
Is The Speed of Light Everywhere the Same? - jc123
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html
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kazinator
> A curvature of rays of light can only take place when the velocity [Einstein
> means speed here] of propagation of light varies with position.

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".

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qwerta
Modern definition of distance is derived from speed of light. So by definition
speed of light is CONSTANT.

[https://en.wikipedia.org/wiki/Metre#Timeline_of_definition](https://en.wikipedia.org/wiki/Metre#Timeline_of_definition)

~~~
partomniscient
It's been defined as a constant to make calculations/measurements via
physics/maths easier/possible, but that doesn't mean defining it as a constant
is correct, or necessarily a good idea.

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.

~~~
jerf
"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](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.

~~~
partomniscient
Thanks for the link. Perhaps the higher order sequel to flatland is looking
for an author?

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cLeEOGPw
> the speed of light is only guaranteed to have a value of 299,792,458 m/s
> when measured by an inertial observer in a vacuum.

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.

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evanb
Ward-Takahashi identities prevent, in some sense, the virtual particles from
screwing up the speed of light.

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huuu
Does anyone know why refraction occurs when light doesn't have mass/weight?

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tbrownaw
It actually does. Kind of. (But it doesn't have anything to do with why it can
be refracted.)

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.

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dnautics
It is possible that the speed of light is anisotropic, thus leads to a theory
formulated by frank tangherlini

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jcromartie
And also creationist nonsense.

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newguy101
I heard this said on NPR this afternoon. I thought I misheard the guest. I
guess I didnt! Interesting

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jbb555
Surely the only answer possible to this is we don't know because we've not
been everywhere?

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nl
Surprised no one has mentioned Vernor Vinge's wonderful, awesome, mind
exploding _Zones of Thought_ novels[1].

[1]
[http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...](http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_series)

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jackgavigan
tl;dr: No.

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DanBC
> Does the speed of light change in air or water?

> 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/...](http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/Case/Speed_of_light)

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.

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MisterMashable
I hope this clarifies the appartent contradiction. Light can't help but move
at any other speed than c = 2.997 x 10^8 m/s. It can't go faster, nor slower.
So how does the speed of light appear to slow down in certain circumstances?
In a vacuum, the speed of light is always c. There is nothing to interfere
with the propogation of the individual photons (little atoms of light) as they
move through the vacuum. However, in a material such as glass the individual
photons of light are absorbed and reemitted many trillions of times by the
molecules making up the glass. The photons do a "stop over" and don't move at
all, the velocity of these photons is zero (actually the photons temporarily
don't exist except as energy absorbed by the atoms in the glass). Between
lattice points or individual atoms the photons travel at velocity c. The
combination of stop overs (absorbtion and reemission events) and free
propogation gives the appearance light is travelling at a slower overall
velocity than c.

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imaginenore
Why do the molecules reemitt the light in the same direction as they absorbed
it?

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pif
They don't. A photon absorbed by an atom/molecule can be re-emitted in any
direction. Imagine a thick piece of glass being traversed by a light beam:
most of the photons will traverse it undisturbed (and thus maintain their
original direction), but a few will be scattered all around (and that's the
light you can see if you watch "inside" the glass from one of its sides). On
the other hand, depending on the light frequency and the crystalline structure
of the material, photons can be scattered by the whole lattice itself, rather
than by the single atoms/molecules. In this case, the direction follows the
"rule of the mirror", which is dictated by quantum mechanics.

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TheLoneWolfling
Problem with that:

When you shine light through glass the maximum propagation speed, the speed at
which you start getting photons out the other side, is about 2/3 the speed of
light. This implies that the overwhelming majority of photons are adsorbed and
re-emitted. So this explanation cannot be correct, as I can look through a
pane of glass, but you're saying any photon adsorbed is re-emitted in any
direction, and the overwhelming majority of photons must be adsorbed and re-
emitted.

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pif
Sorry, my fault! We were talking about "absortion"/"stop over" as a way to
describe in layman's term the interaction of photons with the glass atomic
structure. In this sense, as others pointed out, we are talking about
_scattering_ and the final direction is indeed related to the original one.
Anyway, light scattering is a quantum process and there is no way of observing
the "moment" between "absorption" and "re-emission".

In my comment, I talked about actual _absorption_ , which means that there is
a finite time interval when the photon does not exist and the atom/molecule
who absorbed it can be observed in a different state than usual (electron in a
higher orbital for an atom, different vibrational modes for a molecule).
Later, the atom/molecule will go back to its normal state emitting a photon
with the same energy as the first one, or several lower energy photons. This
actual _re-emission_ will not have a favourite direction. Depending on the
typical time scale of the re-emission, you may call this process fluorescence
or phosphorescence
([http://en.wikipedia.org/wiki/Phosphorescence](http://en.wikipedia.org/wiki/Phosphorescence)).

