Muphrid said:
Even if you shut yourself in a perfectly opaque box in deep space, the box will come in thermal equilibrium with the cosmic microwave background, rise to 3 kelvin, and reradiate that heat continously according to a (mostly) blackbody curve. Even at just 3K, there is some amount of radiant intensity at visible wavelengths.
DaveC426913 said:
All electromagnetic radiation propagates by way of photons.
'Light' is a tricky term; it tends to be human-centric. Bees and birds and many other animals can see well into the IR and UV range, using optical sensors, so that is one way of defining light that is not human-centric.
If you go much farther outside the visible spectrum, you can no longer use optical sensors. You get into microwaves on one end and X-rays on the other.
This is great. I had suspected this would be the case. Thank you all for helping me work through this. Now I must pose yet another question which will take me closer to my end goal:
Since, in a matter of speaking, there is "no true darkness," because there are photons being propagated within every vestige of the known universe, would that imply that the universe as a whole is made entirely from and full of substance?
To qualify, this touches on a still raging debate on whether photons actually have mass or not, or whether photons have both mass and no mass at the same time(in a similar way that photons act as both wave and particle). From the standpoint that photons do indeed have mass, and from the technical definition of substance as the mass or tangibility of a thing, then it would follow that the answer to my above question would be yes. But it could also be argued that as photons have no mass, and hence, no literal substance, and that literal substance is scarce in the universe. This would leave us with a no to my above question.
Now, touching on the metaphysical side, and re-qualifying what substance is outside of it's literal and physical definition, even if photons have no mass, could it be argued that they still have substance? Which would allow for a yes to my [first] above question in either case.
Now to pose my suggested re-qualifying of what substance is: can something which has function, lack substance? For photons, whether mass-less or not, have function. If something has function, would that not imply, philosophically, that it must have substance?
I fear though that I may be posing a question which cannot be properly answered in a general physics forum, so pardon my digression, against my own previous resistance, into human perceptions.