# Waves And Relative Velocity

relativity

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### #1 ralfcis

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Posted 22 May 2019 - 08:02 AM

In 2006 I asked my first question on a physics forum. I couldn't understand the difference between a sound wave and a light wave as it pertained to relative velocity. Of course I got nothing but nasty, snarky, condescending, general and irrelevant answers from Wiki-like sources to my question which set the tone of my attitude towards physics forums to this day. I understood a source could not force a sound or light wave any faster through it's medium, the wave's velocity was always the same relative to its medium. Any velocity of the source or a receiver would manifest itself in the altered frequency of the wave. Years later I learned this was not completely true for sound waves. Although a car approaching you with it's horn blaring couldn't force the sound to reach you any sooner, you could run towards the sound or a strong wind behind the car could make the sound cover the initial distance between you and the start of the sound wave in less time than if you didn't move or if the wind wasn't blowing. Same distance in less time meant the wave's  velocity relative to you increased.

The same was true for the source's relative velocity to the sound wave. Although it couldn't push the sound wave's velocity relative it its medium any faster, the source's relative velocity to the medium would allow it to catch up with the wavefronts or a strong wind behind the car could reduce the car's relative velocity to the wavefronts. The source's velocity relative to the wave altered because its velocity relative to the medium altered while the wave's relative velocity to its medium did not. The key understanding here is there are 2 relative velocities to consider: the relative velocity of the wave to its medium and the relative velocity of the source or receiver to the wave's medium.

The permittivity and permeability characteristics of space make it an electromagnetic medium that allows for the propagation of electromagnetic waves. Sound is a mechanical wave in a material medium. Space, ideally, is a vacuum devoid of material for sound to propagate through but that doesn't mean it is not a medium for light to propagate through. Water has material but light uses the electromagnetic field of water to propagate through it at 75% the speed of light. We will be considering water and vacuum mediums for light to understand the relative velocities of sources and receivers to those mediums to address the main difference between sound and light waves wrt relative velocity.

The main question is this: If you can run towards a sound wave and increase its velocity relative to you, why does relativity say you can't run towards a light wave and increase its velocity relative to you? Just like for the sound wave, your motion towards the wave cuts down the initial distance between you and the wave which allows the wave to reach you in less time just as though you were standing still and the wave's relative velocity had increased to reach you in less time. This addition of velocities definition of relative velocity just ignores the fact not only the time has changed but so has the distance. If instead of choosing the initial distance over final time as the definition of relative velocity you chose final distance over final time, you'd get a completely different answer.

It's the same for light, your velocity towards the light allows the light to reach you sooner so the relative velocity should increase if you use the formula of initial distance over final time. But relativity has changed the rules, there is no relative velocity greater than c so your relative velocity to c can't add or subtract from c. This is enshrined in the relative velocity combination law. But at the same time this does not mean the light doesn't take longer to reach you if you're moving away from it or less time if you're moving towards it.

So how do you resolve this paradox? Well you come up with the idea that the distance between you and the light is contracted by your velocity just as your time is dilated so that the velocity of light remains the same in the spacetime warp your velocity is creating. And from there the history of science fiction begins to the delight of movie goers everywhere.

In the only course I took on relativity, Brian Greene's on worldscienceu.com, the photon is treated like a baseball instead of  a wave. While a wave's velocity can't be affected by the velocity of the source that launched it, a baseball's velocity can be. So using perfect logic, a photon baseball's velocity can't be increased or decreased by a train's motion unlike it would be in a Newtonian universe. Of course this is total nonsense because light doesn't behave like a baseball but it's crucial to selling relativity. Let's look at the train example to see what I mean.

A train is whizzing into a station and as the middle of the platform aligns with the middle of the station, a light beam is sent from the middle to both ends of the train. From inside the train, the light hits both ends simultaneously but from outside the train, the back is moving toward the light so the light reaches it sooner. The front of the train is moving away from the light so the light has to cover more distance in more time to reach the front. I don't see where the train's motion creates a spacetime warp to conserve c from all perspectives when c is simply the distance covered in the time it takes to cover it. No magic here but I'll show you where the magic really is. It's not in the train's relative velocity to light directly, it's in the train's relative velocity to the medium of light.

Let's consider the train example using sound waves. As the middle of the train aligns with the middle of the platform, an air horn goes off inside the train but a firecracker goes off outside the train's middle. Since the air is motionless within the train, the air horn will hit both ends simultaneously from the train's perspective but the sound of the firecracker will hit the back end first because the back end is moving towards it while the firecracker noise will take more time to reach the front because it has to travel a greater distance in more time to reach the front. However if the same experiment had been done using an open boxcar, there would be no difference between when the airhorn or the firecracker noises reach the ends of the train because it's about the train's relative velocity to the medium, not to the sound directly.

