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Author Topic: Why do photons have the same speed regardless of the velocity of their source?  (Read 12724 times)

Offline jeffreyH

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The Pound Rebka experiment used MOTION produced by a speaker cone.  The gamma ray light source was mounted on a MOVING speaker cone, which induced doppler shift.

The argument that doppler shift occurred without motion is flawed.  There was movement.  It didn't require a lot of movement.  Since the distance was in nearly constant flux.  They recorded the phase of the speaker movement to account for doppler shift.

--

Now for my timely wizenhimer remark:  DOH

Joe - Again you entirely miss even the 'intended' purpose of the experiment itself.

For the last time -  A Doppler shift was created by mounting the light source in the centre of the speaker cone.  It is sooo obvious that the gamma ray source will move as per the vibration of the speaker that I thought it unnecessary of mention.  It's the entire premiss of the experiment!  They created the measuring Doppler effect using hertz and the subsequent vibration of the speaker in order to cancel out the blueshift redshift frequencies of the light undergoing gravitational shift.

Now then - listen very carefully...  For a Doppler shift in vibration to cancel out the frequencies of blueshift redshift, the frequencies of blueshift redshift must resemble exactly the vibration of the Doppler shift.  Therefore the 'motion' you insist is occurring for the light source, this motion 'actually being' the motion of the speaker, is resembled exactly 'in' the gravitational shift.  Note:  The gravitational shift distance in this experiment is 'static'.  There is no relative motion between the 'distance' of the gravitational shift and observation 'with respect' to the gravitational field, and therefore no """obvious"""" relative motion for a Doppler shift for light in this experiment in the gravitational field.

Redshift is synonymous with expanding distances, and blueshift with contracting distances, as per space time considerations!!!  Yet, as per Pound Rebka, we see a Doppler shift in the gravitational shift of light, measured by a vibration, caused by motion of a speaker receiving between 10 and 50 hertz, over a constant distance in the gravitational field between a light source and receiver that are experiencing no relative motion between themselves, other than the forward and 'backward' motion of a speakers vibration acting upon the light source.

How can there be a Doppler shift in the gravitational shift of light over a constant distance when the speed of light is constant?  What exactly is causing the 'motion' of this Doppler shift found within the gravitational shift of light in this instance?

P.S Net-Head, I'm sorry if we have digressed.  I'm not entirely sure what your interest is in one photon, but I'm pretty sure knowing about what happens to light in general can only help :)

Now express that in terms even the most pompous can understand. At the moment it is simply unintelligible.
 

Offline timey

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Now express that in terms even the most pompous can understand. At the moment it is simply unintelligible.

Jeff - lol!

And for the pompous:

For a Doppler shift, caused by vibration, to cancel out the frequencies of gravitational blueshift redshift, the frequencies of blueshift redshift must resemble exactly the vibration of the measuring Doppler shift.

The measuring vibration of the speaker, ie: the phase, is 'just measuring' the same effect in the gravitational shift of light.  Except, and this is the important bit, oh pompous one ;), unlike the speaker cone, the gravitational field is NOT in motion.

Now try relating that back to Hubble's law!
 

Offline JoeBrown

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IMO, Hubble's law or constant or parameter is flawed because it simply does not account for gravitational shift.
 

Offline timey

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Joe - As far as I understand, and I am prepared to have my understanding called into question, redshift is thought to be indicative of an expansion between bodies of mass, ie: light source and receiver.  The Doppler shift of redshift is indeed an 'increase' in wave'length'.  Unfortunately GR refuses to provide an absolute reference frame for the universe, due to inconsistencies in measurement under the remit of the speed of light being constant.
IMO, the Pound Rebka proves an absolute reference frame within a gravitational field via redshift blueshift frequencies.  BUT... """Warning""" this requires a rethink on the nature of the phenomenon of time, and it's time dilation, contraction tendencies within the gravitational field.
IMO...the Pound Rebka proves inconsistencies in currently held physics theory.  A Doppler shift of light cannot occur in a gravitational field, unless there is relative motion of the observer, the source, or the field.
If you have got your head around the fact that the motion of the speaker creating a Doppler shift to counter and therefore measure a Doppler shift that is already and naturally occurring in the gravitational field, I'll feel like I've got somewhere in our discussion.
 

