Re: The speed of light is neither constant nor variable
- From: Tom Roberts <tjroberts137@xxxxxxxxxxxxx>
- Date: Mon, 31 Dec 2007 10:41:12 -0600
Albertito wrote:
A photon is emitted from A to B, but if you measure that photon at an
intermediary point C, then that photon was emitted from A to C, not to
B.
This means there is no path for any photon, only two points, (i.e.
emission
and absorption points).
Not quite. Say, rather, that there are an infinite number of paths between a given pair of points (in spaceTIME), and the photon follows them all. There is a probability amplitude associated with each path, and the probability of detecting a photon at the endpoint is the square of the sum of those amplitudes times the probability of the initial emission.
For pairs of points separated by macroscopic distances in vacuum, that probability is essentially zero except for spacetime points separated by a null interval (i.e. for all practical purposes light travels with speed c over macroscopic distances).
If there are charged particles nearby (e.g. in a material medium), then the effective speed will be reduced by an amount related to their density and binding energies -- we normally characterize this by the index of refraction of the medium. This effect is due to the quantum interference among all the possible paths, including those that include interactions with the charged particles.
All this is contained in the theory called QED, the quantum field theory of photons and charged particles. To learn more about the basics, I highly recommend:
Feynman, _QED_. A non-mathematical introduction to quantum
electrodynamics.
> [... much nonsense, because you forgot about the probability
> amplitude associated with each path]
The speed of light in a vacuum, under emission theory, expressed as c'
= c+v is [...]
All such theories have been refuted (including Henri Wilson's "BaTh", even though he won't admit it).
Kapowski wrote:> If you have a reference showing that a photon can travel slower or faster
> than c in a vacuum, I'd like to see it.
The "speed of a photon" does not make sense, due to quantum indeterminacy -- attempting to discuss photons is QUITE DIFFICULT due to subtle quantum effects. One is usually able to discuss light rays and beams much more accurately (this is the large photon number approximation to QED, also known as wave optics or physical optics).
There are a lot of references showing that phenomenon, for instance,
all those concerning Sagnac effect.
SR explains and describes the Sagnac effect without light traveling at any speed other than c in vacuum, RELATIVE TO ANY INERTIAL FRAME. The standard ballistic/emission theory, in which light travels with speed c+v where v is the velocity of the source (relative to an inertial frame), obtain the wrong answer for Sagnac (this theory predicts a null result, but the experiment obtains a non-null result).
Tom Roberts
.
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