# “We don’t really know the speed of light”

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The speed of light is defined as a physical constant.

Therefore it cannot be measured ...it is the measuring stick !

From the original argument then it has to be the same in both directions.

All other measurements , length , time, mass etc "adapt" so that this constant remains the same.

However length, time, mass etc are all relative to the observer who is measuring them.

My head hurts when trying to understand this outcome....

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7 hours ago, Kev M said:

The speed of light is defined as a physical constant.

Therefore it cannot be measured ...it is the measuring stick !

From the original argument then it has to be the same in both directions.

All other measurements , length , time, mass etc "adapt" so that this constant remains the same.

However length, time, mass etc are all relative to the observer who is measuring them.

My head hurts when trying to understand this outcome....

In the current SI system you are correct. It adopted the Einstein synchronisation convention.  However,  this was only done after many measurements found it to be invariant and the strong experimental evidence for SR.

It is a sensible choice but not the only one that could have been made.

The new SI system , unlike the old ones, rests on the validity of both QM and SR if they prove to be wrong things would have to change..

Regards Andrew

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On 24/02/2021 at 22:48, andrew s said:

As you will see in my last post to Robin you can have different theories that can't be experimentally differentiated and you then have to choose a convention. This is normally done to find one that gives the simplest equations. You can write earth centered equations for planetary motion but they are much more complex that for a sun centered system. Newton's law F = ma does not hold in a rotating frame and you have to add additional forces.

I finally found Robin's argument had the isotropy built into the equations he used. Just as you would verify Newton's law in an inertial frame but not in a rotating one (unless you corrected the equations with the pseudo forces needed in a non inertial frame). I would have liked to have found these equations for Robin's experiment but could not find then. However I did find an example from a related area i.e. the Gauss Law.

Regards Andrew

PS To be explicit you would need a correction to the formula  c = fλ with a convention that the one way speeds in the + and - x directions were c/2 and 3c/2 respectively.

Yet this is exactly the problem. You are just claiming that you can do this without any empirical evidence.  We can all do this, as noted before I can likely come up with some reasoning as to why stars are actually glowing marshmallows on the other side of the galaxy.  It sounds unrealistic but I can just keep coming up with ever odd reasons for any argument against the idea but not put forward any way of testing that idea.  It is not scientific method to respond to a criticism of theory by inventing something else to explain the criticism away.

For example at a basic level, if you change c in different directions then the energy conversion between mass and photons changes in different directions.  If energy changes then the quantisation of energy changes which in different directions means that frequencies change for the same energy levels.  As such hydrogen alpha emission/absorption should be different in different directions.  This is simple to measure from a spectrograph.  If energy quantisation levels change then the way that gases act in high g environments changes - and we have standard candles for these sort of things (White Dwarfs).  All of these observations would be slightly out in one direction to another if c is different in different directions.  Just because c = fλ does not mean you can fiddle with c and not have a consequences across a vast number of other fields.

If you don't produce a testable theory then the proposal is not better than a 'flat earth' theory because the same principles used by some to try and explain a flat earth are being used here.

How about another argument against this relating to GPS satellites.  If you stand on the surface and look to one horizon then a GPS would transmit its signal at one speed and then as it travels across you horizon then it would be transmitting in the opposite direction so c would be different.  The 'clock' has not changed only the direction.  Any change in c would be noticeable as you would then see a slight shift in position - so the question is why don't we see this as a changing c should result in a relatively circular and consistent pattern of location changes despite the object being 'bolted' to the ground.

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@Whirlwind you don't  have to take my word for it. Just read the papers I linked to.  Anisotropic values of c are perfectly valid and compatible with SR and QM. Again don't take my word for it look up the peer reviewed papers in the links I gave.

Equally,  there is no empirical evidence the speed of light is the same in both directions. It is, despite Robin's claim, a convention. It was a fundamental insight of Einstein's that led him to SR.