Offline JoeBrown

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Doppler and gravitational shift are two different phenomena we understand cause shift in frequency of light.

Doppler is pretty well understood because it's effects can be heard in sound waves, seen in light waves and works pretty good in radar.

Gravitational shift is a lot harder to detect, which was the purpose of the "Pound Rebka experiment."

They first measured the shift in light between statically separated source and receivers. Took measurements from top and bottom and noted that the shifts were equal and opposite, as predicted by Einstein / gravity shift.

They further metered the change in shift by comparing it to doppler shift produced by the motion of a speaker.  When they compared the phase (of the speaker motion) that canceled the opposing shifts as predicted, the experiment concluded gravity shift is in agreement with the prediction of GR.

GR purposefully does not provide an absolute reference because its premise is that everything is relative, tho the speed of light is about the closest it comes to an absolute frame of reference.
« Last Edit: 22/03/2016 20:43:49 by JoeBrown »
 

Offline timey

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Yawn...

Joe - I too can read the Wiki links and a lot more besides, and have been doing so consistently for over 7 years now.

Good luck in your understanding of physics...

Alan?
Pmb?
Jeff?
 

Offline alancalverd

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IMO...the Pound Rebka proves inconsistencies in currently held physics theory.
In everyone else's opinion, it shows that gravitational redshift can be measured and is exactly as predicted. But you are entitled to your opinion, which is much less boring.
 

Offline timey

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IMO...the Pound Rebka proves inconsistencies in currently held physics theory.
In everyone else's opinion, it shows that gravitational redshift can be measured and is exactly as predicted. But you are entitled to your opinion, which is much less boring.

Alan - you are sooo droll!

Fortunately, being as you are a physicist, and an intelligent one in my non-boring opinion ;), you will be of the irk to push the boundaries of knowledge in the way that people of true thinking capacity are wont.

https://en.m.wikipedia.org/wiki/Doppler_effect

Quoted from link above:
"For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered."

Could you please answer these following questions?

The Doppler shift link above indicates that a Doppler shift cannot be achieved without some relative velocity occurring between observer and source.
Is this true?

The Pound Rebka consists of a man made Doppler shift measuring a naturally occurring Doppler shift within the gravitational shift of light.
Is this true?

The Pound Rebka, apart from the speaker vibration used to create the man made Doppler shift, involves no relative difference in velocity between the observer and the source, and with regards to the phenomenon of natural Doppler shift being measured by the man made Doppler shift, the gravitational field causing the gravitational shift in the light is not experiencing any change in its distance or in its strength of gravitational force other than the changes contained within the remit of the distance of the experiment.
Is this true?

Because if these things are true, then it doesn't matter if the experiment showed that Einstien predicted the gravitational shift correctly, the point of interest is that if a natural Doppler shift of light can occur without any relative motion between observer and source in a gravitational field, then this calls into question the fact of redshift being indicative of an expanding, accelerating universe, because the Pound Rebka has indicated, viewed from this perspective, that there was no expansion or contraction of space time within gravitational shift in light.
If there was no expansion or contraction of the distance, then what has caused the relative motion of the naturally occurring Doppler shift in the gravitationally shifted light?  Both the distance within the gravitational field and the speed of light are constant!
 

Offline JoeBrown

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Okay, now I'm starting to understand your reasoning, tho I don't agree.

You've been terming all shift in frequency of light as doppler shift.  Even though doppler shift is only associated with differing in relative velocities.

It would have been clear if you stated that its "interesting that shift occurred in relatively static source/receiver".  But you did not, you called it doppler shift.

It is interesting that they found shift in light frequency to be equal and opposite in static relative motion in a vertical orientation, more so, because it is what evidence for gravitational shift they sought.

