 sec and 1,5 sec come from ?

Try a little harder mr Rocket you'll figure it out.
Given that the question is coming from someone who manifestly fails to understand the physics involved, I can only guess that you think that

1) The speed of sound in air is 333.333... m/s

and

2) That the speed of sound in air has any relevance to the given problem.

Neither are correct.

It may be possible to deduce the mental mistakes of an idiot, but it is not a particularly good use of time. Adios.

6. without checking the actual calculations involved,
this looks like the velocity of sound in iron vs the velocity of sound in air,
which over a distance would be distinguished as two different signals separated by time of arrival to an observer ...

but why these two different signal travel times should be added together escapes me ...

[eta] I see that Dr Rocket has checked the numbers - and found further errors ...

7. without checking the actual calculations involved
and

I see that Dr Rocket has checked the numbers - and found further errors ...
So how do you know they are errors ?

If you place a giant block of foam somewhere two people can speak "around the corner" just fine. Sound doesn,t follow specific lines that way. So how combine that with speed of sound. The trajektory around the block one way or the other can be different in distance so you hear it twice ? Never noticed such effect simply because it doesn,t happen.

Can imagine trying to use some kind of path-integral method.

8. Originally Posted by Ghrasp
without checking the actual calculations involved
and

I see that Dr Rocket has checked the numbers - and found further errors ...
So how do you know they are errors ?

If you place a giant block of foam somewhere two people can speak "around the corner" just fine. Sound doesn,t follow specific lines that way. So how combine that with speed of sound. The trajektory around the block one way or the other can be different in distance so you hear it twice ? Never noticed such effect simply because it doesn,t happen.

Can imagine trying to use some kind of path-integral method.
Such an effect could very well happen. It's just that you've never been in a situation where you'd notice it. You'd need a very big block of foam (or more likely, a foam covered wall) and a loud enough noise to audibly carry all the way around the longer path.

BTW, why would you think time and the speed of sound are related?

9. If you did not have any other understanding of physics, and placed this idea in a nutshell, you may interpret the 'rate of time' as being different. We do not classify rate of time as being independent within certain material spaces...we classify it as a locally 'global' modifier that can make ANY matter that passes within its space 'run faster'. In other words, rate of time is to only be considered a result of time dilation. Even if every single thing in the planet were to equally begin moving faster, if it is not a result of time dilation, the 'rate of time' did not change' despite being exactly similar to the effects of TD. Energy travels faster within iron than air...and time can be considered just that, the rate of energy exchange and movement. Its just a matter of definition....if you classified the iron as speeding up time, many problems would arise in your ability to describe other facets of the universe.

10. Suppose I was blind I would have no notion of time in any visual sense but still also people who are blind have a notion of time (as anyone they live in their time).

Accoustic you can also lokate objekts and the measurements, strukture, materials etc of those objekts) by making sharp click noices with you,re tongue. Some blind people are very accurate in this. time or timing plays a role in this. You also have clocks that are visual and clocks that are audible or both.

The foam block you may be right but as soon as you have two particular trajektories these are just two examples every other trajektory is also possible then and just as legit. That would lead to the conclusion that the sound will "follow" a third (over it), fourth, fifth etc trajektory so all trajektories (?) according to the path-integral idea.

But then you would in this example not here two sounds but obviously an unendless amount of the same sound from different trajektories and thus different distances ? That,s weird, apart from echos (which you hear distinctive) when someone shouts at one side of a house the sound is clearly heard as one sound for someone on the other side of the house.

11. Originally Posted by Ghrasp
Suppose you tap with a stick on the end of a long bar of iron, 1000 meters long.

speed of sound in the iron is supposed to be 10 km /sec.
So the sound reaches the end of the bar after 0,1 sec.

But as we are not inside the iron we would hear the sound from the back of the bar only after 0,1 + 3 seconds=3,1 sec.

