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MIT Open Access Articles

Mostrando recursos 1 - 20 de 15,094

1.
Power law Polya’s urn and fractional Brownian motion - Hammond, Alan; Sheffield, Scott Roger
We introduce a natural family of random walks S[subscript n] on Z that scale to fractional Brownian motion. The increments X[subscript n] := S[subscript n] − S[subscript n]−1 ∈ {±1} have the property that given {X[subscript k] : k < n}, the conditional law of X[subscript n] is that of X[subscript n−k[subscript n]] , where k[subscript n] is sampled independently from a fixed law μ on the positive integers. When μ has a roughly power law decay (precisely, when μ lies in the domain of attraction of an α-stable subordinator, for 0 < α < 1/2) the walks scale to...

2.
Role of spectral non-idealities in the design of solar thermophotovoltaics - Lenert, Andrej; Nam, Youngsuk; Wang, Evelyn N.; Bierman, David Matthew
To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used...

3.
HEAT TRANSFER FLUIDS - Lenert, Andrej; Nam, Youngsuk; Wang, Evelyn N.
The choice of heat transfer fluids has significant effects on the performance, cost, and reliability of solar thermal systems. In this chapter, we evaluate existing heat transfer fluids such as oils and molten salts based on a new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics, and hydraulic performance of the fluids. Thermal stability, freezing point, and safety issues are also discussed. Through a comparative analysis, we examine alternative options for solar thermal heat transfer fluids including water−steam mixtures (direct steam), ionic liquids/melts, and suspensions of nanoparticles (nanofluids), focusing on the benefits and...

4.
A nanophotonic solar thermophotovoltaic device - Lenert, Andrej; Nam, Youngsuk; Chan, Walker R.; Wang, Evelyn N.; Bierman, David Matthew; Soljacic, Marin; Celanovic, Ivan
The most common approaches to generating power from sunlight are either photovoltaic, in which sunlight directly excites electron–hole pairs in a semiconductor, or solar–thermal, in which sunlight drives a mechanical heat engine. Photovoltaic power generation is intermittent and typically only exploits a portion of the solar spectrum efficiently, whereas the intrinsic irreversibilities of small heat engines make the solar–thermal approach best suited for utility-scale power plants. There is, therefore, an increasing need for hybrid technologies for solar power generation. By converting sunlight into thermal emission tuned to energies directly above the photovoltaic bandgap using a hot absorber–emitter, solar thermophotovoltaics promise...

5.
Origin of Metastable Knots in Single Flexible Chains - Dai, Liang; Doyle, Patrick S.; Renner, Christopher Benjamin
Recent theoretical progress has explained the physics of knotting of semiflexible polymers, yet knotting of flexible polymers is relatively unexplored. We herein develop a new theory for the size distribution of knots on a flexible polymer and the existence of metastable knots. We show the free energy of a flexible molecule in a tube can be mapped to quantitatively reproduce the free energy distribution of a knot on a flexible chain. The size distribution of knots on flexible chains is expected to be universal and might be observed at a macroscopic scale, such as a string of hard balls.

6.
Field-Theory Representation of Gauge-Gravity Symmetry-Protected Topological Invariants, Group Cohomology, and Beyond - Gu, Zheng-Cheng; Wen, Xiao-Gang; Wang, Juven
The challenge of identifying symmetry-protected topological states (SPTs) is due to their lack of symmetry-breaking order parameters and intrinsic topological orders. For this reason, it is impossible to formulate SPTs under Ginzburg-Landau theory or probe SPTs via fractionalized bulk excitations and topology-dependent ground state degeneracy. However, the partition functions from path integrals with various symmetry twists are universal SPT invariants, fully characterizing SPTs. In this work, we use gauge fields to represent those symmetry twists in closed spacetimes of any dimensionality and arbitrary topology. This allows us to express the SPT invariants in terms of continuum field theory. We show...

7.
Poset Vectors and Generalized Permutohedra (Extended Abstract) - Croitoru, Dorian; Oh, Suho; Postnikov, Alexander
We show that given a poset P and and a subposet Q, the integer points obtained by restricting linear extensions of P to Q can be explained via integer lattice points of a generalized permutohedron.

8.
Extending Self-Maps to Projective Space over Finite Fields - Poonen, Bjorn
Using the closed point sieve, we extend to finite fields the following theorem proved by A. Bhatnagar and L. Szpiro over infinite fields: if X is a closed subscheme of P[superscript n] over a field, and φ: X → X satisfies φ∗O[subscript X](1) [~ over _] O[subscript X](d) for some d ≥ 2, then there exists r ≥ 1 such that φ[superscript r] extends to a morphism P[superscript n] → P[superscript n].

