# B Help with understanding the Doppler effect

#### frostysh

Well, you say "classical" in the title, but then in the text you use words like "absolute", "event", "invariant", "frames", and "observer" throughout all of your posts, and you never specify $v<c_s<<c$. All of which essentially require answers to be in the context of relativity if they are to be physically correct answers. I don't know how it is possible to address a question in those terms without relativity.
Grr! What kind of thought lead you to such conclusion? All things you have named is actually have the same meaning in the classical case. I have provided example of the brick wich is splinted into two pieces durring free fall, it is an event, in classic and in the SR kinematics. The relativity exist in classical mechanics too! The different inertial frames exists, so the relativity corresponding to this frames exist too... The classical mechanics relativity in the name of holy Newton, Hamilton and Lagrange can be self-sustained, it's no need this 'relative-relativity' things to describe models. And it's working a very well, and this is actually the case where we totaly no need non-classical things.

As for myself, I am not even started to restudy my university course of classical mechanics which I have some studies long ago, but indeed it is a very fundamental and a very crucial point in learning of physics!

#### Ibix

I am not even started to restudy my university course of classical mechanics which I have some studies long ago
In that case, maybe listen to three of us who are a bit more current on the topic and are saying you have it wrong?

#### Dale

Mentor
What kind of thought lead you to such conclusion?
A couple of decades of experience in discussing such topics.

The language you are using is far more typical of relativity discussions than classical mechanics discussions. I believe that there is a bit of a language barrier here. Your grammar is odd, with for example a lot of usage of “me” where it should be “I”. So the impression may be unintentional, but nonetheless your wording gives the strong impression that you want to discuss relativity. Particularly with the overuse of the word “absolute” towards the beginning of the thread.

In my decades of previous experience discussing these topics I cannot recall ever having someone who asked about “absolute” and “frames” that did not want to discuss relativity. This is a first

#### frostysh

You can detect the motion of the source relative to the medium or the observer relative to the medium. Or you can see it as the motion of the medium relative to the source or the medium relative to the observer.
The relations $\large M \longrightarrow S$ and $\large S \longrightarrow M$ is NOT the same. Assymetry, viola, the end of relativity of inertial frames in case of Doppler's effect.
Get a source and a receiver and set them in motion relative to the medium and record whatever you want to record - wavelengths, frequencies, whatever. Now do the same experiment (same velocities with respect to the medium and everything) in a moving train. You will get the same results, even though the experiment is doing 60mph with respect to the other one.

There is nothing absolute about this experiment. It is only measuring speeds relative to a medium, which may be in motion with respect to other parts of the medium.
The ienrial frame of reference is the inertial frame of reference, the changing of wavelength is an invariant in zillion inertial frame of references. We directly see the direction of this changings! And we directly see it in the all frame of references, in the train, in the car, in the Alpha-Centaura, etc, and we can distinguish the motion! Your bias there...

I understand your position, on the other forum I have pointed that in case of imaginary aquarium with medium, but this is not the point.
Do you understand words 'we have only three inertial frame of references' in the whole Universe? It is an abstraction, no trains, no cars, nothing... This is not he same as free falling in the elevator, we are investigating only the very case of Doppler's effect. Nothing else in world is exist. I am not saying that there is exist an ether, or something that can break relativity as the concept, I am using only the particular model.
A couple of decades of experience in discussing such topics.

The language you are using is far more typical of relativity discussions than classical mechanics discussions. I believe that there is a bit of a language barrier here. Your grammar is odd, with for example a lot of usage of “me” where it should be “I”. So the impression may be unintentional, but nonetheless your wording gives the strong impression that you want to discuss relativity. Particularly with the overuse of the word “absolute” towards the beginning of the thread.
Grammar is yes, of course, my bad, I am apologizing, but the point is the Physics. The Lorentz's transformations is not the same Galelean one...

#### Dale

Mentor
The Lorentz's transformations is not the same Galelean one...
Agreed. I always prefer to start with the general formula I cited, and then explicitly simplify it if $v<c_s<<c$. Mathematically that is how you justify shifting from the correct Lorentz transform to the simplified Galilean transform in the regime where they are approximately equal.

#### Ibix

Do you understand words 'we have only three inertial frame of references' in the whole Universe?
Those words are meaningless. You can define a frame of reference with the words "I consider the medium to be moving with velocity $\vec v$". There does not have to be anything at rest in a frame to define it. Thus there are always an infinite number of inertial reference frames available - even in your case with a medium (and what's it made of, by the way? Are any of its molecules moving with respect to any of these frames you say are the only ones that exist?) and two objects.
The relations M⟶S\large M \longrightarrow S and S⟶M\large S \longrightarrow M is NOT the same.
I don't understand what you mean by this. My guess would be that you mean "source moving with respect to the medium is not the same as medium moving with respect to the source", but this is trivially false. So I'm not sure what you do mean.

#### Ibix

but this is not the point
It is exactly the point.

Edit - to expand on that, I've just shown you (and you agreed) that you are only measuring speeds with respect to a medium, and that there is no violation of Galilean invariance because the two experiments come out the same.

