There is no way to measure the speed of light in a single direction that isn't dependent on how you synchronize two clocks and how you compare the two clocks. Depending on how you establish the synchronization and compare the clocks, you can measure different one way speeds of light from experiment setup to experiment setup and even speeds that different observers of the same experimental setup will disagree on.

The best we can do without introducing the issue of multiple clocks is to use a single clock to measure the time it takes light to travel from the emitter, to some other place, then back to the detector, all as measured by the single clock.

The upshot is that we know very well what the round trip speed of light is, but the one way speed of light is currently undefined and may well depend intimately on the experimental setup with the universe always conspiring to make the round trip time constant no matter what the one way speed measured is.

Einstein recognized this and, by convention, said that the one way speed of light is equal to the round trip speed. However, this is just an arbitrary definition of the one way velocity used in the formulation Einstein published and the actual one way speed may well be unknowable or even not real.

After all, the speed of light isn't special because it's light. It's special because it's the speed of causality in the universe. Talking about a one way speed might not really mean anything since causality just requires a one way flow of information from casual event to caused event. That interaction can happen at any speed and neither the casual event or the caused event can ever know how fast the interaction happened unless the caused event then communicates back to the causal event.

It's only when you try to share information two ways that causality paradoxes can occur, and that's where the well measured two way speed of light (causality) comes in. That two way interaction has a maximum speed that makes sure before and after remain in their proper order.

To illustrate this, let's say that you and I measure the two way speed of light and position ourselves ten light minutes apart, per that two way speed. Let's also say that I'm going to send you a message that says the time on my clock, starting at 0. When you get my message, you immediately send me a message saying YOUR time, again starting at 0. Every time we receive a message, we send our new time out.

Okay, let's assume that light moves the same velocity both ways. I send you "0 minutes". Ten minutes later, you get the message and you send me "0 minutes". Your message gets to me after 10 minutes, so there's been a 20 minute trip and I send you "20 minutes". You get my second message after ten more minutes and send me "20 minutes" and so on.

Now, let's do the same experiment, measure the same round trip speed of light, move the same distance apart, and follow the same rules about the messages we send. The only difference is that when I send you a message, it moves infinitely fast, but your return message only moves half the speed of light. Remember, we agree on the round trip speed that we measured together. I send you "0 minutes". You get the message instantaneously and send me "0 minutes". Now, your message only moves half the speed of light, so when I get your response, 20 minutes have passed, so I send you "20 minutes" and you, again, instantaneously get my message and send back "20 minutes".

In both circumstances, the one way speed of light is different, but the round trip speed is the same and there is absolutely no way for us to know the difference because in the process of us comparing our clocks, the round trip speed of light that we measured and agreed on prevents us from being able to see anything other than the two way speed.

Causality/relativity is wild.

This post was edited on 7/18/22 at 5:19 pm