5,360,660 research outputs found
What is a crystal?
Almost 25 years have passed since Shechtman discovered quasicrystals, and 15
years since the Commission on Aperiodic Crystals of the International Union of
Crystallography put forth a provisional definition of the term crystal to mean
``any solid having an essentially discrete diffraction diagram.'' Have we
learned enough about crystallinity in the last 25 years, or do we need more
time to explore additional physical systems? There is much confusion and
contradiction in the literature in using the term crystal. Are we ready now to
propose a permanent definition for crystal to be used by all? I argue that time
has come to put a sense of order in all the confusion.Comment: Submitted to Zeitschrift fuer Kristallographi
What is a particle?
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 empirically-observed
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
-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.Comment: 19 pages, no figures. Revised version, with a new title, of the 2004
paper "Global particles, local particles
What is a cube?
We give an intrinsic characterization of all subsets of a doubling metric
space that can arise as a member of some system of dyadic cubes on the
underlying space, as constructed by M. Christ.Comment: 6 pages; v3 (Remark 2.14 (on the connection of plumpness to other
geometric notations) and Examples 2.15 of plump sets added
What is a quantum simulator?
Quantum simulators are devices that actively use quantum effects to answer
questions about model systems and, through them, real systems. Here we expand
on this definition by answering several fundamental questions about the nature
and use of quantum simulators. Our answers address two important areas. First,
the difference between an operation termed simulation and another termed
computation. This distinction is related to the purpose of an operation, as
well as our confidence in and expectation of its accuracy. Second, the
threshold between quantum and classical simulations. Throughout, we provide a
perspective on the achievements and directions of the field of quantum
simulation.Comment: 13 pages, 2 figure
What is a premature death?
The one who dies is deprived of goods that this person would have enjoyed if he or she had continued living, according to the popular “deprivation account of harm.” The person who dies “prematurely” is generally thought to suffer the most harm from death. However, the concept of a premature death is unclear, as will be shown. I will evaluate various definitions of a premature death and will argue that the existing definitions are too ambiguous and unreliable to serve as the basis for estimating the degree of harm from death
- …