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Sep 1, 2021, 3:27:09 PM9/1/21

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The original discussion on the question has become muddled by side

paths, so I'll start again.

The assertion was that you can at least in principle use laboratory

measurements of the speed of light to see if it varies.

To see that you can't you need to have at least a vague idea

of how such measurements are done.

A) you build a stable light source.

B) you set up a fixed resonator for it to create a standing wave.

C) using the tricks of the trade you determine

how many wavelength there are in it.

D) idem, and far more difficult, you measure the frequency

of your light source, wrt to an atomic clock.

(frequency dividing, multiplexing, counting etc. very hard)

E) Knowing wavelength and frequency give you speed of light.

All this is the end point of a long evolution.

With a very broad brush:

Starting in the 19th century,

physicists were completely ignorant of the structure of matter,

so units such as platinum bars with scratches

were accepted without further thought.

The speed of light was something you measured with rulers and clocks.

By the end of the 19th people like Rowland

started optical precision measurements,

using interferometry and spectral lines with calibrated wavelengths.

Ultimately all modern precision manufacture came to depend on it.

This soon raised a problem:

the precision of wavelength calibrations inceased to the point

where it came to be limited by the precision to which meter rods

could be reproduced at the site where the wavelength measurements

were done.

So the next step was obvious and inevitable:

the meter was redefined in terms of a suitable stable wavelength,

and the metal bars and blocks became secondary standards.

Next came the precision speed of light measurements, see above.

Again, the same problem arose:

the precision of the light speed measurement was limited

by the accuracy to which that standard wavelength could be reproduced.

So, again, the meter was redefined

now in terms of the frequency of the light source

and a defined value for the speed of light.

This eliminated the meter completely as a fundamental unit,

and all measurements of distance and size

were reduced to measurements of time intervals.

What used to be a measurement of the speed of light

now became a calibration of a standard wavelength,

so of a secondary meter standard.

So the speed of light has dropped out of the story.

If it were to change,

all -measured- lengths of all objects would change in the same way,

and by Alice, this would be unobservable.

Next comes the very good question:

in how far does this capture physical reality?

After all, we can invent other length units.

Would they all vary in the same way? Can we test this?

Going back in history,

the only other length unit that is reproducible enough

for comparison is the metal bar meter.

Supposing some fundemental things are time dependent,

would the platinum meter change in the same way

as the optically defined meter?

(or as I joked several times already,

since precision manufacturing uses optical standards,

would yesterdays pistons fit tomorrows engines)

More practical, in the lab,

would a standing wave that fits an optical resonator

set on a metal frame or granite block go on fitting it forever?

If not, we have a new effect, but what is it,

and how would we interpreted it?

I'll explain in a #2 that the problem could not lie

with the frequency, hence with the clocks,

so we must look at the length units.

Fortuantely, a hundred years of progress

has given us an understanding of the structure of matter,

so we understand our units, at least in principle.

For optical wavelengths the scale is set by the Rydberg unit

(energy, inverse wavelength, frequency)

In crap-free units the Rydberg wavelength is 1/ \alpha^2 m_{electron}

(with a whole slew of higher order corrections)

For dimensions of material objects otoh

the scale is the Bohr radius,

which is (again crap-free) 1/\alpha m_{electron}.

(again with a whole slew of higher order corrections)

So they differ to lowest order by a factor of \alpha.

Now, after all these preliminaries, we can deal with Helbig's question.

The meaningless question: can the measured speed of light vary?

becomes a meaningful question if we rephrase it as:

if things in the universe are variable,

would all possible length units vary in the same way?

(note that this transforms a meaningless question

about dimensioned things into a meaninful question

about dimensionless ratios between units)

From the arguments above the answer is yes,

and we could in principle observe such an effect in the lab.

OTOH there is no way that we would interpret such an observation

as a variable speed of light, supposing we would know what that means.

The prime suspect will be \alpha. (but it might be something higher up)

And for Philip: I hope that this provides the explanation you asked for,

Jan

(about time in #2)

paths, so I'll start again.

The assertion was that you can at least in principle use laboratory

measurements of the speed of light to see if it varies.

To see that you can't you need to have at least a vague idea

of how such measurements are done.

A) you build a stable light source.

B) you set up a fixed resonator for it to create a standing wave.

C) using the tricks of the trade you determine

how many wavelength there are in it.

D) idem, and far more difficult, you measure the frequency

of your light source, wrt to an atomic clock.

(frequency dividing, multiplexing, counting etc. very hard)

E) Knowing wavelength and frequency give you speed of light.

All this is the end point of a long evolution.

With a very broad brush:

Starting in the 19th century,

physicists were completely ignorant of the structure of matter,

so units such as platinum bars with scratches

were accepted without further thought.

The speed of light was something you measured with rulers and clocks.

By the end of the 19th people like Rowland

started optical precision measurements,

using interferometry and spectral lines with calibrated wavelengths.

Ultimately all modern precision manufacture came to depend on it.

This soon raised a problem:

the precision of wavelength calibrations inceased to the point

where it came to be limited by the precision to which meter rods

could be reproduced at the site where the wavelength measurements

were done.

So the next step was obvious and inevitable:

the meter was redefined in terms of a suitable stable wavelength,

and the metal bars and blocks became secondary standards.

Next came the precision speed of light measurements, see above.

Again, the same problem arose:

the precision of the light speed measurement was limited

by the accuracy to which that standard wavelength could be reproduced.

So, again, the meter was redefined

now in terms of the frequency of the light source

and a defined value for the speed of light.

This eliminated the meter completely as a fundamental unit,

and all measurements of distance and size

were reduced to measurements of time intervals.

What used to be a measurement of the speed of light

now became a calibration of a standard wavelength,

so of a secondary meter standard.

So the speed of light has dropped out of the story.

If it were to change,

all -measured- lengths of all objects would change in the same way,

and by Alice, this would be unobservable.

Next comes the very good question:

in how far does this capture physical reality?

After all, we can invent other length units.

Would they all vary in the same way? Can we test this?

Going back in history,

the only other length unit that is reproducible enough

for comparison is the metal bar meter.

Supposing some fundemental things are time dependent,

would the platinum meter change in the same way

as the optically defined meter?

