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Author Topic: Why do photons have the same speed regardless of the velocity of their source?  (Read 11036 times)

Colin2B

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #75 on: 24/03/2016 17:18:37 »
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!
Agreed

You are wrong.
Thanks

There is a Doppler shift in the gravitational shift of light.
No. as I've said before there are 2 different effects at work here

The Pound Rebka proved it!
No it didn't

The wiki Doppler shift link states it!
Where

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.
The time dilation isn't responsible for any relative motion (because there isn't any relative motion) but it does affect the frequency.
Lets take an eg with simple numbers:
Observer high in weak gravity field measures time of 1 cycle as being 1s = 1Hz.
Further down the field clocks run slow (time dilation) so the same cycle could be measured as say 0.5s =2Hz.
This is an increase in frequency - blueshift - and a shortening of wavelength.
No movement needed, no doppler shift, just due to grav field and clocks running slow.

Furthermore:

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

"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."
Although that is true it only applies to the doppler part of the experiment.
The presence of a medium is irrelevant, this is light not sound.

The Pound Rebka measured a Doppler shift in the gravitational shift over a distance of 22.5 metres.
No, they used a doppler shift to measure the gravitational shift.

As I said before, between the top of the tower and the bottom there is a gravitational shift which they wanted to measure. As Pmb says, they could have used an accurate freq counter, but I don't think they had one accurate enough. So, my reading of the experiment is that they looked for a way of creating a null. Using a null reading is extremely accurate and commonly used eg an interference pattern or a bridge cct. In this case they decided to create the null by making a doppler shift using the movement of the loudspeaker, I suspect the other end was a simple scintilation counter. When the doppler shift equals the grav shift they cancel and the counter shows null. The value of the doppler shift = the value of the grav shift and they can work out the doppler value from the frequency fed into the loudspeaker.
At no point are they measuring a doppler shift in the grav shift, just using one to measure the other.

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.
They didn't try to measure this.
Because of time dilation the observer at top of tower and the one at the bottom will measure the same speed of light.

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!
Physics isn't trying to have it's cake and eat it. You are looking for something that isn't there.

Anyway, that's it from me as I suspect you aren't going to believe me. Either Pmb or Alan will no doubt put me right as I've been winging it here.
« Last Edit: 24/03/2016 17:33:15 by Colin2B »

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #76 on: 24/03/2016 18:00:32 »
Colin - I cannot 'do' these multi quote posts on my phone easily at-all, and my phone is all I have at mo...so please excuse me, but can we do this one or two portions at a time?

It states in the Doppler shift link that Both light waves and gravity waves are subject to Doppler shift.  In lights case this is because wavelength is proportional to frequency.  It is the extra, or lesser length in the wavelength that is """synonymous""" to a Doppler shift.

Now - if the man made Doppler shift is cancelling out the natural frequency of the gravitationally shifted light, then perhaps the man made Doppler shift is creating a time variance in accordance with the time drift found in the gravitational field.  If it were then my question is:
Where in the fixed distance of 22.5 metres do the extra or lesser lengths of these wavelengths exist?

However, on the basis that a person living on top floor in a high rise, will, over the course of 79 years, only be 1 minute older than his downstairs neighbour on the ground floor, (or something similarly within this tiny region anyway), I think we can pretty much rule out the fact of the speaker cone creating such a 'subtle' vibration at between 10 and 50 hertz.  The vibration that the speaker vibrated at is more synonymous with a much greater factor of Doppler shift than this.  One that matches the frequency being proportional to the wavelength, I'd imagine.  So again, my question is:
Where in the fixed distance of 22.5 metres do the extra or lesser lengths of these wavelengths exist?
And:
What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?

Btw, Colin, I'm not winging it.  I've had training in sound engineering and have been home studying physics for 7 years.  I appreciate that this does not mean that I'm correct in what I'm observing, I may well be mistaken, but I've racked my brain with it all for many years now, and really, I don't actually think I am mistaken ;)
« Last Edit: 24/03/2016 18:17:05 by timey »

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #77 on: 24/03/2016 19:01:04 »

Colin2B

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #78 on: 24/03/2016 23:25:44 »
Ok, we have some common ground. I worked as a sound/vision engineer (BBC trained) and studied Applied Physics day release/evenings until I went to study full time. When I say winging it, I mean reading the various reports of Pound Rebka experiment to make sense of it, most of it is fairly clear but you need a bit of guess work to fill in the gaps.

