Physics and intrinsic properties

The paper sketches out an ontology of physics in terms of matter being primitive stuff distributed in space and all the properties physics is committed to being dispositions that fix the temporal development of the distribution of matter in space. Whereas such properties can be conceived as intrinsic properties of particles in classical mechanics, in quantum physics, there is a holistic property or structure that relates all matter and that fixes its temporal development. 1. Classical physics and intrinsic properties At the end of the “Opticks” (1704), Newton writes: ... it seems probable to me, that God in the Beginning form’d Matter in solid, massy, hard, impenetrable, moveable Particles ...; no ordinary Power being able to divide what God himself made one in the first Creation. ... the Changes of corporeal Things are to be placed only in the various Separations and new Associations and Motions of these permanent Particles. (Question 31, p. 400 in the edition Newton 1952) According to Newton, matter consists in particles that are distributed in a background space, a particle being a material object that is so small that it is localized at a point in space, thus being indivisible. Hence, some points of space are occupied – a particle is localized at them –, whereas other points are empty. The distribution of matter in a background space develops in a background time. That is to say, as time passes, there is change in which points of space are occupied and which are empty. That change is furthermore such that each particle moves on a continuous trajectory. Consequently, each particle has an identity in time by which it distinguishes itself from all the other particles. If one adopts a sparse view of physical properties, there is no reason to make use of the notion of properties as far as this basic characterization of matter is concerned – the primitive ontology of Newtonian mechanics, to take up an expression introduced by Goldstein (1998) in another context. Matter is primitive stuff, and it is a primitive matter of fact that some points of space are occupied whereas others are not. There is a good reason for conceiving matter in terms of particles, that is, in terms of points of space being occupied or empty. If one considered matter to be a continuous stuff distributed all over space (that is, gunk), then one would have to maintain that there is more stuff at some points of space and less stuff at others in order to be able to accommodate variation. But it could not be a primitive matter of fact that there is more stuff at some points of space and less at others; a property of the stuff would be needed to account for that difference. However, as I shall argue shortly, all the properties that classical mechanics attributes to matter concern its temporal development, not simply the fact that there is matter. The view of matter consisting in particles can easily pay heed to the fact that there is more matter in some regions of space than in others: in some regions of space, more points are occupied than in others. Physics and intrinsic properties 2 Newton’s theory seeks to account for the temporal development of the distribution of matter in space. It does so by starting from a certain stipulation: Newton assumes that a certain form of motion of the particles does not call for an explanation, namely inertial motion, that is, constant motion on a straight line (Newton’s first law). Only change in the state of motion, that is, the acceleration of the particles (change in their velocity) has to be accounted for. Newton’s theory does so by introducing forces (Newton’s second law).1 It hence is a second order theory, being concerned with the temporal development of the velocity of the particles, by contrast to a first order theory that would be concerned with the temporal development of the position of the particles (velocity being the first temporal derivative of position, acceleration being the second temporal derivative). The forces, in turn, are traced back to properties of the particles. Thus, in virtue of possessing a mass, the particles exert a force upon each other, namely a force of attraction, that is, gravitation. Mass manifests itself in the mutual attraction of the particles (gravitational mass) as well as in their resistance to acceleration (inertial mass). In brief, a property is attributed to the particles in the form of mass to account for change in their state of motion. I gloss here over the question of whether or not forces should be admitted as properties in addition to properties of the particles such as their mass.2 If one seeks for a parsimonious ontology, there is no reason to recognize forces over and above properties of the particles such as their mass, since given the distribution of mass in space at a time, a certain change of the state of motion of the particles – their acceleration as described by the law of gravitation – ensues. Is mass an intrinsic property of the particles? A world in which only one point of space is occupied at any time – that is, a world with only one particle – is a possible world of Newtonian mechanics, given that Newton admits a background space and a background time. That one particle would forever continue to be in inertial motion (or to be at rest). Consider the widespread view according to which an intrinsic property is a property that an object possesses independently of being alone in a world or being accompanied by other objects.3 On this view, mass counts as an intrinsic property. However, a world in which there is only one particle with mass would be indiscernible from a world in which there is only one massless particle. In other words, taking the particle to be equipped with the property of mass over and above the fact of a point of space being occupied makes no difference as long as one limits oneself to considering possible worlds in which only one point of space is occupied at a time. But the lack of a difference in the case of a one particle world does not decide against mass being an intrinsic property of the particles. On the way in which I’ve introduced the ontology of Newtonian mechanics above, the theory starts with the distribution of matter in a background space, that distribution consisting in the fact that some points of space are occupied by primitive stuff, whereas others are not. Properties then are called for only to account for the form of the temporal development of the distribution of the primitive stuff in space, more precisely, to account for changes in the state of motion of the particles. Properties that are needed in order to do that job are dispositions in 1 See Maudlin (2012, chs. 1-2) for an excellent recent examination. 2 See notably Bigelow, Ellis and Pargetter (1988), Wilson (2007) and Massin (2009) on the ontology of Newtonian forces. 3 See Langton and Lewis (1998 and 2001), as well as Hoffmann-Kolss (2010, first part) for a detailed