Would be less interesting if they also conducted the experiment in a horizontal orientation and produced the same results.  Then it wouldn't be thought to be the effect of gravitational shift or that it invalidates the principle and/or the experiment and wouldn't be considered evidence of gravitational shift.
« Last Edit: 23/03/2016 02:25:11 by JoeBrown »
 

Offline PmbNEP

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Quote from: timey
Very good Pmb!!!

Now could you please tell me how, within the Pound Rebka experiment, a Doppler shift can be identified within the gravitational shift of light when there is no relative motion between the light source and the receiver other than the oscillations of the speaker cone...
Doppler shift is function of the speed of the source and nothing else. The source of gamma rays is mounted in the center of the loudspeaker and is attached to the cone so that as a signal is sent to the speaker  the source moves back and fourth. Therefore there is a relative motion between source and receiver. There can only be a Doppler shift when the source is moving as is the case here.

Note: This has nothing to do with the original question so this is an off-topic discussion.
 

Offline Colin2B

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Note: This has nothing to do with the original question so this is an off-topic discussion.
It is also moving perilously close to a new theory discussion, which could be split out and moved.
It must also be confusing the socks off the question poster!
 

Offline timey

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Doppler shift is function of the speed of the source and nothing else. The source of gamma rays is mounted in the center of the loudspeaker and is attached to the cone so that as a signal is sent to the speaker  the source moves back and fourth. Therefore there is a relative motion between source and receiver. There can only be a Doppler shift when the source is moving as is the case here.

Note: This has nothing to do with the original question so this is an off-topic discussion.

So Pmb - you are now telling me that the experimenters created a Doppler shift of gamma rays by a mechanism that is clearly obvious, and well documented in Pound Rebka link, (and indeed by most of the posters here, including myself)... and then used this man made Doppler shift to measured WHAT?
Because as far as I understand, they used this man made Doppler shift of the gamma ray, to measure a naturally occurring Doppler shift of light being shifted by the gravitational field!!!
True or false?

Although the man made Doppler shift was created by a slight variance in distance, caused by the speaker oscillations of 'back and forth', the naturally occurring phenomenon of gravitational shift that it was measuring occurred over a fixed distance of 22.5 metres.  There was no relative motion between the source and receiver with regards to the phenomenon being measured.
True or false?

With regards to the original posters question, you answered this question earlier this thread with the """correct""" answer already.  It's called invariance, and no one actually knows """how""" a photon travels at speed of light from a light source that is moving, it is only known what the light will be doing from an observers point of reference.

If you care to explore the parameters of the Pound Rebka experiment with me, as I am doing, I """can""" possibly actually answer the original posters question more precisely, because I think that perhaps I """do""" know how light moves across space in a gravitational field.

Btw...are you Pmb Pete who used to post before?  If so, I'm glad to see you back.

P.S.  Colin, if it's bothering the status quo, by all means split the thread and put it in New Theories.

P.S.S.  Joe - in answer to your post, please read the Wiki link on Doppler shift more carefully.  You will see that it is a documented fact that light undergoes a Doppler shift, that it is referred to as a Doppler shift, and is responsible for lights wavelength which is proportional to frequency.  The man made Doppler shift matched the frequencies of this naturally occurring phenomenon, cancelling out the gravitational shift of the gamma rays, more gamma rays were absorbed by the receiving sample and the number of gamma rays detected by the scintillation counter dropped accordingly, constituting the measurement, via the speaker 'phase', of the naturally occurring phenomenon of a Doppler shift in the gravitational shift of light.
 

Offline PmbNEP

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Quote from: timey
So Pmb - you are now telling me that the experimenters created a Doppler shift of gamma rays by a mechanism that is clearly obvious, and well documented in Pound Rebka link, (and indeed by most of the posters here, including myself)... and then used this man made Doppler shift to measured WHAT?
Because as far as I understand, they used this man made Doppler shift of the gamma ray, to measure a naturally occurring Doppler shift of light being shifted by the gravitational field!!!
If this thread was about the Pound-Rebka experiment then I wouldn't have responded to this thread because I'm not an experimental physicist. As you said, everything you want to know about that experiment is posted in Wikipedia. I only responded to the question I quoted. I simply don't know why they had to have a time varying Doppler shift in the signal. I do, however, know experts who'd know this. I'll ask them about it and get back to you.
 