(or as I joked several times already,

since precision manufacturing uses optical standards,

would yesterdays pistons fit tomorrows engines)

More practical, in the lab,

would a standing wave that fits an optical resonator

set on a metal frame or granite block go on fitting it forever?

If not, we have a new effect, but what is it,

and how would we interpreted it?

I'll explain in a #2 that the problem could not lie

with the frequency, hence with the clocks,

so we must look at the length units.

Fortuantely, a hundred years of progress

has given us an understanding of the structure of matter,

so we understand our units, at least in principle.

For optical wavelengths the scale is set by the Rydberg unit

(energy, inverse wavelength, frequency)

In crap-free units the Rydberg wavelength is 1/ \alpha^2 m_{electron}

(with a whole slew of higher order corrections)

For dimensions of material objects otoh

the scale is the Bohr radius,

which is (again crap-free) 1/\alpha m_{electron}.

(again with a whole slew of higher order corrections)

So they differ to lowest order by a factor of \alpha.

Now, after all these preliminaries, we can deal with Helbig's question.

The meaningless question: can the measured speed of light vary?

becomes a meaningful question if we rephrase it as:

if things in the universe are variable,

would all possible length units vary in the same way?

(note that this transforms a meaningless question

about dimensioned things into a meaninful question

about dimensionless ratios between units)

From the arguments above the answer is yes,

and we could in principle observe such an effect in the lab.

OTOH there is no way that we would interpret such an observation

as a variable speed of light, supposing we would know what that means.

The prime suspect will be \alpha. (but it might be something higher up)

And for Philip: I hope that this provides the explanation you asked for,

Jan

(about time in #2)

Sep 5, 2021, 6:45:50 AM9/5/21

to

J. J. Lodder <nos...@de-ster.demon.nl> wrote:

[Followup to myself, continued with more personal,

and perhaps more controversial ideas]

Let us suppose for the sake of argument

that 'Helbig's nightmare' becomes true,

and that all kinds 'fundamental' things

turn out to be variable.

In particular, let us assume that we have an experimental basis for it,

in that we have several independent length and time units,

all reproducible to adequate precision,

and drifting with respect to each other.

Hence we will have many variable 'speeds of light',

wich we can all measure by hand-picking units.

Will this force us to give up the relativity postulate,

and the idea of a universal absolute speed,

and hence the idea of relativistic space-time?

I think that the answer will be no.

Instead we will say that one should use 'fitting'

pairs of units, with matching space and time units

related by a factor c_{universal}

So in summary: we'll keep the spacetime, (and c=1)

and describe all that variable mess

in terms of new laws of physics in space-time.

Bottom line: we'll give up that space-time

only if all else fails,

Jan

[Followup to myself, continued with more personal,

and perhaps more controversial ideas]

Let us suppose for the sake of argument

that 'Helbig's nightmare' becomes true,

and that all kinds 'fundamental' things

turn out to be variable.

In particular, let us assume that we have an experimental basis for it,

in that we have several independent length and time units,

all reproducible to adequate precision,

and drifting with respect to each other.

Hence we will have many variable 'speeds of light',

wich we can all measure by hand-picking units.

Will this force us to give up the relativity postulate,

and the idea of a universal absolute speed,

and hence the idea of relativistic space-time?

I think that the answer will be no.

Instead we will say that one should use 'fitting'

pairs of units, with matching space and time units

related by a factor c_{universal}

So in summary: we'll keep the spacetime, (and c=1)

and describe all that variable mess

in terms of new laws of physics in space-time.

Bottom line: we'll give up that space-time

only if all else fails,

Jan

Sep 22, 2021, 4:57:16 AM9/22/21

to

On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

>The assertion was that you can at least in principle use laboratory

>measurements of the speed of light to see if it varies.

>To see that you can't you need to have at least a vague idea

>of how such measurements are done. ...
>The assertion was that you can at least in principle use laboratory

>measurements of the speed of light to see if it varies.

>To see that you can't you need to have at least a vague idea

I just wanted to thank the OP for his excellent precis. It has

bothered me for a long time that with defining our length scale in

reference to c-dependent physical outputs, that we've given up an

absolute length scale as a basis of measurement. That is, we've

assumed c to be ever unchanging WRT a physical rod. If that

assumption is wrong, we've disabled our ability to find out. We have

put blinkers on ourselves. It can't be right to do that.

In all the sciences, only astronomy looks directly backwards into

time. We assume that there is no overhead in doing so. And yet there

is the redshift which we interpret as physical recession. But who can

say what exactly separates the present from the past? The redshift

may be a symptom of something else as yet unmodelled.

Normally if we set up an apparatus or a software system and switch it

on, then if its particles/data are seen to be expanding and

accelerating all around, we adjudge that the system is mis-calibrated.

So we look for how to calibrate it. Our "accelerating expansion"

universe may simply be uncalibrated, and a new parameter needed to

calibrate it. I greatly hope that we haven't already blinkered

ourselves in such a way as to make that calibration impossible.

Sep 22, 2021, 9:52:50 AM9/22/21

to

In article <614a76af....@news.aioe.org>, er...@flesch.org (Eric

it was measured before the redefinition of the metre? If you actually

find it to vary, no reasonable person will say that that is wrong since

the speed of light is defined to be a constant. Nature doesn't care how

we define our units.

> In all the sciences, only astronomy looks directly backwards into

> time. We assume that there is no overhead in doing so. And yet there

> is the redshift which we interpret as physical recession. But who can

> say what exactly separates the present from the past? The redshift

> may be a symptom of something else as yet unmodelled.

There is no shortage of alternative theories. There is no shortage of

criticism of them. If you have a good idea, publish it, and let it be

debated.

ANYTHING may be a symptom of something else as yet unmodelled. But

there seems to be no reason to doubt the cosmological redshift.

> Normally if we set up an apparatus or a software system and switch it

> on, then if its particles/data are seen to be expanding and

> accelerating all around, we adjudge that the system is mis-calibrated.

For a normal system in the lab, yes. For the Universe, no. It follows

from GR that it can be static only if infinitely fine-tuned. We have no

reason to doubt the validity of GR on large scales.

