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

[robin_astro]

1 minute ago, andrew s said:

 

1 in 10^4 is good but not conclusive in the esoteric realm of the measurement of fundamental constants 😀 No Nobel prize just yet.😏

Give me a grant and I will build an instrument to improve the precision (The bigger the grant the greater the precision 😀 )

[andrew s]

19 minutes ago, robin_astro said:

Give me a grant and I will build an instrument to improve the precision (The bigger the grant the greater the precision 😀 )

I grant you dominion over the whole observable Universe.  Now deliver. 👹

Regards Andrew 

[Macavity]

Reminded of onetime plot of evolution of the measured speed of light
- Appeared among "constants" in the Particle Data Book. But no more. 😉
(Some things are fixed...  defined... whatever, now.)

https://pdg.lbl.gov/   (Repost -- Obligatory bedtime reading?) 🥳

But the accepted values can tell an interesting story? The measurement
precision gets better with time (usually), but sometimes dramatic STEPS
occur too! Not the speed of light, but the mass of the omega meson: 😎



Different experiments, different techniques, different assumptions... etc.
Sometimes a bit of "psychology" at play?!? Someone wryl remarked on
reluctance of peer review to accept sudden change in accepted things. 🤫

Edited January 21, 2021 by Macavity

[theropod]

Don’t light echos provide us with a one way measurement?

[andrew s]

19 minutes ago, theropod said:

Don’t light echos provide us with a one way measurement?

Can you explain how? Regards Andrew 

[theropod]

From what I understand someone used the detonation of a supernova to measure the time it took the light to bounce off a dust cloud a known distance from the subject star.  Not sure about this as the mechanism, but remember reading about it.

[andrew s]

2 hours ago, theropod said:

From what I understand someone used the detonation of a supernova to measure the time it took the light to bounce off a dust cloud a known distance from the subject star.  Not sure about this as the mechanism, but remember reading about it.

The difference in arrival time is easily measured.  The issue will be how the "distance" is known without assuming the two way measurement  of c.

Regards Andrew 

[vlaiv]

Don't we know the speed of light - exactly?

I mean - we might not know how long is a second or meter - but we do know the speed of light in vacuum exactly to be 299792458 m/s

How can we measure if stick is 1 meter long in both directions?

[andrew s]

10 minutes ago, vlaiv said:

Don't we know the speed of light - exactly?

I mean - we might not know how long is a second or meter - but we do know the speed of light in vacuum exactly to be 299792458 m/s

How can we measure if stick is 1 meter long in both directions?

The issue is we only measure the two way average of the speed of light. The challenge is to measure it just one way. Within relativity the issue of simultaneity makes this theoretically impossible if you use something equivalent to the Einstein synchronisation protocols.

Now c is defined and length and time defined relative to it.

Regards Andrew

[vlaiv]

1 minute ago, andrew s said:

The issue is we only measure the two way average of the speed of light

Yes, I know that, but I'm asking if we need to measure it at all - given that it is "derived" quantity.

We have definition of time and we have definition of light. Currently, distance is derived quantity.

One meter is distance traveled by light in vacuum in one second.

I'm wondering if the fact that we can't measure light in one direction only - now translates to the problem of measuring distance in either direction? Maybe it does not translate - but we can similarly postulate that distance is not the same in both directions - and there would be no way to test it?

[andrew s]

11 minutes ago, vlaiv said:

Yes, I know that, but I'm asking if we need to measure it at all - given that it is "derived" quantity.

We have definition of time and we have definition of light. Currently, distance is derived quantity.

One meter is distance traveled by light in vacuum in one second.

I'm wondering if the fact that we can't measure light in one direction only - now translates to the problem of measuring distance in either direction? Maybe it does not translate - but we can similarly postulate that distance is not the same in both directions - and there would be no way to test it?

Not now it is defined to be c = 3.xxxxxxx m/s exactly.  The metre and second are now derived values from c.

Regards Andrew 

[vlaiv]

2 minutes ago, andrew s said:

Not now it is defined to be c = 3.xxxxxxx m/s exactly.  The metre and second are now derived values from c.

Regards Andrew 

From wiki article on second:

Quote

Since 1967, the second has been defined as exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K)

From wiki on meter:

Quote

Defines the metre as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.^[69][70]

 

[andrew s]

The attached gives the current state of play.Martin-Delgado_2020_Eur._J._Phys._41_063003.pdf

Regards Andrew 

[Whirlwind]

The problem with articles like this are that they are only really pseudoscience.  They postulate something but provide no tests to prove it is incorrect.  

