What does the Doppler effect say about light's motion?

[yknot]

The optical Doppler effect is defined as follows:

 

"A change in the observed frequency of light or otherelectromagnetic radiation caused by relative motion of the source and observer."

 

The problem with this definition is the fact that the light is no longer connected to the source by the time the former is viewed (by observers).

 

 

 

In fact, the source could be long gone or even utterly destroyed by the time its light is finally seen.

 

 

 

I see this light as being analogous to a simple picket fence that is floating in space whilst observers in various (inertial) frames pass by.

 

 

 

The fence does not (and cannot) change (no accelerations here, we are talking special relativity only), so the only thing that can cause a different view of its "frequency" (the passing of its posts) is different observer motions relative to the fence.

 

 

 

In the case of light, the same thing happens, that is, observers in different (inertial) frames see the light differently (different colors) because they are passing the light differently.

 

 

 

And since relative motion is reciprocal, this means that the light is passing the observers differently.

 

 

 

But now we have the problem that this last fact runs counter to special relativity theory.

 

 

 

At least, that's *my* conclusion. What say ye?

[ACG52]

Whether the emitter is still there or not is imaterial. If we are heading towards where the emitter is or was, we will see the light as being blue-shifted, as compared to the spectrum when it was emitted. If headed away from the original position at the time of emission, we see it as red-shifted.

 

 

 

And since relative motion is reciprocal, this means that the light is passing the observers differently.

 

 

 

But now we have the problem that this last fact runs counter to special relativity theory.

 

 

 

No it doesn't. All observers measure the speed of light as c. The frequencies will be different due to relative motion, but this in no way violates SR.

[Ronald Hyde]

We see red shifted galaxies from billions of years ago and our telescopes were not even built then.

 

Obviously there is something wrong with your assumptions, facts or reasoning.

[yknot]

To ACG52:

 

 

 

My point about the source was that motion relative to it cannot be involved if the source is no longer there. The only things that are involved are the light rays and the observers. They pass each other in the night.

 

 

 

My point about light's passing speed still stands because no one has ever measured light's passing (or one-way) speed. Can you possibly show how such a measurement can be even be made?

 

 

 

To Mr. Hyde:

 

I am unsure of your point. Could you possibly clarify?

[Ronald Hyde]

 

To Mr. Hyde:

 

I am unsure of your point. Could you possibly clarify?

 

Well, one assumption that is wrong is that something that happens to the source or sink of the light during transit does affect

the lights progress.

[EquisDeXD]

The optical Doppler effect is defined as follows:

 

"A change in the observed frequency of light or otherelectromagnetic radiation caused by relative motion of the source and observer."

 

The problem with this definition is the fact that the light is no longer connected to the source by the time the former is viewed (by observers).

 

 

 

In fact, the source could be long gone or even utterly destroyed by the time its light is finally seen.

 

 

 

I see this light as being analogous to a simple picket fence that is floating in space whilst observers in various (inertial) frames pass by.

 

 

 

The fence does not (and cannot) change (no accelerations here, we are talking special relativity only), so the only thing that can cause a different view of its "frequency" (the passing of its posts) is different observer motions relative to the fence.

 

 

 

In the case of light, the same thing happens, that is, observers in different (inertial) frames see the light differently (different colors) because they are passing the light differently.

 

 

 

And since relative motion is reciprocal, this means that the light is passing the observers differently.

 

 

 

But now we have the problem that this last fact runs counter to special relativity theory.

 

 

 

At least, that's *my* conclusion. What say ye?

 

Mathematically you can think of it as this: If you move towards light, you run into the "bumps" or sign changes in a sine wave faster, and if you move away from light, you run into the sign changes at a slower rate, though the actual measurement of light is an instant process, but it might have to do with relative kinetic energy as well.

[yknot]

Mathematically you can think of it as this: If you move towards light, you run into the "bumps" or sign changes in a sine wave faster, and if you move away from light, you run into the sign changes at a slower rate, though the actual measurement of light is an instant process,but it might have to do with relative kinetic energy as well.

