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## Main Question or Discussion Point

(I don't have a specific question here, but observations/insights on the above paper or following excerpts are welcome....)For instance an accelerated observer will see more particles (Unruh).

http://arxiv.org/abs/gr-qc/0409054

What is a particle?

Marcus posted the above in another thread...And I do wish he would STOP that type of thing because everytime I read another paper I discover something else I had not thought about and therefore clearly do not understand!!!! (just joking,partly, but it never seems to end...peeling a layer simply reveals yet another layer...)

OMG, not only is space and time frame dependent, seems like particles are too...and in more ways than simply the virtual particle pairs of Unruh...On curved spacetime, in general there is no symmetry group, no preferred set of modes and no preferred decomposition into positive and negative frequency.

As a consequence, there is no preferred vacuum state, and the interpretation of the field states in term of particles appears to be difficult....The defining properties of the particles, mass and spin (or helicity), are indeed the invariants of the Poincar´e group. Now, strictly speaking we do not live in a Poincar´e invariant region of spacetime:does this means that, strictly speaking, the world around us has no particles?

So there are some fundamental differences among theorists about what particles are....Such arbitrariness and ambiguity of the particle concept have led some theoreticians like Davies to affirm that “particles do not exist” , a view shared by several relativists. ... other theoreticians .... who hold that QFT is fundamentally a formalism for describing processes involving particles, such as scattering or decays.... A typical example of this position is Weinberg who cannot certainly be suspected of ignoring general relativity. These difficulties become serious in a background-independent quantum context. For instance, in loop quantum gravity quantum states of the gravitational field are described in terms of a spin network basis.Can we talk about gravitons, or other particle states, in loop quantum gravity.A common view among relativists is that we cannot, unless we consider the asymptotically flat context.

So we are back to the old reliable, "I don't know exactly what it is, but at least I can measure whatever it is....thank heaven for "observables"...."....we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier.

This sounds reminisescent not only of the Unruh effect but also things like "relativity of simultaneity" and the apparent affects of gravity (curved spacetime) on local versus distant time....is nothing constant in this universe except lightspeed????Therefore, strictly speaking there are two distinct notions of particles in QFT. Local particle states correspond to the real objects observed by finite size detectors. .... On the other hand, global particle states....can be defined only under certain conditions. Global particle states are simpler to define and they approximate well the local particle states detected by local measurements. Therefore the global particle states, when they are available, give a good approximate description of the physics of the “real” particles detected by the detectors....In the paper we illustrate the difference between these two classes of states, and discuss their relation. The precise sense in which global states approximate local particle states is subtle.