A little while ago I provided input to a video by Veritasium on the One-Way Speed of Light. You can watch the video __here__ . This is not a new idea - Einstein realised that you cannot measure the one-way speed of light, only the two-way speed, and so we assume that the one-way speed is equal to the two-way speed.

I am amazed by the reaction of most physicists when they encounter this. They are certain it must be wrong because, well, because... and a lot of foot stamping ensues. Once you realise this is a coordinate transformation, and so all the transformations are preserved, so the observables come out to be the same, eventually resignation sinks in.

But I still get emails etc with new and interesting methods to measure the one-way speed. Here's the latest.

So, I went through the equations, and (as expected) the result of the experiment is the same irrespective of whether light is isotropic or anisotropic. Physics wins again!

I don't claim to understand a lot of the math behind this debate, but I wonder if the thought experiment below fits in with the physics behind the unknown one way speed of light.

Imagine a light source traveling towards a black hole that is positioned on a horizontal line. At a safe distance to the left of the black hole is observer A, and at an equal distance to the right of the black hole is observer B. We'll also assume that light in this scenario travels at 1.5c when traveling right to left, and 0.5c when traveling left to right. Both observers watch the light source as it nears the black hole and reaches it's event horizon.

Yet…