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dhd40
2009-Jul-11, 10:02 AM
The rough draft below shows the situation/Gedankenexperiment which I don´t understand.

The (clockwise) rotating object (e.g. a galaxy) emits photons from identical atoms (say H2) at positions A and B, respectively. Photons A and B have the same wavelength/frequency.
There´s an observer at a constant distance from the rotating object who will see photon A blueshifted (relative to the emitted photon), and photon B redshifted.
There´s no doubt about this, we have seen photos of the Andromeda Galaxy which clearly show this effect.

But how can that be explained? There´s no relative motion between the photons (after emission) and the observer (in this Gedankenexperiment!).

I´m obviously missing something fundamental here.

Hornblower
2009-Jul-11, 10:41 AM
A is moving toward the observer at the moment the light is emitted, and B is moving away. That causes the blueshift and redshift respectively. The fact that the sources are moving in circles rather than straight lines does not change that immediately. Of course those shifts will change as the emitters come around in their orbits. Half an orbital period later A will be redshifted and B blueshifted.

dhd40
2009-Jul-11, 10:57 AM
A is moving toward the observer at the moment the light is emitted, and B is moving away. That causes the blueshift and redshift respectively. The fact that the sources are moving in circles rather than straight lines does not change that immediately. Of course those shifts will change as the emitters come around in their orbits. Half an orbital period later A will be redshifted and B blueshifted.

But the frequency shift doesn´t happen at the moment of emission. That´s at least how I understand it. Both photons A and B have exactly the same wavelength when emitted at A and B, respectively.

Amber Robot
2009-Jul-11, 12:26 PM
Both photons A and B have exactly the same wavelength when emitted at A and B, respectively.

Relative to the objects that emitted them. And those objects have different velocities relative to the detector.

When you look at spectra of stars you see absorption lines that are broadened by the speed of rotation of the star.

Spaceman Spiff
2009-Jul-11, 01:47 PM
I think the trouble dhd40 may be having is in reconciling the particle vs. wave nature of light. If thought of as a wave, then it should be easy to picture how the Doppler effect works (and zillions of intro textbooks or popular books in physical science show how this happens using waves (http://en.wikipedia.org/wiki/Doppler_effect)).

While I do not pretend to understand quantum field theory (http://en.wikipedia.org/wiki/Quantum_field_theory) (QFT) at any level but shallow, I do understand that it is sometimes less useful to think of light as (classical) little particles traveling through space. First off, all quanta travel as wave packets (http://en.wikipedia.org/wiki/Wave_packets). One can think of photons as what happens during an interaction between the quantized electromagnetic field and the quantum field of one or more electrons (usually) -- a localized exchange of energy and momentum. (Also -- the EM interaction between charged particles is mediated through exchanges of virtual photons.) An expanded discussion of the nature of photons can be found here (http://en.wikipedia.org/wiki/Photon).

The above description is probably a little squishy, but the gist is probably there.

dhd40
2009-Jul-11, 07:05 PM
Relative to the objects that emitted them. And those objects have different velocities relative to the detector.



Does that mean that the frequency shift happens already during emission? If so, how does the emitter know about the relative velocities between emitter and detector? And there are trillions of detectors with trillions of different relative velocities.

Hornblower
2009-Jul-11, 08:51 PM
Does that mean that the frequency shift happens already during emission? If so, how does the emitter know about the relative velocities between emitter and detector? And there are trillions of detectors with trillions of different relative velocities.

The emitter doesn't know or care what the detectors are doing. It just does its thing, which is to emit light that has a characteristic wavelength as measured by a detector which is stationary relative to the emitter. Detectors which are moving at different velocities get different values when they measure that wavelength.

Tobin Dax
2009-Jul-12, 05:38 AM
Does that mean that the frequency shift happens already during emission? If so, how does the emitter know about the relative velocities between emitter and detector? And there are trillions of detectors with trillions of different relative velocities.
You're putting the detector in the rest frame. Try looking at this from the emitter's point of view. The photon is emitted from point B at its rest frequency. The detector is moving away from the photon, so when the photon reaches the detector it takes a little bit longer for whole wave to enter the detector and the detector therefore sees a lower frequency (blueshifted) photon. A photon leaving from point A enters the detector more quickly due to the relative motion, and is read as having a higher frequency (redshifted).

That description may not be perfect, but Galileo will have to work right now. Adding length contraction and time dilation would probably make the above more accurate.

Hornblower
2009-Jul-12, 08:06 AM
You're putting the detector in the rest frame. Try looking at this from the emitter's point of view. The photon is emitted from point B at its rest frequency. The detector is moving away from the photon, so when the photon reaches the detector it takes a little bit longer for whole wave to enter the detector and the detector therefore sees a higher frequency (blueshifted) photon. A photon leaving from point A enters the detector more quickly due to the relative motion, and is read as having a lower frequency (redshifted).

That description may not be perfect, but Galileo will have to work right now. Adding length contraction and time dilation would probably make the above more accurate.

I'm sorry, but you appear to have it backward. In this line of thought, if a wave takes longer to enter a receding detector, it should appear to be a lower frequency and redshifted.

sirius0
2009-Jul-12, 10:57 AM
Ok so let us imagine that there are two detectors equal in distance from the emitter and directly opposite each other through the centre of the emmiter. Telemetry brings their data back to you. Now for detector 1 everything is as it was for the OP, photons from point A on the rotating object are blue shifted. Whilst those from point B are red shifted.

But for detector 2 photons from point A are red shifted and those from B are blue shifted.

Exactly the same amounts of shift but B swaps with A for detector 2.

So a photon leaving point A and propagating towards detector 1 has gained some energy due to the rotation from detector 1's perspective. Whilst a photon leaving point A and propagating to detector 2 has lost some energy from detector 2's perspective. However this does not mean that the rotation itself is losing energy due to the emmision of photons (this is just a relative effect) because for every higher energy photon emmitted there is a lower energy one to 'compensate' in a sense. It is epecially interesting to put the detectors on the same side but have a mirror where detector 2 was.

sirius0
2009-Jul-12, 12:06 PM
By the way all these effects could be demonstrated with a loud speaker on a spinning wheel and a microphone as a detector.

Jeff Root
2009-Jul-12, 12:54 PM
dhd40,

Here's an animation I made to explain a related (and more complex)
question about Doppler shifts. You can ignore the numbers on the right.
Pay attention to the bullets. Compare the progress of the bullets from
gunman number one and gunman number three. Each bullet can represent
a wavecrest in light emitted from a rotating galaxy. Gunman one is at the
center of the galaxy, not moving relative to us. Gunman three is moving
toward us, like matter at the top of your galaxy diagram.

http://www.freemars.org/jeff2/Doppler3.htm

Strangely enough, because of the strange fact that the speed of light is
the same for everyone, bullets fired by gunman number three are a better
analogy to light waves than are bullets fired by gunman four.

Bullets fired by gunmen one and three move at the same speed, but those
fired by gunman three are closer together because he moves forward
between shots, like the stars at the top of your rotating galaxy.

-- Jeff, in Minneapolis

Tobin Dax
2009-Jul-12, 02:54 PM
I'm sorry, but you appear to have it backward. In this line of thought, if a wave takes longer to enter a receding detector, it should appear to be a lower frequency and redshifted.
You know, that's what I wrote at first, and then I "fixed" it. I knew I should have just gone to bed. Thanks, Horblower.

Ken G
2009-Jul-13, 04:44 PM
Does that mean that the frequency shift happens already during emission? If so, how does the emitter know about the relative velocities between emitter and detector?I think what is bothering you is actually a very deep principle of relativity, that is often not well appreciated. The answer to "when does the frequency shift happen" is, it is not answerable in general. It is simply not a question that "absolute reality" knows the answer to, even though it sounds like it is a perfectly clear question. The only thing that is "absolutely real" is that a given measurement device receives a certain frequency from a given emitter. Just how that happened is a "story" that we tell, and the story can be told in many different ways, all equally valid if the measurables come out the same. In your case, you have been influenced by a particular version of how to tell the story, and because that version does not apply in general (as none of them do), you are confused as to why things don't work out.

Now, one solution some people take, especially in special relativity, is to make statements like "the answer is different for different observers". That's closer to the truth, but is not the truth, because even a given observer is not required to use any particular version of the story. What the "story" really relies on is an arbitrary choice of coordinate system, and if the same observer uses different coordinates, they get a different story about when and why the frequency shift occured. No observer is forced to use any particular coordinates, it is just that there is a common convention used in special relativity, and that convention is often erroneously interpreted as the "actual reality" for that observer. But there is no such actual reality, other than the outcome of the measurement itself. General relativity corrects this mistake, but it also brings in gravity, which makes it so complicated that the important message kind of gets lost.

Sam5
2009-Jul-13, 05:17 PM
I think what is bothering you is actually a very deep principle of relativity, that is often not well appreciated. The answer to "when does the frequency shift happen" is, it is not answerable in general.



Ken,

I think the actual truth is: “Nobody knows the answer yet.”

The same is true with light from the receding galaxies. Does the redshift take place when the light leaves the galaxies, or while it travels through space, or when it enters our galaxy, or as it reaches the earth?

The same applies to light the earth receives from a distant start that is fixed relative to the sun. When we are moving away from the star, during our annual rotation around the sun, we receive the star’s light as being redshifted, but when we move toward the star six months later, we receive its light as being blueshifted. This hints that the frequency shift takes place somewhere near the earth, perhaps inside our own solar system. But what causes it to shift, and where does the shift actually take place? I think the real answer is: “Nobody knows the answer yet”.

This is a simple Doppler effect regarding starlight, which Doppler himself predicted in the 1840s, so there is no need to confuse the issue with mysterious talk of “relativity” and assuming that the question is “not answerable”. The fact is, nobody yet knows how to answer it, but that doesn’t mean it’s not answerable, and I think it would be better to encourage new young astronomers to set a goal of trying to answer it in the future, rather than telling them to just forget about it because it’s “not answerable”.

trinitree88
2009-Jul-13, 06:00 PM
dhd40. It's not just the photons either. Your object rotates in the background neutrino sea, in which we all live and observe. Viewed from your vantage point, the receding edge of a solid object runs into the neutrinos that strike it, effectively blue shifting them. The approaching edge runs away from them, effectively redshifting them. If originally they were relatively isotropic, they now are not. Cross-sections for both charged and neutral currents(the manner in which neutrinos slightly interact with normal baryonic matter) vary as the square of the frequency/energy. So, the effect the receding edge has on the object's "neutrino sea shadow" is more pronounced than the effect of the approaching edge. (Peltoniomi's Ultimate Neutrino Page). What does this do? From your distant vantage point the vector sum of the forces will be skewed slightly from the dead center of the rotating object...towards the receding edge. That means the orbiting satellite will precess it's perihelion ("periobjection") from considerations not including GR deformations of spacetime...curved space, but from considerations of straight-forward particle physics.That's not to say GR is not there, it's just not acting alone....not in our universe. pete

your object:http://www.bautforum.com/attachments/space-astronomy-questions-answers/10425d1247306416-frequency-shift-photons-emitted-rotating-objects-redshift-4.jpg



Ultimate Neutrino Page see:http://cupp.oulu.fi/neutrino/nd-cross.html

Jeff Root
2009-Jul-13, 06:26 PM
Sam,

I disagree. The answer is known and is as Ken says. The Doppler shift
does not occur at a time or a place. It is a relationship between the
motion of the emitter and the motion of the receiver. Which is entirely
relative. Which is what relativity is about.

The basic question here is extremely simple so I am surprised that you
would assert that the answer isn't known.

-- Jeff, in Minneapolis

Ken G
2009-Jul-13, 08:58 PM
I think the actual truth is: “Nobody knows the answer yet.”At some level, that is the answer to all questions. But since it is clear that a better answer is of more value, what we really mean when we ask a question is, "what is the answer according to our current understanding of our reality". The answer to that question is the one I gave-- according to our current understanding, reality does not answer that question, period. Indeed, it is a central tenet of our best theory dealing with the question (relativity) that reality does not answer that question. Certainly, relativity could be wrong, that more or less goes without saying in science.


This is a simple Doppler effect regarding starlight, which Doppler himself predicted in the 1840s, so there is no need to confuse the issue with mysterious talk of “relativity” and assuming that the question is “not answerable”. Even with the "simple" Doppler shift, the fact that light does not have a medium (apparently) is the reason that we cannot say when the shift occured. This holds true even for the simple Doppler shift, there's no need to include time dilation, the absence of an absolute frame is all that is required. Doppler himself would have assumed light had a "ponderable medium" (to use Einstein's words), so would have thought the question had an answer, but he would have been wrong (apparently). That he could get the right prediction, imagining light had a medium, does not mean that light does have a medium-- it was mere happenstance that his prediction did not require that tidbit of knowledge. Yet, today we do have that tidbit.


The fact is, nobody yet knows how to answer it, but that doesn’t mean it’s not answerable, and I think it would be better to encourage new young astronomers to set a goal of trying to answer it in the future, rather than telling them to just forget about it because it’s “not answerable”.Then you misinterpret the key lessons of relativity. I would make an analogy with the uncertainty principle in quantum mechanics-- do you maintain that teaching the uncertainty principle is telling our new young astronomers to "just forget about" trying to get more information than that principle allows, or do you think it is an example of physics education? If you don't understand relativity yourself, that's no reason to not teach it to "young astronomers".

trinitree88
2009-Jul-13, 09:17 PM
At some level, that is the answer to all questions. But since it is clear that a better answer is of more value, what we really mean when we ask a question is, "what is the answer according to our current understanding of our reality". The answer to that question is the one I gave-- according to our current understanding, reality does not answer that question, period. Indeed, it is a central tenet of our best theory dealing with the question (relativity) that reality does not answer that question. Certainly, relativity could be wrong, that more or less goes without saying in science.
Even with the "simple" Doppler shift, the fact that light does not have a medium (apparently) is the reason that we cannot say when the shift occured. This holds true even for the simple Doppler shift, there's no need to include time dilation, the absence of an absolute frame is all that is required. Doppler himself would have assumed light had a "ponderable medium" (to use Einstein's words), so would have thought the question had an answer, but he would have been wrong (apparently). That he could get the right prediction, imagining light had a medium, does not mean that light does have a medium-- it was mere happenstance that his prediction did not require that tidbit of knowledge. Yet, today we do have that tidbit.
Then you misinterpret the key lessons of relativity. I would make an analogy with the uncertainty principle in quantum mechanics-- do you maintain that teaching the uncertainty principle is telling our new young astronomers to "just forget about" trying to get more information than that principle allows, or do you think it is an example of physics education? If you don't understand relativity yourself, that's no reason to not teach it to "young astronomers".