> So we look for how to calibrate it. Our "accelerating expansion"

> universe may simply be uncalibrated, and a new parameter needed to

> calibrate it.

Again, publish a hypothesis and let it be debated.

> I greatly hope that we haven't already blinkered

> ourselves in such a way as to make that calibration impossible.

I don't think so, but in any case it doesn't have anything to do with

the definition of the metre or the constancy of the speed of light, at

least not at this level. (Note that some people have investigated

cosmological models with a varying speed of light.)

Flesch) writes:

> On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> >The assertion was that you can at least in principle use laboratory

> >measurements of the speed of light to see if it varies.

> >To see that you can't you need to have at least a vague idea

> >of how such measurements are done. ...

>

> I just wanted to thank the OP for his excellent precis. It has

> bothered me for a long time that with defining our length scale in

> reference to c-dependent physical outputs, that we've given up an

> absolute length scale as a basis of measurement. That is, we've

> assumed c to be ever unchanging WRT a physical rod. If that

> assumption is wrong, we've disabled our ability to find out. We have

> put blinkers on ourselves. It can't be right to do that.

What is to prevent you from measuring the speed of light in the same way
> On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> >The assertion was that you can at least in principle use laboratory

> >measurements of the speed of light to see if it varies.

> >To see that you can't you need to have at least a vague idea

> >of how such measurements are done. ...

>

> I just wanted to thank the OP for his excellent precis. It has

> bothered me for a long time that with defining our length scale in

> reference to c-dependent physical outputs, that we've given up an

> absolute length scale as a basis of measurement. That is, we've

> assumed c to be ever unchanging WRT a physical rod. If that

> assumption is wrong, we've disabled our ability to find out. We have

> put blinkers on ourselves. It can't be right to do that.

it was measured before the redefinition of the metre? If you actually

find it to vary, no reasonable person will say that that is wrong since

the speed of light is defined to be a constant. Nature doesn't care how

we define our units.

> In all the sciences, only astronomy looks directly backwards into

> time. We assume that there is no overhead in doing so. And yet there

> is the redshift which we interpret as physical recession. But who can

> say what exactly separates the present from the past? The redshift

> may be a symptom of something else as yet unmodelled.

criticism of them. If you have a good idea, publish it, and let it be

debated.

ANYTHING may be a symptom of something else as yet unmodelled. But

there seems to be no reason to doubt the cosmological redshift.

> Normally if we set up an apparatus or a software system and switch it

> on, then if its particles/data are seen to be expanding and

> accelerating all around, we adjudge that the system is mis-calibrated.

from GR that it can be static only if infinitely fine-tuned. We have no

reason to doubt the validity of GR on large scales.

> So we look for how to calibrate it. Our "accelerating expansion"

> universe may simply be uncalibrated, and a new parameter needed to

> calibrate it.

> I greatly hope that we haven't already blinkered

> ourselves in such a way as to make that calibration impossible.

the definition of the metre or the constancy of the speed of light, at

least not at this level. (Note that some people have investigated

cosmological models with a varying speed of light.)

Sep 25, 2021, 3:12:20 AM9/25/21

to

Eric Flesch <er...@flesch.org> wrote:

> On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> >The assertion was that you can at least in principle use laboratory

> >measurements of the speed of light to see if it varies.

> >To see that you can't you need to have at least a vague idea

> >of how such measurements are done. ...

>

> I just wanted to thank the OP for his excellent precis. It has

> bothered me for a long time that with defining our length scale in

> reference to c-dependent physical outputs, that we've given up an

> absolute length scale as a basis of measurement. That is, we've

> assumed c to be ever unchanging WRT a physical rod. If that

> assumption is wrong, we've disabled our ability to find out. We have

> put blinkers on ourselves. It can't be right to do that.

It is not just that we have given up on having an absolute lenght unit,
> On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> >The assertion was that you can at least in principle use laboratory

> >measurements of the speed of light to see if it varies.

> >To see that you can't you need to have at least a vague idea

> >of how such measurements are done. ...

>

> I just wanted to thank the OP for his excellent precis. It has

> bothered me for a long time that with defining our length scale in

> reference to c-dependent physical outputs, that we've given up an

> absolute length scale as a basis of measurement. That is, we've

> assumed c to be ever unchanging WRT a physical rod. If that

> assumption is wrong, we've disabled our ability to find out. We have

> put blinkers on ourselves. It can't be right to do that.

we have understood that we never had one to begin with,

if we look at things to sufficient accuracy.

We have not assumed c to be ever unchanging wrt to a physical rod,

we have defined distances (to much greater accuracy and reproducibility)

by giving the speed of light a defined value.

This has not diminished our ability to measure things in any way,

it only means that we have agreed

to incorporate observed changes somewhere else.

(if there are any)

The effect of this is to replace the physically meaningless question

of whether the -dimensioned- quantity c can vary

with the physically observable and meaningful question

of whether the dimensionless ratios of differently defined length units

are changing wrt to each other.

Jan

Sep 25, 2021, 3:12:21 AM9/25/21

to

Phillip Helbig (undress to reply) <hel...@asclothestro.multivax.de>

wrote:

> In article <614a76af....@news.aioe.org>, er...@flesch.org (Eric

> Flesch) writes:

>

> > On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> > >The assertion was that you can at least in principle use laboratory

> > >measurements of the speed of light to see if it varies.

> > >To see that you can't you need to have at least a vague idea

> > >of how such measurements are done. ...

> >

> > I just wanted to thank the OP for his excellent precis. It has

> > bothered me for a long time that with defining our length scale in

> > reference to c-dependent physical outputs, that we've given up an

> > absolute length scale as a basis of measurement. That is, we've

> > assumed c to be ever unchanging WRT a physical rod. If that

> > assumption is wrong, we've disabled our ability to find out. We have

> > put blinkers on ourselves. It can't be right to do that.

>

> What is to prevent you from measuring the speed of light in the same way

> it was measured before the redefinition of the metre?

Nothing. As a matter of fact these measurement -are- done routinely

in standards laboratories.

Nowadays they serve to calibrate secondary meter standards.

> If you actually find it to vary, no reasonable person will say that that

> is wrong since the speed of light is defined to be a constant.