So, let's concentrate on one question at a time as you suggest.

"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 22.5m. 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). Remember these are nm wavelengths.

"What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?"
That's easy, there is no relative motion, because the gravitational shift is not due to motion.

I don't know why you have quoted the 2 Wiki articles because neither of them suggest that Doppler is involved in gravitational shift, nor that PR found it as a cause of the shift.

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #79 on: 25/03/2016 00:18:52 »
Damian and I did a lot of work for the BBC, back in the day, from Korner studios, Scrubs Lane, when I was in training as his tape hop.  Jingles, radio recordings, etc.  It wasn't just that we were so close to the White City BBC studios, Damian had the best live room ever!  I remember you posting that you are into sound... doing outdoor recordings as I recall, you play guitar as well if I remember rightly?

Ok - so, yes, of course the wavelengths exist at every point in the distance of 22.5 metres.  Now this is where it gets interesting!  Because there is no relative motion caused by changes in the distance in the phenomenon being measured, only a fixed distance of 22.5 metres, we can now 'peg' gravitational shift in light, exactly to a gravitational field, which provides us with an absolute reference frame.  The reason why GR cannot provide us with such, is because a relativistic Doppler shift in light is thought to be synonymous with expansion or contraction of distance.  The PR shows that this is not the case.  There is a relativistic Doppler shift 'effect' within a distance that is not expanding or contracting.

And:
I said - "What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?"
You said "That's easy, there is no relative motion, because the gravitational shift is not due to motion."

Gravitational shift in light is due to an expansion, or contraction, of distance between light source and receiver.  The relative velocity between light source and receiver expands or contracts the gravitational field.  Hence redshift being indicative of an expanding universe.

In the PR, light travels a fixed distance of 22.5 metres, at the speed of light.  A time variance has been added to the test signal via the vibration of the speaker. This constitutes relative motion.  The signal matches the frequency of the gravitational shift.  Does this not indicate that the relative motion of the signal is reflected in the gravitational shift?

Of course, it could be that all these considerations are contained and addressed within the maths of the PR, as Pmb suggests.  I wouldn't have a clue being a non-mathematician, but the written in 'words' dialogue doesn't state this as being the case though, and usually it would...

P.S.  Not sure why you are asking that about the Wiki links.  The Doppler shift link mentions light and gravity.  The relativistic Doppler 'effect' link explains frequency and wavelength, and drops it back to the Lorentz transformations which I have addressed in my "4 a deeper discussion - is distance an absolute invariant" thread.  Are you sure I'm not just confusing you and the issue by my 'shoddy' use of terminology ;) ?
« Last Edit: 25/03/2016 00:26:48 by timey »

JoeBrown

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #80 on: 25/03/2016 02:29:42 »

Pound Rebka did two observations without motion.

Of the 2 still (speaker off or absent) observations one was photons going up and the other with photons down.

The results showed equal but opposite shift:
shift toward red (photon going up)
shift toward blue (photon going down)

Now if this presumed gravity shift has anything to do with cosmological shift, why would the results be the opposite change in shift?

Another problem with cosmological shift and the experiment is that the effect of cosmological shifts are so subtle the same shift would require distances of well over a million light years (which is quite a bit further) instead of 22.5 meters.

But Like you said, I don't get it, so I really shouldn't bother chiming in.  Perhaps *I* should go read it again.  ;)
« Last Edit: 25/03/2016 02:48:34 by JoeBrown »

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #81 on: 25/03/2016 03:07:09 »
Ok Joe - I now formally apologise to you for saying you have short legs.  That which seems obvious to me is clearly not as obvious to others as I had thought it might be!

Yes of course they measured both directions.  Yes redshift is light going away from Earth and blueshift towards the earth, as expected.  It is the 'time drift' that occurs, according to established physics, in the opposite direction.  Faster time away from the earth, and slower time towards the earth.

I personally do not see any difference between redshift occurring near a gravity field or in deep space, it's only that the wavelength will be longer in the weaker gravity field.

