Are the particles real?

Are the particles real? Or are they just useful mathematical constructs? Of course it is not possible to see or perceive elementary particles in the everyday sense of the word. Not in the sense in which it is possible to see a bird or a flower. At the same time, arguably the only things we can really see are photons, which are elementary particles. We know the particles exist in the sense that we can construct machines called particle detectors that make them visible in some way: we can then perceive traces of the particle's interaction with the detector. Some kinds of particles decay too quickly to leave a perceivable trace in detectors, but it may be possible to see their decay products and infer the presence of the particle from them.

The particles are also useful mathematical constructs. From this point of view, a particle is basically a collection of properties (mass, momentum, and various discrete quantities called quantum numbers) that travels and interacts locally in space and time. It is conceivable that the particle picture we have of the fundamental constituents of matter will some day turn out to be flawed and need to be replaced by something else. Perhaps not all properties of matter can be explained by local particles, or by particles with fixed mass but changeable momentum. The latter is proposed by a theory called unparticle physics.