That is precisely what reasonable people will say.

They will ask: varies -with respect to what-?

All that might be observed experimentally

is that the meter, as defined by clock and c,

varies wrt to the meter defined in some other way.

(platinum bar? seconds pendulum? some optical wavelength?)

Instead of people saying that the speed of light

has been observed to be variable

they will ask what the 'right' length unit is.

Jan

wrote:

> In article <614a76af....@news.aioe.org>, er...@flesch.org (Eric

> Flesch) writes:

>

> > On 01 Sep 2021, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

> > >The assertion was that you can at least in principle use laboratory

> > >measurements of the speed of light to see if it varies.

> > >To see that you can't you need to have at least a vague idea

> > >of how such measurements are done. ...

> >

> > I just wanted to thank the OP for his excellent precis. It has

> > bothered me for a long time that with defining our length scale in

> > reference to c-dependent physical outputs, that we've given up an

> > absolute length scale as a basis of measurement. That is, we've

> > assumed c to be ever unchanging WRT a physical rod. If that

> > assumption is wrong, we've disabled our ability to find out. We have

> > put blinkers on ourselves. It can't be right to do that.

>

> What is to prevent you from measuring the speed of light in the same way

> it was measured before the redefinition of the metre?

in standards laboratories.

Nowadays they serve to calibrate secondary meter standards.

> If you actually find it to vary, no reasonable person will say that that

> is wrong since the speed of light is defined to be a constant.

They will ask: varies -with respect to what-?

All that might be observed experimentally

is that the meter, as defined by clock and c,

varies wrt to the meter defined in some other way.

(platinum bar? seconds pendulum? some optical wavelength?)

Instead of people saying that the speed of light

has been observed to be variable

they will ask what the 'right' length unit is.

Jan

Sep 25, 2021, 3:16:40 AM9/25/21

to

On 9/1/21 2:27 PM, J. J. Lodder wrote:

> [...]

Here's a description of a laboratory experiment to measure any variation

in the vacuum speed of light during a year, at the part per billion

level. Please explain why you think that it could not detect such

variations.

The basic idea is to construct a very stable vacuum optical cavity of

length L, and measure any variations in the frequency of its free

spectral range (= c/(2L)). The precise value of L does not matter,

as this is looking at variations.

Construct a temperature-controlled cell a meter or so on a side (c.f.

Kennedy-Thorndike), and inside it construct a vacuum optical cavity

whose length is ~ 0.5 meters, determined by material with essentially

zero coefficient of thermal expansion (e.g. invar). The free spectral

range of such a cavity is c/(~1 meter), which is ~ 300 MHz. Use

Pound-Drever-Hall laser locking to lock two high-quality lasers to

adjacent fringes and count their heterodyne frequency, using at least

four Cs-133 atomic clocks to generate the timebase [#]. By counting for

1000.000000000000 seconds and averaging multiple counts this should

easily have a resolution of ~0.1 Hz (out of ~300 MHz). Make measurements

repeatedly over at least a year.

[#] Don't use GPS, as they will steer its clocks to offset

any variation in c.

This should detect variations in c over one year, at the part per

billion level. In principle it could do better....

Tom Roberts

> [...]

Here's a description of a laboratory experiment to measure any variation

in the vacuum speed of light during a year, at the part per billion

level. Please explain why you think that it could not detect such

variations.

The basic idea is to construct a very stable vacuum optical cavity of

length L, and measure any variations in the frequency of its free

spectral range (= c/(2L)). The precise value of L does not matter,

as this is looking at variations.

Construct a temperature-controlled cell a meter or so on a side (c.f.

Kennedy-Thorndike), and inside it construct a vacuum optical cavity

whose length is ~ 0.5 meters, determined by material with essentially

zero coefficient of thermal expansion (e.g. invar). The free spectral

range of such a cavity is c/(~1 meter), which is ~ 300 MHz. Use

Pound-Drever-Hall laser locking to lock two high-quality lasers to

adjacent fringes and count their heterodyne frequency, using at least

four Cs-133 atomic clocks to generate the timebase [#]. By counting for

1000.000000000000 seconds and averaging multiple counts this should

easily have a resolution of ~0.1 Hz (out of ~300 MHz). Make measurements

repeatedly over at least a year.

[#] Don't use GPS, as they will steer its clocks to offset

any variation in c.

This should detect variations in c over one year, at the part per

billion level. In principle it could do better....

Tom Roberts

Sep 25, 2021, 5:03:58 AM9/25/21

to

Tom Roberts <tjrobe...@sbcglobal.net> wrote:

> On 9/1/21 2:27 PM, J. J. Lodder wrote:

> > [...]

>

> Here's a description of a laboratory experiment to measure any variation

> in the vacuum speed of light during a year, at the part per billion

> level. Please explain why you think that it could not detect such

> variations.

Thank you for this perfect illustration of my point.
> On 9/1/21 2:27 PM, J. J. Lodder wrote:

> > [...]

>

> Here's a description of a laboratory experiment to measure any variation

> in the vacuum speed of light during a year, at the part per billion

> level. Please explain why you think that it could not detect such

> variations.

Supposing there would be an effect, what would we conclude? [1]

Your naive assumption is that blocks of metal must have a constant

length. They feel real solid, don't they?

However, if we assume that fundamental constants

(such as alpha for example) might be variable

there is no reason to believe in constancy of the length of

metal rods.

Given what we know about spacetime, and about the physics of metals,

my guess is that the second interpretation will be the preferred one,

Jan

[1] I ignore the practical point that the limited accuracy

of your setup (a mere 10^-9) will not yield a meaningful result anyway.

We already know that things are far more stable than that.

So the practical conclusion will be some kind of experimental error.

Sep 25, 2021, 1:15:45 PM9/25/21

to

In article <1pfxkcn.125esf5v7jrgeN%nos...@de-ster.demon.nl>,

nos...@de-ster.demon.nl (J. J. Lodder) writes:

> > If you actually find it to vary, no reasonable person will say that t=

vary with respect to ALL possible standards, in which case it wouldn't

make sense to define any sort of length with respect to that speed, just

as one doesn't define any length with respect to the speed of someone

riding a bike, say.

nos...@de-ster.demon.nl (J. J. Lodder) writes:

> > If you actually find it to vary, no reasonable person will say that t=

hat

> > is wrong since the speed of light is defined to be a constant.

