4,190 research outputs found
Quantum Non-Barking Dogs
Quantum weak measurements with states both pre- and postselected offer a
window into a hitherto neglected sector of quantum mechanics. A class of such
systems involves time dependent evolution with transitions possible. In this
paper we explore two very simple systems in this class. The first is a toy
model representing the decay of an excited atom. The second is the tunneling of
a particle through a barrier. The postselection criteria are chosen as follows:
at the final time, the "atom" remains in its initial excited state for the
first example and the particle remains behind the barrier for the second. We
then ask what weak values are predicted in the physical environment of the
"atom" (to which no net energy has been transferred) and in the region beyond
the barrier (to which the particle has not tunneled). Previous work suggests
that very large weak values might arise in these regions for long durations
between pre- and postselection times. Our calculations reveal some distinct
differences between the two model systems.Comment: 8 pages, 3 figure
Evolutionary Transitions and Top-Down Causation
Top-down causation has been suggested to occur at all scales of biological
organization as a mechanism for explaining the hierarchy of structure and
causation in living systems. Here we propose that a transition from bottom-up
to top-down causation -- mediated by a reversal in the flow of information from
lower to higher levels of organization, to that from higher to lower levels of
organization -- is a driving force for most major evolutionary transitions. We
suggest that many major evolutionary transitions might therefore be marked by a
transition in causal structure. We use logistic growth as a toy model for
demonstrating how such a transition can drive the emergence of collective
behavior in replicative systems. We then outline how this scenario may have
played out in those major evolutionary transitions in which new, higher levels
of organization emerged, and propose possible methods via which our hypothesis
might be tested.Comment: 8 pages, 4 figure
Theory and design of quantum cascade lasers in (111) n-type Si/SiGe
Although most work towards the realization of group IV quantum cascade lasers (QCLs) has focused on valence band transitions, there are many desirable properties associated with the conduction band. We show that the commonly cited shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to the (111) growth direction. Specifically, a large band offset and low effective mass are achievable and subband degeneracy is preserved. We predict net gain up to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal waveguide, and show that a Ge interdiffusion length of at least 8 â„« across interfaces is tolerable
The Algorithmic Origins of Life
Although it has been notoriously difficult to pin down precisely what it is
that makes life so distinctive and remarkable, there is general agreement that
its informational aspect is one key property, perhaps the key property. The
unique informational narrative of living systems suggests that life may be
characterized by context-dependent causal influences, and in particular, that
top-down (or downward) causation -- where higher-levels influence and constrain
the dynamics of lower-levels in organizational hierarchies -- may be a major
contributor to the hierarchal structure of living systems. Here we propose that
the origin of life may correspond to a physical transition associated with a
shift in causal structure, where information gains direct, and
context-dependent causal efficacy over the matter it is instantiated in. Such a
transition may be akin to more traditional physical transitions (e.g.
thermodynamic phase transitions), with the crucial distinction that determining
which phase (non-life or life) a given system is in requires dynamical
information and therefore can only be inferred by identifying causal
architecture. We discuss some potential novel research directions based on this
hypothesis, including potential measures of such a transition that may be
amenable to laboratory study, and how the proposed mechanism corresponds to the
onset of the unique mode of (algorithmic) information processing characteristic
of living systems.Comment: 13 pages, 1 tabl
Thermal divergences on the event horizons of two-dimensional black holes
The expectation value of the stress-energy tensor \langleT_{\mu\nu}\rangle
of a free conformally invariant scalar field is computed in a general static
two-dimensional black hole spacetime when the field is in either a zero
temperature vacuum state or a thermal state at a nonzero temperature. It is
found that for every static two-dimensional black hole the stress-energy
diverges strongly on the event horizon unless the field is in a state at the
natural black hole temperature which is defined by the surface gravity of the
event horizon. This implies that both extreme and nonextreme two-dimensional
black holes can only be in equilibrium with radiation at the natural black hole
temperature.Comment: 13 pages, REVTe
Clues to Nuclear Star Cluster Formation from Edge-on Spirals
We find 9 nuclear cluster candidates in a sample of 14 edge-on, late-type
galaxies observed with HST/ACS. These clusters have magnitudes (M_I ~ -11) and
sizes (r_eff ~ 3pc) similar to those found in previous studies of face-on,
late-type spirals and dE galaxies. However, three of the nuclear clusters are
significantly flattened and show evidence for multiple, coincident structural
components. The elongations of these three clusters are aligned to within 10
degrees of the galaxies' major axes. Structurally, the flattened clusters are
well fit by a combination of a spheroid and a disk or ring. The nuclear cluster
disks/rings have F606W-F814W (~V-I) colors 0.3-0.6 magnitudes bluer than the
spheroid components, suggesting that the stars in these components have ages <
1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain
the stellar populations and provide a lower limit on the dynamical mass via
spectroscopy. We also present tentative evidence that another of the nuclear
clusters (in NGC 4206) may also host a supermassive black hole. Based on our
observational results we propose an in situ formation mechanism for nuclear
clusters in which stars form episodically in compact nuclear disks, and then
lose angular momentum or heat vertically to form an older spheroidal structure.
We estimate the period between star formation episodes to be 0.5 Gyr and
discuss possible mechanisms for tranforming the disk-like components into
spheroids. We also note the connection between our objects and massive globular
clusters (e.g. Cen), UCDs, and SMBHs. (Abridged)Comment: Accepted for publication in the A
How to be causal: time, spacetime, and spectra
I explain a simple definition of causality in widespread use, and indicate
how it links to the Kramers Kronig relations. The specification of causality in
terms of temporal differential eqations then shows us the way to write down
dynamical models so that their causal nature /in the sense used here/ should be
obvious to all. To extend existing treatments of causality that work only in
the frequency domain, I derive a reformulation of the long-standing Kramers
Kronig relations applicable not only to just temporal causality, but also to
spacetime "light-cone" causality based on signals carried by waves. I also
apply this causal reasoning to Maxwell's equations, which is an instructive
example since their casual properties are sometimes debated.Comment: v4 - add Appdx A, "discrete" picture (not in EJP); v5 - add Appdx B,
cause classification/frames (not in EJP); v7 - unusual model case; v8 add
reference
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Detecting the rotating quantum vacuum
We derive conditions for rotating particle detectors to respond in a variety of bounded spacetimes and
compare the results with the folklore that particle detectors do not respond in the vacuum state appropriate to
their motion. Applications involving possible violations of the second law of thermodynamics are briefly
addressed
Beyond shareholder primacy? Reflections on the trajectory of UK corporate governance.
Core institutions of UK corporate governance, in particular the City Code on Takeovers and Mergers, the Combined Code on Corporate Governance and the law on directors’ duties, are strongly orientated towards the norm of shareholder primacy. Beyond the core, however, stakeholder interests are better represented, in particular at the intersection of insolvency and employment law. This reflects the influence of European Community laws on information and consultation of employees. In addition, there are signs that some institutional shareholders are redirecting their investment strategies, under government encouragement, away from a focus on short-term returns, in such a way as to favour stakeholder-inclusive practices by firms. On this basis we suggest that the UK system is currently in a state of flux and that the debate over shareholder primacy has not been concluded
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