Well said, Ken. pete

Amber Robot
2009-Jul-13, 11:41 PM
The same is true with light from the receding galaxies. Does the redshift take place when the light leaves the galaxies, or while it travels through space, or when it enters our galaxy, or as it reaches the earth?

This is at least somewhat answerable, based on the excitation of certain molecules by the CMB as a function of redshift.

Ken G
2009-Jul-14, 03:38 AM
This is at least somewhat answerable, based on the excitation of certain molecules by the CMB as a function of redshift.Actually, the question remains unanswered for each of those molecules. The point is, every observer, and every molecule, will see something, for sure, but when and why what they see became that way is never knowable, the result is only a function of the complete process, the null geodesic followed by the light. For example, if I perceive the molecules you describe as moving, or as being in some gravitational field, based on any particular coordinates I'm using to separate space and time in any arbitrary way, I may reason that it wasn't the light at all that had anything happen to it. My picture could as well be that the molecules had something happen to them, while the light was always just the same. There simply is no experiment that comes out A if something happened to the light, and B if it happened to the molecules, and that's why the question goes unanswered in a unique way-- we are free to answer it in any number of equally physically valid ways, as they can all be made entirely equivalent (indeed, the appearance of the word "equivalent" in the equivalence principle is no coincidence here-- equivalence is at the core of general relativity).

To make this more clear, imagine I am in a rocket moving toward the source of the CMB in some direction. Perhaps I'm moving so fast that the CMB looks just like the red light it was when it was first emitted, not the microwaves we see from Earth. Shall I say the light was redshifting all the while, then suddenly got blueshifted just as I observed it, or shall I say nothing changed about the light at all, it was just all those moving intervening molecules that thought the light was redshifting? Granted, the "comoving frame" description has plenty of merit as a convention, but it still is never anything but convention-- the question as posed is still not knowable, because convention is not truth.

dhd40
2009-Jul-14, 11:13 AM
You're putting the detector in the rest frame. Try looking at this from the emitter's point of view. The photon is emitted from point B at its rest frequency. The detector is moving away from the photon, so when the photon reaches the detector it takes a little bit longer for whole wave to enter the detector and the detector therefore sees a lower frequency (blueshifted) photon. A photon leaving from point A enters the detector more quickly due to the relative motion, and is read as having a higher frequency (redshifted).

That description may not be perfect, but Galileo will have to work right now. Adding length contraction and time dilation would probably make the above more accurate.

(my bold)
But that´s exactly what I thought was not happening in my Gedankenexperiment, because the distance between the rotating object and the detector doesn´t change. Therefore, after the photon has been emitted it´s already moving in the detector´s frame (my view), isn´t it?

dhd40
2009-Jul-14, 11:41 AM
As I see from several answers now the conclusion seems to be that we simply don´t know where and when the frequency shift happens. This somehow surprises me because my home-made model, which put the shift-happening (:)) into the detector (or the detector´s frame), worked so well for many years.

Now, that´s a little bit disappointing. On the other hand, the outcome of measurements (of frequency shifts) seems to be very consistent. What I mean by this is it isn´t abitrary. Everybody who runs the same experiment will get the same answer.

Hmm, mother nature seems to know how to run things without telling us her secrets.

Thank you all for your valuable contributions

JohnD
2009-Jul-14, 12:15 PM
I cerytainly can't agree that the place at which the frequency shift occurs is unknown. It occurs at the detector. Or do I misunderstand the question?

Another thought experiment to illustrate. Two experiments.

1/ Galaxy as before. Beam of light from either limb. In setad of single observer, two at same distance, but ravelleing towrads and away from the galaxy, at the velocity of the rim. 'Towards' at the receding limb.
Both will 'see' the light at the same wavelength.
If they stop, relative to the galaxy, they will both see equal frequency shift, in opposite directions.

2/ 'Observer' at galaxy rim.
'Sees' light from approaching atom, notes blue shift.
Turns quickly as the atom goes by, 'sees' red shift, equal amount from same atom, now receding.
But if the observer is in the galaxy , roattaing with the atom - no shift.

Wherever you place the 'observer', whatever their relative velocity to the radiator, the frequency shift occurs at the observer.

John

Ken G
2009-Jul-14, 02:16 PM
I cerytainly can't agree that the place at which the frequency shift occurs is unknown. It occurs at the detector. Not so, and neither of your examples establish your claim. You give two examples that show the motion of the detector affects the result. How does that show when and where the shift occured? Motion is relative, ergo. "relativity". You could just as easily alter the galaxy, and you'll also get a difference in the shift. Does that prove the shift happens at the galaxy?

Tobin Dax
2009-Jul-14, 03:33 PM
(my bold)
But that´s exactly what I thought was not happening in my Gedankenexperiment, because the distance between the rotating object and the detector doesn´t change. Therefore, after the photon has been emitted it´s already moving in the detector´s frame (my view), isn´t it?
The rotating object isn't emitting the photons in question, small parts of the object, which have a relative radial velocity to the detector, are emitting these photons. You cited a galaxy as the rotating object in your OP. A galaxy is made up of many individual stars, most of which will have a relative velocity when compared to the detector. The photons come from the stars, not just from the center of the galaxy.

Amber Robot
2009-Jul-14, 03:50 PM
Actually, the question remains unanswered for each of those molecules. The point is, every observer, and every molecule, will see something, for sure, but when and why what they see became that way is never knowable, the result is only a function of the complete process, the null geodesic followed by the light. For example, if I perceive the molecules you describe as moving, or as being in some gravitational field, based on any particular coordinates I'm using to separate space and time in any arbitrary way, I may reason that it wasn't the light at all that had anything happen to it. My picture could as well be that the molecules had something happen to them, while the light was always just the same. There simply is no experiment that comes out A if something happened to the light, and B if it happened to the molecules, and that's why the question goes unanswered in a unique way-- we are free to answer it in any number of equally physically valid ways, as they can all be made entirely equivalent (indeed, the appearance of the word "equivalent" in the equivalence principle is no coincidence here-- equivalence is at the core of general relativity).

To make this more clear, imagine I am in a rocket moving toward the source of the CMB in some direction. Perhaps I'm moving so fast that the CMB looks just like the red light it was when it was first emitted, not the microwaves we see from Earth. Shall I say the light was redshifting all the while, then suddenly got blueshifted just as I observed it, or shall I say nothing changed about the light at all, it was just all those moving intervening molecules that thought the light was redshifting? Granted, the "comoving frame" description has plenty of merit as a convention, but it still is never anything but convention-- the question as posed is still not knowable, because convention is not truth.

This is why I am not a cosmologist. :lol:

Ken G
2009-Jul-14, 04:06 PM
This is why I am not a cosmologist. I agree at one level it's kind of frustrating, but at another, it's kind of liberating. To me, it's a bit like learning that arithmetic can be done using other bases than base 10, and it still works and still gives equivalent results. We use base 10 so much that we begin to think numbers themselves have to work that way, but numbers are something deeper that transcends the base used to manipulate them. Apparently, our local concepts of space and time are like that too, for observers in relative motion at the same place and time, and this retreat from uniqueness into equivalence introduces an ambiguity in global extensions of our local concepts of space and time. The conventions we use to calculate using those extensions are just conventions like a choice of base. We can't really manipulate and communicate arithmetic quantities without choosing a base, but that doesn't mean the entities we are manipulating give a hoot what base we choose. We have to learn to separate what nature is doing from what we are doing, it's an important lesson that permeates through all of science.

JohnD
2009-Jul-14, 04:57 PM
Not so, and neither of your examples establish your claim. You give two examples that show the motion of the detector affects the result. How does that show when and where the shift occured? Motion is relative, ergo. "relativity". You could just as easily alter the galaxy, and you'll also get a difference in the shift. Does that prove the shift happens at the galaxy?

Ken,
My point was that I compared the detector/observer in motion and at rest, relative to the emitter. Would you accept that EMR from hydrogen atoms (as suggested by the PO) is determined by the electron energy, and so is at a constant frequency? It does so regardless of the velocity of the detector, which will find it is as advertised if there is no relative motion. So it is irrelevant if it is the galaxy or the detector that moves. Indeed, ignoring all other objects, there is no way of knowing which it is that is moving.

The light leaves the emitter at a set frequency - it has not changed until it hits the detector, and any change depends on the relative velocity between emitter and detector. As Lord Meercat says, "!Simples!"

John

Ken G
2009-Jul-14, 06:47 PM
Would you accept that EMR from hydrogen atoms (as suggested by the PO) is determined by the electron energy, and so is at a constant frequency?Both its energy and its frequency depend on the reference frame. But yes, in the frame of the emitting atom, and when it is at the location of that atom (say, as it is emitted), we do know its energy and frequency unambiguously. That is the last place and time for which that statement is true, until we absorb that photon at the location of some other atom and in the frame of that atom, and what happened in between is unknowable and unspecifiable in any kind of absolute way. The reason is because the reference frames in between are completely arbitrary, the choices we make will control all the outcomes, we will be doing it ourselves (just as you are). Hence, we cannot state unambiguously when or where the frequency changed, we control the answer ourselves.

In special relativity, we can say why it changed, which is the "relative motion," but in general relativity, we cannot even say why unambiguously, because there is not even an unambiguous concept of nonlocal relative motion. It's all dependent on the coordinate choice, which introduces an inescapable arbitrariness that is only resolved by restricting to results that are invariants of the theory.


So it is irrelevant if it is the galaxy or the detector that moves. Indeed, ignoring all other objects, there is no way of knowing which it is that is moving.Correct, and that is why your claim above is unsupportable.


The light leaves the emitter at a set frequency - it has not changed until it hits the detector, and any change depends on the relative velocity between emitter and detector. The first conclusion is not supported by anything you've said, and even the second conclusion is not generally true, it is only supportable in the limited viewpoint of special relativity. Personally, I don't think we should even teach special relativity, because it leads so inevitably to misconceptions like that second conclusion, but that's not the key part of what I'm saying, because what I said above is true even in special relativity-- we have no way of knowing when the frequency change occured, reality does not arbitrate the point.

I can prove my assertion quite easily. If you believe that reality does arbitrate when the frequency shift occurs if atom A emits a photon and atom B absorbs it at a different frequency, then tell me an experiment that comes out X if the frequency shift occured only at the point of absorption, and not X if it occured in some other way. My guess is, you would try to say something like, take atoms stationary with respect to A and put them all along the path from A to B. They'd all see the photon as being at the same frequency as A emitted it. But I'll just ask, why those atoms? Why not a chain of atoms stationary with respect to B? Those atoms would "prove" the opposite assertion, that the frequency shift happened immediately upon emission. The real answer is, neither is a supportable assertion, it is unknowable because "it takes two to tango"-- the invariant frequency shift is set by the complete process only.

Sam5
2009-Jul-14, 08:52 PM
The light leaves the emitter at a set frequency - it has not changed until it hits the detector, and any change depends on the relative velocity between emitter and detector.

John

I would say that if there is relative motion between the detector and the emitter, there will eventually be a shift in frequency observed by the detector, but where in space the shift actually takes place isn’t necessarily always at the detector.

Jeff Root
2009-Jul-14, 10:38 PM
Ken,

After reading your reply to John D, I've changed my mind. I agree with
John. The change in frequency occurs when the light is detected.

-- Jeff, in Minneapolis

Ken G
2009-Jul-15, 01:30 AM
[After reading your reply to John D, I've changed my mind. I agree with
John. The change in frequency occurs when the light is detected.
If scientific questions were settled by a vote, then your opinion would be relevant. As they are not, I will offer you the same challenge to make your opinion something that holds up to scientific scrutiny: tell me the experiment that must come out X if the shift occurs then, but must come out not X if it doesn't. And it can't be an experiment that I can just turn around and reach the opposite conclusion.

Amber Robot
2009-Jul-15, 01:41 AM
Ken,

After reading your reply to John D, I've changed my mind. I agree with
John. The change in frequency occurs when the light is detected.

-- Jeff, in Minneapolis

What if the light bounces off a diffraction grating before it hits a detector? And the angle that it bounces off depends on its wavelength? Would you then say the frequency shift occurs when it hits the grating? How far up the chain could we push this?

Ken G
2009-Jul-15, 01:53 AM
This is even easier. Just take the reference frame of the detector. When does the frequency shift occur in that reference frame? That's also unanswerable, but if you imagine it does have an answer, you'll get the other answer than the one given so far. Please people, this is very basic relativity.

Sam5
2009-Jul-15, 01:58 AM
.....I will offer you the same challenge to make your opinion something that holds up to scientific scrutiny: tell me the experiment that must come out X if the shift occurs then, but must come out not X if it doesn't. And it can't be an experiment that I can just turn around and reach the opposite conclusion.

Expressed this way in an earlier post:



If you believe that reality does arbitrate when the frequency shift occurs if atom A emits a photon and atom B absorbs it at a different frequency, then tell me an experiment that comes out X if the frequency shift occured only at the point of absorption, and not X if it occured in some other way.

Ken,

By that, are you saying you want two thought experiments; one that tells how the frequency shift can occur at the observer as the shift is being observed, and a different thought experiment that tells how the frequency shift can occur in space, at or near the emitter, long before the shift is observed by the observer?

