While it has long been observed that hypothetical changes in the speed of light would be difficult to measure if certain other quantities changed with it in lockstep (and by "long" I mean 100+ years, going all the way back to the original debates around relativity), it is still meaningful to ask if the speed of light has changed relative to the quantities that it seems to related to.
If that turns out to be the case, it will mean that our definition is wrong, not that the speed of light isn't changing. Our definition is a constant because we believe it to be a constant. If we're wrong, it will need to change.
I could define Pi as 3 and it would be CONSTANT. We'd then argue over what shape a circle looked like, but my maths would certainly be easier than yours due to lack of irrationality.
Actually that argument was had and ended a long time ago; you can set pi to 3 and you get a thing called a spherical geometry. See, for instance, http://mathforum.org/library/drmath/view/55021.html . Between the word "spherical geometry" and what you find in that link you'll have the keywords to continue digging if you want to.
Perhaps the relevant point is that the speed of light is not a number, but a dimensioned (if that's a word) quantity; and now the question becomes: is the dimensioned quantity (either its numerical value, or the meaning of the units) changing? I agree that it's hard to know how to make sense of the notion of units changing, though.
t' = t √(1-v^2/c^2)
That's pretty clear and easy to interpret. So measurements of c may be messy and subject to controversy, but the geometry expressed in the above equation, and what it tells us about spacetime, is much more clear.
> Yes. Light is slowed down in transparent media such as air, water and glass.
[...]
> Special Relativity
> [...]
> The speed of light does not vary with time or place.
These two different ways of talking about speed of light caused megabytes of meta bickering on the Wikipedia page for speed of light, and a case for Arbitration, with topic bans and warnings. (Also the suggestion that you can measure the speed of light in SI units, because after 1983 the SI units use light to define them.)
http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/...
The SoL page shows some of the problems of WP. Articles should have a simplistic lead (with caveats), a general introduction, and then higher level discussion of the oddities as currently understood. The WP page ignores all that, and leaps in at the deep end which means the article isn't much good for anyone.
When you go through quantum field theory, photons are defined in terms of freely propagating particles, not interacting with other fields. When the disturbance in the EM field propagates into, e.g. a dielectric material, and strongly couples to various nuclear and electronic excitations, it is better described in terms of a massive quasiparticle, the polariton, which is a hybrid of the photon, phonon and electron fields and, being massive, propagates at less than c. You can, of course, describe it in terms of perturbations to the free photon corresponding to various types of virtual absorption and re-emission, but it's a bit misleading to think of it being physically absorbed and re-emitted with little stopovers. If anything, the classical model of a continuously interacting medium interacting with the EM wave creating a coherent response wave which interferes with and appears to slow down the EM wave is more instructive.
That's just an assumption.
> That's just an assumption.
Supported by evidence. (And no counter-evidence is known.)
Light has no rest mass. But, light can never be at rest.
Light does however, have momentum. And when it is reflected/refracted, it will transfer some of that momentum to whatever it hit.
Also remember that forces act on pairs of things. Light can be deflected by gravity (which will change it's momentum); therefore it must produce a gravitational field (which will in return change the momentum of whatever deflected it). And of course gravity is proportional to mass.
Light which is moving towards and object will gain momentum, like a falling rock would. Only instead of moving faster (since it's at a fixed speed), it shifts towards higher frequencies. If you look at the equations slightly differently, you get gravitational time dilation.
If you take a large-scale view, you get really good results by using a mathematical model that says that the light is behaves like a wave with a certain speed, and that the speed is slower in water.
If you take a small-scale view, you get decent results by using a model that says that individual photons have a small chance of being absorbed by the water molecules and then new photons are radiated again.
Supposedly there is a quantum mechanical view which applies on all scales and gives even more accurate values, but I have never seen it worked out for something as complicated as the interaction between light and a surface of water molecules. Both of the others I have worked out myself back when I was a physics student.
In a convex lens, photons take longer to pass through the thickest part, which creates a concave wavefront that naturally converges on a focal point:
http://arachnoid.com/example/index.html#Lens_Example
Photons don't need to have mass for this to happen, because they aren't being deflected like billiard balls, they're being slowed by their interaction with atoms.
This is the average speed of photons in vacuum, right? Because according to the quantum field theory, photons should have a probability of hitting virtual particles that popped into existence, which would slow it down a little.
Einstein does not mean "speed" when he uses the word "velocity" here or anywhere. He means "a vector quantity consisting of a speed and direction".
[1] http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...
