the cyclist and the loiterer

 I am loitering on the side of the road with a  flashlight.  Beagles is standing half a mile further down the road.  From up the road comes Old Dog on his motorcycle.  Just as he passes me he flashes his headlight and at exactly the same time I blink on my flashlight.  The light from my flashlight and from Old Dog's headlight will hit Beagles at the same time.  

I think that is what Beagles is thinking of when he spoke of light from different distances reaching a point at the same time.  He is thinking of something counter intuitive and this certainly is.  One would think that the light from Old Dog's headlight would be moving towards Beagles at the speed of light plus the speed of the motorcycle, but of course that would be impossible.

The thing about the speed of light being constant is that it looks constant to every viewer who all have different viewpoints because they are generally moving relative to each other.  In this instance Beagles and I are moving at the same speed (from the rotation of the earth etc.), but Old Dog is moving at a different speed than us, so Beagles sees him differently.  A body moving at a different speed, appears to be moving slower than a body at rest relative to the observer so Old Dog will appear to Beagles to be moving slower (likewise Beagles will appear to be moving slower to Old Dog), and that will work to make his light appear at the same time as mine.

I think.

That's the best I can do for the nonce without doing any research.  I was going to include a horn among the instruments and have us both blow them as Old Dog roars past me, thinking the sound of Old Dog's horn will arrive sooner, but doing a little research I am not so sure.  There is such a thing as the speed of sound and I am not sure why sound should have a constant speed.  Isn't sound just stuff bumping into other stuff?  Isn't that the way with waves in the sea, and don't they all travel at different speeds?

I don't know.

So one thing I have said, I think is true, but I am not sure, and the other I have to admit I don't know anything about.  Not a very good average I am afraid, but I have stretched my mind out about and I always enjoy that.