In your GPS example the clock has changed it's relative  motion to you and you need to make SR corrections for that which include the Einstein convention.

Regards Andrew

In all your examples the equations have the convention  built in. Look how Gauss's law changes with anisotropic c in the link. It is these changes that ensure the empirical facts stay the same.

Edited by andrew s
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1 hour ago, Whirlwind said:

For example at a basic level, if you change c in different directions then the energy conversion between mass and photons changes in different directions.

However, what is energy? Energy is 'something' that represents a translation from one frame of reference to another. This might be between frames moving at different relative speeds, or frames that are at different positions in a field of force.

Having "different energy conversion between mass and photons changing in different directions" can be perfectly consistent with this.

There's a presumption of isotropy, and no evidence against it, but it can't be ruled out.

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19 minutes ago, Gfamily said:

However, what is energy? Energy is 'something' that represents a translation from one frame of reference to another. This might be between frames moving at different relative speeds, or frames that are at different positions in a field of force.

Having "different energy conversion between mass and photons changing in different directions" can be perfectly consistent with this.

There's a presumption of isotropy, and no evidence against it, but it can't be ruled out.

Yes, I agree, energy is frame dependant for example the red and blue shift of spectral lines emitted by sources moving away or towards you.

Regards Andrew

Edited by andrew s
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Somewhat disappointingly,  both @robin_astro and @Whirlwind have tried to discredit my position by association with the view of Flat Earth proponents.

However, what I am saying is mainstream science see here  for a review .

I am not sure if I have convinced Robin with my recent posts but if not I propose he write up his position and submit it to a physics journal as if accepted he may well get a Nobel prize.

Regards Andrew

Edited by andrew s
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18 hours ago, andrew s said:

Somewhat disappointingly,  both @robin_astro and @Whirlwind have tried to discredit my position by association with the view of Flat Earth proponents.

However, what I am saying is mainstream science see here  for a review .

I am not sure if I have convinced Robin with my recent posts but if not I propose he write up his position and submit it to a physics journal as if accepted he may well get a Nobel prize.

Regards Andrew

That's not what we are saying at all.  What is being said is that the approach you are taking to justify the rationale is the same principle used by people to justify the earth is flat (and also the same approach to make the Earth the centre of the cosmos hundreds of years ago).  What you are doing is taking a principle and trying to use reasons to justify the empirical evidence which is the same approach.  In effect what you are doing is trying to prove the principle correct, whereas science is about trying to prove your theory incorrect.  Any theory can be *potentially* correct but it has to be testable (and not just altered to fit what we observe).  The paper referred to doesn't claim that c varies in different direction it just claims that it in principle special relativity doesn't prevent this from occurring.  That isn't *proof* that it actually is.  To do that the theory must be testable observationally that would differentiate the two possibilities.

You have to prove that why the hydrogen alpha line looks the same regardless of which side of the sun we are on and be able to explain not just the why the frequency has stayed the same but why all the observational physical processes we see (down to basic experiments like a double slit experiment can be explained.

19 hours ago, Gfamily said:

However, what is energy? Energy is 'something' that represents a translation from one frame of reference to another. This might be between frames moving at different relative speeds, or frames that are at different positions in a field of force.

Having "different energy conversion between mass and photons changing in different directions" can be perfectly consistent with this.

There's a presumption of isotropy, and no evidence against it, but it can't be ruled out.

Energy levels are also a fundamental of Quantum mechanics.  If you change energy levels then the way objects would bond, interact and their forces would change (molecules might be more or less tightly bound for example).  Such changes to interactions would be observable.

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I see little point in discussing this further with you @Whirlwind. I have provided links to peer review papers on the topic which clearly show that what I have advocated is as empirically justified as conventional SR. Have you bother to look at any of them in detail?.

The last one I linked to provides  for GPS and Maxwell's equations which covers one of your objections and Robin's.

For what it's worth I agree conventional SR provides the simplest equations and is thus to be prefered.