Ken G
2009-Jul-15, 02:04 AM
By that, are you saying you want two thought experiments; one that tells how the frequency shift can occur at the observer as the shift is being observed, and a different thought experiment that tells how the frequency shift can occur in space, at or near the emitter, long before the shift is observed by the observer?The problem is, it is easy to come up with thought experiments that produce those outcomes-- my challenge to those who believe the time the shift occurs is uniquely determined is to come up with an experiment that demonstrates that uniqueness. There cannot be an analogous experiment that indicates anything else, if their claim is indeed the "truth". I'm not actually waiting for them to deliver, I know they cannot-- it would be tantamount to disproving relativity and earning the Nobel prize.

Tobin Dax
2009-Jul-15, 03:00 AM
The frequency shift occurs at the same time that the light changes reference frames. When does the light change reference frames? That's unanswerable, as Ken G says. In fact, the question is meaningless. Reference frames depend on the observer, not the observed, so the light never actually changes reference frames.

Ken G
2009-Jul-15, 03:27 AM
So far, the arguments that the shift occurs upon absorption have only amounted to the (correct) assertion that we won't know the shift unless the state of the receiver is specified. However, we also don't know the shift unless the state of the emitter is specified. If we receive light and we don't know the source, we don't know if it is blueshifted or redshifted until we discover the source motion. Did that shift happen when we figured out the source motion? When we know something is irrelevant to "what happened". All that can be said is that the emission occurs prior to the reception, that does not say when the shift occured-- the shift depends on the complete process. Of course we don't know the complete process until it is over, but that does not mean the shift happens at the end. Note that the time a journey takes also depends on the complete path taken, shall we use similar logic and say that the time elapsed in a journey happens all at the end, because we cannot know the elapsed time until we reach the end, and the end follows after the beginning?

Let me put this another way if there is still any lingering doubt. One might say that to get to the issue of when the shift happened, we can either ask the photon itself, or we can ask observers of the photon, or we can suggest some experiment to answer it. If we do the first, the photon itself thinks the entire process of emission, propagation, and absorption happened all at the same time, so there's no help there. If we do the second, then we control the answer as soon as we specify what observers we will ask. No help there either. Also, no experiment has thus far been suggested (and none will be) that can answer the question. No help anywhere, the question goes unanswered by the tenets of science. Of course anyone can have a personal belief on the issue, but it's about as scientific as any other unsupported belief.

Jeff Root
2009-Jul-15, 03:28 AM
After reading your reply to John D, I've changed my mind. I agree with
John. The change in frequency occurs when the light is detected.
If scientific questions were settled by a vote, then your opinion
would be relevant.
That's just silly. If scientific questions were settled by a vote, I would
have to meet the qualifications to vote, I'd have to register in some way,
and the question would have to be put before everyone qualified to vote,
not just the people reading one particular thread on BAUT.

Instead, my opinion is relevant because:

1) it is my opinion,
2) it reflects what other people may also think, and
3) it's an indication of how convincing your reply to JohnD was.

-- Jeff, in Minneapolis

Ken G
2009-Jul-15, 03:42 AM
Instead, my opinion is relevant because:

1) it is my opinion,That argument is tautological, so nonsensical.


2) it reflects what other people may also think, andThat is equally nonsensical, as if your opinion is irrelevant, than so can be others. If an opinion is uninformed, it is irrelevant, and if it is informed, then it can be supported. Yours has not been, though I told you exactly how you could support it.


3) it's an indication of how convincing your reply to JohnD was.
Also irrelevant. If you are unconvinced by an argument because you have not the knowledge necessary to understand it, then again your opinion is irrelevant. If instead you are unconvinced because you can cite errors in the argument, then you have not done so. I'm afraid this is all basic logic, but as usual, one can only bring the horse to water.

dhd40
2009-Jul-15, 08:18 PM
(snip)If we receive light and we don't know the source, we don't know if it is blueshifted or redshifted until we discover the source motion. (snip)

Is this really true? We can identify, e.g., frequency shifted Balmer series in a spectrum without knowing the source. And, of course, we know whether it is redshifted or blueshifted.
Well, thinking a while about it, maybe, it depends upon what "knowing the source" means?

JohnD
2009-Jul-15, 09:19 PM
If scientific questions were settled by a vote, then your opinion would be relevant. As they are not, I will offer you the same challenge to make your opinion something that holds up to scientific scrutiny: tell me the experiment that must come out X if the shift occurs then, but must come out not X if it doesn't. And it can't be an experiment that I can just turn around and reach the opposite conclusion.

You also said:
"I can prove my assertion quite easily. If you believe that reality does arbitrate when the frequency shift occurs if atom A emits a photon and atom B absorbs it at a different frequency, then tell me an experiment that comes out X if the frequency shift occured only at the point of absorption, and not X if it occured in some other way. My guess is, you would try to say something like, take atoms stationary with respect to A and put them all along the path from A to B. They'd all see the photon as being at the same frequency as A emitted it. But I'll just ask, why those atoms? Why not a chain of atoms stationary with respect to B? Those atoms would "prove" the opposite assertion, that the frequency shift happened immediately upon emission. The real answer is, neither is a supportable assertion, it is unknowable because "it takes two to tango"-- the invariant frequency shift is set by the complete process only."

Well, you read my mind, because that's exactly what I would say, in different words.
But I draw a different conclusion. Each step in the chain of detectors measures a change in frequency that is proportional to the difference in velocity of emitter and detector. The change is measured at detectors anywhere in the chain, and will be the same however long the chain is. So the distance between emitter and detector is irrelevant. If the change occured somewhere in the dim and mysterious depths of space, then the distance would be relevant, like solar neutrinos oscillating in thier passage from Sun to Earth.

BUT! I think I see your point. A known frequency comes from a moving emitter. It has 'something' superimposed on it to represent the emitters velocity, that is interpreted at the detector as a frequency change. You insist that this does chnage the frequency at some indeterminate point on its path and I that the change does not occur until it is detected.
I am irresistably reminded of Schroedingers Cat, and the collapsing wave function - the properties cannot be known until the particle is detected. Until then, it has duality. So maybe that's true of light. But in this case, there is no either/or - the light WILL have a different frequency, depending on the difference in velocity of emitter and detector. You cannot know when the change has occured, and to be honest, neither can I, any more than could poor Frau Schroedinger know the health of her cat before the box was opened.

John

Jeff Root
2009-Jul-16, 12:26 AM
If we receive light and we don't know the source, we don't know if
it is blueshifted or redshifted until we discover the source motion.
Is this really true? We can identify, e.g., frequency shifted Balmer
series in a spectrum without knowing the source. And, of course,
we know whether it is redshifted or blueshifted.
Well, thinking a while about it, maybe, it depends upon what
"knowing the source" means?
Yes. If you identify a series of lines as the Balmer series, then you
have identified the source for this particular purpose. You might not
know where the hydrogen is or what object it is connected with, but
you know enough to be able to determine the redshift.

-- Jeff, in Minneapolis

Sam5
2009-Jul-16, 01:45 AM
JohnD to Ken:


You insist that this does chnage the frequency at some indeterminate point on its path and I that the change does not occur until it is detected.


Hi John D,

Let’s add a third point of view to the list. I believe that in the case of moving stars and galaxies, the shift occurs in space a long way away from the detector at the earth. But we don’t yet know where and how. Someday I think we will be able to determine where and how.

Ken G
2009-Jul-16, 02:37 AM
Is this really true? We can identify, e.g., frequency shifted Balmer series in a spectrum without knowing the source. And, of course, we know whether it is redshifted or blueshifted.
Well, thinking a while about it, maybe, it depends upon what "knowing the source" means?Right. My point is, there are situations where we would not know, say if it was a purely thermal spectrum. It doesn't matter if we could know, it suffices for my argument that we must be able to handle the possibility that we might not know. The argument is that when we know something is different from when it happens.

Tobin Dax
2009-Jul-16, 02:44 AM
Hi John D,

Let’s add a third point of view to the list. I believe that in the case of moving stars and galaxies, the shift occurs in space a long way away from the detector at the earth. But we don’t yet know where and how. Someday I think we will be able to determine where and how.
Are you claiming that the Doppler effect behaves differently at large scales than it does on Earth?

Ken G
2009-Jul-16, 03:01 AM
Well, you read my mind, because that's exactly what I would say, in different words.
But I draw a different conclusion. Each step in the chain of detectors measures a change in frequency that is proportional to the difference in velocity of emitter and detector. Why do you think that will be true? You must be assuming something about the motion of the detectors. Use different motions, and you'll get a different answer. So your argument, which says the 'real" shift happens all at the last detector, must be independent of these "trial" detectors along the way. But only the resulting shift is so independent, not when it happens, refuting the claim.


The change is measured at detectors anywhere in the chain, and will be the same however long the chain is.The only thing that must be the same for all such chains is the shift observed by the last detector. Everything else depends entirely on what you choose to do with that chain of detectors, how you arbitrarily choose to set them up. That's precisely why you can not demonstrate when the shift occured-- you make it what you want it to be, nature has no unique opinion on the subject.


BUT! I think I see your point. A known frequency comes from a moving emitter.Be careful with relativistically awkward language. There is no such thing as "a moving detector", all we can assert is that we have a relative motion between the emitter and the detector (and even that can only be asserted in special relativity, it loses any absolute meaning in general relativity).


It has 'something' superimposed on it to represent the emitters velocity, that is interpreted at the detector as a frequency change. Yes, though I would have said the "something" involves a relationship between emitter and receiver, not the emitters velocity, though perhaps that's what you really mean.


You insist that this does chnage the frequency at some indeterminate point on its path and I that the change does not occur until it is detected.Actually, you did not say the change does not occur until it is detected, you said it occurs when it is detected. That is quite different. Certainly, one can say nothing about the shift if there is no detector, as the shift involves a relationship between an emitter and a detector. No detector, no relationship, no shift to talk about. I'm saying that if it is detected, that relationship results in a frequency shift, and nothing more whatsoever holds to the standards of scientific demonstrability. In support of that claim, I cite that I have refuted any suggested experiments that claim to demonstrate it, by giving a modification to the same experiment that gives a contradictory result.



I am irresistably reminded of Schroedingers Cat, and the collapsing wave function - the properties cannot be known until the particle is detected. That example does not work, because the wave function of the photon does not depend on the detector, it depends on the choices made by the person writing the wave function. I can write that wave function in a thousand ways, all different (though of course equivalent in the resulting predictions, so they are only different in form), and they will all give the same frequency shift in that detector. This is precisely the ambiguity I am talking about.

This does not have to be complicated. If an emitter is flying toward me at c/2, and emits periodic pulses of light, like a blinking flashlight, then I can stay in my own frame and analyze what happens to those pulses. My analysis will incorporate two elements, one is that between pulse emissions, the source moves closer, so subsequent pulses take less time to reach me than the previous pulse. This causes the pulses to arrive closer together on my clock than they were emitted on the flashlight clock. Also, I can use the standard (awkward) language of special relativity, and say that time is dilated at the flashlight, so I think the pulses had a longer time between them than the person emitting them thinks. When I combine these two effects, I predict the correct observed pulse duration.

Now I have two questions for you:
1) when did that pulse duration change between what the emitting flashlight's clock says, and what my clock says, and
2) how is a pulse duration any different from a wave period?

The answers are:
1) unknowable, as it depends on the chosen coordinatization of the problem (in SR, this can be interpreted as depending on the observer).
2) No difference.

Or let me put this non-relativistically to show your position doesn't work there either. If a race car is driving toward you at high speed, and emitting a whine at a certain pitch, you hear a higher pitch. This is because the wavelength of the sound is actually compressed by the motion of the car in between wavefront emissions. There's no relativity here, so this is just true, the wavelength really is shortened by the car's motion through the air. What's more, that shortening occurs right when the sound is emitted, again there is no ambiguity there. So what about your argument do you think works in the relativistic situation that does not work with a racecar? Do you not still need to know what the detector is doing to know the shift?

The actual difference is that for the race car we have a medium, air, so we can know exactly when the wavelength change occured, but for light, we have no medium and so we cannot know. That all went out with the aether. Note that if you bring back the aether, you can know when the shift occured, but it will not be when you say. So your claim just never works, not for photon wavefunctions, not for race cars, not for an unknown aether, and not in standard relativity.


But in this case, there is no either/or - the light WILL have a different frequency, depending on the difference in velocity of emitter and detector. Now you are saying it right-- but note that it does not support any particular claim about when the shift occurs. I'm not trying to be hard on you, I just want to be completely clear where the errors in both relativity and wave mechanics are.


You cannot know when the change has occured, and to be honest, neither can I, any more than could poor Frau Schroedinger know the health of her cat before the box was opened.
Now you have reached the conclusion I've been saying all along, so perhaps you are seeing the point.

Ken G
2009-Jul-16, 03:15 AM
Let’s add a third point of view to the list. I believe that in the case of moving stars and galaxies, the shift occurs in space a long way away from the detector at the earth. But we don’t yet know where and how. Someday I think we will be able to determine where and how.It's perfectly fine to believe that, and it might even turn out to be true, but at present there is no scientific basis for that belief. It's not like homeopathy, where there's a clear scientific basis for saying it does not work as advertised, but it's more like believing that unicorns which have been invisible for so long will one day be discovered. A perfectly valid personal opinion, but the scientific response is just "show me the evidence".

Sam5
2009-Jul-16, 03:55 AM
It's perfectly fine to believe that, and it might even turn out to be true, but at present there is no scientific basis for that belief. It's not like homeopathy, where there's a clear scientific basis for saying it does not work as advertised, but it's more like believing that unicorns which have been invisible for so long will one day be discovered. A perfectly valid personal opinion, but the scientific response is just "show me the evidence".

The OP referred to “galaxies”, which are distant, and that’s why I mentioned “distant stars and galaxies”.

