What is a particle?

Abstract

Theoretical developments related to the gravitational interaction have questioned the notion of particle in quantum field theory (QFT). For instance, uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity. These difficulties have lead some to suggest that in general QFT should not be interpreted in terms of particle states, but rather in terms of eigenstates of local operators. Still, it is not obvious how to reconcile this view with the empiricallyobserved ubiquitous particle-like behavior of quantum fields, apparent for instance in experimental high-energy physics, or "particle"-physics. Here we offer an element of clarification by observing that already in flat space there exist -strictly speaking-two distinct notions of particles: globally defined n-particle Fock-states and local particle states. The last describe the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are appropriately large, global and local particle states converge in a weak topology (but not in norm). This observation has little relevance for flat-space theories -it amounts to a reminder that there are boundary effects in realistic detectors-; but is relevant for gravity. It reconciles the two points of view mentioned above. More importantly, it provides a definition of local particle state that remains well-defined even when the conventional global particle states are not defined. This definition plays an important role in quantum gravity. * Unité mixte de recherche (UMR 6207) du CNRS et des Universités de Provence (Aix-Marseille I), de la Méditerranée (Aix-Marseille II) et du Sud (Toulon-Var); laboratoire affiliéà la FRUMAM (FR 2291).

How many quanta are there in a quantum spacetime?

“…Notice that the field-theory particles are the analog to the n ± quanta, not the n 1,2 quanta. In this sense they are non local [10].…”

Local quanta, unitary inequivalence, and vacuum entanglement

“…Our approach can be viewed as a modification and further elaboration on a previous work by Colosi and Rovelli [17]. Instead of quantizing the field using the standard stationary modes, we employ non-stationary modes which are, together with their time-derivatives, completely localised within a region of space at some arbitrary chosen time.…”

Towards state locality in quantum field theory: free fermions

“…A more operational approach to this problem would be to study the response of such states to local observables or other models for local (particle) measurement devices. A promising line of inquiry in this direction is suggested by the work of Colosi and Rovelli who introduced a notion of local particle as determined by a localized Hamiltonian operator [24]. A more precise implementation of this idea in bosonic [25] and fermionic [26] quantum field theory was achieved subsequently by León and collaborators, using representations (equivalently: complex structures) inequivalent to the standard one.…”