Speed of light for different observers

[Dale]

You are confusing Doppler redshift with Lorentian redshift. In fact, both are present.

What are you referring to by the term "Lorentian redshift"?

 

[Dale]

In my experiment we observe it the other way round - the observer is stationary and the waves move toward him with speed c+the speed of the train, and that is why the frequency is different for B. Well, this is how I see it. Where am I wrong?

Did I just stupidly broke some law of the physics

As has been mentioned a couple of times the part where you are wrong is the idea that the waves move at c+v instead of c. The fact that light always moves at c regardless of the motion of the emitter is called the second postulate of relativity and has been experimentally confirmed many times to very high precision:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#moving-source_tests

 

[starthaus]

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#moving-source_tests

I gave him the same exact link, he chose to ignore it.

 

[Dale]

My apologies, you certainly did, even to linking right to the same sub-section.

 

[starthaus]

My apologies, you certainly did, even to linking right to the same sub-section.

Nothing to apologise about, it is easy to overlook. :-)

 

[starthaus]

The speed of light is not C+V(train).

True.

The length ot the train would be contracted just enough to make the light travel at C.

Not correct and not relevant to the discussion.

You are confusing Doppler redshift with Lorentian redshift. In fact, both are present.

Huh?

 

[utesfan100]

What are you referring to by the term "Lorentian redshift"?

Lorentzian redshift was ment to refer to the frequency shifting caused by the length contraction of the wave lengths in different reference frames (if viewed from an observer moving in the same direction of the light relative to the original observer).

Doppler redshift is cause by wave lengths expanding as the source moves away from an observer (with the source having a set frequency).

I am thinking that these two are not as distinct as I was thinking yesterday.

 

[starthaus]

Lorentzian redshift was ment to refer to the frequency shifting caused by the length contraction of the wave lengths in different reference frames (if viewed from an observer moving in the same direction of the light relative to the original observer).

That's known under the mainstream name of "time dilation".

Doppler redshift is cause by wave lengths expanding as the source moves away from an observer (with the source having a set frequency).

I am thinking that these two are not as distinct as I was thinking yesterday.

True, they are one and the same effect.

 

[utesfan100]

Reconsidering the original experiment:

The basic experiment is comparing the light emitted from a source and reflected by a mirror from two frames of reference, a comoving reference moving opposite the light, taken to be stationary with the system moving towards the outside observer.

Two values not discussed in your analysis are the possitions of the mirror in the two frames at the moment the light is emitted.

The mirror sees a length equal to the rest length of the train. The observer sees a length contracted by the speed of the train. By the time the train gets to where the light hits the mirror it has moved exactly enough to match this contraction.

The observer will see the frequency of the light blue shifted from the velocity of the source, but the source frequency will be time dilated causing an equal red shift.

Suppose the experiment was ran perpendicular to the motion of the train. Then the doppler effects and length contraction effects would be orthogonal and could not cancel.

Here the differences among frames of reference would be found to be related to the angle of the light's travel relative to the change in angle of the emmiter and mirror at a given instant.

 

[starthaus]

Suppose the experiment was ran perpendicular to the motion of the train. Then the doppler effects and length contraction effects would be orthogonal and could not cancel.

Then you would be observing an effect known as "Transverse Doppler Effect". Length contraction has nothing to do with any of this.

 

[Dale]

Length contraction really doesn't apply for wavelengths of light. Length contraction refers to a reduction in the length of a moving object compared to its length measured at rest. A wavelength of light is not at rest in any reference frame so there is nothing to compare it to.

 

[starthaus]

Length contraction really doesn't apply for wavelengths of light. Length contraction refers to a reduction in the length of a moving object compared to its length measured at rest. A wavelength of light is not at rest in any reference frame so there is nothing to compare it to.

We keep echoing each other, we need to stop that :-)

 

[kamikaze762]

"The speed of light is characterized by the speed, with which every next wave due to the velocity at the moment of the emission approaches the observer. "

Not so. Whether or not we use blue light or red light, the speed of light is the speed at which light propagates through space. The speed of light, as with any wave, is Frequency times Wavelength. It will always come out to C in the case of light. The operational definition of a meter is the space light travels in 1/299,792,458 of a second. Therefore, we are not concerned with how often the peaks of the waves present themselves to us, but how fast the first initial wave arrives at a given destination. You are trying to operationally define speed as frequency, but they are two totally different things.

