My concern here is mainly with the paper
Kastner, R. E. (2014). On Real and Virtual Photons in the Davies Theory of Time-Symmetric Quantum Electrodynamics. Electronic Journal of Theoretical Physics, 11(30).
I should say at the outset that the sensationalist title of this post is to compensate for a corresponding lack of sensationalism in the post, and should not be interpreted as a disparaging (or even negative) view of Kastner’s article. I may not be a proponent of transactional interpretations of quantum physics, but neither am I a detractor (and I certainly find it more plausible than multiverse-type interpretations). Before discussing photons and virtual photons, I need to briefly explain the “Possibilist Transactional Interpretation” (PTI). This is Kastner’s own version of the “Transactional Interpretation” (TI) put forward by Dr. John G. Cramer in the 80s. Luckily, this means that there already exists sufficiently simple and concise summaries of the interpretation that PTI is based upon. See e.g.,
Summary of the Transactional Interpretation
Differences between the Transactional and the Copenhagen Interpretations (both of these are sections from the same paper by Cramer)
Cramer, J. G. (1986). The transactional interpretation of quantum mechanics. Reviews of Modern Physics, 58(3), 647.
Even better, a central (perhaps the central) aspect of the PTI can be understood reasonably well at least for our purposes by understanding the possibilist part without the TI foundation. In Kastner’s words:
I wish to view as physically real the possible quantum events that might be, or might have been, experienced. So, in this approach, those possible events are real, but not actual; they exist, but not in spacetime. The actual event is the one that is experienced and that can be said to exist as a component of spacetime. I thus dissent from the usual identification of “physical” with “actual”: an entity can be physical without being actual.
Kastner, R. E. (2012). The Transactional Interpretation of Quantum Mechanics: The Reality of Possibility. Cambridge University Press.
To many this probably sounds strange or even nonsensical (to those for whom it doesn’t, either you are a physicist or should consult a mental health professional). How on earth can a possibility be physically real, especially if it isn’t actual? Well, it is strange and seemingly nonsensical, but that’s not Kastner’s fault, it’s the universe’s fault (I know, classic “blame it on the cosmos” or “dog ate my homework” excuse we hear so often, but its true!). This is where virtual particles, specifically virtual photons, come in.
In quantum electrodynamics (and quantum field theory and particle physics more generally), “real” particles (or processes) are said to be governed by “virtual” ones. These irritating entities are rather poorly behaved. They will often refuse (without giving any good reason) to adhere to conservation laws, and worse may even decide to snub causality just for kicks!
But if they’re not real, then what are they doing in theories of physics? Can’t we just stick to things that exist and worry about what photons are doing without wondering what virtual ones “do”? (actually, technically we sometimes can, in that certain extensions of quantum mechanics may eliminate them, as with photons in quantum field theory, but in QFT/particle physics even if virtual photons are eliminated by weak gauge conditions, we’ll still have virtual gluons or virtual neutrinos, etc.)
A (very simplistic) way to think of virtual particles is that they are virtual because they can’t be directly observed, but are used to explain real, physical effects on what can be. For photons more specifically, we turn to the main paper of interest:
the situation has a much more natural explanation in the transactional picture. In that picture, the response of the absorber is what gives rise to the ‘free field’ that in the quantum domain is considered a ‘real photon’. So the ‘realness’ of the photon is defined in the transactional picture not by an infinite lifetime – which, in reality, is practically never obtained – but rather by the presence of an absorber response. This response is what can give rise (through an actualized transaction) to a real photon with the ability to convey empirically detectable energy from one place to another, which is the function of the free field. That is, the idea that a ‘real’ photon must always have exactly zero rest mass is an idealization.
My goal is now to show how this means that the idea of a physically real possibility isn’t as bizarre as it sounds in comparison with mainstream, basically universally accepted concepts from modern physics. In fact, it could be argued (and is, actually, by Kastner) that describing possibilities as real is better than describing rather fundamental interactions between the basic constituents of reality as imaginary, virtual, or otherwise not “real.” After all, as Richard Feynman says in his Theory of Fundamental Processes (p. 95), “In a sense every real photon is actually virtual if one looks over sufficiently long time scales.” Is it really better to make a distinction which allows an arbitrary distinction between what is and what isn’t real in physics than to treat entities in the mathematical framework that are said to mediate physical interactions or be otherwise causally efficacious as “real”? Kastner, in a certain sense, is just flipping the reasoning behind the real/virtual distinction on its head. All photons are real, virtual or no, and all began as virtual photons. The difference is that “real” photons are “actualized” or “realized” possibilities, and virtual photons are not:
“The virtual photon, in the transactional picture, is just this nascent possibility of a transaction between two currents—but one that was not realized. A transaction is only attainable for virtual photons that satisfy the energy and momentum conservation constraints for the initial and final states of the system.” (italics in original).
More generally, PTI addresses a serious ontological problem in many interpretations of quantum physics, not to mention the language used in physics literature and discourse. Quantum theory presented (and still does) serious challenges for our notions of what is physical, real, material, etc. In mainstream quantum physics, the relationship between mathematical representations of physical systems and processes is frequently anything but clear. Yet it has been so long since quantum mechanics first begat an epistemological crises that there exists a certain amount of numbness to it. Most physicists today were introduced to at least some quantum mechanics as undergraduates and simply became accustomed to the divide between the formalisms and the physics that posed such problems (and not infrequently contradictory “solutions”) for Planck, Whitehead, Born, Einstein, Bohr, Popper, Heisenberg, and other foundational figures in physics and 20th century philosophy. Otherwise confusing descriptions of virtual particles, inserted into mathematical equations to make physical models work correctly (thereby rendering them causally efficacious), became old hat. Being accustomed to such issues doesn’t solve them, though. Kastner’s paper is an example of one solution to such a problem.
I haven’t done justice to Kastner’s paper, and have done much injustice to PTI, mostly because I wanted to introduce an example of the ontological issues in modern physics and a fascinating resolution with a specific example, but this isn’t easy to do without either a post much longer than this filled with explanations or an assumption of a degree of familiarity with QED that I can’t make. The title of this post is misleading, but I thought that fitting given the misleading nature of physics language. And that is really the point: it takes a fairly technical level of familiarity with quantum physics beyond the level of quantum mechanics just to be familiar with how misleading terms like “particle”, “state”, “observable”, etc., are, and still more to become familiar with what they actually mean. It is easier, then, to point out examples like this one of such issues than to provide the detail necessary for an individual without the requisite background to render an informed opinion. For that, a good start would be Kastner’s book (cited above), which I highly recommend. She has another book that I believe is also on PTI, but I haven’t read it…yet.