13,439 research outputs found
Superluminal Propagation and Acausality of Nonlinear Massive Gravity
Massive gravity is an old idea: trading geometry for mass. Much effort has
been expended on establishing a healthy model, culminating in the current
ghost-free version. We summarize here our recent findings -- that it is still
untenable -- because it is locally acausal: CTC solutions can be constructed in
a small neighborhood of any event.Comment: Contribution to Conference in Honour of the 90th Birthday of Freeman
Dyson -- To Appear in Proceeding. v2: Explicit CTC example, and other
improvements, adde
RNA secondary structure prediction using large margin methods
The secondary structure of RNA is essential for its biological role. Recently, Do, Woods, Batzoglou, (ISMB 2006) proposed a probabilistic approach that generalizes SCFGs using conditional maximum likelihood to estimate the model parameters. We propose an alternative approach to parameter estimation which is based on an SVM-like large margin method
Highly charged ions with E1, M1, and E2 transitions within laser range
Level crossings in the ground state of ions occur when the nuclear charge Z
and ion charge Z_ion are varied along an isoelectronic sequence until the two
outermost shells are nearly degenerate. We examine all available level
crossings in the periodic table for both near neutral ions and highly charged
ions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however level
crossings allow for optical electromagnetic transitions that could form the
reference transition for high accuracy atomic clocks. Optical E1 (due to
configuration mixing), M1 and E2 transitions are available in HCIs near level
crossings. We present scaling laws for energies and amplitudes that allow us to
make simple estimates of systematic effects of relevance to atomic clocks. HCI
clocks could have some advantages over existing optical clocks because certain
systematic effects are reduced, for example they can have much smaller thermal
shifts. Other effects such as fine-structure and hyperfine splitting are much
larger in HCIs, which can allow for richer spectra. HCIs are excellent
candidates for probing variations in the fine-structure constant, alpha, in
atomic systems as there are transitions with the highest sensitivity to
alpha-variation
Multiclass Multiple Kernel Learning
In many applications it is desirable to learn from several kernels. "Multiple kernel learning" (MKL) allows the practitioner to optimize over linear combinations of kernels. By enforcing sparse coefficients, it also generalizes feature selection to kernel selection. We propose MKL for joint feature maps. This provides a convenient and principled way for MKL with multiclass problems. In addition, we can exploit the joint feature map to learn kernels on output spaces. We show the equivalence of several different primal formulations including different regularizers. We present several optimization methods, and compare a convex quadratically constrained quadratic program (QCQP) and two semi-infinite linear programs (SILPs) on toy data, showing that the SILPs are faster than the QCQP. We then demonstrate the utility of our method by applying the SILP to three real world datasets
Optical transitions in highly-charged californium ions with high sensitivity to variation of the fine-structure constant
We study electronic transitions in highly-charged Cf ions that are within the
frequency range of optical lasers and have very high sensitivity to potential
variations in the fine-structure constant, alpha. The transitions are in the
optical despite the large ionisation energies because they lie on the
level-crossing of the 5f and 6p valence orbitals in the thallium isoelectronic
sequence. Cf16+ is a particularly rich ion, having several narrow lines with
properties that minimize certain systematic effects. Cf16+ has very large
nuclear charge and large ionisation energy, resulting in the largest
alpha-sensitivity seen in atomic systems. The lines include positive and
negative shifters
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