: J. Liang, L. Zhu & L.V. Wang, 2018, Light: Science & Applications)

One of the most enduring ideas in all the Universe is that everything that exists now will someday see its existence come to an end. The stars, galaxies, and even the black holes that occupy the space in our Universe will all some day burn out, fade away, and otherwise decay, leaving what we think of as a “heat death” state: where no more energy can possibly be extracted, in any way, from a uniform, maximum entropy, equilibrium state. But, perhaps, there are exceptions to this general rule, and that some things will truly live on forever.

One such candidate for a truly stable entity is the photon: the quantum of light. All of the electromagnetic radiation that exists in the Universe is made up of photons, and photons, as far as we can tell, have an infinite lifetime. Does that mean that light will truly live forever? That’s what Anna-Maria Galante wants to know, writing in to ask:

“Do photons live forever? Or do they “die,” and convert to some other particle? The light we see erupting from cosmic events over a verrrrry long past … we seem to know where it comes from, but where does it…