How fast does light travel? Well, you might say the speed of light, but physicists have only ever measured the two-way speed of light, where light heads out and bounces back. But we have never measured the one-way speed of light and it could be between half "the speed of light" and infinity.

This is not a new insight! Einstein realised it in 1905! So why do so few scientists and students know this?

Somehow I do not want to agree with the conclusion. We can measure one way speed of light.

we need to redefine simultaneity. My proposition is as follows:

if a rigid body AB of length l is moving without any acceleration parallel to X axis and at time t0 its point A is at location x, then simultaneously its point B is at location x+l

Let's now design the experiment to synchronize distant clocks and measure one way speed of light:

Imagine four spaceship flying as a perfect square ABCD towards not moving (at least relative to each other) points EF parallel to AD and distance EF equals AD. Point E should be collinear with points AB and point F…