> But the extra photon wasn’t created by that special process, so instead of disappearing when you turn back time, its wavelength will eventually get impossibly small, concentrating its energy so greatly that the photon collapses into a black hole. This creates a paradox, absurdly implying that — in this fictional, expanding universe — microscopic black holes convert into photons. The thought experiment suggests that a naïve mashup of unitarity and cosmic expansion doesn’t work.<p>The Laws of Thermodynamics state that you can’t create a photon from nothing, so isn’t introducing an “extra photon” into the universe already a paradox? Who would be the least bit surprised that paradoxes create paradoxes?