9.
PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. III. PHOTOCHEMISTRY AND THERMOCHEMISTRY IN THICK ATMOSPHERES ON SUPER EARTHS AND MINI NEPTUNES - Hu, Renyu; Seager, Sara
Some super Earths and mini Neptunes will likely have thick atmospheres that are not H2-dominated. We have developed a photochemistry-thermochemistry kinetic-transport model for exploring the compositions of thick atmospheres on super Earths and mini Neptunes, applicable for both H2-dominated atmospheres and non-H2-dominated atmospheres. Using this model to study thick atmospheres for wide ranges of temperatures and elemental abundances, we classify them into hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO2 rather than CH4 or CO in a H2-depleted water-dominated thick atmosphere and that the preferred loss of...

10.
SUN-TO-EARTH CHARACTERISTICS OF TWO CORONAL MASS EJECTIONS INTERACTING NEAR 1 AU: FORMATION OF A COMPLEX EJECTA AND GENERATION OF A TWO-STEP GEOMAGNETIC STORM - Liu, Ying D.; Yang, Zhongwei; Wang, Rui; Luhmann, Janet G.; Richardson, John D.; Lugaz, Noé
On 2012 September 30-October 1 the Earth underwent a two-step geomagnetic storm. We examine the Sun-to-Earth characteristics of the coronal mass ejections (CMEs) responsible for the geomagnetic storm with combined heliospheric imaging and in situ observations. The first CME, which occurred on 2012 September 25, is a slow event and shows an acceleration followed by a nearly invariant speed in the whole Sun-Earth space. The second event, launched from the Sun on 2012 September 27, exhibits a quick acceleration, then a rapid deceleration, and finally a nearly constant speed, a typical Sun-to-Earth propagation profile for fast CMEs. These two CMEs...

11.
THE ROLE OF STELLAR FEEDBACK IN THE DYNAMICS OF H II REGIONS - Lopez, Laura A.; Krumholz, Mark R.; Bolatto, Alberto D.; Prochaska, J. Xavier; Ramirez-Ruiz, Enrico; Castro, Daniel
Stellar feedback is often cited as the biggest uncertainty in galaxy formation models today. This uncertainty stems from a dearth of observational constraints as well as the great dynamic range between the small scales (lsim1 pc) where the feedback originates and the large scales of galaxies [greater-than or equivalent to]1 kpc) that are shaped by this feedback. To bridge this divide, in this paper we aim to assess observationally the role of stellar feedback at the intermediate scales of H II regions (~10-100 pc). In particular, we employ multiwavelength data to examine several stellar feedback mechanisms in a sample of...

12.
IMAGING THE SUPERMASSIVE BLACK HOLE SHADOW AND JET BASE OF M87 WITH THE EVENT HORIZON TELESCOPE - Lu, Ru-Sen; Broderick, Avery E.; Baron, Fabien; Monnier, John D.; Fish, Vincent L.; Doeleman, Sheperd Samuel; Pankratius, Victor
The Event Horizon Telescope (EHT) is a project to assemble a Very Long Baseline Interferometry (VLBI) network of millimeter wavelength dishes that can resolve strong field general relativistic signatures near a supermassive black hole. As planned, the EHT will include enough dishes to enable imaging of the predicted black hole "shadow," a feature caused by severe light bending at the black hole boundary. The center of M87, a giant elliptical galaxy, presents one of the most interesting EHT targets as it exhibits a relativistic jet, offering the additional possibility of studying jet genesis on Schwarzschild radius scales. Fully relativistic models...

13.
Predicting In-Hospital Mortality of ICU Patients: The PhysioNet/Computing in Cardiology Challenge 2012 - Silva, Ikaro; Moody, George; Scott, Daniel J.; Celi, Leo Anthony G.; Mark, Roger Greenwood
Acuity scores, such as APACHE, SAPS, MPM, and SOFA, are widely used to account for population differ ences in studies aiming to compare how medications, care guidelines, surgery, and other interventions impact mortality in Intensive Care Unit (ICU) patients. By contrast, the focus of the PhysioNet/CinC Challenge 2012 is to develop methods for patient-specific prediction of in-hospital mortality. The data used for the challenge consisted of 5 general descriptors and 36 time series (measurements of vital signs and laboratory results) from the first 48 hours of the first available ICU stay of 12,000 adult patients from the MIMIC II database....