This does bring up a question about the experiment though. I think I already know the answer, but does a pulse emitted from a moving source arrived "squashed?" What I mean by that is will a stationary emitter have a longer pulse length than that of a moving emitter; the two beginning and ending the pulse simultaneously?

This is in the experiment, of course, in which the emitter is moving toward the observer. I suppose to the stationary observer it would have to be shorter because the pulse would be ended closer to the observer. Maybe that is what is causing the confusion on the part of the OP. Light speed would still not be violated.

 

[Dale]

I think I already know the answer, but does a pulse emitted from a moving source arrived "squashed?"

Squashed or stretched depending on the direction, yes. E.g. for an emitter moving at v=0.6 the Doppler shift is a factor of 2. So if it broadcasts a 1 s transmission in front then it will be blue-shifted to a 0.5 s transmission, if it broadcasts the same transmission behind then it will be red-shifted to a 2 s transmission.

 

[utesfan100]

Then you would be observing an effect known as "Transverse Doppler Effect". Length contraction has nothing to do with any of this.

Thank you for the syntactic corrections. I would argue that these effects are all inter related, but using the established names in a conversation is important. Improving this aspect of my understanding is a large part of why I am here.

Your comments here and in other threads have been very helpful to me.

 

[sisoev]

Sure. If the sun disappeared 'right now', we wouldn't know it for about 8 minutes. Nonetheless, to understand how the frequency shifts it's best to imagine the source still there.

That's illustrated in the second animation.

It's not any difference in the speed of the light--the speed of the outgoing waves doesn't change. It's the fact that the source moved while the light was being emitted. Thus the wave crests are closer together.

The second diagram is a quite deceiving.
The source is always in the center of the emission.
To imagine a moving source use the first animation and set the blue observer moving toward the center and the read observer out of the center.
You will see that the blue observer is approached faster by the light wave, while the red observer is approached slower by them.

It is true that the speed of the light waves is always the same in respect to the source (I mentioned it in my OP) but that does not apply to the observers.
Obviously when the source or the observer is in motion, we observe different velocity between the light waves and the observer, and that is what we call "measured speed of light".
We don't measure the speed of light in respect to the source, do we?
Einstein postulated it as measured in respect to the observer, and by my opinion he was wrong, and this is what my experiment shows.

I'll ask again WHERE AM I WRONG but I can see that we are already in a corrupted circle.
How can we get out of it?

 

[Doc Al]

The second diagram is a quite deceiving.
The source is always in the center of the emission.
To imagine a moving source use the first animation and set the blue observer moving toward the center and the read observer out of the center.
You will see that the blue observer is approached faster by the light wave, while the red observer is approached slower by them.

I think this gets at the heart of the problem. The first animation shows what an observer sees when the source is not moving. The second animation shows a moving source. You can't take the first diagram and then superpose a moving observer on it, just using "common sense". Light doesn't work that way. (Otherwise the second animation would be wrong.)

It is true that the speed of the light waves is always the same in respect to the source (I mentioned it in my OP) but that does not apply to the observers.

Actually, just the opposite. The light is always moving at the same speed with respect to the observer, not the source.

Obviously when the source or the observer is in motion, we observe different velocity between the light waves and the observer, and that is what we call "measured speed of light".

It might be 'obvious', but it's not true.

We don't measure the speed of light in respect to the source, do we?
Einstein postulated it as measured in respect to the observer, and by my opinion he was wrong, and this is what my experiment shows.

A thought experiment can only show where your assumptions and your physics lead you. (Garbage in, garbage out.) Only real experiments can determine which assumptions are correct. Real experiments overwhelmingly support Einstein's assumptions and conclusions. (See the link that has been provided several times in this thread and also happens to be contained in a sticky at the top of this forum.)

I'll ask again WHERE AM I WRONG but I can see that we are already in a corrupted circle.
How can we get out of it?