I don’t know of any papers or detailed studies that offer scientific opinions about what happens here at the earth regarding light sources and detectors here at the earth. But we do have some mainstream-accepted statements in the Davis-Lineweaver papers that discuss this topic and that indicate the redshifts of the distant galaxies (that we observe here on earth when their light finally reaches us) take place in deep space, at great distances, all along the way as the photons are traveling the great distances toward the earth. This is essentially stated in various ways in this paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070v2.pdf

And this paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

And in this Scientific American article:

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

So it’s not just my opinion. My opinion is based on the scientific principles contained in these papers.

Jeff Root
2009-Jul-16, 06:17 AM
Lemme very briefly summarize part of this:

Doppler shift of light doesn't occur at a location, it occurs between two
different reference frames-- that of the source and that of the observer.
Sam believes that an additional cosmic shift is caused by the expansion
of space that the light is passing through, so occurs where the light is
as it travels toward the observer.

The chain of detectors that Ken read about in John's mind works. Each
detector is an observer. It doesn't matter what state of motion each
of them is in-- the frequency shift occurs at each observer.

Ken said "to [find out] when the shift happened, ... we can ask observers
of the photon.... then we control the answer as soon as we specify what
observers we will ask." That's it. The observer determines where and
when the redshift occurs.

A spectrograph with a detector or set of detectors is an observer.
The frequency shift occurs when the light reaches the observer.

Oh, yeah: "The frequency change does not occur until the light is
detected" is synonymous with "The frequency change occurs when
the light is detected".

-- Jeff, in Minneapolis

Ken G
2009-Jul-16, 06:35 AM
But we do have some mainstream-accepted statements in the Davis-Lineweaver papers that discuss this topic and that indicate the redshifts of the distant galaxies (that we observe here on earth when their light finally reaches us) take place in deep space, at great distances, all along the way as the photons are traveling the great distances toward the earth. This is essentially stated in various ways in this paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070v2.pdf

And this paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

And in this Scientific American article:

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147No, none of those papers intend to assert that the place or time where the redshifts occured is known in a unique way. They do get a bit imprecise at times, and that's my main criticism of Lineweaver and Davis, though they certainly know how to arrive at a correct calculation. This is quite basic relativity, it is the role of coordinates. I'm certain that those authors understand the role of coordinates, in their case, "co-moving frame" coordinates, but they tend to use those coordinates so much it is easy to mistake them for some kind of physical reality, but it is undemonstrable. Any statements you find that suggest otherwise are being misinterpreted, and I know just how they are being misintepreted. You are mistaking a "story" about the redshifts, which works fine and involves a conventional coordinatization (the comoving frame), with a demonstrable statement about the physical reality of when those redshifts actually occured.


So it’s not just my opinion. My opinion is based on the scientific principles contained in these papers.Misinterpretation of the principles, actually. The first principle you need to understand is the importance of the word coordinate-independent "invariants" in relativity, in contrast to the arbitrariness of coordinates.

Ken G
2009-Jul-16, 06:44 AM
Ken said "to [find out] when the shift happened, ... we can ask observers
of the photon.... then we control the answer as soon as we specify what
observers we will ask." That's it. The observer determines where and
when the redshift occurs.I already pointed out the flaw in this statement, so I'll extend the same question to you: when does the Doppler shift in the sound of a racecar happen? I actually need you to answer this, if you don't, you forfeit the entire argument. (You actually already have forfeited it by not supplying me with any experiment to test your assertions, but now you can forfeit it a second time.) Taking your arguments at face value, exactly as you wrote them, the sound that enters our ear gets shifted at that moment, and the sound that misses our ear has not yet been shifted. This holds because no part of your argument invoked relativity, nor is it true to the symmetries of relativity. Unfortunately, the claim is clearly false in any normal application of scientific realism. As I said, it doesn't work in any of the situations I listed: quantum wave mechanics, classical wave physics, relativity without an aether, or quasi-relativity with an aether. 0 for 4. But you can believe it if you choose to, no one said you had to think scientifically. And who knows, some day a new theory might emerge where it is actually true, but that day is not today.


Oh, yeah: "The frequency change does not occur until the light is
detected" is synonymous with "The frequency change occurs when
the light is detected".Nope, it isn't, because the former contains a way for the frequency change to not occur that the second sentence does not contain. The second sentence implies we know the light would be detected, and we're asking when the shift occurs. The first sentence allows there to be no detection, and hence no shift, which is much closer to correct, essentially because the word "until" can have a similar meaning to the word "unless" (which would be a better way to say it because it avoids unsubstantiated assertions about the time of the shift). In my case, I can clearly differentiate a classical wave in a medium from a relativistic wave without a medium, because I am being true to the symmetries present in the latter that are absent in the former. Those symmetries are why we cannot assert there is a shift if there is no detection for relativistic mediumless waves, and the breaking of those symmetries by the medium are why we can assert that for classical sound waves. Your position breaks the symmetry either way (and in contradictory ways, to boot!), so that's why it is bunk as it lacks any scientific justification for accepting it.

sirius0
2009-Jul-16, 08:03 AM
A red herring perhaps WRT the OP (well dhd40's real question being where does the shift occur) But is the logic behind all these statements about "not being possible to know" some what related to the Wheeler experiment? Now I don't mean the dual nature of light. I mean in the sense of where the detector goes and what type of detector etc. I mean it is the detector that demonstrates either the particle or wave nature for that event perhaps. Is this why the detector is being given an apparent place for the shift over the emitter in the current discussion?

Ken G
2009-Jul-16, 01:19 PM
Is this why the detector is being given an apparent place for the shift over the emitter in the current discussion?This situation is not one where entanglement really has much to say, because we already have complete knowledge about the photon. Quantum mechanics was brought up, but erroneously, because the photon in this experiment is already in a pure state, so it cannot be "collapsed" and it cannot be interesting to entangle it. In the way they imagine it, we just don't have complete knowledge about the detector, and that's why they think something is special about the moment of detection. But I already provided a situation where we have complete knowledge of the detector but not the photon (we don't know anything about the emitter, say), and then it is clearly true that establishing the nature of the emitter is what we need to determine the frequency shift. They ignore this fundamental symmetry of relativity, yet fail to provide an experiment that can break that symmetry. Not surprising, that symmetry is thought to go unbroken, so it would be Nobel-prizeworthy to break it. Indeed, it seems that "symmetries were made to be broken", so perhaps Sam5 will be the one who turns out to be right in the end, but no evidence has been offered that we can ever know when the shift occured.

Sam5
2009-Jul-16, 03:33 PM
No, none of those papers intend to assert that the place or time where the redshifts occured is known in a unique way.


Sure they do, in all three papers. See page 3 of their Scientific American article:

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

“Astronomers say that the galactic light has been redshifted. The explanation is straightforward: As space expands, light waves get stretched. If the universe doubles in size during the waves' journey, their wavelengths double and their energy is halved.”

They clearly state that this takes place all through the distant space through which the light waves travel on their way toward the earth.

Jeff Root
2009-Jul-16, 04:56 PM
Ken said "to [find out] when the shift happened, ... we can ask observers
of the photon.... then we control the answer as soon as we specify what
observers we will ask." That's it. The observer determines where and
when the redshift occurs.
I already pointed out the flaw in this statement,
I was quoting it to say that I disagree that it is flawed. Let me
remind you that I agreed entirely with your thesis before your reply
to JohnD, at which point I changed my mind. I now think that John
was right and you were wrong: It is reasonable and useful to say
that the frequency shift occurs at the place and time of observation.



so I'll extend the same question to you: when does the Doppler shift
in the sound of a racecar happen?
Note that at the start of my last post (which you are replying to
here), I was careful to explicitly say "Doppler shift of light..." since
the frequency shift in sound depends as much on the medium the
sound is traveling through as it does on the observer. It results
from the relative speed of the racecar to the air and the relative
speed of the observer to the air, rather than the relative speed of
the observer to the racecar. So, for an observer motionless wrt
the air, it is reasonable to say that the Doppler shift occurs at the
racecar. For a racecar motionless wrt the straight-line wind it is
driving in, it is reasonable to say that the Doppler shift occurs at
the observer who is bracing against the wind. When an observer
moves along with the wind, too, beside the racecar, there is no
Doppler shift.

-- Jeff, in Minneapolis

Spaceman Spiff
2009-Jul-16, 05:04 PM
Sure they do, in all three papers. See page 3 of their Scientific American article:

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

“Astronomers say that the galactic light has been redshifted. The explanation is straightforward: As space expands, light waves get stretched. If the universe doubles in size during the waves' journey, their wavelengths double and their energy is halved.”

They clearly state that this takes place all through the distant space through which the light waves travel on their way toward the earth.

This quote from the SciAm article by Lineweaver & Davis is an example of a useful analogy -- as aimed at the level of the average SciAm reader. The authors certainly know better, but their job at hand is to communicate knowledge to a specific audience, and so they stretch (heh, heh :whistle:) an analogy in order to do so. To gain a deeper understanding of "what is going on", one must delve deeper into what General Relativity says about 'expansion' or 'cosmological redshift'; see this article (http://arxiv.org/abs/0707.0380), for example.

From section 2.6.4 of the above linked article:

The explanation of redshift is a crucial link that needs to be made between cosmological observations and theory. A derivation of the balloon analogy is often employed in the teaching of this concept; a wave is sketched on a balloon and as it is blown up the wavelength is seen to increase as the sketch is stretched along with the expansion of the underlying space. This is largely uncontroversial, but care must be taken in ensuring that the analogy does not mislead. Since we have shown how bodies held together by electromagnetic forces do not expand with the expansion of space, why should electromagnetic waves be affected? The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied. The difference between frames relates to a changing background metric rather than a differing velocity.(emphasis mine)

But in the end, it all comes down to one's choice of coordinates -- as is always true in GR. Go here (http://blogs.discovermagazine.com/cosmicvariance/2008/10/06/does-space-expand/) for an excellent commentary.

Ken G
2009-Jul-16, 05:11 PM
Sure they do, in all three papers. See page 3 of their Scientific American article:

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147You have indeed misinterpreted what they are saying. It is true that the CMB is a thermal spectrum, which is going to lower temperature with time, but that still does not tell you when the photons you observe got redshifted, or even if they got redshifted-- in some reference frames, for example in a rocket moving at 0.99c, some of the photons will actually be blueshifted. The cooling of the CMB temperature only tells you that the temperature you observe is getting less and less, which is a different statement, because it is frame-independent, whereas "redshifting" implies a frame has been chosen (the co-moving frame). In all cases, the actual frequency shifts are observer-dependent, and when those shifts occured is coordinate-dependent. Of course, the conventional coordinates are comoving-frame, and all those articles are written using those coordinates. They know this, they are just not always careful in making this clear (see Spaceman Spiff's comment for when they do make it clear).


“Astronomers say that the galactic light has been redshifted. The explanation is straightforward: As space expands, light waves get stretched. If the universe doubles in size during the waves' journey, their wavelengths double and their energy is halved.”It is quite undemonstrable to assert that "space expands", there is no model of "space" in modern physics whose expansion is testable. Instead, we can say that distances increase. So that is imprecise language. Nevertheless, their statement does not assert when the redshifts happened, because again, as I said, it is possible to observe blueshifts for some observers and some CMB photons.

Ken G
2009-Jul-16, 05:20 PM
I was quoting it to say that I disagree that it is flawed. As I said, if you have no plans to enter any kind of argument in favor of your arbitrary opinion, then your opinion is of no consequence.


Note that at the start of my last post (which you are replying to
here), I was careful to explicitly say "Doppler shift of light..." since
the frequency shift in sound depends as much on the medium the
sound is traveling through as it does on the observer.I'm afraid there is a difference between simply specifying that it be light, and having some aspect of your argument that actually makes sense for light.

It results from the relative speed of the racecar to the air and the relative
speed of the observer to the air, rather than the relative speed of
the observer to the racecar. So, for an observer motionless wrt
the air, it is reasonable to say that the Doppler shift occurs at the
racecar.And what part of your argument specifically applies for light only, so only holds if there is no medium there? I looked back at it, and saw nothing. That isn't surprising, you don't really have an argument at all, other than "it's my opinion for light, though I cannot point to any particular aspect of my argument that relies on the absence of a medium". Note that in actual relativity, the absence of a medium asserts a symmetry principle that your "opinion" violates, so your position has lost, not gained, the importance of an absence of a medium.

Jeff Root
2009-Jul-16, 05:36 PM
At this point, I'm satisfied with what I've said. Your argument against a
particular place and time for Doppler shifting of light has convinced me that
it occurs when the light is observed.

-- Jeff, in Minneapolis

Sam5
2009-Jul-16, 05:41 PM
You have indeed misinterpreted what they are saying.


No I haven’t. It’s just that you disagree with what they are saying.

In the first D-L paper, see this paragraph at the end:
http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070v2.pdf

“Photons now reaching us from our particle horizon were emitted at the Big Bang.
Since lim t→0 a = ∞ in all viable cosmological models, aXy > c would have been
satisfied and these photons would have initially receded from us. Similarly, the first
photon we receive from any object was emitted from a region with aXy > c, vtot > 0.
In the early universe as time progresses, both Xy and a decrease. Thus, vtot of these
photons evolves from positive to negative, and the teardrop shape of our physical
past lightcone is ubiquitous to all times.”


Where they say the “vtot of these photons evolves from positive to negative”, that means the relative speed of the photons (relative to both the earth and the galaxy that emitted the photons) changes as the photons travel through space. And, according to their theory, this is where in space the redshift begins, which is essentially outside the co-moving space of the galaxy that emitted them and outside our own galaxy.

You might disagree with their interpretation and explanation, but it is theirs, not mine. The professors who wrote the “Root of All Evil” paper might disagree with D&L too. But that is their opinion.

