> There are no photons in free space. Um ... I'd suggest rewording that, as this wording is incorrect. Cosmic background photons would likely want to have a w

> There are no photons in free space. Um ... I'd suggest rewording that, as this wording is incorrect. Cosmic background photons would likely want to have a word with you on this as well. It is true that what a photon is in a physical sense is poorly understood (double slit experiment, etc.). The goal of physics is to try to narrow this range, so that we understand more, and have a smaller null space of "here be dragons and photons" that we don't quite grasp. If you can accept analogies, it would be reasonable to assume that photons are the free space analogs of phonons. Phonons are excitons of an underlying system, with stress/strain tensors. Phonons interact with physical particles, scattering them. Phonon-like things are responsible for Cooper pair formation (effectively a Bose Einstein condensate of two electrons which are fermions, into a pair of fermions with net integer spin). Cooper pairs are responsible for superconductivity. I am not sure if phonons (which I believe are bosons) would be considered particles mediatiating interactions in solids, but I am guessing a good/strong case could be made, along those lines. The analogy may be apt. But the question that immediately arises is, given that phonons are excitons of an underlying material, is there something analogous for electromagnetic waves? Up to the very early 1900's, they called this thing the "aether". Transverse waves (oscillating perpendicular to velocity vector of travel) of phonons are called optical modes. Longitudinal waves (think like sound waves, compression and rarefaction) are called acoustic. I don't know if we have a longitudinal equivalent in photons. There are many questions remaining. We don't completely understand QM, up to, and including if QM is the right theory. These questions and tests are good. The more questions a theory answers correctly, the less likely it is to be incorrect. As Feynman noted, you only need one conclusive irrefutable result running at odds with a theory to render the theory defunct. Even with that said, some "wrong" theories (like the "law" of Newtonian gravity) are immensely useful in practical matters, as long as you pay particularly careful attention to range of applicability. Feel happy you don't normally need to pull out full 10-D general relativity to go to the moon. Newtonian mechanics plus corrections works well enough (for now). This will change the farther we want to go. This all said, there are many open questions in physics, including numerous things that you hear about every day. Anyone claiming, ever, in any field, that the "science is settled" is either not a scientist, a poor scientist, or someone with an agenda. Science is never settled. Good science is looking at data late at night in your office when you should be home with your family, and saying "hmmm, that's funny ..." (recovering computational theoretical physicist here, who used to quantum mechanically simulate semiconductors on supercomputers ... quite inception-movie like)