What is a particle?
Is the answer obvious? It is perhaps not as obvious as you might think. I was in a discussion with a colleague of mine yesterday about the “reality” of fields. I’m not convinced that they’re anything more than a mathematical convenience championed by people wishing to sweep the “action at a distance” problem under a rug. I can’t think of an experiment that can only be interpreted in terms of fields (most interference experiments can be interpreted statistically – let’s ignore the recent PBR paper for the moment). He countered that the same could be said for particles. I suppose I could concede that they could exist, but that we might not be able to prove it since it would be impossible to make a measurement of anything without involving a “particle” interpretation somewhere in the chain of interpretations leading from the measurement process to our conscious brains. In any case, he countered my assertion in the most logical way by asking me: what is a particle?
As a foundationalist, I probably am guilty of over thinking this, but the only thing I could come up with that could differentiate a particle from a field was that a particle is a local concept whereas a field is not. Fields have spatial extension (of course this requires the existence of space and quite possibly time as well, but that’s an argument for another day). Particles – fundamental particles, anyway – are mere points. Note that in my above assertion about the requirement of a particle somewhere in the interpretation chain, I make no distinction between the classical and quantum cases because, like many other quantum foundations and information folks, I take the view that the classical world is a special case of the quantum world. As such, “spooky action at a distance” can be explained by virtual particles (for interactions) or mere correlations (for odder things like entanglement).
At any rate, I am still at a loss in terms of trying to find a better differentiation between a particle and a field that is not, in some way, self-referential (in other words, saying that a particle is the quantization of a field is, in my mind, self-referential to some extent). However, stay tuned for a commentary on the nature of mass that might add a little twist to this whole thing.