My opinion, as I stated earlier, is that “we don’t know for sure.... yet”, but someday I think we will know, as soon as enough experts on this subject figure it out and agree on a solution. I tend to favor the D-L opinion, but that’s not my main contention here on this thread. My main contention is that where the shift takes place is not “un-knowable”, it is simply “not yet known.” :)

Ken G
2009-Jul-16, 06:24 PM
At this point, I'm satisfied with what I've said. Your argument against a
particular place and time for Doppler shifting of light has convinced me that
it occurs when the light is observed.
Then you are just being foolish. I know for a fact that if a moving source is coming toward you, sending out periodic pulses of light like a wave, then you have a concept of the time those pulses were emitted, and you have a concept of the distance between those pulses, even before they arrive at you. If you simply calculate when and where you think those pulses got emitted, you can also calculate the distance between those pulses (that distance is called "wavelength", by the way, and it connects to frequency). It requires a very subtle concept of relativity to not think that, and nothing you've said suggests to me you hold such a subtle concept. Ergo, I claim your positions are inconsistent, the concepts you hold in your mind right now do not agree with each other, and you simply don't realize it. Some might care about that, but it's your choice.

Ken G
2009-Jul-16, 06:33 PM
No I haven’t. It’s just that you disagree with what they are saying.Did you read Spaceman Spiff's post? I find it odd you can think I disagree with what they are saying, when they just said exactly what I've been saying throughout this thread.

In the first D-L paper, see this paragraph at the end:
http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070v2.pdf


Where they say the “vtot of these photons evolves from positive to negative”, that means the relative speed of the photons (relative to both the earth and the galaxy that emitted the photons) changes as the photons travel through space.Sam5, they are using quantities that only have meaning in regard to a particular coordinate system. This is relativity 101. If Davis or Lineweaver say "I got a speeding ticket because I was driving my car at 70 mph", are you going to claim that they must think speed is an absolute, if they said there were driving "at 70 mph"? No, they can say that, because they know the quantity has meaning in regard to an implied coordinate system. It is not intended to be interpreted as a statement of absolute reality about their speed, despite a quote that could be misinterpreted that way. You have to be able to understand what the words mean when you quote them.



You might disagree with their interpretation and explanation, but it is theirs, not mine. Again, you seem to have completely ignored Spaceman Spiff's post. I find that curious. Did you just miss it, or do you routinely ignore facts that contradict your claims?

JohnD
2009-Jul-17, 05:13 PM
Now you have reached the conclusion I've been saying all along, so perhaps you are seeing the point.

Ken,
an admission that there can be two points of view is not an admission of error.

John

Ken G
2009-Jul-18, 12:57 AM
Ken,
an admission that there can be two points of view is not an admission of error.
It's not a contest. What matters is the truth. And the truth is, according to our current understanding of the universe, there is no scientifically demonstrable way to establish when or where a frequency shift of light occurs. I don't believe it is really possible to understand relativity without understanding that point, which is why I embarked on the discourse. It's the jumping off point for understanding even more subtle but amazing things about our reality, such as that there is "really no such thing" as strict simultaneity, nor a unique partition between what is a separation in distance and what is a separation in time (all that is objectively real is what is a "timelike" versus "spacelike" separation, after that, degree of separation is just one number, not two). I think it is clear enough that my statements are supported by those who do indeed understand relativity, so then the issue is only, who wants to know what our world is really like?

Spaceman Spiff
2009-Jul-18, 01:58 PM
It's not a contest. What matters is the truth. And the truth is, according to our current understanding of the universe, there is no scientifically demonstrable way to establish when or where a frequency shift of light occurs..... I think it is clear enough that my statements are supported by those who do indeed understand relativity, so then the issue is only, who wants to know what our world is really like?

(emphases mine)

Without qualification, that's a 'funny' statement coming from you! :)
A typical reader who hasn't followed your many illuminations of what separates scientific from metaphysical statements of or searches for "truth" might remain confused on the matter.

Ken G
2009-Jul-18, 04:06 PM
Without qualification, that's a 'funny' statement coming from you!
A typical reader who hasn't followed your many illuminations of what separates scientific from metaphysical statements of or searches for "truth" might remain confused on the matter.You're quite right, I should have said what our current best understanding of reality is really like!

dhd40
2009-Jul-19, 07:28 PM
My guess is, you would try to say something like, take atoms stationary with respect to A and put them all along the path from A to B. They'd all see the photon as being at the same frequency as A emitted it. But I'll just ask, why those atoms? Why not a chain of atoms stationary with respect to B? Those atoms would "prove" the opposite assertion, that the frequency shift happened immediately upon emission. The real answer is, neither is a supportable assertion, it is unknowable because "it takes two to tango"-- the invariant frequency shift is set by the complete process only.

(my bold)
I´ve tried to illustrate to myself by a diagram your situation "why not a chain of atoms stationary with respect to B"
In this diagram, the relative velocity between all detectors (your B) and the emitters (your A) is the same. Therefore, all detectors will see the same frequency shift.
Forgetting about SR, GR and cosmolgical frequency shift, there are only two options how/where the frequency shift happens: Either at the emitter, or at the detector(s). Somewhere in between doesn´t make any sense. If it happened, e.g., between detector d3 and d4, detectors d1 ... d3 should all see the same frequency shift, but different from the frequency shift which detectors d4 and d5 measure.
This would be a contradiction to the fact that the frequency shift depends only on the relative velocity between emitter and detector.

Is there anything wrong with my logic?

Ken G
2009-Jul-19, 08:08 PM
Forgetting about SR, GR and cosmolgical frequency shift, there are only two options how/where the frequency shift happens: Either at the emitter, or at the detector(s). Somewhere in between doesn´t make any sense.No, you are forgetting an important possibility-- that none of the above make any sense in an absolute way. It simply does not make sense to assert when the shift happened. You see, to assert that it does make sense to make that assertion, you must demonstrate that it makes sense. This is how science works-- we must demonstrate our truths, especially when you are trying to break a basic symmetry. That's all before we get to relativity-- after relativity, we have that the symmetry is elevated to a physical principle, settling the issue in the absence of a better theory.

This would be a contradiction to the fact that the frequency shift depends only on the relative velocity between emitter and detector.

Is there anything wrong with my logic?Yes, two things:
1) the fallacy of the neglected "middle", or the "none of the above" option I outlined above, and
2) I can reach the opposite conclusion from you simply by choosing different detectors. If all my detectors move with B, then they all see the shift. If I remove all but B, suddenly the shift happens at B, when before, it happened at A. This is the argument for "none of the above", even before relativity establishes the symmetry principle that says it must be frame dependent (the shift is an invariant, when it happened is not-- all we can say is we need to know the entire process, emission and absorption, to know the shift).

Jeff Root
2009-Jul-20, 01:39 AM
Ken's argument (I started to call it an "analysis" but it isn't really an
analysis, just an argument) shows that the freqency change occurs at
the detector. A line of detectors all moving with detector B will detect
the same frequency as B. A line of detectors all moving with source A
except for detector B will all detect the original frequency except for
detector B, which detects a shifted frequency. This shows clearly that
the change occurs at the detector.

And I want to repeat that I agreed completely with Ken's description
until his reply to JohnD, which changed my mind.

-- Jeff, in Minneapolis

Ken G
2009-Jul-20, 02:09 AM
Ken's argument (I started to call it an "analysis" but it isn't really an
analysis, just an argument) shows that the freqency change occurs at
the detector. Only if you completely misrepresent it. Which you did.

I'm still waiting for you to answer this scenario, so if you dodge the question yet again, I'll know you have no interest in understanding the truth, and can stop wasting time on helping you do that (perhaps JohnD might wish to also, if he wants to understand too.) . You are in a spaceship. Another is approaching at c/2. In that other is a man with a flashlight. That man is pulsing the flashlight off an on, with very short pulses, to send out periodic bursts of light. Here are my questions:
1) Do you, or do you not, think that from your perspective in your spaceship, that there are identifiable times on your clock when he is sending out those pulses, whether or not you know what they are (pick either)?
2) Do you, or do you not, think that from your perspective in your spaceship, those pulses have an identifiable distance from you, that approaches you (at c)?
3) Can you then put #1 and #2 together and give the location as a function of time of those pulses, from your perspective in your spaceship?

That's all I need to know.

Tobin Dax
2009-Jul-20, 02:40 AM
Ken's argument (I started to call it an "analysis" but it isn't really an
analysis, just an argument) shows that the freqency change occurs at
the detector. A line of detectors all moving with detector B will detect
the same frequency as B. A line of detectors all moving with source A
except for detector B will all detect the original frequency except for
detector B, which detects a shifted frequency. This shows clearly that
the change occurs at the detector.

And I want to repeat that I agreed completely with Ken's description
until his reply to JohnD, which changed my mind.

-- Jeff, in Minneapolis

A baseball is pitched at a speed of 90 mph toward a pickup truck driving away from the pitcher at a speed of 40 mph. When the person sitting n the bed of the truck catches the ball, he says it's going 50 mph. Where did the ball lose speed?

It never did. The pitcher will always measure the ball with a speed of 90 mph. The person in the truck will always measure a speed of 50 mph. It depends on the frame of reference and only on the frame of reference. There is no change, only different frames of reference.

The above also applies to Doppler shifts (of both light and sound). There is no change, only different frames of reference.

Ken G
2009-Jul-20, 02:49 AM
Yes, that's an analogous situation, in terms of speed instead of frequency. Unfortunately, when I mentioned cases that did not involve light, Jeff Root just responded that he was only talking about Doppler shifts of light. So that's why I posed the above questions, which are like your situation, but involve pulses of light.

Sam5
2009-Jul-20, 02:59 AM
A baseball is pitched at a speed of 90 mph toward a pickup truck driving away from the pitcher at a speed of 40 mph. When the person sitting n the bed of the truck catches the ball, he says it's going 50 mph. When did the ball loose speed?

It never did. The pitcher will always measure the ball with a speed of 90 mph. The person in the truck will always measure a speed of 50 mph. It depends on the frame of reference and only on the frame of reference. There is no change, only different frames of reference.

The above also applies to Doppler shifts (of both light and sound). There is no change, only different frames of reference.

So are you saying that in Jeff’s example, the speed of light departing the emitter A is measured by A as being “c”, and that same light is measured by the moving detector B as being less than “c” when it arrives at B, and that is what causes the redshift, at the detector?

Jeff Root
2009-Jul-20, 03:00 AM
Only if you completely misrepresent it. Which you did.
I don't think I misrepresented your argument. I know that my
conclusion is radically different from yours. I think that's the
only part that's different.



I'm still waiting for you to answer this scenario, so if you dodge the
question yet again, I'll know you have no interest in understanding
the truth, ...
I preferred it when you said I'm all wet.



... and can stop wasting time on helping you do that ...
You don't know everything that you are doing. I know some things
that you don't, even if you know a lot more things than I know.



You are in a spaceship. Another is approaching at c/2. In that other
is a man with a flashlight. That man is pulsing the flashlight off and on,
with very short pulses, to send out periodic bursts of light. Here are
my questions:

1) Do you, or do you not, think that from your perspective in your
spaceship, that there are identifiable times on your clock when he is
sending out those pulses, whether or not you know what they are
(pick either)?
That depends on what the meaning of "is" is.

There are certainly identifiable times on my clock when I see the light
pulses. But as I've said previously, I think that the farther apart two
events occur, the less meaning "simultaneity" has for them. I can't
say what time my clock read when he sent the pulses.



2) Do you, or do you not, think that from your perspective in your
spaceship, those pulses have an identifiable distance from you, that
approaches you (at c)?
As long as my spaceship doesn't accelerate and the light doesn't go
through a region of varying gravitational fields or travel cosmological
distances, yes, from my perspective the light has an identifiable
distance from me, approaching me at c.



3) Can you then put #1 and #2 together and give the location as a
function of time of those pulses, from your perspective in your spaceship?
No. I don't have enough information to determine the distance.

-- Jeff, in Minneapolis

Tobin Dax
2009-Jul-20, 03:04 AM
Yes, that's an analogous situation, in terms of speed instead of frequency. Unfortunately, when I mentioned cases that did not involve light, Jeff Root just responded that he was only talking about Doppler shifts of light. So that's why I posed the above questions, which are like your situation, but involve pulses of light.
And so you did. I missed that you were a step ahead of me. I'll disappear from the conversation again after responding to Sam5.

Tobin Dax
2009-Jul-20, 03:10 AM
So are you saying that in Jeff’s example, the speed of light departing the emitter A is measured by A as being “c”, and that same light is measured by the moving detector B as being less than “c” when it arrives at B, and that is what causes the redshift, at the detector?
No, I'm saying that the Doppler shifts don't happen at the detector. Since the speed of light is constant in all frames, it's always measured to be the same. Length and time (or frequency) can vary between reference frames, which is why wavelength and frequency Doppler shift. There is no place where the Doppler shift happens, either, since the measurement of wavelength and frequency depend on the reference frame.

Jeff Root
2009-Jul-20, 03:10 AM
Now, Sam, you know better than that. An analogy is just an analogy.

-- Jeff, in Minneapolis

Spaceman Spiff
2009-Jul-20, 03:16 AM
The above also applies to Doppler shifts (of both light and sound). There is no change, only different frames of reference. (emphasis mine)

That's all there is to it. If this concept is foreign to you, I suggest picking up several books on relativity, read them and begin learning what it has to say.

Tobin Dax
2009-Jul-20, 03:20 AM
(emphasis mine)

That's all there is to it. If this concept is foreign to you, I suggest picking up several books on relativity, read them and begin learning what it has to say.
And let me tell you, that was such a cool "aha! moment" for me, and it happened because of this thread.

Ken G
2009-Jul-20, 03:51 AM
There are certainly identifiable times on my clock when I see the light pulses. But as I've said previously, I think that the farther apart two events occur, the less meaning "simultaneity" has for them. I can't say what time my clock read when he sent the pulses.Good, then you have learned some relativity after all. Unfortunately this answer is not consistent with your next one:


As long as my spaceship doesn't accelerate and the light doesn't go
through a region of varying gravitational fields or travel cosmological
distances, yes, from my perspective the light has an identifiable
distance from me, approaching me at c.If it has an identifiable distance from you, and approaches at c, then it has an identifiable time as well. The calculation of the latter is trivial: t = d/c. You will say that you can't synchronize clocks, so I'll just give you a way to synchronize your clocks, in my next answer.