#### frostysh

Agreed. I always prefer to start with the general formula I cited, and then explicitly simplify it if $v<c_s<<c$. Mathematically that is how you justify shifting from the correct Lorentz transform to the simplified Galilean transform in the regime where they are approximately equal.
Or we can forgot about uncle Lorentz so we no need transform anything to 'approximately equal' and not typing such things, and just pary for holy Galeleo...
Those words are meaningless. You can define a frame of reference with the words "I consider the medium to be moving with velocity $\vec v$". There does not have to be anything at rest in a frame to define it. Thus there are always an infinite number of inertial reference frames available - even in your case with a medium (and what's it made of, by the way? Are any of its molecules moving with respect to any of these frames you say are the only ones that exist?) and two objects.
And this is the reason why 'switch on' of' Doppler's effect in the model destroying relativity. You se the change of the invariant is invariant, and if we measured invariant wavelentghs it will say to us where the movement to all inertial frame of refencenses tha you named.
I don't understand what you mean by this. My guess would be that you mean "source moving with respect to the medium is not the same as medium moving with respect to the source", but this is trivially false. So I'm not sure what you do mean.
The all holy saints! I need to describe this diagram earlier. Actually, on my current level of understanding this phenomena it is the difference! It is the true! Why source moving with respect to the medium is not the same as medium moving with respect to the source, because the creation of the waves is an event, and those beacuse it is not more general than Galelean relativity! So in the model we just using a very special event which killing relativity, nothing special, nothing fundamental. Hoorey!

On primitive level I have described it by this picture,

as we can see the change of wavelength is the physical, this is an event, as the breaking of a brick. The explanation of the picture in the first post. If you will move the medium instead, no any change in wavelength appear, because interraction is instant and the speed of medium related to source have no any connection to the very position of the 'cores' related to each others, which is an anlogy in respect to some points of model of one-dimension wave.

#### Dale

Mentor
Or we can forgot about uncle Lorentz
Best not since he is generally right.

By the way, I am not sure if you missed this post of mine above:

#### Ibix

The all holy saints!
I get the impression you are trying to imply that I'm stupid for not following your argument. Am I correct?
because the creation of the waves is an event
No it is not. It's an extended period of time.
the change of wavelength is the physical
...and it happens the same whether you regard the source as moving with respect to the medium or the medium with respect to the source. Proof:

Let the source emit waves of period T and speed c. The medium is stationary and the source moves in the same direction as the waves at speed v. When the second wave crest is emitted, the first wave crest has travelled $cT$ and the source has moved $vT$. Thus the wavelength is $(c-v)T$.

Now again, but this time we regard the source as stationary and the medium as moving at speed $-v$ (i.e., we've subtracted $v$ from the speed of both medium and source). When the second wave crest is emitted, the first has travelled $(c-v)T$ and the source has not moved. Thus the wavelength is $(c-v)T$.

No difference.

#### frostysh

I get the impression you are trying to imply that I'm stupid for not following your argument. Am I correct?
You are again incorrect, I am saying we have reach the crucial point the investigation of which will decide who is more close to reality!
No it is not. It's an extended period of time.

...and it happens the same whether you regard the source as moving with respect to the medium or the medium with respect to the source. Proof:

Let the source emit waves of period T and speed c. The medium is stationary and the source moves in the same direction as the waves at speed v. When the second wave crest is emitted, the first wave crest has travelled $cT$ and the source has moved $vT$. Thus the wavelength is $(c-v)T$.

Now again, but this time we regard the source as stationary and the medium as moving at speed $-v$ (i.e., we've subtracted $v$ from the speed of both medium and source). When the second wave crest is emitted, the first has travelled $(c-v)T$ and the source has not moved. Thus the wavelength is $(c-v)T$.

No difference.
I will better make a picture, one moment. Just go to the outside of the house for a sec...

#### Ibix

I will better make a picture
It will not make a difference. An honest picture must obey the maths I just laid out.

#### frostysh

It will not make a difference.
Well actually you are saying truth about the similarity of the cases! Wery many thanks! You the first which helped to understand, on other forum me get topic deleted! The was my mistake... Very good, know I can continue the studies. This forum is indeed useful!

Again many-many thanks!

#### frostysh

Best not since he is generally right.

By the way, I am not sure if you missed this post of mine above:
The classical mechanics is good description of realitity too, and there is no reason to give any connections to the non-classical theories, it's just specualtions and play with words. Anyway I have understand my mistake, which was obvious but I have created a new physics on it... Anyway it have NO any conection to the Lorentz's transformation. Well, it's not a point, the point is thanx you for your time, this forum is indeed good in terms of users trying to help by topic at least!

#### Torbert

My last formal education was in the early 1970's, but I always kept current and could consider my speculations valid. So I thought.
I joined Physics Forums with a very important view and got swatted immediately.
A bit upset, I decided to take it, study up a bit, the devastate the narrow minded.

It turned out I could not afford to return to school full time for the years it would take to actually replace my aged and misguided past with the reality of what has been learned since.
Human nature has me holding on to my speculations, but, like Einstein in his later years, I have to just plain ignore many newer and proven observations. So be it.

I empathize with frostysh and am curious if the last of his formal education was anywhere near the same time period as mine.

"Help with understanding the Doppler effect"

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