As someone who is simply trying to follow this discussion and understand as much as I can of what's being said, the distinction between direct and indirect may admittedly not be the best way of explaining the problem under discussion, but what seems to me to be meaningful here is that the photon-photon interactions only happen in particular (and somewhat difficult to create experimentally) circumstances. Particularly (if I understand correctly), at certain energy levels. The explanation for that, as far as I can tell, is that the photon-photon interactions are mediated by virtual particles, and the interaction between virtual electron-positron pairs and photons is dependent upon the energy that the photon carries.
The real problem is the idea of "interact". In the world of particle physics, the word does not have exactly the same meaning, with all the same connotations, that it does in most of everyday life or even everyday physics.
Quantum Electrodynamics stands at the end of a long road of work and investigation; it incorporates results about how certain types of what we call "forces" work and categorizes interactions as interactions through these forces. Photons are associated with a certain type of force as the mediators of that force and thus they are associated with a certain class of interactions; within this class there are no interactions that include interactions where the properties of two or more photons and only two or more photons are directly involved in the nature of some physical event.
When one looks to other classes of interactions, one can find ones where there are physical events that rely only on the properties of two or more photons, but these rely on classes of interactions that do not involve photons alone, classes where the forces are mediated by other particles. These events, though they rely on the properties of the photons, involve the mediation of the properties of other sorts of forces than those associated (directly or solely) with photons.
The details of these interactions are mostly inferred from the observations of interactions and the relationships that one can observe in these interactions. Some of the details are suspected to be wrong. Some of the details are known to be wrong. So determining just how complicated interactions, like the photon-mediated-by-other-forces interactions, are important to explore in isolation, if possible. Hence the importance of this collider mechanism that should encourage these interactions for study, if only to see if they work as predicted.
I can understand that as making perfect sense, but in Farsight's view virtual particles have nothing to do with how photons interact, and he seems to be saying that since you can see such interactions in a photon-photon collider, that demonstrates that there's no virtual particles involved in that interaction.
One thing that Farsight writes is that photons interact directly with themselves in some unknown way that makes them produce all the known particles in the universe. He claims that all matter is photons trapped in some kind of self-interaction. While quantum electrodynamics stands at the end of a lot of hard work and investigation, it is clear that Farsight's position hasn't changed much in about a decade and that his hardest work has gone into denial, a refusal to look into relevant mathematics, and insults.
A recent change to Farsight's position is his claim that the recent paper on photon-photon collisions is somehow evidence for his theory. The recent paper is a completely theoretical paper that details how one should go about building a machine to create photon interactions that are mediated by other forces. So there is not yet such a machine, there is no data about such interactions, and the authors of the paper explicitly reject Farsight's basic premise. This last bit is not new: most of the citations that Farsight provides are from authors and papers that provide positions and evidence contrary to Farsight's claims. Farsight is relying on the use of the word "interaction" in the paper to support the idea that scientists believe in "interaction" in the sort of common-place way that Farsight wishes to use it rather than learn the relevant physics.
It is important to realize that this is part of a pattern of behaviour that Farsight exhibits, that there is a reason that he is speaking the way that he does about things. To not have this information is to miss out important information about the content of the arguments offered.
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) allows short-term "borrowing" of mass, energy and momentum.
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