Offline JoeBrown

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Note: This has nothing to do with the original question so this is an off-topic discussion.
It is also moving perilously close to a new theory discussion, which could be split out and moved.
It must also be confusing the socks off the question poster!

The original poster stated it was an enlightening argument with amusement, here:
quote author=NeT-HeaD link=topic=66096.msg483797#msg483797 date=1458670736

I've found the off topic premise concerning, confusing and aggravating.  Seems timey is enjoying itself inciting it.
 

Offline timey

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I've found the off topic premise concerning, confusing and aggravating.  Seems timey is enjoying itself inciting it.

IMO Joe - 'you' are finding 'on topic' discussion confusing!

It was Colin who, quite appropriately to the topic I might add, brought up the subject of a Doppler shift in light in response to the original posters question.

My contribution has been in response to Colin's commentary!
« Last Edit: 23/03/2016 14:44:57 by timey »
 

Offline timey

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If this thread was about the Pound-Rebka experiment then I wouldn't have responded to this thread because I'm not an experimental physicist. As you said, everything you want to know about that experiment is posted in Wikipedia. I only responded to the question I quoted. I simply don't know why they had to have a time varying Doppler shift in the signal. I do, however, know experts who'd know this. I'll ask them about it and get back to you.

Pmb - Thank you for your honesty... and, to say so, I'd be most grateful, and look forward to hearing back from you!
 

Offline Colin2B

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I simply don't know why they had to have a time varying Doppler shift in the signal.
My understanding is that this was the easiest way of creating a variable speed of the source towards and away from the receiver., change the frequency, change the speed, change the Doppler shift. They found the point at which this Doppler shift canceled out the gravitational shift and the frequency gave them the speed, hence shift. Sorry to labour this, but explanation for those who have not read the detail.

Timey, I find your use of Doppler shift to describe gravitational shift as seen by observers in the gravitational field to be confusing in this thread. Gravitational shift is only seen as a Doppler shift by the free falling observer, ground observers attribute it to the acceleration of the photons increasing photon energy, hence frequency, but due to clock rate differences can only measure std speed of light. Again sorry to labour this, but I'm aware a wider audience is reading this.

Can we separate the terms for clarity.
 

Offline timey

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Colin - To clear this up for any confused readers, the reason* why Pmb does not know why the Pound Rebka had to use a time varying Doppler shift in the signal, (this being the man made Doppler shift used to measure the natural phenomenon) is because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift (this being the naturally occurring phenomenon) in the gravitational shift in light over a fixed distance of the gravitational field.

As per GR and space time, redshift is indicative of an expansion of distance between light source and receiver, and blueshift is indicative of a contraction of distance between light source and receiver.

If anyone is confused by my observation here, really...don't worry, you have very good reason to be...

This being the point I am making!

*Pmb, if this is not the reason, I apologise for my presumptuous-ness, and invite you to correct me.
 

Offline alancalverd

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The Pound Rebka consists of a man made Doppler shift measuring a naturally occurring Doppler shift within the gravitational shift of light.
Is this true?
No, no, no. It uses Doppler shift to measure gravitational shift. They are not the same thing, but both have the same effect on wavelength.

Please read this very carefully

Gravitational shift is NOT Doppler shift.

I say again

Gravitational shift is NOT Doppler shift

which is what everyone else has been saying and Messrs P & R understood from the outset.

Please do not talk about "gravitational Doppler shift" - there is no such thing. Gravitational shift is due entirely to the presence of a gravitational field. Doppler shift is due entirely to relative motion.

There is no confusion in anyone's mind but yours, and that only arises because you have invented a term that nobody else uses and has no meaning! 

