[timey]

What happens for the free falling observer Jeff? I assume time runs at 100%. As she falls (pushed off the tower by those who disagree with her theory) she looks up and sees the source receding- red shifted, looking down she realises the detector is moving up against the light waves - blue shifted. Her view is that there is no gravity, just acceleration and all  effects at top and bottom of tower are due to doppler shift.

Thanks for the push Colin!

Whaaaaargh....  I'm faaaaallliiiing!

...at a rate of 9.8m/s2 acceleration...
As I look behind me, I can see blue shifted light rushing towards me, but from the point that it passes me I cannot see it redshift towards the earth from my position.  My position in the field is changing, but as the light is travelling at the speed of light, it's position in the field changes faster than mine.

Alternatively, the second time you pushed me off the tower, (clearly I hadn't yet learned my lesson )...  light is being sent way from Earth.  I look up behind me as I fall, I cannot see the light redshift away behind me... but looking downwards, up till the point that the light passes my position in the field, I can see it redshift towards me.

Now, then... having been pushed off the tower twice, I think I can be forgiven for a bit of wonky thinking, or could it perhaps be that I have become en-light-ened?

My observations when falling where as such:
Firstly:
A Doppler shift was used to cancel out the redshift and blueshift effects, thereby proving that these effects of redshift and blueshift exist as a result of the changes in the gravitational field.
That as a direct result of the observation above in relation to the light source emitter and receiver being held static relative to each other, there is proven a representation of Doppler shift within the relative motion of light via changes in the gravity field.
That both of the above considered in relation to the constant speed of light and the 'constant distance' between ground and top of tower of the experiment, shows that something is a little amiss.  Light should not have any such Doppler shift relative motion properties over a constant distance, travelling at a constant speed!
And... that taking into account that the light will be travelling at the speed of light under the currently thought remit of the minuscule time drifts occurring within the changes in the gravitational field - 'does not' in fact describe the relative motion of a Doppler shift!

Secondly:
That I was falling at a rate of 9.8m/s2.
That the first time I was pushed off the tower, my mobile phone fell out of my pocket... and in that I arranged my mass as to be as lacking in air resistance as my phones mass, I thought I noticed my phone and I arrived on the ground at the same time.
That the second time you pushed me off the tower Colin, with great presence of mind, to say so, I picked your pocket of your phone and repeated the experiment.  Your phone and my phone, (both smashed to smithereens now of course) did not weigh the same, in weight or worth.  Mine was heavier, yours more expensive!  But, again, I noticed that your phone and I arrived on the ground at the same time.

Why does the light not accelerate at 9.8m/s2?  Answer:  Because the speed of light is constant.
Are we expecting that the light will have taken a couple of a billionth of a second longer time to travel the distance on account of the gravity field slowing time down?  Answer:  Yes we do.
We have attributed light with mass equivalent to its energy.  We state that the gravitational field robs the light of energy when leaving a gravitational field, ie: redshift.  So... what exactly is replenishing lights energy when we observe a blueshift?

The energy of mine and the phones falling is easily explained due to potential energy, via the fact of my mass and my position, or changing position, within the gravitational field.

The fact of the lights increase in energy found in its falling towards a gravitational field is described by adding relativistic mass.  But, by this remit, as my mass, or the mass of the phones falls towards earth it must also increase.  Again... we come back to the question as to why lights mass does not 'accelerate' at 9.8m/s2, like mine does?

Looking at the energy of my mass, we can see that as I fall, the potential energy that my mass experiences is reducing as I fall into the gravitational field.  Therefore, the energy that light experiences as a result of the adding of relativistic mass, also will 'reduce' as it falls towards earth.  But light energy does not reduce as it gets closer to a gravitational field.  This constitutes a breach of logic, unless it is the increase in inherent mass energy as the relativistic mass is increased that describes the increase in energy.  It should be noted, that adding relativistic mass to light as its energy 'increases' would not subject the light to any more, or any less than an acceleration of 9.8m/s2!
Yet light is not subject to any 'further' acceleration of its own speed under the influence of a gravitational field.