 

Since I am still confused re Mr. Hyde's remarks, I shall go ahead and address Mr. EquisDeXD's.

 

 

 

Here is a quick question that may help:

 

As far as relative motion per se is concerned, is there really any difference between that of a passing asteroid and that of a light ray? (Yes, I know that the ray may be traveling a lot faster, but that is beside the point.)

 

 

 

It seems to me that there is no difference at all, and this tells us that light's relative motion is no different from that of any other entity, such as a neutrino or a baseball.

 

 

 

This is probably why no experiment has ever shown a variance in light's one-way speed per two clocks.

 

 

 

The optical Doppler effect is just another example of light's varying one-way speed.

 

 

 

At least that's the way I seeze it. How about you guyzz??

[Ronald Hyde]

It isn't just that the light ray's speed is faster, it is that it is invariant. It's direction may change, and therefore it's velocity, but not its speed.

[EquisDeXD]

 

Since I am still confused re Mr. Hyde's remarks, I shall go ahead and address Mr. EquisDeXD's.

 

 

 

Here is a quick question that may help:

 

As far as relative motion per se is concerned, is there really any difference between that of a passing asteroid and that of a light ray? (Yes, I know that the ray may be traveling a lot faster, but that is beside the point.)

 

 

 

It seems to me that there is no difference at all, and this tells us that light's relative motion is no different from that of any other entity, such as a neutrino or a baseball.

 

 

 

This is probably why no experiment has ever shown a variance in light's one-way speed per two clocks.

 

 

 

The optical Doppler effect is just another example of light's varying one-way speed.

 

 

 

At least that's the way I seeze it. How about you guyzz??

No, I'm pretty sure there's experiments that you can easily do, like let's say you have a light source. One has a stationary measuring device and there's another one moving away. We'd measure the stationary one has having no frequency change while we measured the moving away one as being red-shifted. This may also be due to the warping of space as an object travels through it, or the passage of time. Actually, based on black hole theories, isn't probably time. As light approaches a black hole, it get's red shifted since time slows the more you approach a black hole, but time moves more slowly the faster you travel, so that could be an explanation as well, since time slowing would technically lower the number of oscillations per second, but with the directional thing I'm not quite sure, because if you move towards light it get's blue shifted even though time is still slowing down. Someone with a greater knowledge of relativity should probably address this.

Edited October 8, 2012 by EquisDeXD

[IM Egdall]

The Doppler shift affects our observation of the frequency of light. It does not affect our observation of the speed of that light.

 

 

To model how our relative motion affects the speed of light and the speed of other thing, Einstein came up with a clever formula. See link:

 

http://math.ucr.edu/...R/velocity.html

 

Per this formula, the speed of light is always the same value, c (about 670 million miles an hour), no matter what the speed of the light source or the speed of the observer.

 

The speed of a particle with mass (and objects which they are made up of) is always less than the speed of light, c. And per Einstein's formula, no matter what your speed or the speed of such an object, the combined speed is always less than the speed of light.

 

In other words, no matter what your point of view, your (uniform) reference frame, you will measure the speed of light as the same value c. And no matter what your reference frame, you will measure the speed of particles with mass as less than c.

 

My website: http://www.marksmodernphysics.com/

 

 

Edited October 8, 2012 by IM Egdall

[yknot]

The Doppler shift affects our observation of the frequency of light.It does not affect our observation of the speed of that light.

 

To model how our relative motion affects the speedof light and the speed of other thing, Einstein came up with aclever formula. See link:

 

http://math.ucr.edu/...R/velocity.html

 

Thanks for the replies, folks! I don't mean to sound argumentative, but I must stay with the facts, wherever that may lead!

 

 

 

One fact that pertains to the above is that Einstein's composition of velocities formula was based on his _assumption_ of one-way light speed invariance. Not surprisingly, given this, we end up "getting" "c" for the speed of light (and less than c for all other entities). (Neither Einstein nor anyone else ever actually _measured_ light's one-way speed between two clocks.)