You do bring up a valid point about 'distance' though, this being the point I'm making.  If relativistic Doppler 'effect' can happen in a fixed distance of 22.5 metres, the extra, or lesser wavelength can, (in some way), be pegged directly to the gravitational field.  This would rather mean that estimated cosmological distances based on redshift and parallax method are then rendered questionable though!

I 'can' tell from your commentary that you are still not quite getting some of the basics, but so long as you aren't pointing "Doh's" at 'me' over it, I can forgive you... ;)
« Last Edit: 25/03/2016 03:15:56 by timey »

JoeBrown

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #82 on: 25/03/2016 04:11:06 »
I don't believe cosmological shift is directly related to distance.  To my knowledge, I'm in the minority, in this regard.

Its why I vehemently objected to gravity and doppler being termed the same.  They both relate to GR but are achieved by adjusting different variables.  My thesis depends on this.

While I don't wish my thesis to be proved false, I have yet to determine a method to prove it otherwise.

shameless retort replaced with shameless plug (: please review my hypothesis :)
http://www.thenakedscientists.com/forum/index.php?topic=66046.0
« Last Edit: 25/03/2016 04:38:19 by JoeBrown »

evan_au

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #83 on: 25/03/2016 04:43:52 »
Quote from: puppypower
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 most convenient way of driving a loudspeaker is with a sine wave, which has both negative and positive voltage excursions.

The loudspeaker does not passively fall back to the center position, but is actively accelerated through the center position, and out the other side.

So you can't talk about this experimentin terms of a "ground state" of the loudspeaker, or a "ground state" of the speed of light.

Quote from: Colin2B
rate of clock is slower deeper in the Grav field, which matches the blue shift
There may be a bit of confusion in various posts of the thread about whether the gravitational shift is a red shift or a blue shift.

As JoeBrown mentioned, they actually tried the experiment in both directions (up and down). As I read it:
• with the emitter at the base of the tower, and the detector at the top: In this case, the emitter (loudspeaker) must be moving upwards (towards the detector) for the gamma rays to be absorbed by the detector; the blue shift due to the Doppler effect exactly cancels the gravitational red shift.
• with the detector at the base of the tower, and the emitter at the top: In this case, the emitter (loudspeaker) must be moving upwards (away from the the detector) for the gamma rays to be absorbed by the detector; the red shift due to the Doppler effect exactly cancels the gravitational blue shift.

Mössbauer spectroscopy
The Pound–Rebka experiment made use of Mössbauer spectroscopy. I presume that this technique is normally performed in a horizontal direction, so that gravitational shift does not affect the results.

With 14keV gamma rays from Fe57, you need to have a relative velocity of about 10.6mm/s for maximum absorption, due to factors like the internal magnetic field of an iron nucleus (around 30 Tesla).

The additional gravitational shift due to a tower of 22.5 m is very small - the velocity only differs by 7.5x10-4 mm/s. No wonder they had to run the experiment in both directions, to double their signal-to-noise ratio.

evan_au

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #84 on: 25/03/2016 05:07:19 »
A Question: Why weren't the results of this experiment totally swamped by thermal vibrations of the iron atoms?

Wikipedia suggests that
Quote
Elastic waves of different lengths, frequencies, and amplitudes run through crystalline solids at all times. The typical order of the atomic vibrations frequencies is 1013 Hz, and that of the amplitudes is 10−11 m.

We could express one of these thermal vibrations as a distance displacement s=10-14*sin(2π*1013t) mm
Differentiating, this suggests a velocity of the iron atoms of around v=(2π/10) * cos(2π*1013t) mm/s, which has a peak velocity of around 0.6 mm/s.

How could they detect a difference of only 7.5x10-4 mm/s when the thermal noise floor is about a thousand times larger?

There is a picture of the apparatus here, but I don't see a way of cryogenically cooling the emitter or detector.

Colin2B

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #85 on: 25/03/2016 08:43:57 »
Quote from: Colin2B
rate of clock is slower deeper in the Grav field, which matches the blue shift
There may be a bit of confusion in various posts of the thread about whether the gravitational shift is a red shift or a blue shift.

