# The C is the speed of photon ( light ) ?

Dale
Mentor
So what's shapiro effect ?
I think you mean, how is it consistent with my comment. The Shapiro effect does not occur in an inertial frame, it only occurs where there is tidal gravity and therefore no global inertial frames.

If you really just want to know what it is: http://tinyurl.com/3t5y4gj

So what's shapiro effect ?

Imager
Gold Member
From the FAQ: Do Photons Move Slower in a Solid Medium?

So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

I am confused about this statement from the FAQ mentioned earlier, first it says "the lattice does not absorb this photon" then is says "and it is re-emitted". How can it NOT be absorbed and still be emitted?

Bob

pervect
Staff Emeritus
Given that if you perform a local experiment the speed of light is always "c" , regardless of gravity, whether you're deep in a gravity well or far outside it, what do people mean when they say "the speed of light varies"?

What they mean is that in some particular coordinate system, the rate of change of the position coordinate with respect to the time coordinate isn't always the same.

This really has more to do with the behavior of coordinates than any property of light itself.

It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the lattitude with respect to time is greater.

ghwellsjr
Gold Member
It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the lattitude with respect to time is greater.
Don't you mean longitude?

pervect
Staff Emeritus
Don't you mean longitude?
Yes - good catch...

Given that if you perform a local experiment the speed of light is always "c" , regardless of gravity, whether you're deep in a gravity well or far outside it, what do people mean when they say "the speed of light varies"?
What they mean is that in some particular kind of coordinate system, the rate of change of the position coordinate with respect to the time coordinate isn't always the same.
For one, I don't mean that when I say that.
Instead, I mean that for *any* "universal" coordinate system (that uses single, non-local standards for length and time) the speed of light isn't *everywhere* the same. Only for a particular coordinate system that is kept very small, or one that has "elastic units", the speed of light is "always c".
This really has more to do with the behavior of coordinates than any property of light itself.
It's a bit like saying that naval vessels "move faster" when they are near the north pole, because the rate of change of the [longitude] with respect to time is greater.
The earth is a physical object to which those coordinates relate. Thus, do you mean that:
1. there is a physical equivalent to the Earth, but which we cannot see?
or
2. that it's an artifact of using a certain kind of coordinate system?
- if 1.; what is it?
- if 2.; then Einstein's light bending calculation was based on a mere artifact. Then how do you explain the bending of light?

Harald

WannabeNewton
You seem to keep confusing coordinate systems and what is physically happening. Yes $\frac{\mathrm{d} \phi }{\mathrm{d} y}$ for a photon uses a specific coordinate system but it can be done in any arbitrary coordinate system and so the coordinate value is ambiguous even though physically light does bend. People use the coordinate system that is the easiest to work with and that allows one to define classical parameters. You could use a convoluted coordinate system for the schwarzschild metric that results in some crazy value for $\frac{\mathrm{d} \phi }{\mathrm{d} y}$ for the photon but it is easier to make sense of certain parameters when the coordinate system itself is logical, like the impact parameter for the photon which is $b = L/E$ which would be easily definable in the usual coordinate system for the schwarzschild metric but not so in some other coordinate system. But the physical action of light bending is still happening, we just use a certain system to quantify it. Also, for an arbitrary manifold it is not always possible to construct an atlas with a single, universal coordinate chart. In general, a maximal atlas on a manifold will contain multiple smoothly sewn coordinate charts.

[..] But the physical action of light bending is still happening, we just use a certain system to quantify it. [..]
Perhaps you did not see my post #57? Thus my same question no.2 for you: If you think that Einstein's calculation is based on an artifact from choosing a certain coordinate system which gives a misleading result, then there should be no speed gradient and therefore there can be no light bending based on the wave model of light that he used for GRT. Thus how do you explain the effect? Clearly you do not deny that it occurs.