I hope you agree that time travel is science fiction. It’s because clocks don’t literally measure the flow of time. What they really do is clock up some kind of regular cyclic motion and give a cumulative display that we call the time. That sounds like it’s no big deal. But it is. And it’s a whopper. 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". But what’s the big deal? Where's the whopper? The big deal is he’s talking about optical clocks, 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. And the whopper is this: the speed of light is not constant. 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. But 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 my boy. It’s speed and direction. The velocity changes because the direction changes". Guess what? That’s 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. 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 a car veers when it encounters mud at the side of the road. But those brush-off guys will dismiss this like they’re crazy. Do you want to see just how crazy? In relativity, we use an idealised optical clock called the parallel-mirror light-clock. It’s employed in the simple inference of time dilation due to relative velocity, which uses Pythagoras's theorem. OK, imagine we use two parallel-mirror light-clocks instead of the NIST optical clocks to demonstrate gravitational time dilation. Let’s exaggerate a little to make it more obvious. What you’d see is this: Please Register or Log in to view the hidden image! Image credit: Brian McPherson Yes it's exaggerated, and yes the mirrors should be tilted back a little, and yes the light pulses should curve a little. But those two light pulses aren’t going at the same speed. If they were, the clocks would stay synchronised. You’d have to be crazy to say the two light pulses were going at the same speed. And do you know what else is crazy? Check out the NIST caesium fountain clock. It’s used to define the second, like so: "Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". See that mention of radiation? That’s light. Some will say there’s a peak frequency which is found and measured by the detector. But note that frequency is defined as cycles per second, and the second isn't defined yet. So what the detector effectively does is count incoming microwaves. When it gets to 9,192,631,770, that's a second. After that, the frequency is 9,192,631,770 Hertz by definition. Then we use the second along with light to define the metre, like so: "Since 1983 the metre has been defined as the distance travelled by light in vacuum in 1⁄299,792,458th of a second". We use the motion of light to define the second and the metre. And what do we use them for? To measure the motion of light. That’s why we always measure the speed of light to be 299,792,458 m/s. But it isn't constant. If it was, we wouldn't have a Shapiro delay. Those NIST optical clocks would run at the same rate. And they don't. Because the coordinate speed of light varies in a non-inertial reference frame. Like in a gravitational field. Like in the room you're in. And the coordinate speed of light is the speed of light. The measured speed of light isn't. Like Magueijo and Moffat say, that's just a tautology. And when you appreciate that, and look at the evidence and what Einstein said, it's as plain as day that the speed of light is not constant. As obvious as the nose in front of your face. As obvious as your blind spot. Once you know how to look.