« Last Edit: 24/03/2016 06:42:34 by alancalverd »
 

Offline PmbNEP

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Quote from: timey
Colin - To clear this up for any confused readers, the reason* why Pmb does not know why the Pound Rebka had to use a time varying Doppler shift in the signal, (this being the man made Doppler shift used to measure the natural phenomenon) is because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift (this being the naturally occurring phenomenon) in the gravitational shift in light over a fixed distance of the gravitational field.
That's not why I don't know why the experimenters used Doppler shift in there experiment. The reason is that I simply don't know the theory behind the workings of the experiment itself. For example; it may be that they got a decrease in experimental error by doing it that way. I simply don't know.

However there simply is no theoretical reason why they had to do it like that. Theoretically all they needed was a photon emitter and a photon detector which measures wavelength.

By the way it's not true that the speed of light is constant. That only holds in inertial frames, i.e. in gravitational fields. Here is a derivation for a uniform gravitational field/accelerating frame of reference and around a spherical body (i.e. Shwarzschild spacetime).

Go to www dot newenglandphysics dot org slash physics_world slash gr slash c_in_gfield.htm
 

Offline Colin2B

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Theoretically all they needed was a photon emitter and a photon detector which measures wavelength.
When I read the experiment, this is why I assumed they used the loudspeaker. They didn't have a detector accurate enough to measure the shift, so they used the go/no go absorption to see when the Doppler shift matched the Grav shift, neat.

....because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift.
Ok, now I understand why you keep stressing constant distance. Ignoring the loudspeaker, between the top of the tower there is no motion to cause a Doppler shift - that's because there is no Doppler shift as Alan says, just a gravitational shift, which is not caused by movement.

As per GR and space time, redshift is indicative of an expansion of distance between light source and receiver, and blueshift is indicative of a contraction of distance between light source and receiver.
Only in the case of Doppler shift

If anyone is confused by my observation here, really...don't worry, you have very good reason to be...

This being the point I am making!
The confusion is clearing. I am beginning to understand why you are making your claims and where the errors are. Thanks for the clarification.

The following from Pmb explains.
By the way it's not true that the speed of light is constant. That only holds in inertial frames, i.e. in gravitational fields.
Remember also that as a consequence rate of clock is slower deeper in the Grav field, which matches the blue shift - this plus energy change, etc are all part and parcel of an interlocked group of  phenomena with the same cause, as described in GR.

 

Offline timey

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Ok - in answer to the last 3 posts:

Alan, Pmb, Colin... good, good, now we have all understood that I am talking about a Doppler shift that the Pound Rebka identified in the gravitational shift, and that gravitational shift causes an 'expansion' and 'contraction' of wave'length'... Can you please tell me 'where' in the distance of 22.5 metres  the extra, or lesser length of the waves exists?

I understand that there is an aspect of time dilation that 'will' affect the journey of the light, however this time dilation is contrary to the direction of the contraction and expansion of the wavelength and 'cannot' explain the Doppler shift in the gravitational shift...
 

Offline puppypower

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Consider, as an analogy for a photon, a boat traveling on the water. The boat is analogous to the particle, while the wake created by the boat is analogous to the wave aspect of the photon. If we had two tunnels under the bridge, the boat; particle, can only go through one tunnel at a time, while the wake, if spread out, can go through both tunnels.

The boat needs a constant supply of energy to keep moving at velocity V. This energy is needed to overcome friction between the boat and water. If the boat cuts the engines, the particle will slow, coast and then stop, while the wake will decay and finally stop. The speed of light is needed to maintain the wake/wave of the photon particles.

The question becomes how does a photon move through the medium of space-time and not slow down or speed up? What is the source of the propulsion energy?

If we assume the speed of light is the ground state of the universe, this implies all inertial references will be at higher potential. In this case, the constant speed of light would be connected to photons being constantly induced into the ground state ay C. The photons are constantly induced to higher potential; less than C by inertial. They cyclically drop back to the ground state; speed of light, while the energy differential is given off; motor that generates the wake/wave. 

One analogous way to look at this is connected to a house with a variety of electrical circuits, all using the same ground; earth. The ground is common to all the circuits and is at lower potential than any voltage/current configuration we may use. The potential difference between the inertial states, and the ground, is the energy potential used to drive the particle, which creates an analogous wake in space-time. This wake can become modified with the medium of space-time; red and blue shift, but the ground never changes, so the particles are always moving at C.