I would however propose that lasers can disprove this postulation.  

Firstly lasers are based on exciting atoms and using mirrors the same wavelength/frequency of light to de-excite the atoms in a controlled way.  The atoms have specific energy levels because the electrons can only maintain certain energy levels. As a laser can use a mirror to induce this then the light will be travelling in different directions and hence the assumption would be it would be faster/slower in one direction.  However, wavelength and frequency are linked, so a change in the speed of light would change the frequency/wavelength of the light.  So what you should see over longer distances is a divergence and splitting of spectroscopic observations in one direction compared to the other (which is a lot easier to do than in white light because we are talking about specific discrete frequencies).  Over long distances this should be measurable.  In addition as the energy of the light would be different in the two directions then only in one direction would the interaction between the atoms and the light be most efficient (as the atoms have discrete energy levels).  As such the flux of the laser would be different in the two directions (as one direction has induces energy changes better than another).  

[robin_astro]

1 hour ago, Whirlwind said:

However, wavelength and frequency are linked, so a change in the speed of light would change the frequency/wavelength of the light.  

Indeed.  This is conceptually similar to my suggestion here.

 I am still to hear a convincing argument that this cannot demonstrate that the speed of light is isotropic to any given degree of precision. (My tests have already demonstrated this to 10^4)

Robin

[Whirlwind]

3 hours ago, robin_astro said:

Indeed.  This is conceptually similar to my suggestion here.

 I am still to hear a convincing argument that this cannot demonstrate that the speed of light is isotropic to any given degree of precision. (My tests have already demonstrated this to 10^4)

Robin

Yes, sorry I had seen your comment.  I was trying to produce an experiment that could be tested without the need for measuring the actual wavelength/frequency directly and have something that would fundamentally 'break' without measurement if the speed of light was different in various directions (in this case the excited atom). 

[andrew s]

5 hours ago, robin_astro said:

Indeed.  This is conceptually similar to my suggestion here.

 I am still to hear a convincing argument that this cannot demonstrate that the speed of light is isotropic to any given degree of precision. (My tests have already demonstrated this to 10^4)

Robin

The issue is you have you have to assume that space is isotropic so that the frequency is not modified by the direction of propagation.  This is esoteric (and unlikely)  but so is the issue of the isotropy of the seed if light. 

If space where like some  crystals it would have a different refractive indices depending on both polarisation and and direction.

The current SI system assumes both space and the seed of light is isotropic. 

Regards Andrew 

[Guest]

What kind of thing might make it different in one direction as opposed to another?  Why should it be different?

If you had two observers one north and one south and if they each had a mirror and if each of them sent out a photon towards the mirror at the opposite end. Then suppose they could actually measure the speed of his own photon in both directions separately.  

If the speed did come out different in the two directions would they agree on which direction was fastest. eg north to south is faster than south to north for both observers. Or would they find that the outward journey was faster for both observers for example. I assume that they would agree on the round trip speed.

 

[andrew s]

15 minutes ago, woodblock said:

What kind of thing might make it different in one direction as opposed to another?  Why should it be different?

In modern physics we don't  expect them to be different. However, in aether theory they would be due to motion relative to the aether which is why Michelson and Morrley tried to detect it.

It is easy to assume symmetry is never violated but parity P is violated in the weak  nuclear interaction and time reversal symmetry in general relativity. If things were perfectly symmetrical we would not be here as all matter and anti matter would have annihilated just leaving radiation and no matter ( it did so in about 1 part in 10^9 if I remember correctly)

Regards Andrew 

[robin_astro]

On 18/02/2021 at 17:57, andrew s said:

The issue is you have you have to assume that space is isotropic

True and since my apparatus is made up of solid components the inter-atomic forces which define its dimensions would have to be anisotropic in an anisotropic speed of light universe.  This was not the argument put forward in the original article though which claimed that in a relativistic universe it was impossible to measure the speed of light independently in both directions. I claim that my apparatus can do this without needing to resort to a round trip or  two observers with "synchronised" clocks.

[andrew s]

1 hour ago, robin_astro said:

True and since my apparatus is made up of solid components the inter-atomic forces which define its dimensions would have to be anisotropic in an anisotropic speed of light universe.  This was not the argument put forward in the original article though which claimed that in a relativistic universe it was impossible to measure the speed of light independently in both directions. I claim that my apparatus can do this without needing to resort to a round trip or  two observers with "synchronised" clocks.

