Re: "The Impossibility of Measuring the Velocity of Light"

Tom Roberts wrote:
John Kennaugh wrote:
Tom Roberts wrote: 
There are A LOT more, referenced in the FAQ:
  http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
Maxwell's ether theory predicted that c is the speed of light relative
to the ether. Because of this the speed of the source would not effect
it but the speed of the observer would. This latter prediction proved
false. We tend to attribute that to the MMX but that is really shorthand
for a number of experiments which all said the same thing.

Yes. In 1905 there were about a dozen such experiments; today there are hundreds. 

No doubt.

Following MMX there were choices as to which way to go. The two main
ones being:
Choice 1 - Assume that there is no ether. If no ether then c cannot be
the speed relative to the ether leaving two options c = speed relative
to the source or c = speed relative to the observer.

Arguing by exhaustive enumeration nearly always fails. 

Define 'exhaustive enumeration'?
Please point out the fallacy in my argument.

You failed to consider a third possibility: the speed of light is determined geometrically, as in SR and GR.
This makes the propagation of light be independent of both source and observer.

You really should try and understand the difference between maths and physics. Light is energy. Energy is generated in the source by a process which is at least partially understood. It travels through space and is detected by a physical process which has some similarity to the photoelectric effect. While in transit energy exists in space. Can Geometry store energy? Of course not.

Maths is a man made description of a process in quantifiable terms. It describes physical things taking part in physical interactions as the result of causality. What is there physically is not there because the maths says so. Light does not travel from A to B because of Maxwell's equations. No one ever got sunburnt by looking at a differential equation.

Sometimes equations are not the easiest way to make a mathematical description accessible and one may wish to present it in diagrammatic form. Doing so does not change the nature of the maths you are displaying and it certainly has no effect on the physical processes the maths is describing. Nature can cope quite happily without our maths be they equations or diagrams.

Geometry is the mathematics of shape. If I draw a map of a country then that is a two dimensional representation of something highly complex. It is certainly more informative to the human brain than a list of co-ordinates describing the same thing. If I wished to demonstrate population density I could multiply that by a fixed distance and add that to my map as 'height' to give a 3 dimensional model. No one would be daft enough to suggest that population density was in some way controlled by my diagram.

Einstein produced his theory. *Because* of the second postulate which assumed that the speed of light is the same in every FoR, Minkowski was able to multiply time by this fixed velocity and so change the units of time into length so as to put it onto a diagram with spatial dimensions much as in my example population density was multiplied by distance in order to add it to the existing (in my case 2) dimensions to produce a diagram. Just as that process in no way changed the nature of the data so Minkowski's diagrammatical representation of Einstein's maths in no way altered the nature of them. Changing the name to 'space - time geometry' does not alter anything either. It is hardly surprising that so called 'space - time geometry' shows light travelling in every FoR at c because it was constructed assuming that.

 A criticism I would make is firstly he describes experiments which
legitimately disprove Maxwell's prediction but then he implies that this
is evidence of SR as if the choice is between Maxwell's ether theory and
SR rather than the choice between source dependent and source
independent theories.

You do not understand science. If the prediction of the theory agrees with the experimental observation, then the experiment is said to support the theory (in the sense that it agrees with the theory's predictions for its results). This is not a comparison between two (or a limited set) of theories, this is merely affirmation that these experiments support (i.e. do not refute) SR.

You and I clearly have a different definition of the phrase "experimental basis". To me it implies those experiments on which the theory is based.

What do you give as the experimental basis of the second postulate?
I'll help you because I have studied the history and it clearly bores you.

1/ Light is a wave travelling in the ether (Maxwell).
2/ Therefore the MMX indicates every observer is stationary w.r.t the
ether.
3/ If every observer is stationary w.r.t the ether then any ray of light
moves in the 'stationary' system of co-ordinates with the determined
velocity c, whether the ray be emitted by a stationary or by a moving
body.

But that takes only one specific interpretation of the MMX so you should
explain why the alternative interpretation of the MMX, that there is no ether and light is therefore source dependent was justifiably rejected. I can't help you on that one I'm afraid.

" Bradley (1727) discovered that the images of stars move in small
ellipses. This is explained as aberration due to the earth's motion
around the sun. This is inconsistent with a simple model of light as
waves in an aether which is dragged along by the earth; it is consistent
with SR."
In fact Bradley assumed the speed of light was constant w.r.t the source
and got the first really accurate value of the speed of light. So what
Tom is implying is that because it can be interpreted to be consistent
with relativity it supports relativity while in fact the simplest
interpretation is source dependent theory. A case of 'publication bias'.

This is not "publication bias", this is your lack of understanding of how science works.
The observation is in agreement with the prediction of SR
-- that is _ALL_ that matters when quoting this experiment as support for SR.
That is, any other interpretations of the result are irrelevant (they would be relevant to support for those other theories, but that is not the issue _here_).