 

 

 

Another fact that also pertains to the above is that one-way invariance is an invalid assumption because it cannot occur experimentally (or even theoretically).

 

 

 

Although this second fact may be difficult to believe, it is fairly easy to demonstrate by the following simple challenge:

 

 

 

Show on paper how light's one-way speed between two clocks can be c experimentally in any inertial frame or frames.

[Ronald Hyde]

Another fact that also pertains to the above is that one-way invariance is an invalid assumption because it cannot occur experimentally (or even theoretically).

Although this second fact may be difficult to believe, it is fairly easy to demonstrate by the following simple challenge:

Show on paper how light's one-way speed between two clocks can be c experimentally in any inertial frame or frames.

 

Please explain to me this fixation on the notion that the 'one way speed of light' is immeasurable or different than the

two way speed? If you send a radio signal to a spacecraft orbiting Mars and request that it radio back, you don't get a

two way signal back, it's only a one way signal because it's transmitted and not reflected back. But the timing is in full

agreement with the speed being C in both directions. And if you want you can put a clock on the spacecraft. And if you

do all the math, which is only just above high school algebra level, it's in complete agreement with the Special Theory

of Relativity. So all your arguments, yes you are being argumentative, just fall on their face.

[yknot]

To Hyde:

 

Please point out my argumentative stuff; I do not intend to be so.

 

To explain my "fixation" on light's one-way speed, and why and how it

differs fundamentally from the round-trip speed, I turn to Einstein's own

words, since you seem to think that mine are somehow "argumentative."

(I really should not have to use Einstein's words to explain SR because

they have been readily available to everyone for decades, but since you asked....)

 

The one-way and round-trip cases are different. One-way light speed "invariance"

had to be given by definition, whereas round-trip speed invariance was given

prior to special relativity via experiment, as Einstein said in his 1905 paper

.

 

"In agreement with experience we further assume the quantity

 

2AB/(t'a-ta) = c

 

to be a universal constant - the velocity of light in empty space."

 

http://www.fourmilab...in/specrel/www/

[Einstein's "In agreement with experience" means "In agreement

with experiment." (The Michelson-Morley experiment - MMx)

Einstein's "2AB" means "a round-trip from A to B and back."

Einstein's time period "ta'-ta" is the round trip time per one clock.

(Einstein had to use the word "assume" because the MMx did not

use a clock. Later, the Kennedy-Thorndike experiment did use a

clock, and also got c for light's round-trip speed. Yes, there were

two round-trip null results, meaning that the first one did _not_

prove round-trip invariance.)]

 

But Einstein was _unable_ to point to a one-way light speed experiment

because none existed, and this is still the case.

 

As I said, all he could do was to define "invariance," as is seen clearly

by the following from Einstein's SR paper:

 

"Any ray of light moves in the "stationary"' system of co-ordinates

with the determined velocity c, whether the ray be emitted by a

stationary or by a moving body. Hence

 

velocity = light path/time interval

 

where time interval is to be taken in the sense of the definition in § 1."

 

Since the definition in §1 forces clocks to obtain the value "c" for

light's one-way speed (by presetting them to read the same time

for both one-way trips purely by stipulation), it is clear that this

"invariance" is neither a postulate nor assumption, but merely

a convention like the rather trivial convention that 12"= 1'.

 

The problem with Einstein's definition is that it causes clocks to be

asynchronous. This is why SR has relative time, which is really just

a euphemism for incorrect time since everyone knows that SR's clocks

are not absolutely synchronous. SR does not have absolute time only

because Einstein decided to force clocks to "get" "c" as he wished.

To repeat: No experiment has ever said this (or ever will).

 

Even though Einstein preferred his asynchronous clocks (because

they seemed to give him what he thought he needed, namely, one-way

"invariance," he was unable to prove that truly synchronous clocks

cannot exist because negatives cannot be proved.

 

In fact, Einstein mathematically stated that observers who use the

absolutely synchronous clocks of classical physics will get a _variable_

one-way light speed.