Fair enough PuppyPower... BUT... in Pound Rebka """Where"" in space time does this wave LENGTH exist if the distance between light source and receiver is held 'static', with respect to each other, on the basis that the oscillations of the speaker are creating a Doppler shift that cancels out the gravitational shift of the light?

The oscillations of a speaker, can be traced back to the energy signals being added to the speaker. If we shut off the power to the speaker there are no waves. We need a source of power. The energy input propels the speaker forward, which then flexes back to the rest position, due to the potential energy stored within the elasticity of the speaker, caused by the original forward energy that was added.

If we assume C is the ground state, the power circuit for all energy analogy speakers, is part inertial, which can be any of a variety of inertial states, static or in motion. It is also connected to the C ground state which is the same for all speakers. The speaker is driven forward by the potential energy returning to the C ground state. The return wave of the speaker is connected to the elasticity of the space-time medium speaker. If we took away the C ground state, the speaker would stop and no waves will appear. Energy moving at the speed of light, in all references, means the speaker is plugged in.

The Doppler shift of the speaker is driven in one direction. The rebound, in the other direction, is more reactive and is caused by the potential energy stored in the speaker, due to the forward energy pulse. The wave motion is due to the flow of energy from inertial back to the C ground.

Another visual is a water wheel, where the movement of water from a position of potential back to a ground state, results in the water wheel defining a wave. The water analogy for energy waves is time potential. The stationary water wheel defines a circle in space. However, it will define a wave in time.

 

Offline Colin2B

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now we have all understood that I am talking about a Doppler shift that the Pound Rebka identified in the gravitational shift,
Ok, as long as you understand I'm not talking about a Doppler shift, because that's not what they identified, it doesn't exist.

and that gravitational shift causes an 'expansion' and 'contraction' of wave'length'... Can you please tell me 'where' in the distance of 22.5 metres  the extra, or lesser length of the waves exists?
They exist at every point in the 22m. The gravitational shift varies continuously from top to bottom so the frequency/wavelength also varies continuously, becoming increasingly blue shifted down the tower. You can view this as increasing energy as the photons accelerate, or you can look at the slowing of clocks - because each observer will measure the speed of light as constant, the frequency will be seen as increasing (shorter wavelength).

I understand that there is an aspect of time dilation that 'will' affect the journey of the light, however this time dilation is contrary to the direction of the contraction and expansion of the wavelength and 'cannot' explain the Doppler shift in the gravitational shift...

It's not a Doppler shift.
The time dilation - slower clock - explains the blue shift as I said above, how it is contrary?. The time dilation is what 'causes' the acceleration, it's all interlinked just as in SR - time dilation and length contraction are aspects of the same effect, you can't add both together and get an additional effect.
 

Offline timey

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Colin - For the man made Doppler shift to cancel out the blueshift and redshift frequencies, the man made Doppler shift must match those frequencies exactly!  You are wrong.  There is a Doppler shift in the gravitational shift of light.  The Pound Rebka proved it!  The wiki Doppler shift link states it!

If the time dilation aspect was responsible for relative motion within the gravitational shift of light, then you would see 'longer' wave lengths in blueshift, and 'shorter' wave lengths in redshift.

Furthermore:

https://en.m.wikipedia.org/wiki/Doppler_effect

Quoted from above link:
"For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered."

I repeat: "only the relative difference in velocity between the observer and source needs to be considered."

The Pound Rebka measured a Doppler shift in the gravitational shift over a distance of 22.5 metres.  There was no relative difference of velocity between light source and receiver with respect to the gravitational shift they measured over a distance of 22.5 metres.

Now I'm sorry, but physics cannot have its cake and eat it.  Either there 'must' be relative velocity between light source and receiver that expands or contracts the distance between them, in which a wavelength experiences changes in distance that affords the wavelength's expansion or contraction, or there must be a 'medium' in the gravitational field that explains the relative motion of a Doppler shift in the gravitational shift of light!
« Last Edit: 24/03/2016 12:55:49 by timey »
 

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