The sound from halfway the bar we would hear after 0,05 + 1,5 sec.= 1,55 sec.

(so where someone inside the bar (or a water filled tube, same principle)would hear a distance in time between two events (the sound arriving halfway and arriving at the end) of 0,05 sec) someone outside would meassure the time between the events as 1,65 seconds.

Does that mean inside the bar time runs faster ?

12. If the same athlete runs the 100m faster than he cam swim a 100m, that does not mean that time travels slower in water; it simply means the athlete travls slower through a different medium...as does sound.

13. That,s exactly what I mean happens when you use the term speed you are comparing speed of a swimming athlete in water with wavespeed in water and making conclusions.

If you want a comparison it,s better to take a distinct event like a swimmer jumping in the water of a lake and how fish at the other side of the lake in the water hear it (the hearin gitself is also an event by the way) compared with how a fisherman at about the same lokation and distance but outside the water hears it.

Using the notion of speed of sound the fish would hear the event (fish can hear sounds) after less seconds then the fisherman. So for them the event happen before the fisherman is aware of it.

Lets say the swimmer stays in the water for five minutes and then climbs out.

The sound for the fish takes five seconds to hear and for the fisherman twenty sec.

The swimmer jumps in at t=x (due to fog and troubled water neither the fish nor the fisherman can see the other side of the lake)

The fish would hear this at t=x+5. The fisherman at t=X+20.

The fish hears the swimmer go out of the water at t=x+5 + 300+ 5=x+310
The fisherman at : t=x+20+300+20=x+340

The timedistance for both between the two events is for the fish :

300+5

Fisherman 300+20. So both would audible experience different timedistance between the two events and as they have no sight they can't use vision to orientate.

I know there are lots of flaws in the reasoning for example hearing something is in itself an event and doesn,t happen at a moment of time (every sound would have endless short period / high frecquency) But still.

14. You still haven't said why you think the speed of sound should have anything to do with time.

And yes, in the foam block example, you'd hear all the different paths, but not all separately. If two path times close enough together, you'd hear them together. Plus, the total power of all the paths will add up to less than the total power of the original sound since sound dissipates as is travels and bounces.

For an echo, what you basically have is one small set of paths that loop back (after bouncing off a wall for instance) some time later while all other paths lose too much power before returning.

Oh and one more point. The speed of sound in water is about 5 times faster than the speed of sound in air. Does time run 5 times faster when you're swimming?

15. You still haven't said why you think the speed of sound should have anything to do with time.
You know it is said that if you look at something at far distance what you see happened earlier allready. The distance in seconds from here to the sun is related to C.

So if you would look at something through a glass-cable (C is different) with one eye and under normal conditions with the other eye (or two people standing next to each other) you would see it happen a little later through the glass in seconds as if at bigger distance if you would use C for vacuum. One event at two different times for both eyes.

every event has a beginning and an end. In between there is time, duration, this is even on quantumlevel.

Suppose there is a secquency of events following each other direktly like with a clock. For instance a light shines through a cable and around it. Again one eye could look through the cable and one eye not. The light goes of and on each second (a second of a second on etc). So it funktions as a clock.

For one eye the first second ends at t=1 (as normal) for "the cable eye" that second ends at t=1+x (x depending on length and how c is for the cable).
For "the cable eye" the next second can impossibly start before the first second ends could it ?

So second two has it,s beginning at : t=1+x. then it adds up ; the second second ends at t=2+2x, so the third starts at t=2+2x and ends at 3+3x etc.
60 +60x seconds would have gone by after a minute.
and only 60 seconds for the view with the eye not looking through the cable. Both eyes see the same first second of the second minute start with a difference of allready 60 x (s)

So then one eye "sees time go faster" then the other.

If someone would snap his finger at the end of the cable at the sixtieth second it would also come to the same difference. After the next minute the difference in time for another identical snap has grown to 120 X (s). Why would that be ? Is the cable expanding in length ?

