Re: A little challenge for relativists.

shevek wrote:

John Kennaugh wrote: 
Your posting is full of assertions but you attempt to make no case nor
to fault my previous detailed arguments.


If light is source independent it is because some physical process makes
it so. Maxwell's Equations may be consistent with that physical process
but light is not source independent because Maxwell's equations say so.
You may wish to divorce physical process from the mathematics in order
to preserve your belief system but I see no evidence that Einstein did.

Maxwell believed he had found equations which represented light
travelling in the ether. If you decide there is no ether then you have
to decide what, if anything, those equations are describing. Without an
ether they can only be describing light leaving the source at c relative
to the source. Without an ether they cannot be describing light
travelling w.r.t the observer at c - the mathematical alternative. I say
that for two reasons:


I hate to be the bearer of bad news but you may have to acknowledge
that fluids and fields, massless or not, can go by names other than
"ether".  Without an 'ether' we can use Maxwell's equations to show how
the elctromagnetic 'field' behaves.  Language marches on.


A 'field' could be one of 3 things (maybe you can think of more).

1/ If you consider a cube of space and a short flash of light entering that cube and a short time later exiting it, then in the mean time that cube contains energy. The ether explanation is that all space contains something which can store energy in the form of stress, Maxwell implied that that stress is electrical in nature. A 'field' could therefore be a real physical stress in the ether and light is when that stress propagates through the ether.

2/ The ether was originally proposed (before its use to explain light) to explain 'action at a distance' i.e. a magnet induces stress in the ether that stress moves the pin. One might question whether in view of modern knowledge 'action at a distance' needs an explanation. There is a case to be made which says that it is our intuition that is wrong, that all force ultimately acts 'at a distance' and that we should ditch our intuition and accept that that is the way force acts. If we do this then there is no need for the ether but a field is then no longer physical but metaphysical. It maps out the action at a distance force which *would* occur at a point *IF* there was a charge placed at that point. The problem with this model is that there is no way a 'field' can propagate because it is not physical and light is, or carries with it real physical energy. Thus light might be mathematically modelled in terms of fields but really I see no alternative to simply assuming that light consists of particles shot out by the source and I can see no reason why the speed at which they are shot out should not be dependent solely on the fundamental process shooting them out. This leads to c being the speed relative to the source.

3/ A field may be a physically real emanation. It is certainly more believable than the ether. Being real it might be able to break away and shoot off into space carrying energy with it = light. You could interpret Maxwell's equations as saying that there are two sorts of such real fields and that when they propagate one is transposed into the other and back again and the speed at which this progression takes place is c. In this instance the speed of light is not dependent upon the physical processes taking place in the source but the source is the nevertheless the start of that progression so it will progress away from the source at c relative to the source as there is no other reference point - again this would result in source dependency.

The problem I have is that it always comes back to the fact that if one assumes source independence one is forced to assume an ether of some sort. You do not get rid of the ether by renaming it a 'field' or 'space' or 'an observers FoR' nor by banning the use of the word.
If the speed of light is not dependent on the speed of the source it must be dependent on 'something else' and that 'something else' must take control the moment light leaves the source.

They also implied that the speed of light would be dependent upon the
speed of the observer - that is what the MMX was all about. If you
interpret MMX as showing there to be no ether (which is not what either
Lorentz or Einstein did) then you have to decide what it is that ME are
physically describing before you can use them again. As I point out,
without an ether, ME must be describing light leaving the source at c.

The only physical model which will give you source independence is the
ether. If the speed of light is not dependent upon the source, either in
terms of the physical processes taking place in the source, or simply as
a reference point from which a natural progression starts to take place
at constant speed - then it must be dependent upon something else and
that something else must take charge of it the moment it leaves the
source. That 'something else' is called the ether and it has to be
physical as it has to take part in a physical process.  It requires a
physical process which Maxwell's equations may (or may not) model within
their sphere of applicability.



I agree - replace the word 'ether' with 'electromagnetic field' and
you'll get a lot more agreement as well without really changing the
content.


See above.


[...]

While Lorentz's theory gave symmetry in the system of experience
Einstein was not happy with that and tried to remove the asymmetry in
the theoretical structure of Lorentz's theory. The difference between
the two theories is that while Lorentz says it *appears* to every
observer that he is stationary w.r.t the ether Einstein said "Lets just
assume every observer IS stationary w.r.t the ether".



Not really.  They weren't talking here about the same 'ether' that you
were talking about before, the electromagnetic field.  The difference
is subtle; here they are talking about coordinate systems and how to
implement them.