Do you object to the Hamiltonian or Lagrangian formulations of classical mechanics because they cannot be empirically differentiated from Newtons theory?  They are equivalent as are anisotropic versions of SR equivalent to Einstein's .

If that is not enough then nothing I can say will be.

Regards Andrew

Edited by andrew s
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7 hours ago, Whirlwind said:

Energy levels are also a fundamental of Quantum mechanics.  If you change energy levels then the way objects would bond, interact and their forces would change (molecules might be more or less tightly bound for example).  Such changes to interactions would be observable.

Indeed, by why do think they would change?  Non standard SR (i.e. anisotropic  c) does not impact this any more than a Lorentz boost (change of inertial frame)  where the frequency of the observed light will be red or blue shifted. This doesn't impact the atom In its rest frame.

Energy is frame dependant. Being hit by a "stationary" car does not hurt as much as one doing 40mph!

In fact anisotropic c is equivalent to a change of reference frame. The underlying Minkowski spacetime is unaffected.

Regards Andrew

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Most of this thread went right over my head... but one thing at has always puzzled me  with the speed of light thing is that if it were possible to be travelling at the speed of light in a straight line and I had a torch with me, if I pointed it in the direction of travel and turned it on would I see a beam of light ?  - Some say yes as it's relative to me, but I've always thought that the answer would be no I wouldn't see the beam as the photons are doing the same speed as me (light speed) and as such I would catch then up the moment they leave the filament because nothing can travel faster than the speed of light... or so we are told.

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54 minutes ago, malc-c said:

Most of this thread went right over my head... but one thing at has always puzzled me  with the speed of light thing is that if it were possible to be travelling at the speed of light in a straight line and I had a torch with me, if I pointed it in the direction of travel and turned it on would I see a beam of light ?  - Some say yes as it's relative to me, but I've always thought that the answer would be no I wouldn't see the beam as the photons are doing the same speed as me (light speed) and as such I would catch then up the moment they leave the filament because nothing can travel faster than the speed of light... or so we are told.

While you can't do what you propose light pules transmitted together in the same direction would stay in lock step so in a sense yes you would not see a beams move ahead of each other.

Regards Andrew

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22 hours ago, andrew s said:

I see little point in discussing this further with you @Whirlwind. I have provided links to peer review papers on the topic which clearly show that what I have advocated is as empirically justified as conventional SR. Have you bother to look at any of them in detail?.

The last one I linked to provides  for GPS and Maxwell's equations which covers one of your objections and Robin's.

For what it's worth I agree conventional SR provides the simplest equations and is thus to be prefered.

Do you object to the Hamiltonian or Lagrangian formulations of classical mechanics because they cannot be empirically differentiated from Newtons theory?  They are equivalent as are anisotropic versions of SR equivalent to Einstein's .

If that is not enough then nothing I can say will be.

Regards Andrew

The difference is how it is being interpreted.  You are using it as evidence that it does exist.  The papers demonstrate that mathematically the laws we have in place cannot differentiate between the two possibilities.  Therefore this suggests that there may be something about the laws that needs to be refined.  The observations suggest that there is no difference in whether you look left or right.  Whether that is experiments that measure radiation pressure, to spectrum of emissions and so forth.

Mathematical formulations are just a different way of describing the same principle but it doesn't change fundamental parameters.  We know Newton's theory was wrong from measurable evidence (e.g. Mercury's orbit) whereas relativity does resolve this and we continue to test it all the time.  We know something is 'wrong' as we can't combine relativity with quantum mechanics but observationally both fit the theories.  This is why we test ever more refined experiments (e.g. CERN, gravitational wave detectors).  What we didn't do when we had a problem with Newton's laws was to arbitrarily adjust some of the parameters to make things 'fit' to the observations.  We could have done this but we would not understand the universe any better (and in fact science would stagnate).  This is why just arguing that you could adjust the factors in an equation between c and frequency isn't sound.  It's in no way testable, has no physical rationale behind it and drives us down a dead end.