That's impossible offcourse. For the sound example it,s more or less similar.

16. Originally Posted by Ghrasp
You still haven't said why you think the speed of sound should have anything to do with time.
You know it is said that if you look at something at far distance what you see happened earlier allready. The distance in seconds from here to the sun is related to C.

So if you would look at something through a glass-cable (C is different) with one eye and under normal conditions with the other eye (or two people standing next to each other) you would see it happen a little later through the glass in seconds as if at bigger distance if you would use C for vacuum. One event at two different times for both eyes.

every event has a beginning and an end. In between there is time, duration, this is even on quantumlevel.

Suppose there is a secquency of events following each other direktly like with a clock. For instance a light shines through a cable and around it. Again one eye could look through the cable and one eye not. The light goes of and on each second (a second of a second on etc). So it funktions as a clock.

For one eye the first second ends at t=1 (as normal) for "the cable eye" that second ends at t=1+x (x depending on length and how c is for the cable).
For "the cable eye" the next second can impossibly start before the first second ends could it ?

So second two has it,s beginning at : t=1+x. then it adds up ; the second second ends at t=2+2x, so the third starts at t=2+2x and ends at 3+3x etc.
60 +60x seconds would have gone by after a minute.
and only 60 seconds for the view with the eye not looking through the cable. Both eyes see the same first second of the second minute start with a difference of allready 60 x (s)

So then one eye "sees time go faster" then the other.

I
I'm sorry, but this is horribly confused. The delay does not compound. If the light turns on and off at 1 sec intervals then the "cable eye" also sees it turn on and off at I second intervals just displaced by "x" from what the other eye sees. In other words it sees the light turn on a t=1+x, 2+x, 3+x ...

The fact that the light takes time to travel the cable has no bearing on how fast the eye sees the light turn on and off.

17. Originally Posted by Ghrasp
That,s exactly what I mean happens when you use the term speed you are comparing speed of a swimming athlete in water with wavespeed in water and making conclusions.

If you want a comparison it,s better to take a distinct event like a swimmer jumping in the water of a lake and how fish at the other side of the lake in the water hear it (the hearin gitself is also an event by the way) compared with how a fisherman at about the same lokation and distance but outside the water hears it.

Using the notion of speed of sound the fish would hear the event (fish can hear sounds) after less seconds then the fisherman. So for them the event happen before the fisherman is aware of it.

Lets say the swimmer stays in the water for five minutes and then climbs out.

The sound for the fish takes five seconds to hear and for the fisherman twenty sec.

The swimmer jumps in at t=x (due to fog and troubled water neither the fish nor the fisherman can see the other side of the lake)

The fish would hear this at t=x+5. The fisherman at t=X+20.

The fish hears the swimmer go out of the water at t=x+5 + 300+ 5=x+310
The fisherman at : t=x+20+300+20=x+340

The timedistance for both between the two events is for the fish :

300+5

Fisherman 300+20. So both would audible experience different timedistance between the two events and as they have no sight they can't use vision to orientate.

I know there are lots of flaws in the reasoning for example hearing something is in itself an event and doesn,t happen at a moment of time (every sound would have endless short period / high frecquency) But still.
As Janus noted, horribly confused.

The speed of light in a medium has nothing to do with the nature of time.

There is only one speed for photons, and that is c. The "speed of light in a medium" is not c.

c is a fundamental constant of physics. It is the speed at which photons move in all inertial reference frames. It does not matter whether the photon is moving between atoms of some medium or moving through outer space. It still moves at c, which is about cm/sec.

In a medium the process of light propagatin involves the absorption and re-emission of photons by atoms and there an associated time delay in that process that results in a perceived lower "speed of light" but the photons still move at c in the vacuum between atoms.

Watching a pulse of light through air, through glass, or through a vacuum has no effect on time.