>

> That is precisely what reasonable people will say.

> They will ask: varies -with respect to what-?

> All that might be observed experimentally

> is that the meter, as defined by clock and c,

> varies wrt to the meter defined in some other way.

> (platinum bar? seconds pendulum? some optical wavelength?)

>

> Instead of people saying that the speed of light

> has been observed to be variable

> they will ask what the 'right' length unit is.

Except that (as in varying-speed-of-light cosmological models) it might
> > is wrong since the speed of light is defined to be a constant.

>

> That is precisely what reasonable people will say.

> They will ask: varies -with respect to what-?

> All that might be observed experimentally

> is that the meter, as defined by clock and c,

> varies wrt to the meter defined in some other way.

> (platinum bar? seconds pendulum? some optical wavelength?)

>

> Instead of people saying that the speed of light

> has been observed to be variable

> they will ask what the 'right' length unit is.

vary with respect to ALL possible standards, in which case it wouldn't

make sense to define any sort of length with respect to that speed, just

as one doesn't define any length with respect to the speed of someone

riding a bike, say.

Sep 26, 2021, 3:47:12 AM9/26/21

to

Paper is cheap, and people can write all kinds of ds^2 = ...

It is then up to those authors to explain what their models

mean in terms of observation and measurement.

We were discussing the measurability of the speed of light,

and perhaps of changes in it.

(or equivalently, calibration of length standards)

By the very nature of these experiments they can only be done

with any accuracy in the rest frame of standards laboratories.

By the relativity postulate the results must be the same

for all inertial observers.

So we can tell those LGM what our length and time units are.

Astronomically speaking, hence cosmologically

you are completely powerless to begin with,

for all astronomical distances can be known only

in terms of (light)seconds.

They are all based on the AU, which can only be measured accurately

in terms of (light)seconds.

To such an extent even that it became necessary

to give the AU a defined value in terms of meters, hence seconds.

So, if you leave the context of precision laboratory measurement

it is completely unclear to me what you are trying to argue,

Jan

Sep 27, 2021, 1:39:54 PM9/27/21

to

On 21/09/26 9:47 AM, J. J. Lodder wrote:

> Phillip Helbig (undress to reply) <hel...@asclothestro.multivax.de>

> wrote:

>

>> In article <1pfxkcn.125esf5v7jrgeN%nos...@de-ster.demon.nl>,

>> nos...@de-ster.demon.nl (J. J. Lodder) writes:

>>

>>>> If you actually find it to vary, no reasonable person will say that t=

>> hat

>>>> is wrong since the speed of light is defined to be a constant.

>>>

>>> That is precisely what reasonable people will say.

>>> They will ask: varies -with respect to what-?

>>> All that might be observed experimentally

>>> is that the meter, as defined by clock and c,

>>> varies wrt to the meter defined in some other way.

>>> (platinum bar? seconds pendulum? some optical wavelength?)

>>>

>>> Instead of people saying that the speed of light

>>> has been observed to be variable

>>> they will ask what the 'right' length unit is.

>>

>> Except that (as in varying-speed-of-light cosmological models) it might

>> vary with respect to ALL possible standards, in which case it wouldn't

>> make sense to define any sort of length with respect to that speed, just

>> as one doesn't define any length with respect to the speed of someone

>> riding a bike, say.

>

> I'm sorry to say, but you are moving goalposts.

And you are more and more reverting to rhetoric instead of physics..
> Phillip Helbig (undress to reply) <hel...@asclothestro.multivax.de>

> wrote:

>

>> In article <1pfxkcn.125esf5v7jrgeN%nos...@de-ster.demon.nl>,

>> nos...@de-ster.demon.nl (J. J. Lodder) writes:

>>

>>>> If you actually find it to vary, no reasonable person will say that t=

>> hat

>>>> is wrong since the speed of light is defined to be a constant.

>>>

>>> That is precisely what reasonable people will say.

>>> They will ask: varies -with respect to what-?

>>> All that might be observed experimentally

>>> is that the meter, as defined by clock and c,

>>> varies wrt to the meter defined in some other way.

>>> (platinum bar? seconds pendulum? some optical wavelength?)

>>>

>>> Instead of people saying that the speed of light

>>> has been observed to be variable

>>> they will ask what the 'right' length unit is.

>>

>> Except that (as in varying-speed-of-light cosmological models) it might

>> vary with respect to ALL possible standards, in which case it wouldn't

>> make sense to define any sort of length with respect to that speed, just

>> as one doesn't define any length with respect to the speed of someone

>> riding a bike, say.

>

> I'm sorry to say, but you are moving goalposts.

> Paper is cheap, and people can write all kinds of ds^2 = ...

> It is then up to those authors to explain what their models

> mean in terms of observation and measurement.

>

> We were discussing the measurability of the speed of light,

> and perhaps of changes in it.

> (or equivalently, calibration of length standards)

> By the very nature of these experiments they can only be done

> with any accuracy in the rest frame of standards laboratories.

(RĂ¸mer) was actually done on an astronomical scale. Also, planned

experiments like LISA will be space-based and they certainly rely

very strongly on knowing the exact speed of light, even more so

then equivalent earth-based setups. Some of your arguments against

the measurability of c may be sound, but this singling out of some

rest frame isn't!

> By the relativity postulate the results must be the same

> for all inertial observers.

> So we can tell those LGM what our length and time units are.

>

> Astronomically speaking, hence cosmologically

> you are completely powerless to begin with,

--

Jos

[[Mod. note -- LISA does not rely on knowing the exact speed of light.

Instead, it relies on (among other things) knowing the inter-satellite

distances in *light-seconds*. See section 7 of

https://www.livingreviews.org/lrr-2014-6

for details.

