[jeffreyH]

Hint: you cannot use the ratio itself to answer this question. That would be circular reasoning.

[geordief]

Here is another point to ponder. The speed of light through a medium is dependent upon the refractive index of the medium. What is the refractive index of the vacuum? The vacuum is an absence of anything. So why is the value of c the speed of light in the vacuum? Why not 10c or 1000c or 602.5c? What exactly is limiting the speed of light?

Mass seems to make things move more slowly so massless objects should move fastest.

Since things must move at a finite speed (the alternative would be that everything would happen at the same time and causality would  collapse),massless objects must move at this maximum  but finite speed.

This speed is found experimentally to be c.

So nothing is limiting the speed of light  and it just moves at the maximum speed.

That is my take.I hope it is right ;-)

[Colin2B]

Mass seems to make things move more slowly so massless objects should move fastest.

That logic doesn’t work here. Mass makes things accelerate more slowly for a given force, but final speed is dependent on both force and time, both of which are variable.

Since things must move at a finite speed (the alternative would be that everything would happen at the same time and causality would  collapse)

Again this  isn’t logical. Even if everything could move at infinite speed it doesn’t mean everything would happen at once.

This speed is found experimentally to be c.

It hasn’t been found experimentally, it is a consequence of SR. No experiments to date have taken objects with mass to c, but the experiments below c show that the speed/energy relationship follows the predictions of SR and therefore c appears to be the limiting speed for objects with mass.

[Bogie_smiles]

Here is another point to ponder. The speed of light through a medium is dependent upon the refractive index of the medium. What is the refractive index of the vacuum? The vacuum is an absence of anything. So why is the value of c the speed of light in the vacuum? Why not 10c or 1000c or 602.5c? What exactly is limiting the speed of light?

Mass seems to make things move more slowly so massless objects should move fastest.

Since things must move at a finite speed (the alternative would be that everything would happen at the same time and causality would  collapse), massless objects must move at this maximum  but finite speed.

This speed is found experimentally to be c.

So nothing is limiting the speed of light  and it just moves at the maximum speed.

That is my take. I hope it is right ;-)

There is no perfect vacuum, and in that context, there is never an “absence of anything”. So can we say that the refractive index is variable, relative to the medium?

Refractive index
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how light propagates through that medium. It is defined as.

From Wikipedia, the free encyclopedia



A ray of light being refracted in a plastic block
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as
n=c/v

where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water.

Edit: See Wiki on Refractive index

[Colin2B]

There is no perfect vacuum, and in that context, there is never an “absence of anything”. So can we say that the refractive index is variable, relative to the medium?

You have copied a large section of Wiki but don’t seem to have answered the question or shown it’s relevance to the topic.
Although there is rarely a perfect vacuum, space is extremely close (even the speed of light in air is not far off vacuum) and CERN is able to produce pressure of the order of 10^-10 to 10^-11 mbar - a vacuum almost as rarefied as that found on the surface of the Moon. At these pressures there is little practical difference between the theoretical speed of light in vacuum and that measured in the accelerator.

So, within the limits we can measure SR hangs together and tests in particle accelerators confirm this.

[geordief]

Mass seems to make things move more slowly so massless objects should move fastest.
That logic doesn’t work here. Mass makes things accelerate more slowly for a given force, but final speed is dependent on both force and time, both of which are variable.

Quote from: geordief on Yesterday at 22:17:42
Since things must move at a finite speed (the alternative would be that everything would happen at the same time and causality would  collapse)
Again this  isn’t logical. Even if everything could move at infinite speed it doesn’t mean everything would happen at once.

I accept that.

Is there any reason to suppose that the maximum speed limited is connected to the size of the observable plus unobservable universe?

Have I gone off topic now?

[Colin2B]

Is there any reason to suppose that the maximum speed limited is connected to the size of the observable plus unobservable universe?

No, size never came into the equation.
It has to do with electrodynamics, specifically the physics of electric and magnetic fields as derived by Faraday, Amper, Coulomb and Gauss, and what happens when they move. Einstein just followed it through to its logical conclusion. I say ‘just’, but in reality it was a big leap solving problems which had been puzzling scientists for quite a while.
This is also why you can’t derive SR from Newtonian mechanics.

