arXiv subjects

Xiaolei Zhang

Publications and source records attributed to Xiaolei Zhang.

105 records · Page 6Linked to original sources

arXiv2006

A Realist Interpretation of the Quantum Measurement Problem

A new, realist interpretation of the quantum measurement processes is given. In this scenario a quantum measurement is a non-equilibrium phase transition in a ``resonant cavity'' formed by the entire physical universe including all its material and energy content. Both the amplitude and the phase of the quantum mechanical wavefunction acquire substantial meaning in this picture, and the probabilistic element is removed from the foundations of quantum mechanics, its apparent presence in the quantum measurement process is viewed as a result of the sensitive dependence on initial/boundary conditions of the non-equilibrium phase transitions in a many degree-of-freedom system. The implications of adopting this realist ontology to the clarification and resolution of lingering issues in the foundations of quantum mechanics, such as wave-particle duality, Heisenberg's uncertainty relation, Schrodinger's Cat paradox, first and higher order coherence of photons and atoms, virtual particles, the existence of commutation relations and quantized behavior, etc., are also presented.

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arXiv2005

A New Ontological View of the Quantum Measurement Problem

A new ontological view of the quantum measurement processes is given, which has bearings on many broader issues in the foundations of quantum mechanics as well. In this scenario a quantum measurement is a non-equilibrium phase transition in a ``resonant cavity'' formed by the entire physical universe including all of its material and energy content. A quantum measurement involves the energy and matter exchange among not only the system being measured and the measuring apparatus but also the global environment of the universe resonant cavity, which together constrain the nature of the phase transition. Strict realism, including strict energy and angular momentum conservation, is recovered in this view of the quantum measurement process beyond the limit set by the uncertainty relations, which are themselves derived from the exact commutation relations for quantum conjugate variables. Both the amplitude and the phase of the quantum mechanical wavefunction acquire substantial meanings in the new ontology, and the probabilistic element is removed from the foundations of quantum mechanics, its apparent presence in the quantum measurement being solely a result of the sensitive dependence on initial/boundary conditions of the phase transitions of a many degree-of-freedom system which is effectively the whole universe. Vacuum fluctuations are viewed as the ``left over'' fluctuations after forming the whole numbers of nonequilibrium resonant modes in the universe cavity. This new view on the quantum processes helps to clarify many puzzles in the foundations of quantum mechanics.

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arXiv2004

Large Scale CO and [CI] emission in the rho Ophiuchi Molecular Cloud

We present a comprehensive study of the rho Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 40'x40' maps in CO(4-3) and [CI](3P1-3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), and pointed observations in the CO(7-6) and [CI](3P2-3P1) lines. Within, smaller spectral line maps of the cloud in CO, CS, HCO+ and their rare isotopomers are made at the Heinrich Hertz Submillimeter Telescope Observatory (HHT) in Arizona. Comparison with far-infrared and submillimeter continuum emission, and near-infrared H2 emission allows clearer determination of the physical and chemical structure of the rho Oph photodissociation region (PDR). The excitation conditions needed to produce the observed HCO+ and [OI] emission directly imply inhomogeneous structure. Strong chemical gradients are observed in HCO+ and CS; the former is ascribed to a local enhancement in the H2 ionization rate, the latter is principally due to shocks. The distribution of [CI] is very similar to C18O, and generally consistent with illumination from the 'far' side of the cloud. A notable exception is found at the the western edge of the cloud, where UV photons create a PDR viewed `edge-on'. The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD147889. In contrast to conclusions of other studies, we find that no non-equilibrium chemistry is needed to enhance the atomic carbon abundance.

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arXiv2004

On the Generation of the Hubble Sequence through an Internal Secular Dynamical Process

The secular evolution process, which slowly transforms the morphology of a galaxy over its lifetime, could naturally account for observed properties of the great majority of physical galaxies if both stellar and gaseous accretion processes are taken into account. As an emerging paradigm for galaxy evolution, its dynamical foundation had been established in the past few years, and its observational consequences are yet to be fully explored. The secular evolution picture provides a coherent framework for understanding the extraordinary regularity and the systematic variation of galaxy properties along the Hubble sequence.

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arXiv2003

Galaxy Scaling Relations as a Result of Secular Evolution

The secular evolution process which slowly transforms the morphology of a given galaxy over its lifetime through mostly internal dynamical mechanisms could naturally account for most of the observed properties of physical galaxies (Zhang 2003a). As an emerging paradigm for galaxy evolution, its dynamical foundations had been established in the past few years (Zhang 1996, 1998, 1999). In this paper, we explore further implications of the secular morphological evolution process in reproducing the well-known scaling relations of galaxies.