-- jt]]

Oct 7, 2021, 9:52:29 AM10/7/21

to

Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

To get an idea about how a resonator works follow this link:

https://en.wikipedia.org/wiki/Resonator#Explanation

The condition for resonance in a resonator is that the round trip distance,

2 d, is equal to an integer number of wavelengths lambda of the wave:

2 d = N * lambda , N { 1,2,3, ... }

If the velocity of a wave is c the frequency is f = c / lambda,

so the resonant frequencies are:

f = N * c / 2d with N { 1,2,3, ... }

The question is how do you exactly build this rectilinear oscillator.

The problem: what you want to calculate is c = f *2d / N

Suppose you know f and you want to try N=10.

What should now be d, the distance between the sides?

You can start with d = 0.5m as Tom Roberts suggests 25/9/2021

But most probably that value is wrong.

That means you should not try 500mm but for example 501mm

Also that value I expect is wrong.

I have no idea what a correct value is, such that you get a stable resonator.

I also have no idea how "stable" your stable resonator is.

i.e. 1 hour? 1 day? 1 month?

The whole point is how accurate is this experiment i.e. calculation of c?

You can also rephrase this sentence:

How valid is your claim that when you resonator is not stable

that the cause is a variable speed of light?

(I disagree with the remark of Tom Roberts 25/9/2021:

I agree with his remark:

> Don't use GPS, as they will steer its clocks to offset any variation in c.)

Nicolaas Vroom

> The assertion was that you can at least in principle use laboratory

> measurements of the speed of light to see if it varies.

>

> To see that you can't you need to have at least a vague idea

> of how such measurements are done.

> A) you build a stable light source.

> B) you set up a fixed resonator for it to create a standing wave.

> C) using the tricks of the trade you determine

> how many wavelength there are in it.

> D) idem, and far more difficult, you measure the frequency

> of your light source, wrt to an atomic clock.

> (frequency dividing, multiplexing, counting etc. very hard)

> E) Knowing wavelength and frequency give you speed of light.

IMO item B seems to me very tricky.
> measurements of the speed of light to see if it varies.

>

> To see that you can't you need to have at least a vague idea

> of how such measurements are done.

> A) you build a stable light source.

> B) you set up a fixed resonator for it to create a standing wave.

> C) using the tricks of the trade you determine

> how many wavelength there are in it.

> D) idem, and far more difficult, you measure the frequency

> of your light source, wrt to an atomic clock.

> (frequency dividing, multiplexing, counting etc. very hard)

> E) Knowing wavelength and frequency give you speed of light.

To get an idea about how a resonator works follow this link:

https://en.wikipedia.org/wiki/Resonator#Explanation

The condition for resonance in a resonator is that the round trip distance,

2 d, is equal to an integer number of wavelengths lambda of the wave:

2 d = N * lambda , N { 1,2,3, ... }

If the velocity of a wave is c the frequency is f = c / lambda,

so the resonant frequencies are:

f = N * c / 2d with N { 1,2,3, ... }

The question is how do you exactly build this rectilinear oscillator.

The problem: what you want to calculate is c = f *2d / N

Suppose you know f and you want to try N=10.

What should now be d, the distance between the sides?

You can start with d = 0.5m as Tom Roberts suggests 25/9/2021

But most probably that value is wrong.

That means you should not try 500mm but for example 501mm

Also that value I expect is wrong.

I have no idea what a correct value is, such that you get a stable resonator.

I also have no idea how "stable" your stable resonator is.

i.e. 1 hour? 1 day? 1 month?

The whole point is how accurate is this experiment i.e. calculation of c?

You can also rephrase this sentence:

How valid is your claim that when you resonator is not stable

that the cause is a variable speed of light?

(I disagree with the remark of Tom Roberts 25/9/2021:

> The precise value of L does not matter, as this is looking at variations.

But ofcourse there can be a misunderstanding from my side.
I agree with his remark:

> Don't use GPS, as they will steer its clocks to offset any variation in c.)

Nicolaas Vroom

Oct 13, 2021, 3:48:10 AM10/13/21

to

On 10/7/21 8:52 AM, Nicolaas Vroom wrote:

> Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

>> The assertion was that you can at least in principle use laboratory

>> measurements of the speed of light to see if it varies.

>>

>> To see that you can't you need to have at least a vague idea of

>> how such measurements are done. A) you build a stable light

>> source.

Better, build TWO that have variable frequency & wavelength such that
> Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

>> The assertion was that you can at least in principle use laboratory

>> measurements of the speed of light to see if it varies.

>>

>> To see that you can't you need to have at least a vague idea of

>> how such measurements are done. A) you build a stable light

>> source.

you can lock their wavelengths to the optical resonator of (B). This

requires high coherence and narrow linewidth, so lasers are needed.

>> B) you set up a fixed resonator for it to create a standing wave.

both lasers to the cavity, on adjacent fringes [Pound-Drever-Hall laser

locking].

Note that for stability, the cavity must be temperature controlled and

in vacuum.

>> C) using the tricks of the trade you determine how many wavelength

>> there are in it.

many wavelengths are in it. Because with TWO light sources locked to

adjacent fringes of the cavity you will measure its Free Spectral Range

frequency = c/(2L) (in vacuum).

>> D) idem, and far more difficult, you measure the frequency of your

>> light source, wrt to an atomic clock. (frequency dividing,

>> multiplexing, counting etc. very hard)

to 10-20GHz is straightforward, while building highly stable, highly

performant optical cavities is not.

>> E) Knowing wavelength and frequency give you speed of light.

the FSR frequency far more accurately than you will know the length of

the cavity, so it's better to look for variations in FSR frequency, and

thus variations in c, rather than attempt a direct measurement of c.

[We know the lengths of our optical cavities to at best

a few parts per thousand. We measure FSR frequencies a

billion times more accurately.]

> IMO item B seems to me very tricky. [...] The question is how do you

> exactly build this rectilinear oscillator.

It's called a Fabry-Perot interferometer, aka optical cavity, aka F-P
etalon. Building one is simple, making it exceptionally stable is not.

> [... naive discussion omitted] (I disagree with the remark of Tom

> Roberts 25/9/2021:

>> The precise value of L does not matter, as this is looking at

>> variations.

For the experiment I discussed, this is obvious -- you clearly do not
>> The precise value of L does not matter, as this is looking at

>> variations.

understand what I was describing.