As I said before this is the approach we took when we assumed the Earth was the centre of the universe.  We devised ever more elaborate ways to explain the patterns of the orbits but with no physical reality behind it.  It failed because it could not predict the next object found.

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On 28/02/2021 at 12:42, Whirlwind said:

The difference is how it is being interpreted.  You are using it as evidence that it does exist.

Not at all. The only claim I am making is that you can't empirically tell them apart.

It is because of this Robin's experiment can't prove the isotropy of the one way speed of light.

We have for good reason adopted, by convention, the simplest symmetrical solution.

You were claiming  you could empirically differentiate between them which I hope the papers convinced you you can't.

I can transform a Sun centered model of the solar system to an Earth centred one. The equations of motion would be complex and give little or no insight into the dynamics. However, it would be empirically identical to the Sun centered system. You can't  prove one is right and one wrong. They are more or less useful in different contexts. If I am felling a tree in my garden the Earth centred system is the one I use.

On Newtons laws, the MOND theories are doing exactly what you are claiming is not done. Also the neutrino was predicted to patch up the conservation of energy and momentum long before it was found.

One last point might help. No observation is ever theory independent. Making any observation relies on a framework of theory. To a child the Sun obviously transits the earth from the eastern horizon  to the western horizon. It is only in a wider theoretical model that you observe otherwise and "see" it is obviously due to the rotation of the earth.

Regards Andrew

PS it was Newtons theory of gravity that allowed the prediction of the "unseen" planets not the adoption of a Sun centered system which is incidentally only an approximation. It also was, wrongly as it turns out, used in exactly  the same way to predict the planet Vulcan to explain the deviation of Mercury's orbit. Science is much more messy than you appear to think it is.

Edited by andrew s
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I am getting old. I intended to say that section A2  of this , that I posted before, shows the dispersion relation in anisotropic coordinated is equivalent to the isotropic one. It does so by derivation from Maxwell's equations in anisotropic form.

As they are equivalent no experiment can differentiate them. However, as the authors point out the equations are symmetric and more elegant in isotropic form.

All everyday science adopts the symmetric convention and it is built into the current SI units.

Regards Andrew

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• 1 month later...
Posted (edited)

Oh dear, I have a conundrum,

if a photon, a bit of light, traveled at a different speed this way from that, ie. not isotropically, it would need to 'know' which way it was supposed to be going ?

Without a universal frame of ref. how would it know it was traveling from me to you and that it must change its speed when returning ?

Edited by Corncrake
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8 hours ago, Corncrake said:

Oh dear, I have a conundrum,

if a photon, a bit of light, traveled at a different speed this way from that, ie. not isotropically, it would need to 'know' which way it was supposed to be going ?

Without a universal frame of ref. how would it know it was traveling from me to you and that it must change its speed when returning ?

Anisotropy makes sure of that - photon does not need to "know" anything.

Imagine you are swimming in a river that is flowing - you have a blind fold on your eyes. To you - it does not really "feel" any different depending on whether you are swimming up or down stream. For someone on the shore - it is rather obvious that you are swimming faster down the stream compared to the other direction.

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6 minutes ago, vlaiv said:

on the shore

Thank you for attempting to lighten my darkness ! , yes, exactly my problem, where is (or what is) the shore for a photon ?

(Perhaps I was being a bit silly to muddy the waters with "knowing")

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Posted (edited)

But but but but .... is this the same really?  Suppose light travels slower on the outward journey and faster on the return.  The outward and return directions are different depending on whether the light starts from Alice and returns via Bob to Alice, or starts with Bob and returns via Alice to Bob. A to B and back to A or B to A and back to B.  So what would tell the photon to behave differently on the outward and return journeys?

Edited by Ouroboros
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9 minutes ago, Corncrake said:

Thank you for attempting to lighten my darkness ! , yes, exactly my problem, where is (or what is) the shore for a photon ?