There is a difference between 'support' and 'is consistent with' - the former suggests that it is 'in favour' of something i.e. it is the simplest, least convoluted explanation, 'is consistent with' simply that it does not rule it out.

In this particular case you say "This is inconsistent with a simple model of light as waves in an aether which is dragged along by the earth; it is consistent with SR."

Which gives the lie to your statement. You are implying that it is 'supports' relativity and is 'inconsistent' with another theory and have been highly selective about which theory you put it up against. You are trying to give a false impression. Bradley assumed source dependence and that is the simplest, least convoluted explanation. It therefore 'supports' Ballistic theory but 'is consistent with' SR.

"Round-Trip Tests of Light-Speed Isotropy" do not test SR against Ritz's
theory, they are testing the consistency of c.

The webpage is not attempting to test all possible theories, it's title is "What is the experimental basis of Special Relativity?", and that defines its scope. <shrug> 

'Basis' is misleading IMO.

[comments on Alvaeger et al omitted, as you don't have a clue...]

I suspect that you have only studied the abstract of what it is supposed to show. Snipping rather than answering criticism is an old trick which fools no one.

4. Tests of Time Dilation and Transverse Doppler Effect.
"The Doppler effect is the observed variation in frequency of a source
when it is observed by a detector that is moving relative to the source.
This effect is most pronounced when the source is moving directly toward
or away from the detector, and in pre-relativity physics its value was
zero for transverse motion (motion perpendicular to the source-detector
line). In SR there is a non-zero Doppler effect for transverse motion,
due to the relative time dilation of the source as seen by the detector.
Measurements of Doppler shifts for sources moving with velocities
approaching c can test the validity of SR's prediction for such
observations, which differs significantly from classical predictions;
the experiments support SR and are in complete disagreement with
non-relativistic predictions".
Sorry Tom but that is rubbish. They are completely consistent with
source dependent theory.

Your analysis is wrong, as your situation is not transverse. 

Source dependent theory description - Speed of light c w.r.t the
source:
    S ->v ------------------X---X'--------
                            |
                            |y
                            |
                            |
                            O
With ballistic theory a flash at X will expand in a circle which will
remain centred on the source. When the light reaches O the centre of the
circle has reached X'.

And the observer will ascribe to it a direction X'->O which is not transverse to v. For this ballistic theory, to make a transverse measurement, the light must be emitted ahead of X, and reaches O when the center of the circle is at X. O sees no Doppler shift for this transverse situation.

Stated differently: for light emitted far to the left, O will observe blueshift; for light emitted far to the right, O will observe redshift; all theories with any chance of being valid predict this. For what direction does O observe the zero that comes between them? SR predicts the direction will be from slightly to the left of X (by an amount that depends on v), and this ballistic theory predicts the direction will be directly from X (independent of v). Note that O is observing both red/blue-shift AND the direction of the light. Actual observations confirm SR and refute this ballistic theory. 

Have you not come across aberration?



Note that the affect of aberration is that the apparent direction of the
light is XO not X'O as shown below.

You are in essence "double counting". The direction that O observes for the light wave is perpendicular to the wavefront (circle) at O, and for your case that is X'->O, not X->O. At the instant this circular wavefront reaches O, the direction of motion of the wavefront that O observes is moving perpendicularly to the circle, as observed locally by O.

If you do the experiment and the light passes through an aperture when it is orthogonal to the observer then whichever theory you use you will measure the same frequency. There is no aberration if you assume SR because the aperture is in the FoR of the observer so you point the telescope at the aperture.

There is aberration in the ballistic case which means you point the telescope at an angle from the true direction. It turns out that you point it directly at the aperture. If you study the diagrams WHICH YOU SNIPPED you will see that is the case. You cannot construct an experiment which measures transverse Doppler which can distinguish between the two theories.

5. Tests of the "Twin Paradox"
Haefele and Keating, - Another experiment which got through 'peer
review' and is flawed.

You read the idiots too much.

I quote from a detailed analysis by Dr Scott Murray. I can email you a copy is you wish to study it.

The criticisms about H&K "modifying" their data are baseless (c.f. another ongoing thread in this newsgroup). Those criticisms did not get through peer review at all.

I did not claim they 'modified' their data. They corrected their data for gravitational potential using the wrong value of h in the formula. According to Cocke they should have used the height above a notional sphere of polar radius while in fact they used height above sea level. Murray points out that the formula THEY USED says that if they left two planes at sea lever at two different latitudes the clocks would drift apart. i.e. if you make h = 0 and u = 0 then (delta t)/t is proportional to (Cos (angle of latitude)) rather than 0 which is what Cocke's way of specifying height gives.

There are two things which conflict. 

Only due to your lack of understanding.