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arXiv2003

A Large Scale Energy Source For Feeding ISM Turbulence in Spiral Galaxies

One of the important consequences of a newly discovered secular dynamical evolution process of spiral galaxies (Zhang 1996,1998,1999) is that the orbiting disk matter receives energy injection each time it crosses the spiral density wave crest. This energy injection has been shown to be able to quantitatively explain the observed age-velocity-dispersion relation of the solar neighborhood stars. We demostrate in this paper that similar energy injection into the interstellar medium could serve as the large-scale energy source to continuously power the observed interstellar turbulence and to offset its downward cascade tendency.

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arXiv2003

Secular Evolution of Spiral Galaxies

It is now a well established fact that galaxies undergo significant morphological transformation during their lifetimes, manifesting as an evolution along the Hubble sequence from the late to the early Hubble types. The physical processes commonly believed to be responsible for this observed evolution trend, i.e. the major and minor mergers, as well as gas accretion under a barred potential, though demonstrated applicability to selected types of galaxies, on the whole have failed to reproduce the most important statistical and internal properties of galaxies. The secular evolution mechanism reviewed in this paper has the potential to overcome most of the known difficulties of the existing theories to provide a natural and coherent explanation of the properties of present day as well as high-redshift galaxies.

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arXiv2002

The Fizeau Interferometer Testbed

The Fizeau Interferometer Testbed (FIT) is a collaborative effort between NASA's Goddard Space Flight Center, the Naval Research Laboratory, Sigma Space Corporation, and the University of Maryland. The testbed will be used to explore the principles of and the requirements for the full, as well as the pathfinder, Stellar Imager mission concept. It has a long term goal of demonstrating closed-loop control of a sparse array of numerous articulated mirrors to keep optical beams in phase and optimize interferometric synthesis imaging. In this paper we present the optical and data acquisition system design of the testbed, and discuss the wavefront sensing and control algorithms to be used. Currently we have completed the initial design and hardware procurement for the FIT. The assembly and testing of the Testbed will be underway at Goddard's Instrument Development Lab in the coming months.

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arXiv2002

Wide-field Imaging Interferometry Testbed II: Implementation, Performance, and Plans

The Wide-Field Imaging Interferometry Testbed (WIIT) will provide valuable information for the development of space-based interferometers. This laboratory instrument operates at optical wavelengths and provides the ability to test operational algorithms and techniques for data reduction of interferometric data. Here we present some details of the system design and implementation, discuss the overall performance of the system to date, and present our plans for future development of WIIT. In order to make best use of the interferometric data obtained with this system, it is critical to limit uncertainties within the system and to accurately understand possible sources of error. The WIIT design addresses these criteria through a number of ancillary systems. The use of redundant metrology systems is one of the most important features of WIIT, and provides knowledge of the delay line position to better than 10 nm. A light power detector is used to monitor the brightness of our light sources to ensure that small fluctuations in brightness do not affect overall performance. We have placed temperature sensors on critical components of the instrument, and on the optical table, in order to assess environmental effects on the system. The use of these systems provides us with estimates of the overall system uncertainty, and allows an overall characterization of the results to date. These estimates allow us to proceed forward with WIIT, adding rotation stages for 2-D interferometry. In addition, they suggest possible avenues for system improvement. Funding for WIIT is provided by NASA Headquarters through the ROSS/SARA Program and by the Goddard Space Flight Center through the IR&D Program.

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arXiv2001

Secular Evolution of Disks

It is found that a previously thought-to-be well established result of density wave theory, that there is no interaction between a quasi-stationary spiral density wave and the basic state (i.e. the axisymmetric part) of the galactic disk, is in fact false. When solved as a nonlinear and globally self-consistent problem, the presence of non-axisymmetric patterns such as spirals or bars is shown to lead to significant interaction of the basic state and the density wave, with the net result being the simultaneous acquirement of a quasi-steady wave amplitude and the secular redistribution of disk matter. The secular operation of this dynamical mechanism leads to the evolution of the Hubble type of a galaxy from late to early.

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arXiv2001

The Wide Field Imaging Interferometry Testbed

We are developing a Wide-Field Imaging Interferometry Testbed (WIIT) in support of design studies for NASA's future space interferometry missions, in particular the SPIRIT and SPECS far-infrared/submillimeter interferometers. WIIT operates at optical wavelengths and uses Michelson beam combination to achieve both wide-field imaging and high-resolution spectroscopy. It will be used chiefly to test the feasibility of using a large-format detector array at the image plane of the sky to obtain wide-field interferometry images through mosaicing techniques. In this setup each detector pixel records interferograms corresponding to averaging a particular pointing range on the sky as the optical path length is scanned and as the baseline separation and orientation is varied. The final image is constructed through spatial and spectral Fourier transforms of the recorded interferograms for each pixel, followed by a mosaic/joint-deconvolution procedure of all the pixels. In this manner the image within the pointing range of each detector pixel is further resolved to an angular resolution corresponding to the maximum baseline separation for fringe measurements. We present the motivation for building the testbed, show the optical, mechanical, control, and data system design, and describe the image processing requirements and algorithms. WIIT is presently under construction at NASA's Goddard Space Flight Center.