As JoeBrown mentioned, they actually tried the experiment in both directions (up and down). As I read it:
• with the emitter at the base of the tower, and the detector at the top: In this case, the emitter (loudspeaker) must be moving upwards (towards the detector) for the gamma rays to be absorbed by the detector; the blue shift due to the Doppler effect exactly cancels the gravitational red shift.
• with the detector at the base of the tower, and the emitter at the top: In this case, the emitter (loudspeaker) must be moving upwards (away from the the detector) for the gamma rays to be absorbed by the detector; the red shift due to the Doppler effect exactly cancels the gravitational blue shift.
It was the 2nd case I was refering to, moving from weaker field at the top of the tower giving a blue shift. I was trying to show Timey, with the simplified eg I gave, that the time dilation between top and bottom of the tower can explain a blue shift, whereas she says it should give a redshift, and a faster rate of clock at the tower bottom would be necessary for a blue shift. I havent seen her calculation for this. (quote from Timey -"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.")

In the PR, light travels a fixed distance of 22.5 metres, at the speed of light.  A time variance has been added to the test signal via the vibration of the speaker. This constitutes relative motion.  The signal matches the frequency of the gravitational shift.  Does this not indicate that the relative motion of the signal is reflected in the gravitational shift?
No, there is no evidence that the gravitational shift contains any relative motion or doppler shift. This would be like saying a gravitational wave contains an interference pattern because that was used to measure it.

Not sure why you are asking that about the Wiki links.  The Doppler shift link mentions light and gravity.  The relativistic Doppler 'effect' link explains frequency and wavelength,
I mentioned them because you provided links to them and said "There is a Doppler shift in the gravitational shift of light.  The Pound Rebka proved it!  The wiki Doppler shift link states it!"
I have read both these links and although they do, as you say, mention light and gravity, nowhere does it state that there is a Doppler shift in the gravitational shift of light or that Pound Rebka proved it!  Indeed it provides  a link to gravitaional time dilation which describes the effect almost exactly as I did.

I don't feel I am confused by your terminolgy, as you seem to consistently link frequency shift to being a doppler effect only.
But I am confused as to how you calculate that time dilation should result in longer wavelengths. I've shown above how it doesn't and can do no more.

puppypower

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #86 on: 25/03/2016 11:08:41 »
Red and blue shift of energy due to the Doppler shift can also be explain with the concept of entropy. If entropy is defined as increased disorder, two references moving apart at velocity V, will increase entropy. Since an increase of entropy needs to absorb energy, the observed red shift reflects the movement toward lower potential energy quanta to satisfy the needs of the higher entropy; energy conservation.

When two references approach, they are moving toward a state of higher order and therefore lower entropy. The loss of entropy will give off energy potential, which is reflected in the blue shift, where photons appear to gain the energy potential. This is also due to energy conservation.

If we expand a gas, all the gas molecules will see higher entropy with the gas cooling; red shift in the IR. If we compress the gas, the entropy is reduced, energy is given off and the gas gets warmer; blue shifted in IR. If we focus on only two molecules either coming together or moving apart; doppler shift explanation, we will still get blue and red shift in the heat signature; IR.
« Last Edit: 25/03/2016 11:12:25 by puppypower »

agyejy

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #87 on: 25/03/2016 12:00:35 »
Red and blue shift of energy due to the Doppler shift can also be explain with the concept of entropy. If entropy is defined as increased disorder, two references moving apart at velocity V, will increase entropy. Since an increase of entropy needs to absorb energy, the observed red shift reflects the movement toward lower potential energy quanta to satisfy the needs of the higher entropy; energy conservation.

When two references approach, they are moving toward a state of higher order and therefore lower entropy. The loss of entropy will give off energy potential, which is reflected in the blue shift, where photons appear to gain the energy potential. This is also due to energy conservation.

If we expand a gas, all the gas molecules will see higher entropy with the gas cooling; red shift in the IR. If we compress the gas, the entropy is reduced, energy is given off and the gas gets warmer; blue shifted in IR. If we focus on only two molecules either coming together or moving apart; doppler shift explanation, we will still get blue and red shift in the heat signature; IR.

I don't think you really understand entropy all that well. Also you seem to be under the impression that a gas always cools when expanding and heats up when compressed. This is obviously not the case or we wouldn't have things like isothermal expansion or isothermal compression which are specifically expansion and compression at a constant temperature.

