You mean entities like jellyfish in the deep ocean that use light or wholly light based entities. If the luxon theory is right, and I'm not saying it is, we are composed of light. (not trying to start a thread on luxon theory.) ;)
I think it is probably more correct to say that to easily produce a sustained note one can use occillation. One can also produce a sustained note by rolling a steel clyinder across a plane for example perhaps.
Is a planet orbiting in a perfect non decaying orbit around a black hole a perpetual motion machine? Or perhaps an electron orbiting an proton in a vacuum a perpetual motion machine? (End of the universe defining the limits on perpetual if there is such a thing.)
Could not also the total velocities cancel out? I.e. for every galaxy heading in one direction at x speed, there is an equal and opposite galaxy traveling at -x speed. I'm not an expert on physics though.
size of photons
I assumed, perhaps in error, that in its particle state a photon would occupy a region of space of relatively regular dimension for all photons. If you think I'm an idiot, I'm only a student of physics not a master you know so asking me instead of someone else seems to...
Perhaps space cannot expand at greater than the speed of light, but if we take an arbitrary bit of space, say one cubic centimeter, and have it and all the cubic cetimeters adjacent to it expand in each axis at the speed of light, then each side of the cubic centimeter will expand at...