17,280 research outputs found
Quantum objects are vague objects
[FIRST PARAGRAPHS]
Is vagueness a feature of the world or merely of our representations
of the world? Of course, one might respond to this question by asserting
that insofar as our knowledge of the world is mediated by our
representations of it, any attribution of vagueness must attach to the latter.
However, this is to trivialize the issue: even granted the point that all
knowledge is representational, the question can be re-posed by asking
whether vague features of our representations are ultimately eliminable or
not. It is the answer to this question which distinguishes those who believe
that vagueness is essentially epistemic from those who believe that it is,
equally essentially, ontic. The eliminability of vague features according to
the epistemic view can be expressed in terms of the supervenience of
‘vaguely described facts’ on ‘precisely describable facts’:
If two possible situations are alike as precisely described in terms of
physical measurements, for example, then they are alike as vaguely
described with words like ‘thin’. It may therefore be concluded that the facts
themselves are not vague, for all the facts supervene on precisely
describable facts. (Williamson 1994, p. 248; see also pp. 201-
204)
It is the putative vagueness of certain identity statements in
particular that has been the central focus of claims that there is vagueness
‘in’ the world (Parfit 1984, pp. 238-241; Kripke 1972, p. 345 n. 18). Thus,
it may be vague as to who is identical to whom after a brain-swap, to give
a much discussed example. Such claims have been dealt a forceful blow
by the famous Evans-Salmon argument which runs as follows: suppose for
reductio that it is indeterminate whether a = b. Then b definitely possesses
the property that it is indeterminate whether it is identical with a, but a
definitely does not possess this property since it is surely not
indeterminate whether a=a. Therefore, by Leibniz’s Law, it cannot be the
case that a=b and so the identity cannot be indeterminate (Evans 1978;
Salmon 1982)
A discussion on particle number and quantum indistinguishability
The concept of individuality in quantum mechanics shows radical differences
from the concept of individuality in classical physics, as E. Schroedinger
pointed out in the early steps of the theory. Regarding this fact, some authors
suggested that quantum mechanics does not possess its own language, and
therefore, quantum indistinguishability is not incorporated in the theory from
the beginning. Nevertheless, it is possible to represent the idea of quantum
indistinguishability with a first order language using quasiset theory (Q). In
this work, we show that Q cannot capture one of the most important features of
quantum non individuality, which is the fact that there are quantum systems for
which particle number is not well defined. An axiomatic variant of Q, in which
quasicardinal is not a primitive concept (for a kind of quasisets called finite
quasisets), is also given. This result encourages the searching of theories in
which the quasicardinal, being a secondary concept, stands undefined for some
quasisets, besides showing explicitly that in a set theory about collections of
truly indistinguishable entities, the quasicardinal needs not necessarily be a
primitive concept.Comment: 46 pages, no figures. Accepted by Foundations of Physic
Optical read out and feedback cooling of a nanostring optomechanical cavity
Optical measurement of the motion of a 940 kHz mechanical resonance of a
silicon nitride nanostring resonator is demonstrated with a read out noise
imprecision reaching 37 dB below that of the resonator's zero-point
fluctuations. Via intensity modulation of the optical probe laser, radiation
pressure feedback is used to cool and damp the mechanical mode from an initial
room temperature occupancy of (K)
down to a phonon occupation of , representing a
mode temperature of mK. The five decades of cooling is enabled
by the system's large single-photon cooperativity and high
quantum efficiency of optical motion detection ().Comment: 13 pages, 13 figure
The effects of parasitism and body length on positioning within wild fish shoals
The influence of body length and parasitism on the positioning behaviour of individuals in wild fish shoals was investigated by a novel means of capturing entire shoals of the banded killifish (Fundulus diaphanus, Lesueur) using a grid-net that maintained the two-dimensional positions of individuals within shoals.
Fish in the front section of a shoal were larger than those in the rear.
Individuals parasitized by the digenean trematode (Crassiphiala bulboglossa, Haitsma) showed a tendency to occupy the front of shoals. Parasitized fish were also found more in peripheral positions than central ones in a significant number of shoals.
Shoal geometry was affected by the overall parasite prevalence of shoal members; shoals with high parasite prevalence displayed increasingly phallanx-like shoal formations, whereas shoals with low prevalence were more elliptical.
There was no relationship between body length and parasite abundance or prevalence in the fish population which suggests body length and parasite status are independent predictors of positioning behaviour.
Solitary individuals found outside shoals were both more likely to be parasitized and had higher parasite abundance than individuals engaged in shoaling.
Differences in the shoaling behaviour of parasitized and unparasitized fish are discussed in the context of the adaptive manipulation hypothesis
The relation between magnetic and material arms in models for spiral galaxies
Context. Observations of polarized radio emission show that large-scale
(regular) magnetic fields in spiral galaxies are not axisymmetric, but
generally stronger in interarm regions. In some nearby galaxies such as NGC
6946 they are organized in narrow magnetic arms situated between the material
spiral arms. Aims. The phenomenon of magnetic arms and their relation to the
optical spiral arms (the material arms) call for an explanation in the
framework of galactic dynamo theory. Several possibilities have been suggested
but are not completely satisfactory; here we attempt a consistent
investigation. Methods. We use a 2D mean-field dynamo model in the no-z
approximation and add injections of small-scale magnetic field, taken to result
from supernova explosions, to represent the effects of dynamo action on smaller
scales. This injection of small scale field is situated along the spiral arms,
where star-formation mostly occurs. Results. A straightforward explanation of
magnetic arms as a result of modulation of the dynamo mechanism by material
arms struggles to produce pronounced magnetic arms, at least with realistic
parameters, without introducing new effects such as a time lag between Coriolis
force and {\alpha}-effect. In contrast, by taking into account explicitly the
small-scale magnetic field that is injected into the arms by the action of the
star forming regions that are concentrated there, we can obtain dynamo models
with magnetic structures of various forms that can be compared with magnetic
arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered
as coherent magnetic structures generated by large-scale dynamo action, and
associated with spatially modulated small-scale magnetic fluctuations, caused
by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&
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