However... I observed that a Doppler shift cancelled out the redshift blueshift observations from top and bottom of tower perspectives.  These perspectives of the light source emitter and receiver having been held static relative to each other!!!
This alerts me to these 5 considerations:
a) That there is a relative motion in light representing a Doppler shift that does not incorporate any relative motion of distance.
b) That on the basis of there being a relative motion in light representing a Doppler shift that did not incorporate any relative motion of distance, that redshift considerations are not necessarily indicative of an expanding universe.
c) That the relative motion of Doppler shift found in light that is apparent over a 'constant distance' does not add up to gravitational shift, plus time drift consideration, plus the aspect of potential energy reduction in light caused by decreasing distance between its mass and the mass of the earth as it enters further into the gravitational field.  (or at least not without rendering distance as a variable.)
d) That if I were to look upon an increasing gravitational field increasing the rate of time... that the fact of a Doppler shift cancelling out redshift blueshift frequencies indicates that light, travelling at the speed of light, is accelerated towards a gravitational field via an increase in the rate of time that directly relates to the increase in a gravitational field via a representation of Doppler shift.  That light is simply getting its energy directly from the gravitational field, it's frequency is indicative of the rate of time, and it's additions or reductions in wavelength are not 'distance' related, but 'time' related.
e) That perhaps there is no acceleration of gravity, perhaps it is instead only an acceleration of time.

So... what is happening when I am falling?  The difference between light and I being that light has no mass... and, more importantly, it is in fact proven that my clock will run faster in a reduced gravitational field, not slower.  As me and my clock fall, it's rate of time will register ever so slightly slower.
Ok, my mass is subject to gravity potential, but if light 'gets' its energy from a gravitational field, do I also 'get' energy from the gravitational field?  And if I do, then does this effect the status of my e=mc2 inherent mass energy?  ie: having rid ourselves of the idea of relativistic mass for light, does my mass 'change' in a gravitational field, changing my inherent mass energy?

If I state my inherent mass energy as constant, and add on the aspect of potential energy, plus gravitational field energy... given that this describes an 'energy level' that will produce frequency and wave length, does my atomic clocks caesium atoms inherent mass energy, plus the potential energy, plus the gravitational field energy that it is experiencing at elevation, describe the fact of the frequency of its cycles being increased?
Can the aspect of the kinetic energy of a moving body of 'mass' be deducted from the additions of inherent mass energy with potential energy and gravitational field energy, to then describe a slowing of time for both co-moving, and more obviously, non co-moving objects?

And finally...  If so, then could the phenomenon of time itself be 'energy' related?

[alancalverd]

Keep it simple.

Doppler shift is due to relative speed of source and receiver. Gravitational shift is due to relative gravitational potential of source and receiver. They are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other.

As for the original question, there is no "why". It just happens that an awful lot of observations become interconsistent if you assume that c is constant for all observers, and as far as we can ascertain, it is. The task of teaching physics is to explain why everything else is as it is, given only that c is constant. 

[timey]

Keeping it simple...

Doppler shift is due to relative speed of source and receiver.  But... if there is no relative speed between source and receiver, then any representation of Doppler shift 'must' be due to relative motion within the propagation of light within a gravitational field.

Keeping it simple...

Gravitational shift is due to relative gravitational potential of source and receiver. Doppler shift and gravitational shift are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other...  But... If there is no relative expansion of distance via a gravitational shift, there is 'again' no relative motion for a Doppler shift to occur in.

Keeping it simple...

How can lights frequency be cancelled out by a Doppler shift between a light source emitter and a light source receiver that are both held static of motion relative to each other - and consist of a 'constant' distance apart?

[evan_au]

How can lights frequency be cancelled out by a Doppler shift between a light source emitter and a light source receiver that are both held static of motion relative to each other - and consist of a 'constant' distance apart?

If two objects are not moving relative to each other, their Doppler shift is zero.
The only way for Doppler shift to equal gravitational shift is if the gravitational shift is also zero.
Gravitational shift is zero if the two observers are at the same gravitational potential, eg both standing on the surface of the Earth.

Doppler shift and gravitational shift are not the same phenomenon but have the same effect on wavelength, so you can use one to measure the other

This is true in one direction only.

If you observe the moving light source from multiple directions (or watch it as it passes you), you will see that gravitational shift is cancelled in one direction, but not others.