You are wrong because you are using the first animation and trying to apply it to a moving source. That implicitly assumes that the light is moving outward at a constant speed with respect to some medium (what used to be called the 'aether'). Then you add a moving observer traveling through that medium towards the light source. You merely assume that by moving towards the incoming light the speed of that light with respect to you must be faster. That was shown to be a false assumption many years ago.

Light doesn't work in that 'common sense' way. Actually, space and time do not work that way. You can't take reasoning that works well enough in everyday, low-speed situations and apply it to things moving at high speeds (like light).

 

[sisoev]

I think this gets at the heart of the problem. The first animation shows what an observer sees when the source is not moving. The second animation shows a moving source. You can't take the first diagram and then superpose a moving observer on it, just using "common sense". Light doesn't work that way. (Otherwise the second animation would be wrong.)

Sir, I was hoping that in forum like this I'd receive more detailed answers than in the books.
I think this is the reason people come here to ask for help ;)
Now your answer "the light doesn't work that way" made me lough, thinking of a priest who told me that God doesn't work that way
Shall we put our scientific knowledge on faith?

What is the logical reason to present the moving light source off the center of its emission.
I think that the second animation shows how the observer sees the light waves but that is not the actual way of the light propagation.
If you say that I'm wrong, please explain why, so I can understand the core of the problem.

Actually, just the opposite. The light is always moving at the same speed with respect to the observer, not the source.

Well, if the light is not always moving at the same speed in respect to the source, one cannot claim that it always move with the same speed in respect to the observer.
It is simple logic which involves velocity.

A thought experiment can only show where your assumptions and your physics lead you. (Garbage in, garbage out.) Only real experiments can determine which assumptions are correct. Real experiments overwhelmingly support Einstein's assumptions and conclusions. (See the link that has been provided several times in this thread and also happens to be contained in a sticky at the top of this forum.)

The experiment I present is thought experiment until someone decides to perform it.
Its based on logic and it predicts results based on logic.

You are wrong because you are using the first animation and trying to apply it to a moving source. That implicitly assumes that the light is moving outward at a constant speed with respect to some medium (what used to be called the 'aether'). Then you add a moving observer traveling through that medium towards the light source. You merely assume that by moving towards the incoming light the speed of that light with respect to you must be faster. That was shown to be a false assumption many years ago

.

I only assume that the light is moving with the same speed in all directions in respect to the source. If this assumption is wrong then Einstein is also wrong.
Ironically if my assumption is right, Einstein is wrong again.

I'm sure you have a logical explanation of why the source should be off the center of the emission like in the second animation (I hope you are not saying that, and this is only misunderstanding)

Light doesn't work in that 'common sense' way. Actually, space and time do not work that way. You can't take reasoning that works well enough in everyday, low-speed situations and apply it to things moving at high speeds (like light).

Sir, you understand that I am here because I have problem taking this on faith, and I'm hoping for some logical answers.
Help me, please.
I created my experiment as a simple way to show the problem and there should be simple way to give me the answer.
So, follow my questions and give me the answers.

That would be of great help of all that come here with the same question.
Just pin this topic and send them here to read how you explained to that stupid Bulgarian the simple way of understanding Einstein and his theory.

 

[starthaus]

Well, if the light is not always moving at the same speed in respect to the source, one cannot claim that it always move with the same speed in respect to the observer.

Light is always moving with the same speed wrt the source.
What does this tell you about the speed of light wrt the observer?

 

[Doc Al]

Sir, I was hoping that in forum like this I'd receive more detailed answers than in the books.
I think this is the reason people come here to ask for help ;)
Now your answer "the light doesn't work that way" made me lough, thinking of a priest who told me that God doesn't work that way
Shall we put our scientific knowledge on faith?

"Faith" has nothing to do with it. You make assumptions, draw conclusions, then compare to experiment. Experiment rules!

You seem to have "faith" that your common sense reasoning is an accurate depiction of how things work. Alas, not so!

What is the logical reason to present the moving light source off the center of its emission.

It's moving! Note the key assumption of relativity: The speed of light is the same with respect to any observer.