 

[Quoting Einstein:]

"w is the required velocity of light with respect to

the carriage, and we have

 

w = c - v.

 

The velocity of propagation of a ray of light relative

to the carriage thus comes out smaller than c.

 

But this result comes into conflict with the principle

of relativity.... For, like every other general law of

nature, the law of the transmission of light in vacuo

must, according to the principle of relativity, be the

same for the railway carriage as reference-body as when

the rails are the body of reference."

http://www.bartleby.com/173/7.html

 

This is not a closing velocity because Einstein claims

that it conflicts with the principle of relativity, and

a closing velocity would not do this.

 

Also, no mere closing velocity could have given Einstein

a headache.

 

And no mere closing velocity could have caused the creation

of the theory of special relativity.

 

All of the above tells us that even though the round-trip light speed case

was closed (experimentally) prior to SR, the one-way case remains open.

All we need are a pair of (truly) synchronous clocks to get a variable one-way

light speed (w = c ± v, per Einstein himself), which would detect our absolute

motion, just as happened in Einstein's own example/experiment above.

 

So, if you think that I am just being "argumentative," or that my arguments

are merely "falling flat," then maybe you will listen to Einstein's own words.

[Ronald Hyde]

 

But Einstein was _unable_ to point to a one-way light speed experiment

because none existed, and this is still the case.

 

 

This is totally false, I just gave you an example of a dual 'one way speed' experiment that is probably performed daily by people who track

interplanetary spacecraft. This isn't some highfalutin obscure theory that people don't know about, it's something that some people use

every day in their line of work.

[yknot]

First of all, it is funny that Einstein did not mention

your "one-way experiment," and second of all, it is not

possible to measure light's one-way speed without using

two clocks, and third of all, why did you ignore Einstein's

simple equation saying that given the clocks of classical

physics, light's one-way speed must vary with frame velocity?

 

And please show the math (you said it was simple math) for

your experiment to show how light's one-way speed can be c

in any inertial frame.

 

Your claim that my above was "totally false" is itself totally

false, as you will soon discover by trying to do the math.

[Ronald Hyde]

First of all, it is funny that Einstein did not mention

your "one-way experiment," and second of all, it is not

possible to measure light's one-way speed without using

two clocks, and third of all, why did you ignore Einstein's

simple equation saying that given the clocks of classical

physics, light's one-way speed must vary with frame velocity?

 

And please show the math (you said it was simple math) for

your experiment to show how light's one-way speed can be c

in any inertial frame.

 

Your claim that my above was "totally false" is itself totally

false, as you will soon discover by trying to do the math.

 

I'm not aware of any probes that were orbiting Mars or the Moon in Einsteins time.

 

And Einstein was in the process of proving that the classical notion of time as displayed by

moving clocks was not correct.

[IM Egdall]

I think the first verification of the absolute speed of light was by DeSitter in 1913. He examined binary star systems ( two stars revolving around a common center of gravity). His observations and analysis agreed with Einstein's prediction. And this was a one-way light experiment.

 

See for example http://en.wikipedia.org/wiki/De_Sitter_double_star_experiment

[swansont]

Wouldn't GPS fail if there was a variable sped of light? The time of flight from the satellites would be off, leading to a positioning error.

[yknot]

To Mr. Hyde:

 

 

Let me for a minute return to my original post. What do you,

 

Mr. Hyde, think causes observers to see different colors when

 

viewing the same passing light ray? Since the light itself

 

cannot and does not change color, we are left with only one

 

answer, namely, the different speeds of the observers relative

 

to the passing light. This simple experiment alone is enough

 

to prove that light's speed relative to observers varies.

 

 

But let me also return to Einstein's simple equation, w = c - v.

 

Regardless of your statement that Einstein was in the process of

 

"proving" that classical time was incorrect, he _did_ in fact

 

derive the equation w = c - v. Can you show us how he derived it?

 

 

And as for your statement that there were no Mars or Moon probes

 

in Einstein's day, this is irrelevant because Einstein could have

 

used a thought experiment, as he often did (even in the "important"

 

case of his derivation of the relativity of simultaneity.)

