Brian,
Thanks for the great comments.
> - I'm confused right away by "taxonomic and statistical" vs.
> "procedural." How are the life sciences not procedural? There may be
> a technical sense in which you're using this word that I'm unfamiliar
> with, or I might just be processing this wrong. Anyway, it confused
> me. I know you clarify this somewhat in the next sentence, but it was
> still an unclear distinction to me. Is this a common distinction in
> philosophy of science?
I wrestled with that word 'procedural' for a long time but couldn't come up with anything better. The life sciences are certainly partly procedural, but they are very clearly primarily taxonomic. I've been discussing this point with one of our biologists (who is actually a biochemist and thus straddles the boundary) and he agrees with me. In fact we're trying to implement this into our core curriculum. This is why, in some philosophy of science circles, physics is included in what is called the 'exact sciences.'
> - You use "theory of everything" in a way counter to the prevailing
> sense one finds it in the literature. Usually people take it to mean
> a strictly microscopic description, a la the usual reductionist bias
> in particle physics. I think it's important to make a distinction
> here between a "theory of everything" in the microscopic sense (which
> how I think most folks use this term), and one in the more general
> sense you use, in which problems of emergent phenomena and strong
> coupling physics have been solved.
Aha! My point exactly (though I clearly didn't articulate it well enough - so much for following my own advice). How can we use that term if it's only on a microscopic level that we truly mean it? And how, then, is it a fair representation of what we do if people like Brian Greene become household names in the process of muddling the distinction, i.e. Is it truly *everything* or just everything microscopic? Popularizers of science are not always clear on this point. Again, it's all about consistency in the language we use.
> Indeed, it may be useful to
> distinguish between problems of extrapolating macro from micro, and
> problems of understand strongly coupled systems. These are different
> (though sometimes related) problems. Would you say that QCD is not a
> theory of the strong interactions because we don't know the dynamics
> of confinement? Most people I know would say that it is, and that we
> are just still trying to understand the low-energy dynamics.
Hmmm. That's a really good question. I'd have to ponder it before I say one way or the other. But I will say that this relates to your next point a bit...
> - At the top of p. 3, you call the strong nuclear force "higher
> order," and say that its mediator is the meson. This is a little
> misleading -- the mediator of the strong force is the gluon (as you
> say earlier). Sometimes people use "nuclear force" to describe the
> force between hadrons, and this force can effectively be described as
> being mediated by pi and rho mesons. There are lots and lots of
> mesons in the world (most of which are very unstable, of course), so
> saying that this force is mediated by "the meson" is confusing to me.
> Also, you describe this force as "higher order," which sounds like a
> technical claim but is not precise. It's not like this is a
> subleading force in any expansion parameter, it's merely the avatar
> of the strong force once quarks have confined. I would remove the
> words "higher order" and replace them with something a la "low-energy
> effective."
This is one of my biggest pet peeves. Forget the names for the moment. We have interaction A that mediates between quarks and exchanges gluons. This is the interaction that holds nucleons together individually (not to mention mesons themselves), i.e. the reason nucleons exist to begin with. Interaction B mediates *between* the nucleons and exchanges mesons and thus binds the nucleons *to each other.* These are not only two different (though related) things, they are of different orders (or on different levels, if you will). Interaction B involves particles (including the exchange particles) that are *not* fundamental and that are held together by interaction A. Think of it in terms of Legos. Interaction A uses all the base Legos. Interaction B builds up something bigger using pre-assembled pieces. I call interaction A the color interaction while I call interaction B the strong nuclear interaction. Physicists tend to either intermingle these terms *or* use 'strong nuclear' to refer to both (or to gloss over the fact that there are actually *two* things going on here).
It's all about being as consistent in our description as we can be and what I'm saying is that, while nothing will ever be perfect, there is room for improvement and, in pushing ahead, we need to consider these points. I took on Frank Wilczek in Physics Today about this a few years ago regarding the definition of mass. I forgot to add to my letter, though, that my description of mass in that letter (as the magnitude of the four-momentum vector) is the only self-consistent definition for mass if we want to use the term for both point particles as well as extended objects (atoms, boxes, desks, etc.).
> - In Section III, you use the term "interaction picture." I think you
> just mean that this is the model where forces are described via
> exchanging messenger bosons. However, "interaction picture" is a
> technical term used to describe as particular picture of time
> evolution, just like the Schroedinger picture or Heisenberg picture.
> Of course, it's important for building up the exchange picture you're
> talking about, but I don't think they're synonymous. Are they?
> Anyway, this was a bit confusing to me.
Oops. Well, there goes any chance I had at winning a prize. Doh!