I think that the second animation shows how the observer sees the light waves but that is not the actual way of the light propagation.

So you claim.

If you say that I'm wrong, please explain why, so I can understand the core of the problem.

Been trying to.

Well, if the light is not always moving at the same speed in respect to the source, one cannot claim that it always move with the same speed in respect to the observer.
It is simple logic which involves velocity.

That 'simple logic' is really a claim about how light works and how velocities add. And it's been shown to be false.

The experiment I present is thought experiment until someone decides to perform it.
Its based on logic and it predicts results based on logic.

Again, what you are calling "logic" is an expression of how you think the world works. It just doesn't work that way.

I only assume that the light is moving with the same speed in all directions in respect to the source. If this assumption is wrong then Einstein is also wrong.
Ironically if my assumption is right, Einstein is wrong again.

Nonsense.

I'm sure you have a logical explanation of why the source should be off the center of the emission like in the second animation (I hope you are not saying that, and this is only misunderstanding)

The explanation is perfectly logical--and I am saying just that and the misunderstanding is on your end. Light always moves with constant speed with respect to the observer. As the source moves from spot 1, to spot 2, to spot 3, etc, expanding circles of light will be centered on spot 1, spot 2, spot 3, etc. Just like the second animation shows.

Sir, you understand that I am here because I have problem taking this on faith, and I'm hoping for some logical answers.
Help me, please.
I created my experiment as a simple way to show the problem and there should be simple way to give me the answer.
So, follow my questions and give me the answers.

The answers have all been given. I think what you're really looking for is a way to keep your (mistaken) common sense views of how light works and velocities add and still understand relativity. Sorry, that's not going to happen.

That would be of great help of all that come here with the same question.
Just pin this topic and send them here to read how you explained to that stupid Bulgarian the simple way of understanding Einstein and his theory.

If you're really interested in understanding Einstein and his theory, then you need to be willing to start fresh. There are many excellent books that explain how it all works, but again the bottom line is always: Does it agree with real experiment? Yes!

 

[Dale]

Well, if the light is not always moving at the same speed in respect to the source, one cannot claim that it always move with the same speed in respect to the observer.

Actually, that is exactly what one can claim, it is the whole point of the http://en.wikipedia.org/wiki/Lorentz_transform" to show that it is logically possible for something to move at c in one reference frame and also to move at c in another reference frame, despite the fact that the two reference frames are moving at some velocity v<c with respect to each other.

A pulse of light will move at c in any and all inertial reference frames including the inertial reference frame where the inertial emitter is at rest as well as any inertial reference frame where any arbitrary inertial observer is at rest. Such reference frames are related to one another via the Lorentz transform rather than via the Galilean transform, and in the Lorentz transform velocities add as follows:
s = \frac{v+u}{1+vu/c^2}
Which is equal to c if u is equal to c regardless of v.

 

[Frame Dragger]

"Faith" has nothing to do with it. You make assumptions, draw conclusions, then compare to experiment. Experiment rules!

That needs to be on T-Shirts and bumper stickers NOW. Hell, sell them from PF!

You could even have warring messages: "Experiment rules!" and then on the back "Dagon Shall Rise Again".

Wait... that last bit makes no sense at all...

 

[sisoev]

Thanks for the answers, guys.
I have one more thing to ask and tomorrow I'll post my last comment on the topic.

In my experiment you have one source and one emission with one wave length, observed with two different frequencies.
How do you intend to measure same speed for the light?

I guess some of you won't understand the question, so I'll clarify for you.

Every light source will emit light with certain wave length(λ).
The frequency will equal c/λ

When the frequency changes because of different velocity that means that the time for "passing through the wave length" has changed. The distance between the crests are the same, but they approach the observer faster or slower.
In that case we see yellow light as blue or red depending on the velocity.
Note that the light is still yellow, but we observe it as different.
That is seen on stars that approach Earth with certain speed. We see their light in the blue spectrum, but that doesn't mean that their light is blue.
By calculating their wave length without knowing the velocity we can never know their original light color. They might as well be blue stationary stars or red stars traveling with high speed, or ultraviolet stars traveling away from us.