In general, experiments looking for variations in some quantity can be

MUCH more sensitive than measuring the quantity. That is true here --

variations in FSR frequency can be accurate to a few parts in 10^12,

perhaps better; measurements of c are at best a few parts per billion

(using a pre-1983 definition of the meter, limited by the ability to

apply such a definition).

> I agree with his remark:

>> Don't use GPS, as they will steer its clocks to offset any

>> variation in c.)

lab. But we don't have any optical cavity that is nearly stable enough,

because our research does not require it. Obtaining funding to build an

exceptionally stable cavity is unlikely; nor are we particularly interested.

Tom Roberts

Oct 15, 2021, 3:47:29 PM10/15/21

to

Nicolaas Vroom <nicolaa...@pandora.be> wrote:

[repost of another vanished posting, some minor edits]

> The condition for resonance in a resonator is that the round trip distance,

> 2 d, is equal to an integer number of wavelengths lambda of the wave:

> 2 d = N * lambda , N { 1,2,3, ... }

> If the velocity of a wave is c the frequency is f = c / lambda,

> so the resonant frequencies are:

> f = N * c / 2d with N { 1,2,3, ... }

> The question is how do you exactly build this rectilinear oscillator.

Move mirrors on an optical bench, and count fringes. (in principle)

> The problem: what you want to calculate is c = f *2d / N

> Suppose you know f and you want to try N=10.

> What should now be d, the distance between the sides?

> You can start with d = 0.5m as Tom Roberts suggests 25/9/2021

> But most probably that value is wrong.

> That means you should not try 500mm but for example 501mm

> Also that value I expect is wrong.

> I have no idea what a correct value is, such that you get a stable resonator.

> I also have no idea how "stable" your stable resonator is.

The best you can do at present is 2 x 10^-11,

which is the accurracy to which the secondary meter standard

can be defined. (stabilised He-Ne laser at 632.99121258 nm)

> i.e. 1 hour? 1 day? 1 month?

> The whole point is how accurate is this experiment i.e. calculation of c?

> You can also rephrase this sentence:

> How valid is your claim that when you resonator is not stable

> that the cause is a variable speed of light?

See above.

What such an experiment would be really testing

is variability of \alpha.

As such PhH's proposal is completely useless.

He has been beaten beforehand

by the astronomers with their huge telescopes,

(such as the Keck)

They have fixed the variability of \alpha

to be less than 10^-5 over ten billion years, so 10^-15/year,

assuming linearity.

May be better by now, I haven't kept up with the latest numbers.

This is far out of reach of anything you can hope to do

in a laboratory in a few years.

Jan

[repost of another vanished posting, some minor edits]

> Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

> > The assertion was that you can at least in principle use laboratory

> > measurements of the speed of light to see if it varies.

> >

> > To see that you can't you need to have at least a vague idea

> > of how such measurements are done.

> > A) you build a stable light source.

> > B) you set up a fixed resonator for it to create a standing wave.

> > C) using the tricks of the trade you determine

> > how many wavelength there are in it.

> > D) idem, and far more difficult, you measure the frequency

> > of your light source, wrt to an atomic clock.

> > (frequency dividing, multiplexing, counting etc. very hard)

> > E) Knowing wavelength and frequency give you speed of light.

>

> IMO item B seems to me very tricky.

> To get an idea about how a resonator works follow this link:

> https://en.wikipedia.org/wiki/Resonator#Explanation

Yes, it is a poor way to go about it.
> > The assertion was that you can at least in principle use laboratory

> > measurements of the speed of light to see if it varies.

> >

> > To see that you can't you need to have at least a vague idea

> > of how such measurements are done.

> > A) you build a stable light source.

> > B) you set up a fixed resonator for it to create a standing wave.

> > C) using the tricks of the trade you determine

> > how many wavelength there are in it.

> > D) idem, and far more difficult, you measure the frequency

> > of your light source, wrt to an atomic clock.

> > (frequency dividing, multiplexing, counting etc. very hard)

> > E) Knowing wavelength and frequency give you speed of light.

>

> IMO item B seems to me very tricky.

> To get an idea about how a resonator works follow this link:

> https://en.wikipedia.org/wiki/Resonator#Explanation

> The condition for resonance in a resonator is that the round trip distance,

> 2 d, is equal to an integer number of wavelengths lambda of the wave:

> 2 d = N * lambda , N { 1,2,3, ... }

> If the velocity of a wave is c the frequency is f = c / lambda,

> so the resonant frequencies are:

> f = N * c / 2d with N { 1,2,3, ... }

> The question is how do you exactly build this rectilinear oscillator.

> The problem: what you want to calculate is c = f *2d / N

> Suppose you know f and you want to try N=10.

> What should now be d, the distance between the sides?

> You can start with d = 0.5m as Tom Roberts suggests 25/9/2021

> But most probably that value is wrong.

> That means you should not try 500mm but for example 501mm

> Also that value I expect is wrong.

> I have no idea what a correct value is, such that you get a stable resonator.

> I also have no idea how "stable" your stable resonator is.

which is the accurracy to which the secondary meter standard

can be defined. (stabilised He-Ne laser at 632.99121258 nm)

> i.e. 1 hour? 1 day? 1 month?

> The whole point is how accurate is this experiment i.e. calculation of c?

> You can also rephrase this sentence:

> How valid is your claim that when you resonator is not stable

> that the cause is a variable speed of light?

What such an experiment would be really testing

is variability of \alpha.

As such PhH's proposal is completely useless.

He has been beaten beforehand

by the astronomers with their huge telescopes,

(such as the Keck)

They have fixed the variability of \alpha

to be less than 10^-5 over ten billion years, so 10^-15/year,

assuming linearity.

May be better by now, I haven't kept up with the latest numbers.

This is far out of reach of anything you can hope to do

in a laboratory in a few years.

Jan

Oct 19, 2021, 1:59:46 AM10/19/21

to

On 21/10/15 9:47 PM, J. J. Lodder wrote:

> Nicolaas Vroom <nicolaa...@pandora.be> wrote:

> [repost of another vanished posting, some minor edits]

>> Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

> Nicolaas Vroom <nicolaa...@pandora.be> wrote:

> [repost of another vanished posting, some minor edits]

>> Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

>>> The assertion was that you can at least in principle use laboratory

>>> measurements of the speed of light to see if it varies.

