[thedoc (OP)]

Muhammad Al-Hakeem  asked the Naked Scientists:
   
What does the variable speed of light mean?

Regards,
Muhammad Al-Hakeem.

What do you think?

[evan_au]

Whenever someone measures the speed of light in a vacuum, so far they come up with basically the same answer. So scientists think that the speed of light in a vacuum is not variable. In fact, this is so stable that it is now used as the definition of time and length, so by definition, now it cannot change.

If light travels through some medium other than a vacuum, the electric and magnetic fields of the light interact with the electrons in the material it is traveling through, slowing down the light.

The speed of light in this material can be calculated from the permeability and permittivity of the material. For visible wavelengths, the changed speed of light can be measured directly by the refractive index of the material.

The speed of light varies at different wavelengths, which separates out light of different wavelengths to produce the familiar phenomenon of the rainbow.

[Soul Surfer]

There are some theories that the speed light may change under extreme conditions.  physicist  João Magueijo is one of the people working on them.  they have no effect on everyday life but could be important in relation to "theories of everything  look him up or the wikipeadea article.

http://en.wikipedia.org/wiki/Variable_speed_of_light

to find out more.   They are however fringe concepts and not part of the accepted core theories.

[Geezer]

In fact, this is so stable that it is now used as the definition of time

That seems to be a common misconception. The definition of time is not based on the speed of light. It's based on atomic activity.

[JP]

However, the meter is based on the speed of light in vacuum.

[Geezer]

However, the meter is based on the speed of light in vacuum.

Well yes. If time was also a function of c, you'd be pretty much up the creek.

[evan_au]

The SI definition of the second also depends on the electromagnetic force, being measured by the frequency of a transition in caesium 133. http://en.wikipedia.org/wiki/Caesium_standard

If something changed the velocity of light (electromagnetic radiation) in a vacuum, then it may potentially also modify this definition of time.

[Geezer]

The SI definition of the second also depends on the electromagnetic force, being measured by the frequency of a transition in caesium 133. http://en.wikipedia.org/wiki/Caesium_standard

If something changed the velocity of light (electromagnetic radiation) in a vacuum, then it may potentially also modify this definition of time.

Are you sure about that? Caesium clocks don't care about the velocity of light.

[JP]

I think what he's getting at is that if the speed of light varies, our measurements of the atomic clock (which rely on using EM radiation) might be inaccurate if we didn't properly account for the speed of light variation.  This is more of a problem with how we go about measuring it than with the definition itself (which doesn't depend on EM radiation). 

[Geezer]

But the atomic clock tunes the frequency of a microwave source to maximize the number of photons produced, so it's really only recovering the natural frequency of the atomic transitions. If c varied, why would it have any effect on the frequency of the atomic transitions?

BTW - I think we beat this to death at least once before

[JP]

I recall lots of hand waving on this issue, but no rigorous answer.  IMHO, if you allow natural laws to vary in ways we don't anticipate, then relying on those laws to take measurements is probably fishy... I'm not sure how to think about it rigorously though. 

If the speed of light varied with space or time, for example, then clocks at different places or times wouldn't agree with each other, though they'd probably do just fine locally.

[Geezer]

My only point is that an atomic clock is just that - it's actually an oscillator that is locked to the atomic transitions of stimulated atoms, and the current definition of time is based on that technique. The adjustment process maximizes photon production, and that's the only connection there is with light, and it has nothing to do with the speed of light. I think there is a lot of confusion on this subject because most of the atomic clock descriptions I've seen don't do a very good job of explaining how they actually work!

There may be some interesting connection between c and atomic activity of course, but if there is, that's a different thing entirely.

[bizerl]

If light travels through some medium other than a vacuum, the electric and magnetic fields of the light interact with the electrons in the material it is traveling through, slowing down the light.

This could be a whole new thread here, but I was under the impression that a "vacuum" is actually a seething mass of particle/ antiparticle pairs forming then annihilating, and that this was the mechanism for Hawking radiation (although I've never understood why the black hole should prefer antimatter over matter). Do these pairs interfere with light in the same way matter does, and therefore "slow" it down?

[syhprum]

Please quote the source that says that blackholes prefer anti matter to matter I thought that gravity made no distinction.
 

[Pmb]

The speed of light in a flat spacetime in an inertial frame is always the same, i.e. v = c. However in non-inertial frames such as in a gravitational field, the speed of light will depend on the gravitational potential in the region of space where the light is passing through.

[bizerl]

Please quote the source that says that blackholes prefer anti matter to matter I thought that gravity made no distinction.
 

I'm basing this thought on the fact that black holes appear to "radiate" when a pair of virtual particles is formed near the event horizon. The matter particle escapes and the antimatter one doesn't, thus reducing the mass of the black hole. For the black hole to keep losing mass, wouldn't that mean that more antimatter particles would be trapped by the black hole than matter?

at this website it is said

A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle.

I don't really understand why the particle that fell in the black hole has to have negative energy, I may just have to put this down to "stuff I'm not really going to understand unless I go study it at university!"

Also, I think we've gone way off subject - sorry Muhammad!

[evan_au]

Addressing two tangents:
1. @Geezer & JP: The speed of light in a vacuum is derived from the permittivity and permeability of free space. It is reasonable to assume that if the speed of light in free space changed, then the permittivity and permeability of space in inside an atom would also change, and this could reasonably affect the difference in electromagnetic potential energy of the electrons in the hyperfine ground states of caesium-133 atoms which is used to set the time standard. But as I said at the start, scientists do not think that the speed of light is changing at energy levels that we can observe (and that means these other constants are not changing either).

2. @bizerl: There is no suggestion in the quoted text that it is always the antiparticle which is absorbed by the black hole - it is probably 50% chance. However, we haven't yet collected enough anti-matter to accurately measure the acceleration due to gravity of anti-particles. Current theories suggest that gravity should attract particles & antiparticles equally (one theory many years ago suggested that anti-particles should have 2/3 the gravitational attraction of "normal" particles; this could result in an excess of antiparticles near a black hole).

[Geezer]

The speed of light in a vacuum is derived from the permittivity and permeability of free space. It is reasonable to assume......

It's also reasonable to assume that it would not make the slightest difference.

The point is that the speed of light is not used as the definition of time as was tangentially proposed.

[Lmnre]

Water is a transparent substance than slows the speed of light enough that fast subatomic particles in water can travel faster than the speed in that substance (but not in a vacuum). When they do this, they emit light waves called Cherenkov radiation, and the water seems to glow a nice, light blue color — just like in the photographs.

[yor_on]

'c' is a constant having validity in both SR and GR, as far as I can see?
SR is a construction without Gravity (aka flat SpaceTime), but it does not exclude accelerations as far as I know? That Einstein later defined the equivalence between acceleration and gravity in GR doesn't state that you can't describe a acceleration in flat SpaceTime? http://en.wikipedia.org/wiki/Proper_acceleration

Baez discuss both SR and GR here, and?

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

It is a constant in both SR and GR, as he seems to see it, as well as I.
You only need to adjust for mass-energy and curved SpaceTime. A photon don't have a (rest)mass, but it has a equivalence to mass in its energy.
==

Further more, assuming that it, 'c', in fact is a valid constant, then assuming that 'information' is carried by 'photons' as being 'force carriers', now ignoring the idea of virtual photons, instead assuming indeterminacy as the proper description for QM?  Then, as far I can see all interactions we measure involve radiation? and if you know a better measurer of 'time', please tell