True but you don't measure the speed of light with your apparatus.  You deduce it's isotropic but that deduction relies on other assumptions of isotropy. 

Your null result could be due to a change in c with a corresponding change in frequency or length. 

We seem to be talking past each other on this. 

Formally, I think  the issue is that you can't measure the one way speed of light if you adopt the Einstein clock synchronisation protocol with assume it isotropic and is built into the current IS system of units.

I will post your method on Physics Forums and see if the concur with you.

Regards Andrew 

 

[robin_astro]

On 23/02/2021 at 12:54, andrew s said:

Your null result could be due to a change in c with a corresponding change in frequency or length. 

Yes the dimensions  are assumed to remain constant under rotation but the mean frequency of the photons is based on transitions in a large population of randomly oriented atoms so is not orientation dependent. 

These kinds of arguments could equally be applied to almost any observation, even those of flat earthers. 😃. In the absence of evidence, only Ocam's razor can save us from these.  Relativity explained  anomalous observations. I would argue unless there is evidence to the contrary there is "nothing to see here".

Cheers

Robin

[Whirlwind]

On 23/02/2021 at 12:54, andrew s said:

True but you don't measure the speed of light with your apparatus.  You deduce it's isotropic but that deduction relies on other assumptions of isotropy. 

Your null result could be due to a change in c with a corresponding change in frequency or length. 

We seem to be talking past each other on this. 

Formally, I think  the issue is that you can't measure the one way speed of light if you adopt the Einstein clock synchronisation protocol with assume it isotropic and is built into the current IS system of units.

I will post your method on Physics Forums and see if the concur with you.

Regards Andrew 

 

I'm sorry but this argument simply isn't science.  You can create any number of arguments to argue against the evidence to hand (e.g. the spectroscopy doesn't work because you change frequency / length) and hence you end up with increasing divergence of physics just to 'prove' that a theory could be correct.  This is the same as saying that all stars on the other side of the galaxy are made of bright glowing marshmallows because of any number of concoctions that can be thought up.  It *could* be possible, because we can't physically check but all evidence suggests that it is not the case and hence an experiment that distinguishes the two possibilities.  

To argue your case you need to provide a test that would definitively test or disprove the arguments that are brought forward.  You have two here - one that spectroscopy measures wavelength/frequency which are related to c; the second that lasers would not work in the way they do if c changed in different directions.  You now have to put forward tests that break these experiments but at the same time still allow c to change in different directions.  Otherwise we will just go round and round.  

[andrew s]

@robin_astro ok you are unconvinced. I suspected you might be, but how about this.

 After some research I have found that you can make SR work with any convention on the one way speed of light from c/2 to infinity provided the average is c for the two way average. See  One way speed of light. for example. Since these theories give the same observed outcomes as with the Einstein convention where the speeds are equal and both equal to c they can't be experimentally differentiated.  

 I have not been able to find a formulation of your experiment but I found this paper On the Conventionality of Simultaneity which  explains that while the experimental results don't change you need to change the equations. It gives an example of the Gauss law of electromagnetism.

So the response to your challenge is that the equations you use i.e. the grating equation and c = fλ have the Einstein simultaneity convention built in i.e.  c is isotropic. 

I am not sure this will convince you but I think this is the best I can do.

Regards Andrew

[andrew s]

26 minutes ago, Whirlwind said:

I'm sorry but this argument simply isn't science.  You can create any number of arguments to argue against the evidence to hand (e.g. the spectroscopy doesn't work because you change frequency / length) and hence you end up with increasing divergence of physics just to 'prove' that a theory could be correct.  This is the same as saying that all stars on the other side of the galaxy are made of bright glowing marshmallows because of any number of concoctions that can be thought up.  It *could* be possible, because we can't physically check but all evidence suggests that it is not the case and hence an experiment that distinguishes the two possibilities.  

To argue your case you need to provide a test that would definitively test or disprove the arguments that are brought forward.  You have two here - one that spectroscopy measures wavelength/frequency which are related to c; the second that lasers would not work in the way they do if c changed in different directions.  You now have to put forward tests that break these experiments but at the same time still allow c to change in different directions.  Otherwise we will just go round and round.  

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.

Edited February 24, 2021 by andrew s