Have a look on arXiv for papers with variable light speed or varying speed light or VSL in the title. There's a fair few. One of my favourites is Comments on "Note on varying speed of light theories" where Joao Magueijo and John Moffat were having a bit of a dispute with George Ellis. They said this:
"The unit of time is defined by an oscillating system or the frequency of an atomic transition, and the unit of space is defined in terms of the distance travelled by light in the unit of time. We therefore have a situation akin to saying that the speed of light is “one light-year per year”, i.e. its constancy has become a tautology or a definition"
It's true. And in truth, the speed of light is not constant at all. It's easy to appreciate this once you appreciate that time travel is a fantasy. Because clocks don’t literally measure the flow of time. A clock isn't some kind of cosmic gas meter with time flowing through it. Instead a clock "clocks up" some kind of regular cyclical motion and gives a cumulative display that we call the time. It's a simple little observation, but it leads you places.
You’ll know about gravitational time dilation: clocks go slower when they’re lower. We allow for this in GPS, and it’s even detectable in a lab. See this interview with David Wineland of NIST: "if one clock in one lab is 30cm higher than the clock in the other lab, we can see the difference in the rates they run at". The clocks he's talking about are optical clocks. And like other clocks, optical clocks go slower when they're lower. And when a clock goes slower it’s because the regular cyclic motion inside that clock is going slower. So what sort of regular cyclic motion might you find inside an optical clock? The motion of cogs? The motion of a quartz crystal? No. The motion of light. I know that goes against the grain of what people say about relativity. People say things like "Einstein showed us that the speed of light is constant". He did in 1905, when he was doing special relativity. But check this out:
That’s Einstein talking about the speed of light varying in a gravitational field. If you ask around about all this, some will brush it off by pointing to the word velocity. They’ll say "It’s a vector quantity. It’s speed and direction. The velocity changes because the direction changes". But they're wrong. Go back to the original German, and what Einstein actually said was that a curvature of rays of light can only take place when "die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert". That translates to "the propagation speed of the light with the place varies". The word "velocity" in the English translations was the common usage, as in "high velocity bullet". This is crystal clear because Einstein referred to c which is the speed of light, and to "one of the two fundamental assumptions". That’s the special relativity postulate of the constant speed of light.Originally Posted by Einstein
Have a read of Ned Wright’s Deflection and Delay of Light and note this: "In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light”. Light doesn’t curve because it curves, and it doesn’t curve because spacetime is curved. Einstein never said that. It curves because the speed of light varies with position. Like sonar waves curve. Like this:
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