They weren't talking about the same ether as each other certainly
but my 'definition' of what I am talking about when I talk about the ether is simply whatever it is which provides the physical process which wrests control of the speed of light from any influence the source may have - which covers both a traditional Maxwell type ether and Einstein's ether without the immobility of Lorentz's.

The clearest statement of his justification for the second postulate
comes in 1938

"Light is a propagated wave propagated by a medium called the Aether.
The velocity of a wave is a function of the medium which propagates it
and its velocity can only be effected by the source if the movement of
the source causes movement of the medium. Aether drag experiments,
passing light close to heavy rotating flywheels has shown that they had
no effect on the light passing close to them hence the speed of light
cannot be effected by the speed of the source.

Although the speed of light might be expected to vary with the speed
of the observer Michelson and Morley had shown that not to be the case
so it is a strange but indisputable fact that the velocity of light is
constant independent of the velocity of the source or the observer."
Infeld/Einstein  - The Evolution of Physics. 1938

Later in the book he suggests, possible as a sop to people like you who
hate the E-word that perhaps something which cannot be detected cannot
be said to exist but this is false logic. It is only our speed relative
to the ether which is undetected. Having justified source independence
as being a property of the ether any experiment showing source
independence has confirmed the existence of the ether.


Standards of detection, direct and indirect, change as people become
more comfortable with concepts.


>> "The speed of light is always constant w.r.t the source emitting it"
>>  is completely consistent with the PoR and had not been ruled out by
>> experiment.
>
>It could be consistent with the PoR, but not with the M.E.

It is consistent with the PoR and to get Maxwell's equations to describe
light travelling away from the source at c relative to the source all
you have to do is assume that the source is stationary w.r.t Maxwell's
ether.


Or that that the source is movnig w.r.t. Maxwell's ether, and clocks
and meter sticks change their tick rates and lengths when at motion in
the ether according to the Lorentz transformations.


>
>       Just because Einstein used the word "ether" in one paper
>       IN A FORUM ON ETHER does not mean that GR is an ether theory.
>       Moreover, in that quote he explicitly states that what he
>       called "ether" has essentially no relationship to the usual
>       meaning of the word (in particular, it is not possible to
>       measure "speed" relative to what he called "ether").

The emphasis tries to add a bit of spin 

Perhaps in more ways than you intended..

but that statement is totally
consistent with my understanding. Einstein was after a different sort of
ether which did not imply a unique FoR stationary w.r.t it but it still
needs to provide a physical process to wrest control of the speed of
light from any influence the physical processes taking place in the
source would otherwise have.



He was also after a consistent idea of what we mean by extension and
duration.

Until he came along we thought we knew. Universal time and fixed distance were axioms of physics. It is only if you want the PoR AND want to retain the idea of source independence that you have to ditch the axioms and redefine time and space. As he himself said source independence is "...apparently irreconcilable" with the PoR.

[...]

> IOW: the only viable ether theories known today are those that are
>experimentally indistinguishable from SR/GR.

Quite so. The only physical theories consistent with the mathematical
models of SR/GR are ether theories.

What physical process makes the speed of light constant w.r.t the
observer observing it?



The physical process is the observer defining his system of units,
defining what he means by "extension" and "duration".

You'll have to explain what you mean. It sounds to me as if you are reversing cause and effect.

That is a valid 'physics' question.

"the speed of light is constant in the observers FoR" is a description
of the maths and a re-statement of the second postulate. The observers
FoR is a mathematical abstraction mapping out real physical space. What
is it in real physical space which provides the properties assumed in
that mathematical description?


The light waves propagate through something that allows propagation. 

define 'propagation' - it means different things to different people.

From there we can build up the mathematical description of a 
relativistic/modern coordinate system - you don't need to know much
more about the medium for SR, until you start looking at E&M, quantum,
or GR that is.




>> I would be less reluctant to accept the Maxwell/Lorentz/Einstein
>>ether  theory if I was certain that the alternative no ether, source
>>dependent  option had been properly evaluated.
>
>It has been tried and found wanting.

Please explain. Note that the success of a different theory does not
rule out the possibility that this theory would not be equally
successful given the necessary resources to develop it. Waldren thought
so. The success of relativity could be put down to the fact that that is
where all the effort has gone.



Do you think a source-dependent SOL / corpuscular theory still survives
the experimental record?  (see link at end)


I will have a look and make a separate posting this one is long enough

--
John Kennaugh
to email convert the number from hex to decimal
.

Relevant Pages

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