PmbNEP

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #88 on: 25/03/2016 12:54:33 »
Quote from: timey
Btw...are you Pmb Pete who used to post before?  If so, I'm glad to see you back.
Yep. That's me. I was unable to figure out how to login with my old account so I had to create a new one. Thanks for the warm welcome back.

Again, please take note that I simply don't know why they use Doppler shift. And I only trust textbooks and journal articles for thinks like that.

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #89 on: 25/03/2016 13:05:50 »
Evan - (Colin this also applies to you). I have no confusion over the direction of the time drift, or that redshift makes for longer waves travelling from bottom to top of tower, and blueshift shorter waves from top of tower to bottom.

You said:
""with the emitter at the base of the tower, and the detector at the top: In this case, the emitter (loudspeaker) must be moving upwards (towards the detector) for the gamma rays to be absorbed by the detector; the blue shift due to the Doppler effect exactly cancels the gravitational red shift.""

To be clear, it was the gamma rays that were emitted from the speaker and traveled upwards, the speaker did not move, apart from the movement of the fabric of the cone caused by vibration.

""for the gamma rays to be absorbed by the detector; the blue shift due to the Doppler effect exactly cancels the gravitational red shift.""

A man made Doppler shift was created via the vibration of the speaker constituting, as Pmb suggested, a time variance in the signal that exactly matched the Doppler 'effect'... of... what did you say there Evan?  That it cancels out the gravitational redshift?  There was no contrary signal of gamma rays being emitted from bottom of tower when they conducted the experiment from the top of tower, only the receiver was at the bottom of the tower at the time.  What the signal travelling from top of tower cancelled out was the Doppler 'effect' in the gravitational shift when the test signal exactly matched this naturally occurring Doppler 'effect'.  This being the relativistic Doppler 'effect' that light experiences in a gravitational field.
Same for the redshifted gamma rays from bottom of tower to top.
Yes - within the calculations, the top of tower and bottom of tower results were correlated with each other.

(Edit:  Evan - having re-read what you said here again, I find that you have indeed explained the situation correctly...although perhaps my commentary clears up any possible misinterpretation of moving speakers and contrarily 'emitted' signals...anyway, I apologise, because your commentary is correct.)

My point being that a relativistic Doppler 'effect' of the gravitational shift in light is NOT supposed to occur, unless the observer is moving towards the light source, or away from the light source, or the light source is moving away from the observer, or towards the observer, or both light source and receiver are moving relative to each other with a difference in relative velocity.
The Pound Rebka obtained the results of a relativistic Doppler 'effect' in the gravitational shift over a fixed distance of 22.5 meters!

My point about the time drift was, that IF the time drift were to have caused any relative motion within this fixed distance, as to then be the cause of 'relative motion' within the fixed distance of 22.5 metres, then as the time drift is contrary to the direction of the expansion and contraction of the waves, ie: redshift occurs travelling into a weaker gravity field, but in a weaker gravity field time runs faster, this - IF we were looking for 'relative motion' within the fixed distance, which of course a 'time variance' would provide us with - would in fact cause redshifted wavelength to become shorter.  ie: that it should take a shorter amount of time for the light to cover the same unit of distance.
The time variance in the time drift is also far too subtle to be replicated by the vibration of the speaker cone that is creating a time variance in the test signal.
Therefore we can rule out the factor of the time drift being responsible for the 'matching time variance' of the test signal found within the naturally occurring phenomenon being measured.

That the Doppler shift, time variated test signal matched the relativistic Doppler 'effect' of gravitational shift over a fixed distance constitutes an illogicality within the remit of GR.  Where, within the fixed distance of 22.5 metres, in space time are the extra or lesser lengths of wavelength existing.  Either distances expand or contract as per the Lorentz transformations, or they don't!!!
Within the Pound Rebka it would appear that they don't!!!

What is causing the 'relative motion' in light over this fixed distance???