(Perhaps I was being a bit silly to muddy the waters with "knowing")

In shore example - it is quite obvious since we can see the flow of the water.

Suppose that EM quantum field is "flowing" in arbitrary direction in space. We can't see actual quantum field except for vibrations in it that we identify as photons. Could we able to tell anything at all?

Maybe flowing analogy is not as good - maybe we can have additional field that photons couple to and it is anisotropic - and photons couple more or less to it depending on direction. Again - no way to tell.

12 minutes ago, Ouroboros said:

But but but but .... is this the same really?  Suppose light travels slower on the outward journey and faster on the return.  The outward and return directions are different depending on whether the light starts from Alice and returns via Bob to Alice, or starts with Bob and returns via Alice to Bob. A to B and back to A or B to A and back to B.

It really does not make a difference for original question - which ever way anisotropy is pointing - we won't be able to tell since we can't measure single leg of the journey.

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21 minutes ago, Ouroboros said:

So what would tell the photon to behave differently on the outward and return journeys?

Exactly so.

9 minutes ago, vlaiv said:

which ever way anisotropy is pointing - we won't be able to tell since we can't measure single leg of the journey.

I think we just went round in a circle, from the photon not knowing to us not knowing ! So, a single leg measurement is a meaningless question ? I think that is what Andrew S was saying somewhere ??

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Posted (edited)

What I was saying the isotropy is built into the assumtions in the current SI system and via the Einstein clock synchronisation protocol into relativity and QED.

You can perfectly well have an anisotropic speed of light but all the equations are much more complex.

Experimentally you can't tell them apart.

Photons etc don't need to know what to do. If you pass light through an anisotropic crystal like calcite you naturally get the splitting into the ordinary and extraordinary rays due to the interaction of the EM field of the crystal to the of the light.

If space were anisotropic the different speeds would just happen due to the anisotropy.

I am very happy to go with it being isotropic as there is no indication it's not.

Regards Andrew

Edited by andrew s
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Yes I know about birefringence, (it is entertainingly pretty in my microscope) but that is just replacing the shoreline of the river with a crystal.

Where is the reference for a photon in free space ? Would that not need a universal direction to be defined somehow ?

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3 minutes ago, Corncrake said:

Where is the reference for a photon in free space ? Would that not need a universal direction to be defined somehow ?

That is the meaning of anisotropy.

Isotropic space means that it is behaving equally irrespective of chosen direction. Anisotropic space means that it is behaving differently depending on chosen direction.

I don't think that Anisotropic space implies global reference frame, but it does define preferred direction and as such - any reference frame would have preferred direction.

We don't need to specify reason for it behaving differently - in much the same way we can't specify the reason why it is behaving the same in every direction. It is just the fact.

Whole problem is that given our current state of understanding - we accepted cosmological principle - which comprises of isotropy and homogeneity (on large scales) and it is build in our formulas. This thread has been about pointing out that if anisotropy exists that effects the speed of light - we would not be able to tell from experiments.

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Posted (edited)
9 minutes ago, vlaiv said:

That is the meaning of anisotropy.

Isotropic space means that it is behaving equally irrespective of chosen direction. Anisotropic space means that it is behaving differently depending on chosen direction.

I don't think that Anisotropic space implies global reference frame, but it does define preferred direction and as such - any reference frame would have preferred direction.

We don't need to specify reason for it behaving differently - in much the same way we can't specify the reason why it is behaving the same in every direction. It is just the fact.

Whole problem is that given our current state of understanding - we accepted cosmological principle - which comprises of isotropy and homogeneity (on large scales) and it is build in our formulas. This thread has been about pointing out that if anisotropy exists that effects the speed of light - we would not be able to tell from experiments.

Exactly. Regards Andrew

PS It might be worth recalling spacetime has properties and in GR they are dynamic.

Edited by andrew s

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