14.
Making Big Data Useful for Health Care: A Summary of the Inaugural MIT Critical Data Conference - Badawi, Omar; Brennan, Thomas Patrick; Celi, Leo Anthony G.; Feng, Mengling; Ghassemi, Marzyeh; Ippolito, Andrea; Johnson, Alistair; Mark, Roger Greenwood; Mayaud, Louis; Moody, George; Moses, Christopher; Naumann, Tristan Josef; Nikore, Vipan; Pimentel, Marco; Pollard, Tom J.; Santos, Mauro; Stone, David J.; Zimolzak, Andrew
With growing concerns that big data will only augment the problem of unreliable research, the Laboratory of Computational Physiology at the Massachusetts Institute of Technology organized the Critical Data Conference in January 2014. Thought leaders from academia, government, and industry across disciplines--including clinical medicine, computer science, public health, informatics, biomedical research, health technology, statistics, and epidemiology--gathered and discussed the pitfalls and challenges of big data in health care. The key message from the conference is that the value of large amounts of data hinges on the ability of researchers to share data, methodologies, and findings in an open setting. If...

15.
Approximate Bayesian Image Interpretation using Generative Probabilistic Graphics Programs - Mansinghka, Vikash Kumar; Kulkarni, Tejas Dattatraya; Perov, Yura N.; Tenenbaum, Joshua B.
The idea of computer vision as the Bayesian inverse problem to computer graphics has a long history and an appealing elegance, but it has proved difficult to directly implement. Instead, most vision tasks are approached via complex bottom-up processing pipelines. Here we show that it is possible to write short, simple probabilistic graphics programs that define flexible generative models and to automatically invert them to interpret real-world images. Generative probabilistic graphics programs (GPGP) consist of a stochastic scene generator, a renderer based on graphics software, a stochastic likelihood model linking the renderer’s output and the data, and latent variables that...

16.
Automatic Calibration of Multiple Coplanar Sensors - Brookshire, Jonathan; Teller, Seth
This paper describes an algorithm for recovering the rigid 3-DOF transformation (offset and rotation) between pairs of sensors mounted rigidly in a common plane on a mobile robot. The algorithm requires only a set of sensor observations made as the robot moves along a suitable path. Our method does not require synchronized sensors; nor does it require complete metrical reconstruction of the environment or the sensor path. We show that incremental pose measurements alone are sufficient to recover sensor calibration through nonlinear least squares estimation. We use the Fisher Information Matrix to compute a Cramer-Rao lower bound (CRLB) for the...

17.
A Duality Principle for Selection Games - Sheffield, Scott Roger; Levine, Lionel; Stange, Katherine E.
A dinner table seats k guests and holds n discrete morsels of food. Guests select morsels in turn until all are consumed. Each guest has a ranking of the morsels according to how much he would enjoy eating them; these rankings are commonly known.
A gallant knight always prefers one food division over another if it provides strictly more enjoyable collections of food to one or more other players (without giving a less enjoyable collection to any other player), even if it makes his own collection less enjoyable. A boorish lout always selects the morsel that gives him the most...

18.
Deterministic Approximations of Random Reflectors - Sheffield, Scott Roger
Within classical optics, one may add microscopic "roughness'' to a macroscopically flat mirror so that parallel rays of a given angle are reflected at different outgoing angles. Taking the limit (as the roughness becomes increasingly microscopic) one obtains a flat surface that reflects randomly, i.e., the transition from incoming to outgoing ray is described by a probability kernel (whose form depends on the nature of the microscopic roughness).
We consider two-dimensional optics (a.k.a. billiards) and show that every random reflector on a line that satisfies a necessary measure-preservation condition (well established in the theory of billiards) can be approximated by deterministic...

19.
Disjoinable Lagrangian spheres and dilations - Seidel, Paul
We consider open symplectic manifolds which admit dilations (in the sense previously introduced by Solomon and the author). We obtain restrictions on collections of Lagrangian submanifolds which are pairwise disjoint (or pairwise disjoinable by Hamiltonian isotopies) inside such manifolds. This includes the Milnor fibres of isolated hypersurface singularities which have been stabilized (by adding quadratic terms) sufficiently often.

20.
A fully anisotropic mechanism for formation of trapped surfaces in vacuum - Klainerman, Sergiu; Luk, Jonathan; Rodnianski, Igor
We present a new, fully anisotropic, criterion for formation of trapped surfaces in vacuum. More precisely we provide local conditions on null data, concentrated in a neighborhood of a short null geodesic segment (possibly flat in all other directions) whose future development contains a trapped surface. This extends considerably the previous result of Christodoulou (Monographs in Mathematics. European Mathematical Society, Switzerland, 2009) which required instead a uniform condition along all null geodesic generators. To obtain our result we combine Christodoulou’s mechanism for the formation of a trapped surface with a new deformation process which takes place along incoming null hypersurfaces.