>>>

>>> To see that you can't you need to have at least a vague idea

>>> of how such measurements are done.

>>> A) you build a stable light source.

>>> B) you set up a fixed resonator for it to create a standing wave.

>>> C) using the tricks of the trade you determine

>>> how many wavelength there are in it.

>>> D) idem, and far more difficult, you measure the frequency

>>> of your light source, wrt to an atomic clock.

>>> (frequency dividing, multiplexing, counting etc. very hard)

>>> E) Knowing wavelength and frequency give you speed of light.

>>

>>> measurements of the speed of light to see if it varies.

>>>

>>> To see that you can't you need to have at least a vague idea

>>> of how such measurements are done.

>>> A) you build a stable light source.

>>> B) you set up a fixed resonator for it to create a standing wave.

>>> C) using the tricks of the trade you determine

>>> how many wavelength there are in it.

>>> D) idem, and far more difficult, you measure the frequency

>>> of your light source, wrt to an atomic clock.

>>> (frequency dividing, multiplexing, counting etc. very hard)

>>> E) Knowing wavelength and frequency give you speed of light.

>>

>> IMO item B seems to me very tricky.

>> To get an idea about how a resonator works follow this link:

>> https://en.wikipedia.org/wiki/Resonator#Explanation

>

> Yes, it is a poor way to go about it.

And anyhow, the above says that D) is in fact far more difficult.
>> To get an idea about how a resonator works follow this link:

>> https://en.wikipedia.org/wiki/Resonator#Explanation

>

> Yes, it is a poor way to go about it.

What would actually be the first divider? Are there injection-locked

dividing lasers nowadays?

--

Jos

Oct 22, 2021, 3:07:41 PM10/22/21

to

No point in me repeating Google on this,

Jan

Oct 23, 2021, 5:03:08 PM10/23/21

to

[missingin action, presumed lost, repost]

Some of the > >> stuff is by me, some of it is by others]

> The experiment I described is something we routinely do in our optical

> lab. But we don't have any optical cavity that is nearly stable enough,

> because our research does not require it. Obtaining funding to build an

> exceptionally stable cavity is unlikely; nor are we particularly interested.

As I implied in previous postings,

the revolutionary step in the redefinitions of the meter

was to replace material standard meter

(lines on a metal bar) with an optical standard meter.

(the wavelengty of a krypton line, later a laser)

There is no good a-priory reason to assume that the two must be,

and will forever remain the same thing.

That experiment that 'we are not particularly interested in'

could in principle test the question.

(but there are good reasons to believe

that it could not possibly yield a useful result)

The speed of light in all this is merely a red herring.

All there is to it is that experimentally

one can maintain an optical frequency standard

to much greater precision than a wavelength standard.

(by several orders of magnitude)

If we wouldn't care about best reproducibility

we could in principle go back to a wavelength standard

to make c measurable again, but to no better than

the reproducibility of the wavelength standard.

Jan

Tom Roberts <tjrobe...@sbcglobal.net> wrote:

> On 10/7/21 8:52 AM, Nicolaas Vroom wrote:

> > Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

> >> The assertion was that you can at least in principle use laboratory

> >> measurements of the speed of light to see if it varies.

[snipped a mess of garbled attributions without further comment.
> On 10/7/21 8:52 AM, Nicolaas Vroom wrote:

> > Op woensdag 1 september 2021 om 21:27:09 UTC+2 schreef J. J. Lodder:

> >> The assertion was that you can at least in principle use laboratory

> >> measurements of the speed of light to see if it varies.

Some of the > >> stuff is by me, some of it is by others]

> The experiment I described is something we routinely do in our optical

> lab. But we don't have any optical cavity that is nearly stable enough,

> because our research does not require it. Obtaining funding to build an

> exceptionally stable cavity is unlikely; nor are we particularly interested.

the revolutionary step in the redefinitions of the meter

was to replace material standard meter

(lines on a metal bar) with an optical standard meter.

(the wavelengty of a krypton line, later a laser)

There is no good a-priory reason to assume that the two must be,

and will forever remain the same thing.

That experiment that 'we are not particularly interested in'

could in principle test the question.

(but there are good reasons to believe

that it could not possibly yield a useful result)

The speed of light in all this is merely a red herring.

All there is to it is that experimentally

one can maintain an optical frequency standard

to much greater precision than a wavelength standard.

(by several orders of magnitude)

If we wouldn't care about best reproducibility

we could in principle go back to a wavelength standard

to make c measurable again, but to no better than

the reproducibility of the wavelength standard.

Jan

Oct 24, 2021, 5:12:09 PM10/24/21

to

out the signal with the frequency divided by N?

> No point in me repeating Google on this,

should only allow 'original research' to be posted here. I doubt

whether that would be a useful strategy.. Especially in a "tutorial"

it would be quite unnatural!

--

Jos

[[Mod. note -- Yes, tutorial material is (may be) ok for the newsgroup.

For this topic,

https://en.wikipedia.org/wiki/Optical_frequency_comb

might be a starting point. The link (#3 of 5 in the "External Links" section)

"Optical frequency comb for dimensional metrology, atomic and molecular

spectroscopy, and precise time keeping" looks quite relevant, but the

linked-to archive.org copy is "temporarily offline" right now. :(

-- jt]]

Oct 25, 2021, 10:49:25 AM10/25/21

to

> > No point in me repeating Google on this,

>

> If repeating published material is of no use, then our moderators

> should only allow 'original research' to be posted here. I doubt

> whether that would be a useful strategy.. Especially in a "tutorial"

> it would be quite unnatural!

of equally spaced lines in the frequency domain.

This can be done in several way,

using ultra-short laser pulses for example.

High precision is achieved by counting the 'low' frequency difference

between the elements of the comb.

Such a frequency comb functions as a ruler

from which you can read off an unknown optical frequency.

The point for the defined light speed discussion is that you can measure

light frequencies with far greater precision

than you can define a lengthh standard,

Jan

For example:

<https://www.nist.gov/topics/physics/optical-frequency-combs>

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