The reason is that the Doppler effect can produce red and blue shift. However, gravitational frequency shift produces the same shift whether the object is moving towards you or away from you. This is called Relativistic Beaming.

[timey]

Evan - my point is that within the Pound Rebka, a Doppler shift was used to cancel out the frequency of redshift from bottom of tower to top of tower.  And again to cancel out the frequency of blueshift from top of tower to bottom.

There was no relative speed between the top of tower and bottom of tower in either instance.  There was also no relative expansion or contraction of the distance between top of tower and bottom of tower in either instance.

Yet - a Doppler shift cancelled the blueshift redshift frequencies between the 2 'static in motion' and 'constant in distance' locations... How did it do this?

[JoeBrown]

Yet - a Doppler shift cancelled the blueshift redshift frequencies between the 2 'static in motion' and 'constant in distance' locations... How did it do this?

They gravitational shifted the height of the tower.  -- errr.  Trying, to be  a wisenhymer.

They moved the light source and/or receiver, which produced doppler shift to cancel gravitational shift.  I don't believe they had to change the height of the tower to accomplish the feat.

[timey]

Joe - I can't believe I am actually dignifying your post of absolute and total ignorance with an answer... But here goes...

They did not 'move' the light source or the receiver.  They did not extend or shorten the 'distance' from the bottom of tower to the top of tower.

Why don't you read up on the subject 'before' commenting?

https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

[Colin2B]

.

There was no relative speed between the top of tower and bottom of tower in either instance. 

But there was, the source was mounted on a loudspeaker cone so it moved towards and then away from the receiver.

[timey]

Colin - they created a Doppler shift by vibrating the speaker at frequencies between 10 and 50 hertz to cancel out the redshift  blueshift frequencies.  The distance between the speaker and the receiver remained at 22.5 metres.  Therefore the distance between the emitter and receiver remained at 22.5 metres and the distance between the top of tower and bottom of tower remained at 22.5 metres.  The gamma rays travelled 22.5 metres 'only'.

When the Doppler shift canceled out the gravitational blueshift, the receiving sample absorbed gamma rays and the number of gamma rays detected by the scintillation counter dropped accordingly. The variation in absorption could be correlated with the phase of the speaker vibration, hence with the speed of the emitting sample and therefore the Doppler shift.

A Doppler shift in light was identified in this experiment whereas there was no relative motion between the emitter of the gamma rays and the receiver, and no contraction or expansion between the top of tower and bottom of tower.

How did it do this?

Clearly redshift is thought indicative of an expanding universe, whereby the light emitter and receiver are experiencing an expansion of distance between themselves, at an accelerated rate.

The Pound Rebka indicates that redshift from bottom of tower to top of tower shows no such thing!  The light source is not accelerating away. There is no expansion of distance.

[timey]

Therefore the only explanation for any relative motion is within the movement of the light!  But this is not possible, because the speed of light is constant.  The only other aspect in this instance that would affect relative motion is 'time', and I'm sorry, but a Doppler shift of time in accordance with redshift blueshift frequencies is entirely contrary to currently held theory!

This being my point...

[JoeBrown]

Perhaps they selectively applied cosmological shift to the tower?

The wiki article you linked to states:

By vibrating the speaker cone the gamma ray source moved with varying speed, thus creating varying Doppler shifts. When the Doppler shift canceled out the gravitational blueshift, the receiving sample absorbed gamma rays and the number of gamma rays detected by the scintillation counter dropped accordingly. The variation in absorption could be correlated with the phase of the speaker vibration, hence with the speed of the emitting sample and therefore the Doppler shift.

The vibration of the speaker caused a change in relative motion.  They weren't quite statically separated.

However, as you pointed out, shift has nothing to do with the subject.  So why are we beating this poor horse? To give it some cosmological shift?

[timey]

Yup - you are definitely a little on the dense side to be sure!

The definitive statement here being "When the Doppler shift canceled out the gravitational blueshift, the receiving sample absorbed gamma rays"

For the gravitational blueshift to be cancelled out by the phase of the speaker, the blueshift of the light would have to match the phase of the speaker.

Does this compute?

[JoeBrown]

Timey, I must be dense.