Einstein predicted that a clock on the equator would go slower than a
clock at the pole. Implication is that two clocks at different latitudes
will not keep time. Experiment shows that two clocks at sea level keep
the same time independent of latitude.

Yes. Einstein was considering clocks on the surface of a sphere,

Rubbish. He gave it as an example of 'any closed path'. It is true to say that when he made that prediction he had not formulated GR and was not taking any account of gravitational effects. I have not studied the literature sufficiently to see whether he subsequently corrected it himself but I am NOT suggesting that he be held to account for his earlier prediction without correcting it in light of his subsequent theory.

and mean sea level is not a sphere, and the difference accounts for this exactly. This is well known and unremarkable. <shrug>


Cocke explained this [...]

I don't know who Cocke is,

He was the chap who explained why Einstein's prediction appeared not to be true.

Footnote in Hafele & Keating, Op.cit.(4): p.168 (9). says:

"Clocks at rest on the earth's surface (at average sea level) keep the same relativistic time independently of latitude differences. The effect of the difference in surface speed at different latitudes is cancelled to lowest order by a corresponding effect from the difference in surface potential owing to the oblate figure of the earth."

The reference they give is "Cocke. W.J. Relativistic corrections for terrestrial clock synchronisation; Phas.Rw.Lett. vol.l6, 1966"
Murray followed up all references in the H&K paper.

but that is essentially the standard analysis in GR, using the usual approximation for this physical situation.

It may be so NOW but in 1971 when the H&K experiment was done it was a recent 'clarification' of GR only having been published 5 years earlier.
It was only when it started to become clear that clocks at mean sea level do not behave as Einstein predicted that anyone tried to come up with a reason why. I suspect, although admittedly I cannot prove, that gravitational potential was 'clarified' if not redefined. Prior to Cocke I am sure that everyone would have assumed that mean sea level is an equipotential surface both for water and clocks and DID expect Einstein's prediction to be correct.

H & K did their corrections for height using the height above sea level
[...] [if] H&K were correct to take
height above an equi-potential surface [then]
the fact that clocks at sea level keep time is
a violation of SR.

You are confused.

No H&K were confused. They accepted Cocke's explanation of why clocks at sea level kept time but they failed to take on board that his definition of height was not height above sea level. They used height above sea level while Cocke showed that in order to get the right answer - clocks at sea level staying synchronised - you must calculate gravitational potential using height above a notional sphere not height above sea level.

The fact that they seem to have got the 'right' answer having used the wrong equation to correct the data may be seen by some as indicating that they must have 'modified' or been selective with their data.

SR is IRRELEVANT for this, as the point at issue is the gravitational effect. Clocks at rest on the geoid remain in sync -- this is well known and indisputable, 

Quite so.

and is fully consistent with GR. H&K used an old-fashoned description that separates the computation of GR into "two effects" which they call "SR effect" and "GR effect"

They were trying to demonstrate 'time dilation' an SR effect. They had to correct their data for GR effects. That may be an old fashioned or even wrong way of looking at it - I do not know. The fact is they used the wrong formula in their GR correction.

(meaning the term due to speed and the term due to altitude; in this approximation the GR computation separates into these two terms for this physical situation). Altitude above mean sea level is the correct altitude to use in that correction (so altitude=0 and v=0 give a correction of zero for a clock at rest on the geoid).

And as Murray points out if you set altitude = 0 and v = 0 in the formula THEY used (delta t)/t is proportional to Cos(angle of latitude)
not 0.

Murray's article is interesting historically as at that point there were still many aspects of SR and GR which had not been resolved from an interpretation point of view. Things which you think of as 'standard' today had to be resolved.

For example take Einstein's prediction - Dingle asked "What entitled Einstein to conclude from his [special relativity] theory, that the equatorial, and not the polar clock worked the more slowly?" Today you may see that as a silly question because there is a standard way of looking at it but at the time Dingle asked, there were several widely different answers. When H&K did their experiment there was no standard answer so they chose the one which was gaining acceptance. They decided to accept a view put forward in : Builder, G. Ether and relativity, Aust..l. Phys. vol. 11, 1958.

"It is important to emphasise that special relativity purports to describe certain physical phenomena only relative to (or from the point of view of) inertial reference systems, and the speed of a clock relative to one of these systems determines its timekeeping behaviour (G. Builder, 1958)."

Murray writes "[Builders Paper] proposes a re-interpretation of one aspect of special relativity which it suggests has been universally misunderstood"

You would now consider that interpretation as standard and falsely assume it has always been so. H&K were in effect basing their experiment on what then was Builder's version of relativity.

Murray followed up that paper and although that bit of Builders SR theory is now standard Murray notes that in the same paper Builder argues:

"Thus we conclude that the relative retardation of clocks predicted by the restricted theory does indeed compel us to recognise the causal significance of absolute velocities."

--
John Kennaugh
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