"You can't measure one-way speed of light" is formal doctrine in relativity cant, and "true" as far as it goes, in that anything you manage to measure makes no

"You can't measure one-way speed of light" is formal doctrine in relativity cant, and "true" as far as it goes, in that anything you manage to measure makes no difference without reference to an absolute rest frame. It hinges on formally failing to synchronize clocks at a distance from one another, and people make hay about that, rather pointlessly. They are really talking about "speed of causality", not light, which happens to go at the same speed; and one-way speed of causality doesn't mean anything. But the cosmic microwave background defines an absolute rest frame that we happen to be measured going 600km/s against. And you can measure one-way speed relative to that, provided you admit that, yes, you really can synchronize displaced clocks entirely adequately for the purpose, as we do absolutely routinely for GPS satellites in wacky orbits thousands of miles apart. Your measurement had better have the time-to-traverse from you to a clock indicating an extra 600km/s in that direction, and short the same in the other direction, and equal toward clocks placed at right angles to those directions. If it doesn't turn out to match CMB asymmetry, you probably get a Nobel prize. On the subject of the CMB, some of the measurements show a bias exactly aligned with the plane of the solar ecliptic. This is called the "Axis of Evil" in astronomy circles, and is rarely mentioned as it is deeply embarrassing to cosmologists. "Cosmologists are often in error, but never in doubt." -- Lev Landau.