No. I don't have enough information to determine the distance.
Let's say you know precisely the complete motion of the rocket, because the rocket starts at a galaxy, say Andromeda. You send out a light pulse that says "when you receive this, accelerate instantly to c/2 toward me, and also start flashing your flashlight once per second." Since you have said in other threads you know the distance to Andromeda, you know how long it will take for him to get your signal (on your clock). Now, please try your answer again: will you know the distances he is at when he sends the pulses? Will you know the times, on your clock, when he sends the pulses?

Ken G
2009-Jul-20, 03:55 AM
And so you did. I missed that you were a step ahead of me. Don't get me wrong, I appreciate the help-- perhaps your way of saying it might have clicked better. Apparently it did not.

Tobin Dax
2009-Jul-20, 04:32 AM
Don't get me wrong, I appreciate the help-- perhaps your way of saying it might have clicked better. Apparently it did not.
And I haven't had the time to pay attention to this thread, but decided to throw another post in after dhd40 had posted. Hopefully my analogy will help him even though it's not working for the others.

dhd40
2009-Jul-20, 01:21 PM
There is no change, only different frames of reference.



Yes, that´s exactly how I see it. But it doesn´t solve the problem of "where" the shift happens (I think this has been said before by someone else). How (or when) does the photon know that it changes frames of reference? My naive thinking is: only when it interacts with something in the "new" reference frame. And that "something in the new reference frame" is called a detector (or observer, ...), isn´t it?

Jeff Root
2009-Jul-20, 02:01 PM
dhd40,

What makes you think that the photon has to change in order for it to
look different to different observers?

-- Jeff, in Minneapolis

Ken G
2009-Jul-20, 03:22 PM
My naive thinking is: only when it interacts with something in the "new" reference frame. And that "something in the new reference frame" is called a detector (or observer, ...), isn´t it?This is pretty much the perspective that was put forward by JohnD and Jeff Root. The problem with it is that it breaks a symmetry that reality does not break, and relativity is built to respect that symmetry as a result. To know the outcome of the observation, you need to know two things, not one-- you need both the reference frame (the motion) of the emitter and the detector. Relativity treats those two motions with complete symmetry, but saying the "shift occurs during the detection" breaks that symmetry, and it is never demonstrably the case. If it were, relativity, built to respect that symmetry, would be falsified.

It may help you to understand that in relativity, there is no demonstrably real sense to which the emission of light temporally precedes its absorption. The path followed by light is a "null geodesic", which means the proper time (the sole invariant time) between the emission and absorption is zero. The emission causes the absorption, but there's no unique, absolute, or meaningful sense to which the emission precedes the absorption in time. For the photon, zero time separates those events, and for anyone else, the time difference is arbitrary, it is a coordinate thing.

Furthermore, there is a principle that says any process that can emit a photon can also absorb one. So there really is no fundamental difference between an emission and an absorption of light, and the two happen at fundamentally the same time. We just choose to label one as coming first, so the time sequence, based on the logic of causality. But causality is a relationship between two events, like the emission and absorption, and it can explain a frequency difference-- but it does not tell you when the frequency difference occured, it just explains why it occured (the difference in reference frames between emission and absorption). It's not that we "just don't know" when it occured, it is that the entire question is apparently quite meaningless.

Jeff Root
2009-Jul-20, 03:34 PM
To know the outcome of the observation, you need to know two things,
not one-- you need both the original frequency and the relative motion
between the emitter and the detector.

-- Jeff, in Minneapolis

Ken G
2009-Jul-20, 04:48 PM
To know the outcome of the observation, you need to know two things,
not one-- you need both the original frequency and the relative motion
between the emitter and the detector.
Right, that's a better way to put it. Now, are you planning on answering my question above, or have you vacated your original untenable argument?

Spaceman Spiff
2009-Jul-20, 05:18 PM
Yes, that´s exactly how I see it. But it doesn´t solve the problem of "where" the shift happens (I think this has been said before by someone else). How (or when) does the photon know that it changes frames of reference? My naive thinking is: only when it interacts with something in the "new" reference frame. And that "something in the new reference frame" is called a detector (or observer, ...), isn´t it?

This question is equivalent to asking, regarding two events, "whose ruler measures the real spatial interval?" or "whose clock measures the real temporal interval?". Does that help at all?

dhd40
2009-Jul-20, 05:32 PM
dhd40,

What makes you think that the photon has to change in order for it to
look different to different observers?

-- Jeff, in Minneapolis

Oh, I really don´t think the photon has to change. It doesn´t change at all. It simply "looks as if it changed" in a different frame.

dhd40
2009-Jul-20, 05:45 PM
This question is equivalent to asking, regarding two events, "whose ruler measures the real spatial interval?" or "whose clock measures the real temporal interval?". Does that help at all?

Well, yes and no.
Thinking about your and others´ comments (I´m simply not competent enough to reply to all of them) a faint idea comes to my mind that I may identify "my personal reference frame" as the "true" reference frame. :silenced:

Tobin Dax
2009-Jul-20, 06:01 PM
Yes, that´s exactly how I see it. But it doesn´t solve the problem of "where" the shift happens (I think this has been said before by someone else). How (or when) does the photon know that it changes frames of reference? My naive thinking is: only when it interacts with something in the "new" reference frame. And that "something in the new reference frame" is called a detector (or observer, ...), isn´t it?
There is no "where." dhd, put yourself with your detector in a reference frame. Put me next to the star in the distant galaxy (which magically is only orbiting, not spinning, for simplicity). There is a relative radial velocity between these two frames.

There is no "old" reference frame and there is no "new" reference frame, there's just mine and yours. We each stand in our frames with our metersticks and our stopwatches making our measurements. I measure every photon emitted by the star to be unshifted because my frame is the star's frame. Your frame is different: it has a relative velocity to the star, so you will measure a shifted frequency. If you, still in your reference frame, are dropped right next to me, you will still measure the shifted frequency, because your are still in a different reference frame than the star.

This works both ways. (This is the symmetry Ken has been discussing.) If you point a laser toward me, you'll measure the unshifted frequency, but I'll measure the shifted frequency, no matter where we are with respect to each other. Also note that it doesn't matter which one of us is "stationary" and which one is "moving," this will still be true. As we stand there, in our reference frame, we will always make our measurements based on that frame. You will always see a shifted photon, no matter where you measure it. I will always see an unshifted photon, no matter where I measure it.

I understand why you might be having a hard time with this, since it's not quite an every-day thing. The "joy" of relativity is that it's weird as all heck, but it works.

Ken G
2009-Jul-20, 06:15 PM
Well, yes and no.
Thinking about your and others´ comments (I´m simply not competent enough to reply to all of them) a faint idea comes to my mind that I may identify "my personal reference frame" as the "true" reference frame.Interestingly, that is very close to the resolution that relativity chooses. It says that all reference frames are equally valid descriptions of "reality", this is a symmetry principle. Personally, I think it goes a step further, and says that there is a difference between a description of reality and actual reality, such that only the constructs that are invariant (the same for all observers) count as constructs pertaining to actual reality. The actual reality is "less than" our descriptions of it. A classic example of this is the illusion that if an astronaut on the Moon speaks into a radio, then I receive the signal some time later-- we use language like this all the time as if it were an expression of "actual reality", but the reality does not assert that, only our choice of how we think about things (our coordinates) does.

Tobin Dax
2009-Jul-20, 06:20 PM
Well, yes and no.
Thinking about your and others´ comments (I´m simply not competent enough to reply to all of them) a faint idea comes to my mind that I may identify "my personal reference frame" as the "true" reference frame. :silenced:
That's what I think a lot of people do, and that's part of what makes relativity hard to grasp at first. The kicker is that there is not "true" reference frame, just "this one," "that one," "mine," and so on.

And pardon me for going into "crazy teacher mode" above. :silenced:

trinitree88
2009-Jul-20, 06:52 PM
That's what I think a lot of people do, and that's part of what makes relativity hard to grasp at first. The kicker is that there is not "true" reference frame, just "this one," "that one," "mine," and so on.

And pardon me for going into "crazy teacher mode" above. :silenced:

Tobin Dax..."crazy teacher mode" is the new normal in this millenium. :shifty::lol: pete

Jeff Root
2009-Jul-21, 12:40 AM
Oh, I really don´t think the photon has to change. It doesn´t change
at all. It simply "looks as if it changed" in a different frame.
Okay, in that case, why did you ask, in post 85:


How (or when) does the photon know that it changes frames of reference?
Rather than asking, for example, how or when an observer knows that
the photon's frequency has changed?

-- Jeff, in Minneapolis

dhd40
2009-Jul-21, 01:21 PM
If you, still in your reference frame, are dropped right next to me, you will still measure the shifted frequency, because your are still in a different reference frame than the star.
(my bold)

I´m in 100% agreement with what you say. And I understand that with "dropped right next to me" you mean that I´m still in relative motion to you, i.e., I can pass as close as possible to you, and there´s still the same frequency shift. Yes!


I understand why you might be having a hard time with this, since it's not quite an every-day thing. The "joy" of relativity is that it's weird as all heck, but it works.

If I only knew more about it! It´s still a "joy" to follow all the comments of you experts. And I´m always wondering how patient you are with those who don´t understand it, but who still continue to ask the same questions over and over again. :clap:

dhd40
2009-Jul-21, 01:30 PM
That's what I think a lot of people do, and that's part of what makes relativity hard to grasp at first. The kicker is that there is not "true" reference frame, just "this one," "that one," "mine," and so on.

And pardon me for going into "crazy teacher mode" above. :silenced:

(my enhancement)
You certainly should not apologize for going into "crazy teacher mode". I´m pretty sure there are a lot of readers, like me, here on this board who expect you to do this.

That´s why I´m here (and, of course, to have fun)

dhd40
2009-Jul-21, 01:39 PM
Okay, in that case, why did you ask, in post 85:

Rather than asking, for example, how or when an observer knows that
the photon's frequency has changed?

-- Jeff, in Minneapolis

You´re right. I used a bad wording.

dhd40
2009-Jul-21, 01:44 PM
Tobin Dax..."crazy teacher mode" is the new normal in this millenium. :shifty::lol: pete

Investment advisers !? :mad:

Jeff Root
2009-Jul-21, 02:55 PM
Okay, in that case, why did you ask, in post 85:


How (or when) does the photon know that it changes frames of reference?
Rather than asking, for example, how or when an observer knows that
the photon's frequency has changed?
You´re right. I used a bad wording.
No, I wasn't making an assertion! I was asking a question!

If you asked the wrong question, or used bad wording, I want to
know why you asked the wrong question, or what caused you to use
bad wording. My suggested replacement isn't necessarily any better.
It was only an example of one sort of answer you could give.

-- Jeff, in Minneapolis

Ken G
2009-Jul-21, 03:13 PM
Speaking of questions and answers, I asked you the most important one, which can clarify the flaw in the idea that the shift occurs when absorbed. Instead of just answering it, you chose to simply restate the original question. If you have forgotten, the question was, in post #82:

Let's say you know precisely the complete motion of the rocket, because the rocket starts at a galaxy, say Andromeda. You send out a light pulse that says "when you receive this, accelerate instantly to c/2 toward me, and also start flashing your flashlight once per second." Since you have said in other threads you know the distance to Andromeda, you know how long it will take for him to get your signal (on your clock). Now, please try your answer again: will you know the distances he is at when he sends the pulses? Will you know the times, on your clock, when he sends the pulses?

It's even cleaner if you just wait for the ship to pass you, use the pulses you saw before to calculate when and where they will be emitted on the outbound leg. So can you, or can you not, calculate (in your reference frame, a la that whole "distance to Andromeda" business, remember that, hmm?) where and when those pulses will be emitted on the outbound route, if the ship stays inertial and continues to pulse the flashlight?

This is how you can actually understand why there is no such thing as "the distance between yourself and where those light pulses were emitted." In the process, you can recognize the inconsistent ideas you hold in your head right now. If you care, of course-- the mind is quite capable of holding contradictions, and a lot of people simply don't care.

Tobin Dax
2009-Jul-21, 05:24 PM
I´m in 100% agreement with what you say. And I understand that with "dropped right next to me" you mean that I´m still in relative motion to you, i.e., I can pass as close as possible to you, and there´s still the same frequency shift. Yes!
Yes, that's exactly what I meant: two observers right beside each other in space, but still having the relative velocity which defines the two reference frames. (Position can also define reference frames, but it's not important in this case.)


If I only knew more about it! It´s still a "joy" to follow all the comments of you experts. And I´m always wondering how patient you are with those who don´t understand it, but who still continue to ask the same questions over and over again. :clap:

(my enhancement)
You certainly should not apologize for going into "crazy teacher mode". I´m pretty sure there are a lot of readers, like me, here on this board who expect you to do this.

That´s why I´m here (and, of course, to have fun)
Thank you. (Also note that you didn't hear me scream [a little] and run through the argument aloud in my apartment, including hand gestures. :lol: That could be a good thing.)

dhd40
2009-Jul-21, 08:18 PM
I want to
know .... what caused you to use
bad wording.
-- Jeff, in Minneapolis

Bad language instinct ? (Steven Pinker)

Seriously: English isn´t my first language. Therefore, I sometimes feel really lost/helpless when posting

Ken G
2009-Jul-21, 08:27 PM
English isn´t my first language. German isn't even my second, so your sig translates online to "The gap, which we leave, we completely replaced". I presume something is getting lost there.

dhd40
2009-Jul-21, 08:48 PM
... there is a difference between a description of reality and actual reality...

If I understand correctly what you mean, I can give an other example (which, perhaps, doesn´t contribute to this thread´s topic): looking northwards from my village, I see a rainbow. I call my friend, living at the far north, to look at the beautiful colours of the rainbow.
His reply: No rainbow, no colours up there.

