If light does not age and is timeless. Does it not contradict the principle of perpetual motion? I read a previous question that asked if it is possible to date ancient light that has been emitted from a distant star. The best answer, explained that a photon does not ‘experience’ time as it is travelling the speed of light. Therefore old light is the same as new light. So my question is, if this light continues to travel unaffected by time, is it not compromising the idea that there is no such thing as perpetual motion ?
 A: If you define perpetual motion in this way, then Newton's first law says that perpetual motion is possible, just throw a rock in empty space and look as it flies away at constant speed forever.
Actually, when one talks about perpetual motion, the point is often to try to extract infinite energy from it. And this cannot be done with the photon, nor with the floating rock, nor with anything else.
When you read that perpetual motion is impossible, it doesn't mean that nothing can move forever. It means that you cannot extract infinite energy from it.
A: Perpetual motion is a bit of a misnomer - a poor choice of terminology.
When people discuss perpetual motion, they mean a system which can do work to a point that it breaks the Law of Conservation of Energy - meaning you get more energy out than in. Which shouldn't be possible, as far as we know.
Most perpetual motion ideas,the work they do is against friction, by simply continuing to run for a long time despite friction losses. But again,when studied carefully, they dont really do more work than energy in. Its an illusion.
So lets call these systems, not "perpetual motion" but "perpetual free work". Its more accurate.
Now, a photon in motion, in space, is doing no work. None at all. So it isn't one of these. It isn't breaking the Law of Conservation of Energy. It just continues in motion until something happens - it collides or whatever.
So you can see, we are discussing two very different things here. The terminology is the problem.