When a speaker vibrates, it is considered motion.  Motion causes doppler shift.

You indicated that doppler shift occurred without motion, when motion was clearly stated to be involved.

I have no idea what's supposed to compute. I calculate you have completely read past the word "motion"

--
I've read these sentences, completely ignoring the word motion.  Makes me kinda sea sick, like your argument, whatever that might be.

[timey]

Dense was perhaps too mild a term...

The speaker phase consists of the motion of a Doppler shift.  The light matches the motion of a Doppler shift, proven by the phase of the speaker cancelling out the frequency of the light.  However, the light emitter and the light receiver are NOT in motion.  Neither does the distance between the light emitter and the receiver contract or expand.  Therefore the Doppler shift 'motion' identified by the experiment is due to a Doppler shift in the light itself.

On the basis that the premiss of the experiment has clearly gone over your head, never mind the point I'm making.  Legs on you, an egg wouldn't crack if it fell out of your arse!

I look forward to a proper physicists comments...

[JoeBrown]

I look forward to a proper physicists comments...

I fear you are incorrect.  You misunderstand what others comprehend, then argue a misguided point.  That does not make for "proper" comment of physics or anything in general.  Your posts are intentionally rude and ignorantly inappropriate.

I'm dense for trying to explain.  For that I apologize.

--

If you truly wish to prove your erroneous point:

the gamma ray source moved

Kindly explain how that phrase does not equate to motion.



Perhaps it isn't the word "moved" that's causing the issue.

The gamma ray source, is the light source that moved.

[alancalverd]

Colin - they created a Doppler shift by vibrating the speaker at frequencies between 10 and 50 hertz to cancel out the redshift  blueshift frequencies.  The distance between the speaker and the receiver remained at 22.5 metres.

Either the speaker was vibrating, in which case the distance between source and detector was varying, or it wasn't vibrating and the distance was constant.

Is that enough "proper physics"?

[Colin2B]

Colin - they created a Doppler shift by vibrating the speaker at frequencies between 10 and 50 hertz to cancel out the redshift  blueshift frequencies.  The distance between the speaker and the receiver remained at 22.5 metres.

Either the speaker was vibrating, in which case the distance between source and detector was varying, or it wasn't vibrating and the distance was constant.

Is that enough "proper physics"?

Thanks Alan, my point exactly.

[timey]

Is that enough "proper physics"?

Oh fer goodness sake!!!

My answer:  No, not really!

Yes - the Doppler shift is created by the vibration of the speaker...but the speaker isn't creating the gravitational shift in the light that the vibration of the speaker cancels out.  The speaker cone fabric is moving back and forth, as speakers do, to create vibration, but neither the light source or the receiver are in relative motion with respect to each other, such as a natural light source and receiver.  The distance between the light source and the receiver, apart from this vibration of the speaker is 'static'.

What the experiment shows is that there is a Doppler shift in the motion of light that is not associated with an expanding distance, or an acceleration of the speed of travel of the light source.

How can there be 'relative motion' in light over a distance that is constant when the speed of light is constant?

[timey]

I fear you are incorrect.  You misunderstand what others comprehend, then argue a misguided point.  That does not make for "proper" comment of physics or anything in general.  Your posts are intentionally rude and ignorantly inappropriate.

Actually I am not incorrect.  And I am not being rude.  If you want to accuse someone of being a weisenhiemer, and make idiotic comments about cosmological shift, and intimate that the tower itself suffered a gravitational shift, without actually attempting to understand what the poster is in fact talking about, then you are fair game as far as I'm concerned.  Short legs being an apt description!

[McQueen]

Here is the quote on the location of the gamma ray source:

  A solid sample containing iron (57Fe) emitting gamma rays was placed in the center of a loudspeaker cone which was placed near the roof of the building. Another sample containing 57Fe was placed in the basement.

Since the gamma ray source was placed in the centre of the loudspeaker cone, I assume that the gamma ray source was moving up and down with the vibration of the speaker, causing the distance over which the light had to travel to vary. But in which case where does the doppler shift come in ? In order to establish validity (i.e., to prove that the gravitational blue shift would act as was supposed resulting in non-absorption of the gamma ray) shouldn't a test have been carried out without the speaker, or is that taken for granted ?