But "my" rainbow is real, isn´t it? (well, I know the answer to this)


The actual reality is "less than" our descriptions of it. A classic example of this is the illusion that if an astronaut on the Moon speaks into a radio, then I receive the signal some time later-- we use language like this all the time as if it were an expression of "actual reality", but the reality does not assert that, only our choice of how we think about things (our coordinates) does.

(my enhancement)
Shouldn´t it read the actual reality is "more than" our descriptions of it?

Ken G
2009-Jul-21, 09:08 PM
But "my" rainbow is real, isn´t it? I think a closer analogy might be, if you call your friend in a distant country where it is 3 a.m., and he groggily answers "do you know what time it is", and you say "sure, it's 9 p.m., what are you doing in bed?" These times have no independent meaning, they are arbitarily coordinatized relationships with the position of the Sun in the sky, and we have no business extending our personal coordinates to other aspects of reality. Similarly, a frequency shift can be arbitrarily coordinatized to occur in many ways, all that is "real" is the outcome.


Shouldn´t it read the actual reality is "more than" our descriptions of it?This is what you would normally expect, and no doubt there are many aspects of reality that do not appear in our scientific descriptions. I just mean that for those elements that do, our scientific descriptions often create a pretense that we can say more than we really can demonstrate. A classic example is, if you imagine a one-dimensional universe containing only two particles and nothing else, it would be customary to imagine that we have two separate numbers there, identifying the location of each particle along that line, and two separate speeds, one for each particle. However, the laws of physics are built around the proposition that there would actually only be one number there, the distance between the particles, and one speed, their relative motion. Some might say, "but surely both particles have a location, even if we have no way to measure it", but I think it would be truer to the assumptions of objective science to say that if there is no demonstrable meaning to a second number there, then no such second number is real. The actual reality would be less than the way we imagine it, when we imagine that all particles have a property of "location".

Jeff Root
2009-Jul-22, 11:41 AM
English isn´t my first language. Therefore, I sometimes feel really
lost/helpless when posting
I don't believe your question in post 85,


How (or when) does the photon know that it changes frames of reference?
was the result of any difficulty with English. You express yourself
very well. I don't recall ever noticing any clumsinesses at all.
The anthropomorphization of the photon is obviously absurd, but is
a common way to express such questions, and really doesn't get in
the way of my understanding what you are asking at all. Instead, I
think the question is the same as your original question. You had two
photons travelling toward an observer. One is stretched out and the
other is compressed. You don't understand how that is possible,
considering that the two photons are travelling at the same speed.

I tried to answer that question with my animation:

http://www.freemars.org/jeff2/Doppler3.htm

I said that the bullets fired by gunman number 3 are the best analogy
to wavecrests of light, although the atoms in your galaxy are of course
in continuous motion like gunman number 4, not stopping and starting
like gunman number 3. You didn't reply to that so we didn't get into a
discussion of why 3 works better than 4. But as I said, it has to do
with light, unlike bullets, always moving at constant speed.

Did my animation do anything for you?

Your more recent question is --ahem-- framed in terms of reference
frames. You acknowledge that there is no change, only different
frames of reference, yet you still think the photon changes. That is
clearly NOT a problem expressing yourself in English, but a problem in
understanding what is happening.

Please tell us what you think about this.

-- Jeff, in Minneapolis

dhd40
2009-Jul-22, 01:23 PM
German isn't even my second, so your sig translates online to "The gap, which we leave, we completely replaced". I presume something is getting lost there.

The online translation doesn´t make any sense to me. My translation would be

The gap, which we leave, is a perfect replacement of ourselves (most probably bad English)
or
The gap, which we leave, completely replaces us

i.e., nothing´s really lost when we leave this world :sad:

a little bit sarcastic, but most probably true

Ken G
2009-Jul-22, 01:24 PM
Did my animation do anything for you?
That animation is just like a question I posed to you in another thread about a flashlight being turned on an off suddenly, periodically, by a moving person. The pulses might be thought of as being just like the bullets in your animation #3. The problem is, if, as you claim, animation #3 is a good description of a Doppler shift, then it is clear the Doppler shift can successfully be thought of as occuring when the bullets (analogous to wave fronts) are emitted. So I'm confused-- do you still claim the shift occurs only upon detection of those bullets (wavefronts)? This would seem to confirm my claim that you are holding contradictory notions in your head, and so I think you owe it to the other readers not to tutor about Doppler shifts until you have worked through that contradiction yourself.

dhd40
2009-Jul-22, 01:27 PM
A classic example is, if you imagine a one-dimensional universe containing only two particles and nothing else, it would be customary to imagine that we have two separate numbers there, identifying the location of each particle along that line, and two separate speeds, one for each particle. However, the laws of physics are built around the proposition that there would actually only be one number there, the distance between the particles, and one speed, their relative motion. Some might say, "but surely both particles have a location, even if we have no way to measure it", but I think it would be truer to the assumptions of objective science to say that if there is no demonstrable meaning to a second number there, then no such second number is real. The actual reality would be less than the way we imagine it, when we imagine that all particles have a property of "location".

Ha, nice example!

dhd40
2009-Jul-22, 02:22 PM
Please tell us what you think about this.

-- Jeff, in Minneapolis

Now, that´s a lot of questions.

My personal view of (Doppler) frequency shift is this: Each photon, sent out by an atom, has a well defined energy E = h*f (f=frequency), which is independent of the velocity v (relative to what?) of the atom. Because of c= f*L = const, this results also in a well defined wavelength L, also independent of v. Let´s assume the photon "passes" an observer who can measure the photon´s frequency and wavelength exactly at the moment of passage. (no idea, how this is possible without interating with the photon)

The observer would measure f´> f and L´ < L (still, f´*L´=c). But nothing would have happened to the photon. The same would be true if the photon would hit the observer (detector). But now, of course, the energy would be E´=h*f´ > h*f .

This view of mine says that the frequency shift "happens" at the detector, although the photon itself didn´t experience any frequency shift at all

I know that this view is not in accord with our relativy experts´ view.

Does that answer your question?

Concerning your animation: Honestly, I was too lazy to walk through it, mentally (although it was a nice animation). Maybe, I will do it in the weekend. But no guaranty that I understand it!

And yes, thank you for your contributions.

Spaceman Spiff
2009-Jul-22, 02:32 PM
This view of mine says that the frequency shift "happens" at the detector, although the photon itself didn´t experience any frequency shift at all

I know that this view is not in accord with our relativy experts´ view.


Actually, it's just fine, as long as what you mean by this is that the observer, in a different reference frame from the emitter, measures a shift in the frequency with his/her detector, with the reason being the differences in their references frames. :)

Ken G
2009-Jul-22, 02:49 PM
Because of c= f*L = const, this results also in a well defined wavelength L, also independent of v.But after the bird leaves the nest, it's frequency is no longer well defined, but that's OK-- the state of the photon is less than we imagine!

Let´s assume the photon "passes" an observer who can measure the photon´s frequency and wavelength exactly at the moment of passage. (no idea, how this is possible without interating with the photon)
It's possible, you just set up a "sieve" that would absorb photons at any other frequency. Incidentally, that is a device for exposing the flawed ontology of claiming that the shift "happens" when detected, the detector could just as easily be replaced by an absence of a detection.


The observer would measure f´> f and L´ < L (still, f´*L´=c). But nothing would have happened to the photon.Correct, the energy of a photon does not just depend on the photon, that's why it isn't strictly an "attribute" of the photon. As Spaceman Spiff said, the difference is in identifying what is happening to the photon, versus what is happening in a measurement. Obviously, you can't know the outcome of a measurement until you specify the measurer (note my use of "until" synonymously with "unless", an issue that cropped up earlier).
This view of mine says that the frequency shift "happens" at the detector, although the photon itself didn´t experience any frequency shift at all.If something didn't "happen" to the photon, then it is irrelevant to identify a time when the non-happening happened. All we can say is the observed frequency is an invariant of the relationship between emitter and detector. That relationship is manifestly symmetric. Don't break symmetries that reality does not break, it's bad science!

dhd40
2009-Jul-22, 07:20 PM
Actually, it's just fine, as long as what you mean by this is that the observer, in a different reference frame from the emitter, measures a shift in the frequency with his/her detector, with the reason being the differences in their references frames. :)

Yes, that´s what I mean

dhd40
2009-Jul-22, 07:50 PM
If something didn't "happen" to the photon, then it is irrelevant to identify a time when the non-happening happened.

But something happened to the observer (in the observer´s frame). When a picket fence passes me, or I pass the picket fence (100% symmetric), I can touch the pickets and feel very different ticks, depending on the relative velocity between the fence and me. But the distance between the pickets doesn´t change (we are not talking about relativistic length contraction).
So, nothing is happening to the picket fence. But I still can identify when and how fast the pickets pass my fingers.
No matter, how fast the fence passes by, the pickets´ distances don´t change in the fence´s frame, but the ticks in my frame do change.

Ken G
2009-Jul-22, 09:20 PM
But something happened to the observer (in the observer´s frame).Most definitely, as long as the event is described in terms of something happening to the observer, instead of something happening to the photon, then there's no problem with the language. This is made clear by noting the clear difference between the statement "the frequency shifted when it was observed" versus "the frequency was observed to be shifted when it was observed to be shifted." The latter is unarguable, but it is also not saying much because it is a tautology; the former is the type of statement I objected to earlier.

Sam5
2009-Jul-24, 12:12 AM
dhd40,

Here's an animation I made to explain a related (and more complex)
question about Doppler shifts.

http://www.freemars.org/jeff2/Doppler3.htm

What do you think of Ned Wright’s Doppler-shift illustration for light on this web page of his?

http://www.astro.ucla.edu/~wright/doppler.htm

Jeff Root
2009-Jul-24, 05:48 AM
What do you think of Ned Wright’s Doppler-shift illustration for light
on this web page of his?

http://www.astro.ucla.edu/~wright/doppler.htm
It looks fine. (I didn't evaluate the final paragraph in any depth,
but it seems okay, too.) I hope you aren't going to tell me that
something in my animation or analysis thereof conflicts with what
Ned Wright said on that page. It shouldn't.

-- Jeff, in Minneapolis

dhd40
2009-Jul-24, 07:26 PM
What do you think of Ned Wright’s Doppler-shift illustration for light on this web page of his?

http://www.astro.ucla.edu/~wright/doppler.htm

If you talk about his sketch (see attached thumbnail) then I have some difficulties because it doesn´t say in which frame this is valid, the emitter´s frame or the observer´s frame?
Obviously, it can´t be the emitter´s frame, because in this frame the circles should be concentric. Think of a brick thrown into a pool.
Then, it must be the observer´s frame. But which observer´s frame? If there are n observers approaching the emitter at n different velocities (or should I say n observers with n relative velocities between emitter and observer), then there are n sketches? How does the emitter know "which sketch to use"?

This is the biggest problem I have with Ken G´s and others´ statement that we don´t know where the shift happens, that it might as well happen at the emitter.

Tobin Dax
2009-Jul-24, 09:00 PM
If you talk about his sketch (see attached thumbnail) then I have some difficulties because it doesn´t say in which frame this is valid, the emitter´s frame or the observer´s frame?
Obviously, it can´t be the emitter´s frame, because in this frame the circles should be concentric. Think of a brick thrown into a pool.
Then, it must be the observer´s frame. But which observer´s frame? If there are n observers approaching the emitter at n different velocities (or should I say n observers with n relative velocities between emitter and observer), then there are n sketches? How does the emitter know "which sketch to use"?

This is the biggest problem I have with Ken G´s and others´ statement that we don´t know where the shift happens, that it might as well happen at the emitter.
Your first paragraph tells us that the shift cannot happen at the emitter, and that is correct. The shift does not happen anywhere. It cannot happen at a location, because there is no location (and not even a gradual change with location) that can account for the shifts detected by all n observers.

As to the image, it's a classical description, IMO. I tried going down that road earlier (http://www.bautforum.com/space-astronomy-questions-answers/90581-frequency-shift-photons-emitted-rotating-objects-3.html#post1532414), but got put down for it by Jeff Root. But your thoughts in the first paragraph are correct. Keep following that train of thought.

Ken G
2009-Jul-24, 10:11 PM
If there are n observers approaching the emitter at n different velocities (or should I say n observers with n relative velocities between emitter and observer), then there are n sketches? Yes, there are n sketches. There could even be n million sketches, because you can use any coordinates you like to depict the motion of the wavefronts, but in the spirit of that particular sketch's coordinates, there would be n sketches.


How does the emitter know "which sketch to use"?The emitter uses yet another sketch! The one that looks like a stone dropped in a pond, with approaching observers "cutting through the waves" like speedboats on that pond.


This is the biggest problem I have with Ken G´s and others´ statement that we don´t know where the shift happens, that it might as well happen at the emitter.But you are right at the brink of understanding. A sketch is just a sketch, a way of picturing the reality. The sketch is not the reality! Reality is less than the sketch, because nothing in that sketch can actually be verified in any absolute way, other than the outcome of the calculation any of the n million sketches would suggest.

Jeff Root
2009-Jul-25, 11:26 AM
As to the image, it's a classical description, IMO.
I tried going down that road earlier (http://www.bautforum.com/space-astronomy-questions-answers/90581-frequency-shift-photons-emitted-rotating-objects-3.html#post1532414), but got put down for it
by Jeff Root.
I didn't reply to that post. Ken explained that unfortunately I had
specified that I was talking about light, not sound, but that certainly
wasn't a put-down! I didn't reply because you didn't ask a question
and I didn't disagree with anything you said.

-- Jeff, in Minneapolis

Tobin Dax
2009-Jul-25, 12:00 PM
I didn't reply to that post. Ken explained that unfortunately I had
specified that I was talking about light, not sound, but that certainly
wasn't a put-down! I didn't reply because you didn't ask a question
and I didn't disagree with anything you said.

-- Jeff, in Minneapolis
You may not have replied to the post, but I'm pretty sure that you referred to it. If I misinterpreted your response to Sam5's reply ("An analogy is just an analogy."), I apolgize.

