[Lewis Thomson (OP)]

Mark would like some answers to this question.

"If light travels forever, is that perpetual motion? Isn’t that illegal?"

What do you think? Leave your answers in the comments below...

[Halc]

If light travels forever, is that perpetual motion? Isn’t that illegal?

No more than a rock, in the absence of a force acting on it, moving forever in a straight line per Newton's first law.

What is illegal is perpetual motion that does work (typically in the form of friction) without input of energy. Neither the light nor the rock is performing work, so it must go forever per energy conservation laws.
Light can be made to do work without hitting anything, but then the light loses energy in the process. Ditto with the rock.

[yor_on]

It depends on definitions. A field with a one way arrow in where all SpaceTime positions are interrelated is one way to see it, and then you have observer dependencies to add to it. It makes time/change very important though if you think of it that way. That's the biggest problem with this interpretation, observer dependencies. How of much of reality do they represent? It's about reality all of it, and how you want to define it.
=

It's about ones local definition versus a 'objective global', and how they fit together. To simplify it you can think of SpaceTime as a plane in where everything consisting of positions in that plane being interrelated under a same arrow. As they change you get to 'c' (locally defined) when seen from our modern four dimensional model. It makes observer dependencies very interesting. And 'laws'