Time is not defined through c, but rather c is a speed whose definition depends on time and space, One can use that invariant relationship to define distance in terms of time, using c, or time in terms of distance, also using c. But that does change the nature of space, time, or spacetime.

None of this has anything to do with sound, or the speed of sound. Sound is nothing more than a propagating stress wave in a medium that is governed by the interatomic and intermolecular forces of electromagnetism. The speed of sound is not a fundamental constant of physics and has nothing whatever to do with the nature of time.

18. I'm sorry, but this is horribly confused. The delay does not compound. If the light turns on and off at 1 sec intervals then the "cable eye" also sees it turn on and off at I second intervals just displaced by "x" from what the other eye sees. In other words it sees the light turn on a t=1+x, 2+x, 3+x ...
I had a duration for the "no light second" of 1+x I think that may have confused you.

But if the "no light" second would take a second for both eyes (instead of a difference) the difference of the first second is not gone. If both take a second brake during the no-light second the bake for the glaseye begins later but he will have the same time for lunch.

The only difference with my former post would be that the difference runs up half as fast.

19. There is only one speed for photons, and that is c. The "speed of light in a medium" is not c.
That,s apart from this, it just takes a visible signal to come through a medium for an observer from a distance a certain time and that time is different when you look through water, glass, air, even a foggy day.
If the fotons take a cup of coffee in a cafe halfway and run a little harder in between is not relevant.

20. Originally Posted by Ghrasp
I'm sorry, but this is horribly confused. The delay does not compound. If the light turns on and off at 1 sec intervals then the "cable eye" also sees it turn on and off at I second intervals just displaced by "x" from what the other eye sees. In other words it sees the light turn on a t=1+x, 2+x, 3+x ...
I had a duration for the "no light second" of 1+x I think that may have confused you.

But if the "no light" second would take a second for both eyes (instead of a difference) the difference of the first second is not gone. If both take a second brake during the no-light second the bake for the glaseye begins later but he will have the same time for lunch.

The only difference with my former post would be that the difference runs up half as fast.
It makes no difference how long the No-light period is. If the light is on for 1 sec at the source and off for two, [i] both eyes will see the light for 1 sec and of for 2 sec. The only difference will be that they will be out of phase by the difference in propagation.

Example, Let's say that it take 1 millisecond for light to travel the distance through the vacuum. We'll assume that the cable has a index of refraction of 1.5

In this case, the light traveling through the cable takes 1.5 ms to make the trip.

The light turns on.
1ms later, the light arrives at eye 1
0.5 ms later, the light reaches eye 2.
At 1 sec, the light turns off.
1ms later at T=1.000001 sec the tail end of the light pulse reaches eye 1 and it see the light turn off.
At T=1.0000015 sec, eye 2 sees the light turn of

At T=3, the light turns back on
at T=3.000001, Eye one sees the light.
at T=3.0000015 sees the light.

At T=4. the light turns off again.
At T=4.000001, Eye 1 sees the light turn off
At T=4.0000015 Eye 2 see the light turn off.

The times between seeing the light on and off is the same as seen by both eyes, Eye 2 just sees the light turn or off 0.5 ms later than Eye 1. This difference remains constant. Eye 1 sees flash 1 0.5 ms before Eye 2 sees the same flash and Eye 1 sees the 1,000,000th flash 0.5ms before Eye 2 sees it.

21. Originally Posted by Ghrasp
There is only one speed for photons, and that is c. The "speed of light in a medium" is not c.
That,s apart from this, it just takes a visible signal to come through a medium for an observer from a distance a certain time and that time is different when you look through water, glass, air, even a foggy day.
If the fotons take a cup of coffee in a cafe halfway and run a little harder in between is not relevant.
So what ?

Consider a car coming at you at 20 feet per second from a point 100 feet away. The car is 20 feet long. The rear bumper passes the initial point at t= 1 second. The front bumper reaches you at t=5 seconds, and the rear bunper at t=6 seconds. The difference is one second.