Jeff Root
2009-Jul-25, 12:07 PM
What do you think of Ned Wright’s Doppler-shift illustration for light
on this web page of his?

http://www.astro.ucla.edu/~wright/doppler.htm
If you talk about his sketch (see attached thumbnail) then I have
some difficulties because it doesn´t say in which frame this is valid,
the emitter´s frame or the observer´s frame?
It DOES say, both in the surrounding text and right in the picture!
Ned says:


Observers looking at an object that is moving away from them see
light that has a longer wavelength than it had when it was emitted
(a redshift), while observers looking at an approaching source see
light that is shifted to shorter wavelength (a blueshift).
An observer on the left sees the object receding and the waves
stretched apart; an observer on the right sees the object approaching
and the waves squished together. Observers in other directions will
see in-between amounts of motion and Doppler shifting. Which is
exactly what the diagram shows.



Obviously, it can´t be the emitter´s frame, because in this frame
the circles should be concentric. Think of a brick thrown into a pool.
In that case, of course, the waves are waves of the medium, and
the brick is not moving along the surface of the water-- just sinking!



Then, it must be the observer´s frame. But which observer´s frame?
If there are n observers approaching the emitter at n different velocities
(or should I say n observers with n relative velocities between emitter
and observer), then there are n sketches?
The sketch actually shows two cases: An observer on the left from
whom the source is receeding, and an observer on the right toward
whom the source is approaching. In between is a range of other
velocities, with the complication that there is also a component of
motion perpendicular to the line of sight.

But yes, draw the sketch to show the relative speed between the
source and the observer, as well as the original frequency of the
waves from the source.

Ned's diagram shows the object moving to the right. He could as well
have made the object stationary, the circles concentric, and the two
observers moving to the left. But that would be more difficult for most
people to comprehend.



How does the emitter know "which sketch to use"?
What makes you think that the emitter has to know what the
observers are doing in order for its light to look different to
different observers?

I cut and pasted my earlier question to you and edited it to fit the
new context. Here's the previous go-round:



What makes you think that the photon has to change in order for
it to look different to different observers?
Oh, I really don´t think the photon has to change. It doesn´t change
at all. It simply "looks as if it changed" in a different frame.
If you agree that the photons don't need to change, why would you
think the emitter needs to do anything different?

It is the original frequency, plus the relative motion between the
emitter and observer, which together determine which sketch applies.
The emitter can change which sketch applies by changing the frequency
of its emission or by moving relative to the observer. The observer can
change which sketch applies by moving relative to the emitter. So the
emitter gets the first say but the observer gets the final say.

-- Jeff, in Minneapolis

Jeff Root
2009-Jul-25, 12:43 PM
You may not have replied to the post, but I'm pretty sure that you
referred to it. If I misinterpreted your response to Sam5's reply
("An analogy is just an analogy."), I apologize.
Oh THAT! No, I must apologize to you! That did look like a
put-down, but it was absolutely not intended to be. Sam was
drawing the inference from your analogy that motion of an observer
relative to the source changes his speed relative to the light from
the source. That is a shortcoming of the analogy, but of course
Sam knew that.

-- Jeff, in Minneapolis

dhd40
2009-Jul-25, 06:52 PM
The emitter uses yet another sketch! The one that looks like a stone dropped in a pond, with approaching observers "cutting through the waves" like speedboats on that pond.


That´s exactly how I see it. And those speedboats, they all have their own, individual sketch.
I´m really happy with this view (if I understand you correctly). The only problem I still have is to understand why we (GR?) don´t know where the "cutting through the waves (=frequency shift)" happens.

Sam5
2009-Jul-25, 07:15 PM
If you talk about his sketch (see attached thumbnail) then I have some difficulties because it doesn´t say in which frame this is valid, the emitter´s frame or the observer´s frame?
Obviously, it can´t be the emitter´s frame, because in this frame the circles should be concentric. Think of a brick thrown into a pool.

I agree.

According to the 1989 university-level textbook titled “Physics”, by John Wiley and Sons, the drawing of the kind you posted actually applies to SOUND, with a moving sound emitter, stationary air, and a stationary observer.
http://www.physics.brocku.ca/Courses/1P22_Crandles/images/f16031.jpg

This is one of the two main causes of the Doppler shift for sound. According to the textbook, the sound waves in stationary air are either compressed (in front of) or stretched-out (behind) the moving sound emitter, causing a physical wavelength change in the air which is eventually observed by the stationary observer. In this case the circles of waves are not concentric around the moving sound emitter.

According to the same textbook, the other main cause of the Doppler shift for sound occurs at the moving observer, with a stationary sound emitter and stationary air. In this case the expanding circles of sound waves are shown as being concentric and centered on the sound emitter, and there is no wavelength change in the air around the emitter. The cause of the frequency shift is due to the observer-relative speed of each observed sound wavecrest, as it is received at the observer. In this case, more or fewer wevecrests arriving at the observer per second causes the frequency shift.
http://www.physics.brocku.ca/Courses/1P22_Crandles/images/f16033.jpg

The text that explains the second drawing is here:

http://i26.tinypic.com/2rhab5t.jpg

http://i30.tinypic.com/ay3tj6.jpg

I don’t think either of these classical drawings adequately illustrate the Doppler shift of light, which is supposed to be due only to the “relative motion” of the emitter and observer, while keeping in mind the relativity requirement of the constant “c”, both at the emitter and at the observer, and while considering no known propagating “medium” for light.

dhd40
2009-Jul-25, 07:27 PM
It DOES say, both in the surrounding text and right in the picture!
Ned says:

An observer on the left sees the object receding and the waves stretched apart;
Then, why doesn´t that observer to the left of the receding object "see" the waves to the right of the receding object also stretched?


Ned's diagram shows the object moving to the right. He could as well
have made the object stationary, the circles concentric, and the two
observers moving to the left. But that would be more difficult for most
people to comprehend.
That could be an interesting sketch. What would it look like? (BTW, please don´t see my comments as an affront against Ned)


What makes you think that the emitter has to know what the
observers are doing in order for its light to look different to
different observers?
My view of this is simply that if the shift happens at the emitter, how can observers with different relative velocities (relative to the emitter) "see" their appropriate ( individual, peculiar, proper) shifts?


If you agree that the photons don't need to change, why would you
think the emitter needs to do anything different?
Not sure what you mean. Neither the photon, nor the emitter "need to change" from my point of view


The emitter can change which sketch applies by changing the frequency
of its emission or by moving relative to the observer.
(my bold)
Now, that really sounds strange. How can an atom change the frequency of its emission(s)? (remember, we talk about Doppler shifts!)


So the
emitter gets the first say but the observer gets the final say.

I´m an (almost) perfect SR/GR layman, but this seems to contradict basic SR/GR rules

-- Jeff, in Minneapolis[/QUOTE]

dhd40
2009-Jul-25, 07:37 PM
I don’t think either of these classical drawings adequately illustrate the Doppler shift of light, which is supposed to be due only to the “relative motion” of the emitter and observer, while keeping in mind the relativity requirement of the constant “c”, both at the emitter and at the observer, and while considering no known propagating “medium” for light.

That´s a good point which I was always wondering about. How, or why, do we use an analogy between sound and light propagation? Medium vs no medium, longitudinal waves vs transversal waves, mechanical vs electromagnetic waves, etc, etc.

Do we fall victim to wrong analogies?

Sam5
2009-Jul-25, 08:00 PM
Do we fall victim to wrong analogies?


I think we do. :)

Although the mechanics of sound-related Doppler shifts are very much understood today, there are still some mysteries regarding light-related Doppler shifts.

Tobin Dax
2009-Jul-25, 09:03 PM
According to the 1989 university-level textbook titled “Physics”, by John Wiley and Sons, . . . .
It looks like the textbook for that course is College Physics: A Strategic Approach by Knight, Jones and Field. Wiley & Sons is a publisher. While those diagrams may not from the College Physics book, note that your citation is lacking. Though considering how common a title Physics is, it would take some work to find the authors.

Sam5
2009-Jul-25, 09:20 PM
It looks like the textbook for that course is College Physics: A Strategic Approach by Knight, Jones and Field. Wiley & Sons is a publisher. While those diagrams may not from the College Physics book, note that your citation is lacking. Though considering how common a title Physics is, it would take some work to find the authors.


The title of the textbook is “Physics”. The credited authors/editors are John D. Cutnell and Kenneth W. Johnson. Published by John Wiley and Sons, 1989. ISBN 0-471-89850-3.

I bought it used at a University book store in the mid-1990s.


http://www.directtextbook.com/9780471898504/0471898503

Tobin Dax
2009-Jul-25, 10:06 PM
The title of the textbook is “Physics”. The credited authors/editors are John D. Cutnell and Kenneth W. Johnson. Published by John Wiley and Sons, 1989. ISBN 0-471-89850-3.

I bought it used at a University book store in the mid-1990s.


http://www.directtextbook.com/9780471898504/0471898503
I suspected it was that text, but didn't have time to check. The current edition still uses the same diagram for sound. It doesn't have a diagram for light because the text only discusses the Doppler effect in terms of sound waves, but the same equation can be used for non-relativistic Doppler shifting of light waves.

Nearly every astronomy textbook on a similar level to Cutnell & Johnson (aimed at students who are physics majors) uses a diagram very similar to Ned Wright's.


Jeff Root, sorry about the misunderstanding and thanks for the apology. I seem to be a bit snippy lately, and I could have handled that better.

Ken G
2009-Jul-26, 08:15 AM
That´s exactly how I see it. And those speedboats, they all have their own, individual sketch.
I´m really happy with this view (if I understand you correctly). The only problem I still have is to understand why we (GR?) don´t know where the "cutting through the waves (=frequency shift)" happens.
The way to see the problem with your perspective is to consider regular old sound waves. Here we do have a medium, so we can ask which is really moving relative to that medium. If the emitter, then the wavelength is clearly altered during the emission process. Nevertheless, we cannot tell what frequency will be observed until we also specify the motion of the observer, relative to the medium. So if we have a medium, we have no problem associating a certain fraction of the observed shift with the motion of the emitter, and a certain fraction with the motion of the receiver. So why, when we remove the medium, do you feel it is appropriate to suddenly shift the whole process to the absorber? Your arguments, and those of Jeff Root, always amounted to "we can't tell what the final shift will be until we know the whole process, so it all has to happen at the end." But the first part of that sentence also holds for sound waves, while the second part does not. No part of your argument has ever satisfactorily accounted for why your argument only applies to mediumless waves.

In summary, your position breaks a symmetry, that between the contribution of the emitter and the absorber to the ultimate shift, that the absence of a medium should preserve. The presence of a medium breaks the symmetry in such a way that makes the partition easy to identify, so the absence of a medium should not break that symmetry and make it impossible to identify such a partition.

Sam5
2009-Jul-26, 04:36 PM
Here is an interesting Java applet for sound that everyone can play around with:

http://www.compadre.org/Astronomy/items/Load.cfm?ID=7938

It helps if you use your cursor to drag the ear and place it over the flashing siren.

Then, in the upper left corner of the window, in the category titled “source motion”, click on “linier”. Then click on “play” in the upper right corner of the window. That will start the siren moving.

Later stop the siren from moving and click on “receiver motion” and “linier”. That will start the ear moving.

This applet shows the two main causes of the Doppler effects with sound:

1: emitter moving through a stationary medium, with the observer stationary

2: observer moving through a stationary medium, with the emitter stationary

Note that you can also speed up motion of the siren and/or the ear, and you can change the speed of the waves through the medium, and you can change the frequency of the waves.

dhd40
2009-Jul-26, 07:05 PM
In summary, your position breaks a symmetry, that between the contribution of the emitter and the absorber to the ultimate shift, that the absence of a medium should preserve.

Yes, I understand what you say concerning the symmetry aspect. And I´m pretty sure it´s in full agreement with GR (because GR has never been falsified).
But, obviously, it´s simply beyond me.
A medium! A medium! My kingdom for a medium! :doh:

(To all who offer a medium: My kingdom is extremely small!)

Ken G
2009-Jul-26, 07:26 PM
But, obviously, it´s simply beyond me.
Don't be beguiled by the jargony sound of the word "symmetry", this isn't beyond you. Indeed, I'd say the whole reason symmetries are viewed as important things to keep track of in physics is that they are arguably the simplest possible things we can notice in our environment. If someone said that all mammals are left/right symmetric (I'm not claiming that's true, I just can't think of any counterexample), then you can know something about all mammals, even if you've never seen one, or have any idea what a mammal even is. The symmetry in the frequency shift is essentially a time-reversal symmetry-- the emission and absorption of a photon is a process that could play out identically the same in every way but with time going in the opposite direction, but a claim that the shift "happens" at the absorption would be inconsistent with the time-reversed situation.

Note you also can't really say that upon emission, the photon frequency goes into a kind of "limbo" state where all information is lost about it, and this information is then "resurrected" upon absorption, because even though that would not break the time-reversal symmetry, it would give us no way to predict the observed frequency. The photon (or more correctly, the complete situation) must retain the information of the motion of the emitter, and it must react to the information of the motion of the absorber, in a symmetric way that ends up only depending on their relative motion (as that is also the only form of "motion" that can be demonstrated). How that actually works is as completely mysterious as motion itself, we really can say nothing at all about it that we can demonstrate to be true other then the outcome of the experiment.

dhd40
2009-Jul-27, 03:40 PM
The photon (or more correctly, the complete situation) must retain the information of the motion of the emitter, and it must react to the information of the motion of the absorber, in a symmetric way that ends up only depending on their relative motion (as that is also the only form of "motion" that can be demonstrated). How that actually works is as completely mysterious as motion itself, we really can say nothing at all about it that we can demonstrate to be true other then the outcome of the experiment.

Sounds kinda ENTANGLEMENT (which is also beyond me)

Ken G
2009-Jul-27, 06:21 PM
Sounds kinda ENTANGLEMENT (which is also beyond me)
I suspect entanglement might give us an experimentally convincing way to make my point, but I haven't thought about how to do that because I'm pretty content with the conclusion already.