(Colin - the fact that my concept of an inverted time dilation, for massless entities, within the remit of my Inverted Time Theory, exactly explains this 'time variance' matched in the naturally occurring phenomenon by the test signal, is of course what is drawing me to the premiss of this experiment)

P.S.  Pete - good, good!  As far as I'm concerned the forum has been feeling a little empty without your contributions.  I know we've had our little run in's, ;), but your knowledge and clarity of explanation is without question.
« Last Edit: 25/03/2016 18:13:35 by timey »

alancalverd

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #90 on: 27/03/2016 11:06:44 »

To be clear, it was the gamma rays that were emitted from the speaker and traveled upwards, the speaker did not move, apart from the movement of the fabric of the cone caused by vibration.

The cone fabric is irelevant. The gamma source was attached to the speaker core, so the source moved.

And since no part of the speaker can move faster than the speed of sound, the relativistic Doppler effect is irrelevant too - the result can be calculated adequately from the classical Doppler equations.

Keep it simple, chaps!
« Last Edit: 27/03/2016 11:10:42 by alancalverd »

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #91 on: 27/03/2016 13:28:10 »
Yes - but it is not 'sound' that is causing there to be a time variance of the emission of the gamma rays within the distance of 22.5 metres between the speaker and the receiver.  It is the motion of the speaker cone that the gamma ray emitter is mounted upon that is doing this.  It's the back and forth vibration that moves the light source back and forth, not the sound itself.

The point being though, Alan, is that a natural relativistic Doppler 'effect' found within a fixed distance matched this time variated signal exactly!

Therefore:
Do distances expand and contract via the Lorentz transformations or not?
« Last Edit: 27/03/2016 18:09:15 by timey »

evan_au

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #92 on: 27/03/2016 23:27:48 »
Quote from: Timey
Do distances expand and contract via the Lorentz transformations or not?
Yes, distances are affected noticeably at relativistic speeds (eg velocity > c/10).

But the speeds in this experiment were around 11mm/s, ie velocity < c/109, which is nowhere near fast enough for relativistic Doppler effects to be visible. The non-relativistic Doppler effect will be sufficient to interpret the effect of the loudspeaker in the Pound-Rebka experiment.

So this non-relativistic Doppler shift was enough to cancel the relativistic Gravitational shift.

Quote
Doppler 'effect' of the gravitational shift in light
- Gravitational shift is a frequency shift.
- Doppler shift is a frequency shift.
- But Gravitational shift is not a Doppler shift.
- This description is similar to the logic which says "1 is a number; 2 is a number; therefore 1=2."

alancalverd

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #93 on: 28/03/2016 00:03:38 »
The point being though, Alan, is that a natural relativistic Doppler 'effect' found within a fixed distance matched this time variated signal exactly!
No, a thousand times no.

Gravitational redshift is NOT Doppler shift.

For the last time before I lose all patience with you, they measured gravitational shift  by comparing it with a known Doppler shift which they produced by moving the detector at a known speed. That's all.

alancalverd

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #94 on: 28/03/2016 00:11:57 »
A Question: Why weren't the results of this experiment totally swamped by thermal vibrations of the iron atoms?
Because thermal vibration is random. It's fairly easy to extract a signal of known phase and frequency from random noise.

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #95 on: 28/03/2016 00:37:20 »
Evan - good, your description is very concise!

So... Where within the gravitational shift measured is the relative motion of the test signal replicated?  In other words, where in space time are the extra, or lesser lengths of wave located?

If they are located within the fixed distance of 22.5 metres, this means that either the speed of light is not constant over the naturally occurring variance of the gravitational shift, or there is a time variance within the gravitational shift matching the time variated test signal... (The gravitational shift being the cause of the experiments observed redshift, blueshift results, not the test signal.)

Alan - yes... Agreed!!!  And I don't see why you keep banging on about the fact.  I'm saying that a Doppler shift was used to identify something within a fixed distance of gravitational shift that 'resembled' a Doppler shift.  It had to 'resemble' a Doppler shift because a Doppler shift was used to measure it... ???

alancalverd

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #96 on: 28/03/2016 00:48:27 »
If they are located within the fixed distance of 22.5 metres, this means that either the speed of light is not constant over the naturally occurring variance of the gravitational shift, or there is a time variance within the gravitational shift matching the time variated test signal

Or that photons demonstrate a blueshift on moving through a gravitational gradient, which is what everyone else thinks.