I hope you understand my point.

Now, in my experiment we have one only wave length (color) because it is one only emission.
Having two different frequencies, we must have two different speed of the light.
But because SR does not agree with that we end up with the paradox of having two different wave lengths for one emission.

 

[Nickelodeon]

Now, in my experiment we have one only wave length (color) because it is one only emission.
Having two different frequencies, we must have two different speed of the light.
But because SR does not agree with that we end up with the paradox of having two different wave lengths for one emission.

I think that you can't pin the wave length. The wavelength and the frequency are (edit: appear to be) modified by the relative speed of the source and observer so that c stays the same. On the other hand, if the observer was accelerating either away or towards the light source then the time interval from the emission to the observation would reflect a speed greater or lesser than c.

 

[Doc Al]

In my experiment you have one source and one emission with one wave length, observed with two different frequencies.
How do you intend to measure same speed for the light?

The observed frequency shift of the light depends on the speed of the source, not the speed of the light. Your arrangement leads to multiple frequencies observed because the source is moving and some of the light is reflected from a moving mirror (which acts as a secondary source). In all cases the speed of light is the same.

Now, in my experiment we have one only wave length (color) because it is one only emission.
Having two different frequencies, we must have two different speed of the light.
But because SR does not agree with that we end up with the paradox of having two different wave lengths for one emission.

What paradox? The different frequencies observed are easily explained within SR. Again, it has nothing to do with 'different speeds of light'. You realize, of course, that the same source of light could be seen as having as many different observed frequencies are there are observers moving at different speeds with respect to the source. Why stop at two?

Here's some homework for you. Give the train some relativistic speed (say, 0.9c). And give the light source some original frequency in its own frame (600 nm, say). Why don't you predict what frequencies will be observed?

 

[dx]

Here's a spacetime diagram for your situation as I understand it, in a frame in which B is at rest:

[PLAIN]http://img11.imageshack.us/img11/1161/expnk.jpg

Now can you explain what your problem is with reference to this image? (Keep in mind the time dilation for the moving observers. That affects the frequency that they observe.)

 

[Frame Dragger]

Isn't this the precise conundrum that is the basis for SR?... that the geometry of spacetime is altered because "c" represents that limit however one chooses to think of it, or arrive at the conclusion?

I'm not trying to be sarcastic... I just want to make sure that I'm not missing more than usual. To me, this has always been where you start with the notion of Relativity.

 

[sisoev]

Here's some homework for you. Give the train some relativistic speed (say, 0.9c). And give the light source some original frequency in its own frame (600 nm, say). Why don't you predict what frequencies will be observed?

This "home work" just shows how deceived people are about wave length and wave frequency.
600 nm in the "home work" is the wave length and it never changes.
The frequency is measured in Hz and it depends on the speed of the light - how fast the wave length is observed, and that depends on the additional speed added to it by the moving source or moving observer.

When we observer different frequency, we must be able to calculate the wave length (which never changes), but that is impossible without knowing the velocity and add it to the speed of the light.
I already pointed to this in my previous comment but you chose not to comment on it.

I still have my last comment coming.

 

[sisoev]

Here's a spacetime diagram for your situation as I understand it, in a frame in which B is at rest:

[PLAIN]http://img11.imageshack.us/img11/1161/expnk.jpg

Now can you explain what your problem is with reference to this image? (Keep in mind the time dilation for the moving observers. That affects the frequency that they observe.)

I'll answer to this in my last comment.

 

[sisoev]

Isn't this the precise conundrum that is the basis for SR?... that the geometry of spacetime is altered because "c" represents that limit however one chooses to think of it, or arrive at the conclusion?

I'm not trying to be sarcastic... I just want to make sure that I'm not missing more than usual. To me, this has always been where you start with the notion of Relativity.

The geometry of space-time will be valid only if the speed of light is constant.
In this discussion it is in question and the geometry of space-time cannot be an argument.
It doesn't stand logical in the context of the experiment since one light emission is seen simultaneously with two different frequencies.
The calculation will show either different wave length (which is a paradox) or different speed of light (which will clear the paradox).

Read my above comments