So how is light different? If the train was a giant vacuum bottle travelling through a vacuum, there would be no difference whether the train was a vacuum bottle or open to the outside vacuum. There's no medium to move relative to. There's no vacuum wind. This is what the MMX proved. If the train let off a red light inside the train and a blue light outside the train, the outside perspective would see both the blue and red lights hit the back simultaneously before they both hit the front simultaneously later. From inside the train's perspective, both red and blue lights would hit the front and back of the train simultaneously. This could never happen in the airhorn and firecracker example because of the relative velocity of the train to the sound medium.

Let's do the intermediate example of a water filled train going through water. (This is actually the Fizeau experiment.) Light is no longer going at c through this medium but 75% c. The train's velocity will actually be a factor this time between an open box car and a sealed train version because it can have a relative velocity to the medium. However, the addition of the velocities is not straight Newtonian additive but the combined relative velocity is according to the relativistic combo law. So there's nothing magical about the nature of light at all, the magic happens when things bump up against the universe's max speed limit.

Edited by ralfcis, 22 May 2019 - 08:10 AM.

### #2 Dubbelosix

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Posted 22 May 2019 - 08:37 AM

Ignoring the rest of your post, could you first give us the formula for relative sound and relative light first.

I could probably find them, but when you open with a question, it would be really nice if you could present some equations so that we can follow properly.

In 2006 I asked my first question on a physics forum. I couldn't understand the difference between a sound wave and a light wave as it pertained to relative velocity.

Edited by Dubbelosix, 22 May 2019 - 08:37 AM.

### #3 ralfcis

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Posted 22 May 2019 - 10:11 AM

v=x/t

### #4 ralfcis

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Posted 22 May 2019 - 10:24 AM

Ok so here's the crux of the matter, how is it possible the red light dot and the blue light dot arrive simultaneously from any perspective regardless of the motion of the train. The red dot inside the train travels half a train length while the blue dot outside the train has less distance to travel since the back of the train moves towards the dot. Relativity explains this through length contraction, the train in motion contracts such as the distance the light travels outside the train equals the distance the light travels within the train from the outside perspective. Case closed.

But I see a far simpler explanation for the blue and red dots to hit the back of the train simultaneously from all perspectives. The train's velocity is v=x/t = .6c for example. The time within the train is dilated. Using invariant distance over this dilated time is v'=x/t' = Yv = .75c. The distance the light travels under this dilated time is subject to the formula Yc = x/t' = 1.25 c, it covers a longer inside distance in less time from an outside perspective. So instead of using length contracted distance over time, I use normal distance over dilated time. The result is the same. Time within the train is dilated from an outside perspective instead of assuming length within the train is contracted from an outside perspective. Yv and Yc do not break the rule that nothing goes faster than light which only applies to v and c. Now the case is closed.

Edited by ralfcis, 22 May 2019 - 10:25 AM.

### #5 Dubbelosix

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Posted 22 May 2019 - 10:28 AM

I don't mean to be harsh, but you don't seem to understand what a relative velocity entails. It is certainly not $v = \frac{x}{t}$, simply because it specifies nothing relative to it. If we properly speak about a relative velocity, we must measure it in context of something else. A more appropriate formula would be

$\frac{f}{v(r)} = \frac{f_0}{v(s)} = \frac{1}{\lambda}$

In which $v(r)$ is the wave's velocity relative to the receiver and $v(s)$ is the wave's velocity relative to the source. $\lambda$ is naturally the wavelength.

Edited by Dubbelosix, 22 May 2019 - 10:28 AM.

### #6 ralfcis

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Posted 22 May 2019 - 10:36 AM

So you prefer the reciprocal doppler shift ratio formula. Who cares. That's not part of this discussion which is dumbed down but still accurate for the audience here. I won't be dragged sideways off the point. Read the rest of the post or not, you'll see your point has no relevance. I'm talking about relative velocity combination.

Edited by ralfcis, 22 May 2019 - 10:40 AM.

### #7 ralfcis

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Posted 22 May 2019 - 02:11 PM

006, I do mean to be harsh. Are you going to respond or just slink away like everyone else. Do you stand by your statement that I don't understand relative velocity?

### #8 GAHD

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Posted 22 May 2019 - 02:25 PM

there is no relative velocity greater than c

Wrong, No relative velocity greater than 2 C. 2 objects can easily be moving towards each other at C, and C + C = 2C. The LHC regularly smashes things into each other at Relatively 1.9999C.

Trying to figure out why you think otherwise?

### #9 ralfcis

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Posted 22 May 2019 - 02:29 PM

Wrong, that is not relative velocity that is a scalar speed. Adding c+c using the relative velocity combo equation results in c. You're talking about closing speed. Just ask Popeye, he went over this with me in my relativity and algebra thread #369657.

Edited by ralfcis, 22 May 2019 - 02:38 PM.

### #10 ralfcis

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Posted 23 May 2019 - 04:18 AM

Weird I would have thought that maybe one person on this forum would have been finally enlightened as to how relativity works but nothing.

### #11 VictorMedvil

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Posted 23 May 2019 - 04:26 AM

Weird I would have thought that maybe one person on this forum would have been finally enlightened as to how relativity works but nothing.