I'm not a great proponent of consensus in science, but this does seem to be the simplest description of what happens.

timey

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #97 on: 28/03/2016 01:15:43 »
Well yes of course everyone thinks that Alan, including me... but are you not at all curious as to how photons do this?
Does it not raise any questioning in you as to how photons can do this within a fixed distance of gradient in the gravitational field?
Surely over a fixed distance there would be no need to add a time variance to the test signal in order to measure the gravitational shift...why would it not work by just emitting the gamma rays from a static position?

alancalverd

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #98 on: 28/03/2016 09:42:20 »

The Mossbauer effect has an amazingly narrow bandwidth. You can demonstrate nuclear resonant fluorescence if the source and detector are fixed in a horizontal plane, but the effect disappears if either moves at more than about 1 cm/second with respect to the other. This is a classical resonance phenomenon involving conservation of energy and momentum, nothing to do with Doppler or gravitation.

Now when the P-R experimental plane was vertical, the incoming signal was red or blue shifted so could not produce resonance in a stationary receiver - they had to move the source or receiver to compensate for the gravitational shift.

One advantage of using a clean sinusoidal movement such as a loudspeaker coil is that you can phase-lock your detector to the source motion and thus infer the speed at which the source is moving when the detector resonates. Remember that if displacement = sin ωt, then velocity = cos ωt at any given time t.

www.tau.ac.il/~lab3/MOSSBAUER/Literature/Mossbauer_Harvard.pdf describes a standard teaching experiment and is well worth reading.

evan_au

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #99 on: 28/03/2016 10:07:28 »
Quote from: Timey
where in space time are the extra, or lesser lengths of wave located?
You have to observe the experiment from some frame of reference - preferably one which is static relative to the surface of the Earth (or your own acceleration will further confuse the discussion).

Probably the most useful frame of reference is adjacent to the detector at the top of the tower, with the emitter at the bottom of the tower.
• The detector sees gamma rays arriving from the bottom of the tower.
• The frequency of the gamma rays varies sinusoidally at a frequency (say 50Hz), determined by the audio frequency fed into the speaker
• Most of the time (including the entire downwards half-cycle), the gamma ray frequency is below the resonance frequency, and go straight through the detector, firing the scintillation counter.
• Some  of the time during the upwards stroke, the gamma ray frequency is above the resonance frequency, and go straight through the detector, firing the scintillation counter.
• At two moments in each cycle of the audio feeding into the speaker, while the speaker is moving upwards at about 10.6258 mm/second, the gamma rays will have just the right frequency to be absorbed by Fe57 in the detector. For these moments, the output of the scintillation counter drops, because more of the gamma rays are being absorbed.
• At these moments, the Doppler shift and gravitational shift cancel, and you get the same result as if you conducted the experiment horizontally.
Gamma Rays emitted near the highest point of the speaker cone travel many wavelengths less distance than gamma rays emitted near the lowest point of the speaker cone. But the effect of gravitational shift, which happens over 22.5m dwarfs the effect of the speaker moment (which is probably 0.01m).

(I hope this helps - just reverse the logic if you want to consider gamma rays emitted at the top and absorbed at the base of the tower.)

Quote
why would it not work by just emitting the gamma rays from a static position?
There is a very narrow window of frequencies/wavelengths that an Fe57 nucleus will emit or absorb. You have to get the frequency just right (to within ≈1011), or it will go straight through the detector plate and hit the scintillation counter behind it.

The nucleus which emits the gamma ray is in a metal matrix with other metal atoms, but there is still a microscopic recoil which loses some energy from the photon. There are other effects in the emitting and absorbing nucleus that means you need to have the emitter approaching the detector at 10.6258 mm/second to be absorbed. And that is if you are doing the experiment in a horizontal orientation.

Repeating the experiment in a vertical orientation produces an additional tiny gravitational frequency shift, which is what Pound & Rebka were trying to measure.

See: https://en.wikipedia.org/wiki/M%C3%B6ssbauer_spectroscopy#57Fe_M.C3.B6ssbauer_spectroscopy

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Re: Why do photons have the same speed regardless of the velocity of their source?
« Reply #99 on: 28/03/2016 10:07:28 »