 

 

Perhaps if you just show how Einstein derived his equation w = c - v,

 

you will see what I have been talking about (non-argumentatively).

 

 

But I _will_ give you the following little hint: Einstein did not

 

really prove that the clocks of classical physics were wrong, but

 

simply discarded such clocks in lieu of the really wrong clocks

 

(asynchronous clocks) of special relativity. As we know, he replaced

 

good clocks with bad clocks (absolutely synchronous clocks with

 

absolutely asynchronous clocks) in order to (try to) get all clocks

 

to record "c" for the one-way speed of light. Of course, he very

 

conveniently did _not_ show how this can be done experimentally.

 

But you, Mr. Hyde, are welcome to try.

 

 

Well, I have blabbered enough for now.

 

 

(Oh, I almost forget to mention that you have yet to show how your

 

probe experiment yields the value "c" for light's one-way speed.)

 

I think the first verification of the absolute speed of light was by DeSitter in 1913. He examined binary star systems ( two stars revolving around a common center of gravity). His observations and analysis agreed with Einstein's prediction. And this was a one-way light experiment.

 

See for example http://en.wikipedia....star_experiment

 

Well, this is only about light's source independency, not its one-way speed.

 

Wouldn't GPS fail if there was a variable sped of light? The time of flight from the satellites would be off, leading to a positioning error.

 

All clocks today are synchronized per Einstein's definition, and this

definition assumes "c" for light's one-way speed.

 

Obviously, truly or absolutely synchronous clocks would be better for GPS,

but it can get by with Einstein's slightly-off clocks because of the following:

 

On the satellite side, timing is almost perfect because

they have incredibly precise atomic clocks on board.

 

But what about our receivers here on the ground?

 

Remember that both the satellite and the receiver need

to be able to precisely synchronize their pseudo-random

codes to make the system work.

 

If our receivers needed atomic clocks (which cost upwards

of $50K to $100K) GPS would be a lame duck technology.

Nobody could afford it.

 

Luckily the designers of GPS came up with a brilliant

little trick that lets us get by with much less accurate

clocks in our receivers. This trick is one of the key

elements of GPS and as an added side benefit it means

that every GPS receiver is essentially an atomic-accuracy clock.

 

The secret to perfect timing is to make an extra satellite measurement.

That's right, if three perfect measurements can locate a point in

3-dimensional space, then four imperfect measurements can do the

same thing.

 

By using an extra satellite range measurement and a little algebra

a GPS receiver can eliminate any clock inaccuracies it might have.

 

If you, Mr. Swan..., think that GPS clocks are absolutely synchronous,

then please tell us how that was done. According to current theory,

it cannot be done, and should not be done ("cause time is relative, not

absolute per Albert E).

 

What you all are overlooking here is the simple fact that Einstein himself

said mathematically that the one-way light speed will vary given the

clocks of classical physics, clocks which he was unable to obtain probably

because he wanted to discard them.

[ACG52]

Since the light itself

 

cannot and does not change color, we are left with only one

 

answer, namely, the different speeds of the observers relative

 

to the passing light. This simple experiment alone is enough

 

to prove that light's speed relative to observers varies.

 

 

The different FREQUENCY of the light relative to the observers, not the speed.

[Ronald Hyde]

To Mr. Hyde:

 

 

 

Let me for a minute return to my original post. What do you,

 

Mr. Hyde, think causes observers to see different colors when

 

viewing the same passing light ray? Since the light itself

 

cannot and does not change color, we are left with only one

 

answer, namely, the different speeds of the observers relative

 

to the passing light. This simple experiment alone is enough

 

to prove that light's speed relative to observers varies.

 

 

 

But let me also return to Einstein's simple equation, w = c - v.

 

Regardless of your statement that Einstein was in the process of

 

"proving" that classical time was incorrect, he _did_ in fact

 

derive the equation w = c - v. Can you show us how he derived it?

