# One-way speed of light

Measuring the one-way speed of light is currently thought to be impossible with clock synchronization due to two misunderstandings:

- Time dilation is not reversible
- Time dilation stays in effect after the second clock comes to rest

In my previous notes I showed that time dilation is simply the result of light having to travel a larger distance when an object is moving away. So it appears that a clock moving away goes at a slower rate because the distance is increasing. If the clock then comes to rest relative to the observer, the distance between the two will be constant and time dilation will cease to be. The rate will catch up and only a difference in time will remain - that which light takes to travel the distance.

Conversely, if the clock is moving towards the observer, time contraction will ensue. The observed rate of the clock will be higher because the distance is getting smaller. This is very similar to the Doppler effect.

With this explanation it's easy to see how one might measure the one-way speed of light. The simplest method is to have two clocks synchronized locally then sending the second clock somewhere far away and taking it to rest relative to the first clock. You would wait a bit for the rate to catch up and observe the time the second clock shows from the frame of reference of the first clock. The difference will be the time light takes one-way. Divide the separation distance by that difference in time and you get the one-way speed of light in the direction from the second clock to the first.

If you place three clocks along some x,y and z axis like this, you'll be able to measure the how the observer clock is moving relative to light itself. This won't be the same as absolute motion in the classical sense but it's pretty similar.