Topics Physics

Question This may be a silly question displaying only my ignorance on the subject. My question has to do with point-particles and spatiality. Physicists say that point particles have causal powers, i.e. photons striking someone’s eyes at certain wavelengths cause them to see. Perhaps photons are only contributory causes to one’s seeing. Physicists also say point particles are objects that are both concrete and physical. That is, they can be located in space which entails they are spatial objects too. However, by definition a point-particle lacks width, length, and depth, the three spatial dimensions. My question is how can this be? Is this a conceptual incoherence, or am I missing something? Does spatiality entail physicality or conversely, does physicality entail spatiality? Alternatively, is it that these two concepts have no intimate connection? Please explain. Thanks.

Accepted:
February 8, 2007

Comments

Your question seems to concern the connections between being spatial, being concrete, and being physical. Part of your question seems also to concern the idea of point particles.

Now it might be that ordinary material objects really do consist of point particles. If that's true, then point particles are concrete, physical, spatial entites. (That a point body has no dimensions does not keep it from being spatial: it has a location in space.)

On the other hand, it might be that the fundamental, elementary objects that physics seeks are not point sized. (Perhaps they are strings or whatever.) In that case, it might still be that certain physical theories that invoke point bodies do a pretty good job for certain purposes. Point bodies would then be idealizations -- useful ones. They would be abstract rather than concrete objects, in one sense of "abstract." (It could work the other way too: hydrodynamical theories that treat water as a continuous fluid may work very well even if a body of water is, in fact, a collection of mass points, or at least molecules rather than a continuous substance.)

Of course, an abstract object can still be spatial. For instance, a perfect geometrical sphere is an abstract object, unrealized (I presume) in the actual world -- merely ideal. However, it is spatial.

The notion of a thing being "physical" is not easy to unpack. Some objects that might seem to qualify as 'physical' do not have causal powers. If Newton was correct, then there was absolute space and absolute time. They would be described by theories in physics, so to that extent, they would be physical. But they would have no causal powers. The same applies to the laws of nature. They are not causes, but they help to explain facts and events. Are they "physical"? They are not concrete, of course, since they do not have spatiotemporal locations.