I am not alone in wondering what we mean by the term “physical”. Chomsky pointed out recently that the term is anything but simple or clear (ref lost: see articles sent around before his UCD visit in 2009).  I have previously pointed out that unreflective use of the term seems to confuse two senses.  The first is exemplified by the insistence of common sense, where one bangs on the table to emphasize its solidity and says “This, this is physical”.  That might be termed Phenomenal-physical, and the best known example is Doubting Thomas, who wants to put his finger in Christ’s wounds before he can accept the resurrection.  The Phenomenal-physical has time and space coordinates centered at the Now and the I, respectively, or with a spatial coordinate system centered somewhere behind the eyes, and a temporal coordinate system centered at the present.

We can contrast this with the more usual use of the term Physical to refer to a universe of kickable objects.  This universe depends on a naive understanding of Newton, and a physics of particles in motion.  Its temporal scale is measured in seconds, and has no center, but extends from minus to plus infinity.  This is the realm in which masses are acted upon by forces, and it provides the framework within which we can discuss measurements.  If we can build a meter, and can agree on what it is that that instrument measures, then it is probably a physical quantity.  Though this is problematic. We might measure enthusiasm using the intensity of applause as a proxy, but we would be reluctant to admit “enthusiasm” to the set of physical variables.  The relationship between the use of measuring instruments and the set of concepts assumed to underlie those observations is anything but simple.  Let us call this Newton-Physical.

Since the early 20th Century, we must add a third kind of Physical to this menagerie: the Theoretical-Physical.  This is simply the domain of theoretical physics. I have no desire to talk further about it, except to say that our best account of the Theoretical-physical is constantly changing, and it can be weird.  Interesting issues such as the role of the observer, the directional arrow of time, and such like arise here.

The Theoretical-Physical routinely violates common sense, and is very distant from the Phenomenal-Physical.  Interestingly, the domain of Newton-Physical can be understood as a bridge between the two.  Newtonian physics works best for mid-sized objects at moderate time-scales, where the reference scale for defining mid-sized and moderate is the phenomenal world, and its best known exemplar: the apple that falls on Newton’s head.  Theoretical physics originally strove to underpin our knowledge of the phenomenal world, and it did a fantastic job.  As Theoretical Physics has diverged from Newtonian Physics, so the kind of phenomenon to be accounted for has moved further and further away from the phenomenal, strictly considered.  The immensely huge and the very tiny, the extremely long and unimaginably short, these provide the realms of discouse for Theoretical Physics, and as we approach the mid-sized and mid-durational, so Newtonian Physics does a better and better job, at the expense of a proliferation of basic entities.  A simple and beautiful physical theory will be impossibly removed from the world of apples and teapots.

Theoretical physics thus approaches the R-world, albeit in terms that starkly drive home the distance between us and our familiar worlds, and the underlying Noumenal realm.