 

 

 

And as for your statement that there were no Mars or Moon probes

 

in Einstein's day, this is irrelevant because Einstein could have

 

used a thought experiment, as he often did (even in the "important"

 

case of his derivation of the relativity of simultaneity.)

 

 

 

Perhaps if you just show how Einstein derived his equation w = c - v,

 

you will see what I have been talking about (non-argumentatively).

 

 

 

But I _will_ give you the following little hint: Einstein did not

 

really prove that the clocks of classical physics were wrong, but

 

simply discarded such clocks in lieu of the really wrong clocks

 

(asynchronous clocks) of special relativity. As we know, he replaced

 

good clocks with bad clocks (absolutely synchronous clocks with

 

absolutely asynchronous clocks) in order to (try to) get all clocks

 

to record "c" for the one-way speed of light. Of course, he very

 

conveniently did _not_ show how this can be done experimentally.

 

But you, Mr. Hyde, are welcome to try.

 

 

 

Well, I have blabbered enough for now.

 

 

 

(Oh, I almost forget to mention that you have yet to show how your

 

probe experiment yields the value "c" for light's one-way speed.)

 

 

 

http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

This is a reference to Einsteins first paper, I can't find your little formula in it, maybe you can point it out or quote it.

 

The color of the light and also it's energy depends on its frequency, it's speed is independent of its color.

Another replier has already told you this.

This is pretty basic stuff, I can't imagine it being left out of a high school science book.

 

The satellite experiment works because they are tracking the satellite and can predict its exact position and speed

relative to the transmitting and receiving antennas.

 

Einstein was right and you are wrong, simple as that. And you are the one obligated to prove your assertions in a case like this

where you are saying that well established theories are wrong.

[LaurieAG]

Long-slit Spectroscopy provides a 90 degree twist on the doppler effect.

http://en.wikipedia.org/wiki/Long-slit_spectroscopy

in an expanding universe?

[yknot]

 

The different FREQUENCY of the light relative to the observers, not the speed.

 

 

 

 

 

 

You must have forgotten about or somehow overlooked my fence analogy

 

(given in my first post).

 

 

 

Let's say that a picket fence is floating in space (inertially, no acceleration).

 

Let's say that you and I see different frequencies for this fence. That is, we

 

observe different numbers of fence posts passing us each second. Is not

 

the only way that this can occur is for us to be moving at different speeds

 

relative to said fence?

 

 

 

And, as I said in my original post, since relative motion is reciprocal, we

 

must say that the fence is moving at different speeds relative to each of us.

 

 

 

Methinks that you fellows are (for some reason that I cannot yet fathom) fighting

 

too strongly against the simple equation w = c - v. It's time to face the facts

 

and get on with life.

[Ronald Hyde]

 

Methinks that you fellows are (for some reason that I cannot yet fathom) fighting

 

too strongly against the simple equation w = c - v. It's time to face the facts

 

and get on with life.

 

You've never provided us with the context that equation is used in.

 

Why are you wasting people's time with this post?

 

Why are you wasting you own time, if you study and understand Einstein's paper you will have the correct view,

and not the erroneous one that you are espousing?

 

Nature simply doesn't choose to work according to your line of reasoning, so your reasoning is just plain wrong.

[ACG52]

 

 

 

 

 

You must have forgotten about or somehow overlooked my fence analogy

 

(given in my first post).

 

 

 

Let's say that a picket fence is floating in space (inertially, no acceleration).

 

Let's say that you and I see different frequencies for this fence. That is, we

 

observe different numbers of fence posts passing us each second. Is not

 

the only way that this can occur is for us to be moving at different speeds

 

relative to said fence?

 

 

 

And, as I said in my original post, since relative motion is reciprocal, we

 

must say that the fence is moving at different speeds relative to each of us.

 

 

 

Methinks that you fellows are (for some reason that I cannot yet fathom) fighting

 

too strongly against the simple equation w = c - v. It's time to face the facts

 

and get on with life.

 

Except that light is not a fence post. The speed of light is invarient in all inertial frames, regardless of their relative motion.

 

You do not seem to have heard about Relativity.