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arXiv2001

CO (J = 4-3) and [C I] Observations of the Carina Molecular Cloud Complex

We present large area, fully-sampled maps of the Carina molecular cloud complex in the CO (J = 4 - 3) and neutral carbon [CI] 3P1 - 3P0 transitions. These data were obtained using the 1.7 meter diameter Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The maps cover an area of approximately 3 square degrees with a uniform 1' spatial sampling. Analysis of these data, in conjunction with CO (J = 1 - 0) data from the Columbia CO survey and the IRAS HIRES continuum maps for the same region, suggests that the spiral density wave shock associated with the Carina spiral arm may be playing an important role in the formation and dissociation of the cloud complex, as well as in maintaining the internal energy balance of the clouds in this region. Massive stars form at the densest regions of the molecular cloud complex. The winds and outflows associated with these stars have a disrupting effect on the complex and inject mechanical energy into the parent clouds, while the UV radiation from the young stars also heat the parent clouds. The present set of data suggests, however, that massive stars alone may not account for the energetics of the clouds in the Carina region. The details of the data and the correlation among the various data sets hint at the possible role that the spiral density wave shock plays in feeding interstellar turbulence and in heating molecular clouds.

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arXiv2000

The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO)

AST/RO, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 microns, was installed at the South Pole during the 1994-1995 Austral summer. The telescope operates continuously through the Austral winter, and is being used primarily for spectroscopic studies of neutral atomic carbon and carbon monoxide in the interstellar medium of the Milky Way and the Magellanic Clouds. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Four heterodyne receivers, an array receiver, three acousto-optical spectrometers, and an array spectrometer are installed. A Fabry-Perot spectrometer using a bolometric array and a Terahertz receiver are in development. Telescope pointing, focus, and calibration methods as well as the unique working environment and logistical requirements of the South Pole are described.

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arXiv2000

First Detection of Submillimeter [C I] Emission in the Small Magellanic Cloud

We report the first detection of [CI] (3P1-3P0) emission at 609 um in a region of the Small Magellanic Cloud (N27). Environments poor in heavy elements and dust such as the SMC are thought to be dominated by photodissociation regions. This is the lowest metallicity source where submillimeter neutral carbon emission has been detected. Studying the [CI]/CO intensity ratio in several sources spanning more than an order of magnitude in metallicity, Z, we find that the [CI]/CO ratio increases for decreasing Z. The existence of such a trend points to a photodissociation origin for most of neutral carbon in molecular clouds, in agreement with standard PDR models. We also report ISO FIR spectroscopic observations of N27, and use them to derive its physical properties. Comparison between the density and radiation field revealed by FIR diagnostics (n~300-1000 cm-3, Xuv~30-100 Xo) and those derived from millimeter and submillimeter data (n~10^5 cm-3, Xuv<30 Xo) suggests that the FIR lines originate in more diffuse gas, and are perhaps dominated by the interclump medium. Regardless of the cause, analysis of the FIR and mm-submm data produces a discrepancy of two orders of magnitude for the density of this source.

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arXiv2000

Carbon in the N159/N160 Complex of the Large Magellanic Cloud

We present a study of carbon in N159/N160, an HII region complex in the low metallicity Large Magellanic Cloud. We have mapped this region, which comprises four distinct molecular clouds spanning a wide range of star-formation activity, in four transitions: 13CO (1-0) CO (2-1) and (4-3), and [CI] (1-0). Combining these data with existing [CII] observations provides a complete picture of the predominant forms of carbon in the gas phase of the ISM. The new CO (2-1) data show that the complex is immersed in an envelope of extended, low level emission, undetected by previous (1-0) mapping efforts. The CO (2-1)/(1-0) ratio in this envelope is >~3, a value consistent with optically thin CO emission. The envelope is also relatively bright in [CI] and [CII], and calculations show that it is mostly photodissociated: it appears to be translucent (Av<1). Neutral carbon emission in the complex unexpectedly peaks at the quiescent southern cloud (N159S). In the northern portion of the map (the N160 nebula), the HII regions prominent in [CII] correspond to holes in the [CI] distribution. Overall we find that, while the [CII]/CO ratio is enhanced with respect to similar complexes in the Milky Way, the [CI]/CO ratio appears to be similar or reduced.

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