Transcript:

Hi, so yesterday somebody asked me a question: what is the speed of light between A and B? And I honestly could not give them an answer.

The speed of light that we know of — 3 × 10⁸ meters per second — is a two-way speed of light. It is from A to B and back. It is always two-way. There is no experiment that can be done to measure the one-way speed of light. We do not know if light travels from A to B equally fast as it goes from B to A. There is no guarantee. There is no experiment that can be done to measure a one-way speed of light.

It is always two-way speed of light divided by two is assumed to be the one-way speed of light, but it is an assumption. This is very important — the speed of light that we know of is a two-way speed of light. We cannot measure one-way speed of light.

It is amazing.