There has been a lot of discussion recently on the topic of fluctuations in spacetime and whether, ''they cause a heat signature.'' A heat can only be generated by a system, if the system contains some kind of motion in its elementary/fundamental structure. As I explained also, there are no two ways around it, if you speak about heat energy you must be referring to motion within the system!!! Now, if you remove all visible matter and energy from a square measure of space, you will find that zero point energy fields persist: in other words, the heat cannot be ONLY generated by the visible matter in the universe, there must be residual motion of other systems ie. fluctuations. There are subtle but very important differences between off-shell (virtual) to on-shell (visible) matter. I always found it interesting to note that while virtual particles do exist, they cannot be described by the same operators we use to describe ordinary matter - ordinary matter consists of Hermitian matrices making them observables, while fluctuations, exist for such a short time, we would be forced to find some other description for them. So this is really a no-brainer - the question is largely rhetorical based on what I just said, but here we go: ''if you removed all visible matter and energy from a system, but you still find a residual heat, what could be causing a heat signature if not caused by vacuum fluctuations.'' Be careful in your replies, remember, heat is motion.