You have a interesting perception of how relativity works but I have yet to decide whether your right or not give me several months to continue to examine your posts, Link me to a paper you created and I will peer review it within a week to see if it matches with known physical evidence and theory.

Edited by VictorMedvil, 23 May 2019 - 04:31 AM.

### #12 ralfcis

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Posted 23 May 2019 - 04:48 AM

My only paper to link to is on this forum as the relativity and simple algebra thread. It's not crucial you understand my Yv position. I was just showing here that relativity is about how light inside the train must meet up simultaneously with light outside the train  from all perspectives because of relative velocity to vacuum and how it does so using either length contraction or time dilation or Yv. (My position is length contraction is just a superfluous explanation so it's not necessary to explain why c is invariant.) Thanks for responding.

Edited by ralfcis, 23 May 2019 - 04:54 AM.

### #13 ralfcis

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Posted 23 May 2019 - 11:42 AM

You don't think the doppler effect is anything to do with what different observers would see.

I never ever said that in fact the doppler shift ratio is solely responsible for age difference. What I said to 006 was the doppler effect has nothing to do with this particular thread and the argument I'm posing here.

Einstein's train is the only example outside of the twin paradox (age difference). Every other example is the same ( and the topic is time dilation and the constancy of c) but set in a different scenario. Nothing in this argument has to do with the doppler effect even though I mention it in passing. This is about relative velocity. The doppler effect for sound is about how a source's relative velocity has no effect on the wave's relative velocity to the medium only on its frequency. A receiver's velocity has an effect on the relative velocity to the wave (because there's a relative velocity to the medium) and the freq received. For light, the source or receiver does not affect the relative velocity to light in a vacuum so why would I discuss their effect on the light's freq at all. You are not following my argument because you've chosen to discuss the doppler effect just because I mentioned it. It's not the point at all, stop skimming for key words and don't reference me general irrelevant wiki type stuff.

Edited by ralfcis, 23 May 2019 - 11:47 AM.

### #14 ralfcis

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Posted 23 May 2019 - 01:06 PM

Would this wiki link not be to the point https://en.wikipedia...ddition_formula .

Yes absolutely to the point but what is your point in referencing it? If I've made a mistake in interpreting it, that would be a good point but just referencing general knowledge without having a specific point has no point at all, in fact it's very dull.

Yes that was explained quite clearly as a viable but supefluous explanation of why the red and blue dot hit simultaneously.

Edit you mentioned the train example  http://galileoandein...dding_vels.html

Is that my specific train example, no. My words do not match this train example even though every example is essentially the same but this is not the same scenario. It would require a different description.

The principle of relativity is the laws of physical phenomena must be the same for a stationary observer as for one carried along in a uniform motion of translation.

Yes but the principle of relativity is not the theory of relativity nor is it the facts of relativity. Words matter. In fact I dont dispute the principle of relativity because the people inside the train dont know theyre moving. They cant know they`re moving because the red and blue dots hit simultaneously which would not be true for a closed train in the airhorn and firecracker example. They would be able to detect they were moving because the sounds would not hit simultaneously.

By medium do you mean ether, space, quantum foam what ?

Do you read every 5th word or 20th word? I'm not going to spoonfeed you. I spoke of electromagnetic and material mediums. A vacuum is lack of material medium but not lack of electromagnetic medium and water has both types. Do you have a question?

Edited by ralfcis, 23 May 2019 - 01:15 PM.

### #15 ralfcis

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Posted 23 May 2019 - 02:40 PM

Of course it does, just ask Maxwell who defined those two characteristics of an electromagnetic medium and how it limits c. Did you also realize Maxwell's equation for this is similar to the equation that defines mechanical wave speed through a mechanical medium. Permittivity and permeability in an electromagnetic medium  correspond to inertia and elasticity in a mechanical medium.

### #16 ralfcis

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Posted 23 May 2019 - 03:57 PM

Why are you confusing things. Whether Maxwell also believed in the easter bunny has about as much relevance. Please try to read what's written not what you'd like it to say so you can create a straw man. I went through open and sealed trains for sound in air and light in water. For a closed and open train there would be differences if ether existed. The light inside the open train would hit the back before the front but more importantly for a closed train with an ether, the light inside would hit the front and back simultaneously but  not hit the back simultaneously with the light outside as it does without ether. Can you understand that or has your mind just glossed over what that means. Just as in the light with water example, the velocity of the train has an effect on the inside and outside light simultaneity where there is none in the vacuum train example. Don't mix up inside and outside light with front and back of the train like you're doing. The light in water is how the MMX experiment would have found ether but it still would have been able to deduce the same relativistic combination equation to determine the max speed in the universe. You're just spewing wiki postings without understanding what they mean like everyone else.

Edited by ralfcis, 23 May 2019 - 04:16 PM.

### #17 ralfcis

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Posted 23 May 2019 - 05:09 PM

Yes that is exactly how you read my posts, trying to make sense out of what's left after your mind has redacted most of the post just isn't going to work. See when I don't understand something, I ask questions for clarification. Not only do you not do that, you also ignore my questions to you.