[jeffreyH]

The question of why the speed limit of light in a vacuum is c is one of those unanswerable questions, as Feynman pointed out in interviews. The problem with posters who pose questions such as the one in this thread is that they do not appreciate these types of issues.

When you argue against established knowledge you need to be able to support your position. You cannot simply guess that you have found a 'loophole'. That benefits no one.

Newton was around at a time when Galilean relativity was still the norm. He was a very clever man but was limited in his knowledge nonetheless.

[pensador (OP)]

Here is another point to ponder. The speed of light through a medium is dependent upon the refractive index of the medium. What is the refractive index of the vacuum? The vacuum is an absence of anything. So why is the value of c the speed of light in the vacuum? Why not 10c or 1000c or 602.5c? What exactly is limiting the speed of light?

Mass seems to make things move more slowly so massless objects should move fastest.

Since things must move at a finite speed (the alternative would be that everything would happen at the same time and causality would  collapse),massless objects must move at this maximum  but finite speed.

This speed is found experimentally to be c.

So nothing is limiting the speed of light  and it just moves at the maximum speed.

That is my take.I hope it is right ;-)

You can view c as being a fixed constant that goes in exactly straight lines.

But

It is reasonable to assume light is limited by the medium it travels through "in all cases". For space c=1/ 

Light speed has been measured to be c, as it travels through space which is full of virtual particles, CBR, and various other little bits of matter which probably dont matter too much in the scheme of things. It will be absorbed and reemitted and interact with virtual particles it passes through. Similiar to the way it moves through other materials.

Space has no refractive index and is transparent to electromagnetic waves.

Generally the speed of light is thought to be c, but these guys think they might be able to go faster

https://www.livescience.com/29111-speed-of-light-not-constant.html 

[Colin2B]

Light speed has been measured to be c, as it travels through space which is full of virtual particles, CBR, and various other little bits of matter which probably dont matter too much in the scheme of things. It will be absorbed and reemitted and interact with virtual particles it passes through. Similiar to the way it moves through other materials.

Please don't put speculation and New Theories into this section of the forum.
Thank you
We'll answer on this occasion but bear in mind that in future posts may be moved or you could be limited to 'The Lighter Side' 

The amount of matter in space is very sparse and the main interactions are scattering and absorption. Even air, which is relatively dense makes only a small decrease in the speed compared to vacuum and to suggest light is slowed significantly by matter – virtual or otherwise – implies very dense matter.
One interesting aspect is dispersion, which occurs when light travels through a medium. This can be seen with pulsars where higher frequencies arrive earlier than lower frequencies which interact with free electrons. However, in the sort of vacuum we see in, say, CERN accelerators, the speed is as close to theoretical as is measurable.

References to virtual particles can be very misleading and are often taken out of context.
I know I keep banging on about people misinterpreting jargon and shorthand terms, but it’s worth reading eg https://www.physicsforums.com/insights/misconceptions-virtual-particles and https://www.physicsforums.com/insights/physics-virtual-particles also https://www.physicsforums.com/insights/vacuum-fluctuation-myth
This is also why virtual photons don’t become real photons in the near field, and why photons can’t ‘bang into’ virtual particles. It’s also why pop science descriptions of the Casmir effect which quote virtual photons are misleading.

Generally the speed of light is thought to be c, but these guys think they might be able to go faster
https://www.livescience.com/29111-speed-of-light-not-constant.html

Very new theory and 2 separate views. As quoted in the article “Some scientists are a bit skeptical, though. Jay Wacker, a particle physicist at the SLAC National Accelerator Laboratory, said he wasn't confident about the mathematical techniques used, and that it seemed in both cases the scientists weren't applying the mathematical tools in the way that most would. "The proper way to do this is with the Feynman diagrams," Wacker said. "It's a very interesting question [the speed of light]," he added, but the methods used in these papers are probably not sufficient to investigate it.”