Not consider a car coming at you at 10 feet per second from the same pont 100 feet away. This car is ten feet long. The rear bumper passes the initial point at t=1 second/ The front bumper reaches you at t=10 sedonds and the reat bumper at t=11 seconds. The difference is one second, as in the first case.

Exactly the same thing happens with your light signal -- one when the front bumper leaves the initial pont and off when the rear bumper leaves.

22. But still Truman Burbank (Truman show) saw the sun come up a little later then you or you must be in the same situation. Either the universe started earlier for him or the time between the start of the universe and the start of a day was different for him.

23. Originally Posted by Ghrasp
But still Truman Burbank (Truman show) saw the sun come up a little later then you or you must be in the same situation. Either the universe started earlier for him or the time between the start of the universe and the start of a day was different for him.
What in the world does a movie have to do with this?

It very simple. Eye 1 sees the light at T= 0.000001 sec, but the light took 0.000001 sec to reach him from the source. Thus according to Eye 1's clock the light at the source turned on at t= 0 sec.

Eye 2 sees the light at T = 0.0000015 sec, but the light took 0.0000015 sec to reach him, meaning by its clock, the light left the source at t=0. Both eyes agree as when the light turned on at the source.

It is no different than two cars leaving a city 100 mi away at the same instant but driving at different speeds. If they both leave at 1:00 by the clock at the city and one drives at 50 mph and the other at 25 mph, then:
One car will arrive at 3:00 by your clock and the other will arrive at 5:00. But taking account their speeds., you know that the first car left 2 hrs ago and the other 4 hrs ago. Thus you know that the both cars left at 1:00 by your clock, the same as they did by the city's clock.

24. What in the world does a movie have to do with this?
That could be a good question to the direktor.

Art and science are stronger connected then you might think. Einstein played a violin. Is it coincidence that with that instrument you can change a note freely different then with a guitar or most other instruments ? Playing different notes you change wavelengths. It could have influenced and help him develop his imagination and intuition.

To me the connection with the film is that Truman is seeing part of the world through glass as he was born in a glass studio. Allthough lengthwise it is short part of the trajektory in essence it is the same. But he can,t compare, he doesn,t even know the glass is there between him and the clouds and stars. Little like people in a stationary driving train not knowing the train is driving without a reference.

Allthough I admit I was mistaken I still have doubts (not about you,re calculations as much.

To give a hint to why :

If you would do a comparison with sound travelling through different media would the sound stay the same sound or be heard as a different sound ? Even with foggy weather a cow sounds allready different then when it is a bright sun-shiny day.

If this case (if the foggy air is a different medium like glass is different) you are in the same medium as the cow.

In my example that isn,t the case. So if I meassure the wavelength of the sound behind the glass and before it (the same medium) how can I know that wavelength doesn,t adapt inside and to the glass. The sound coult start outside the glass as particular wavelength, in the glass it changes and outside it changes back again each time according to the medium and the relation meters per second as is expressed with Cs.

That way you don,t need the notion of different speed. A meter meassurement and assuming the same wavelength all along the trajektory between observer and source would not do this fenomena justice. If the wavelength changes inside the glass (where you can,t meassure) maybe you (or we) should adapt the meter to meassure distance inside the glass.

That way you would have a different meter for inside the glass meassuring distance (glass as medium) and meassuring length (which is a meassurment not different then with or thickness (in case of truman the thickness of the glass is the length as a trajektory for the light of the sun).

Or length contraction of spacetime locally for a different medium ? that way a difference between the two meassurements (the thickness, width or length of the glass as an objekt and the glass as medium) could be integrated.

And it,s not a weird idea as for waves in water we can see it daily. A wave at sea from something like a boat changes it,s wavelength if the water is locally muddy, if the depth changes etc.

Also if spacetime deforms nearby objekts wouldn,t it be logic if it deforms even more inside it ?

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