I will explain how any canonical quantum theory can be understood to arise from the compatibility of the statistical geometry of distinguishable observations with the canonical Poisson structure of Hamiltonian dynamics. This geometric perspective offers a novel, background independent non-perturbative formulation of string theory. I will invoke a quantum version of the equivalence principle, which requires both the statistical and Poisson geometries of canonical quantum theory to be fully dynamical quantities. This approach sheds new light on such basic issues of quantum gravity as the nature of observables, the problem of time, and the physics of the vacuum. In particular, the observed numerical smallness of the cosmological constant can be rationalized in this approach.

Since we will be celebrating the 100th birthday of Einstein's Special Relativity next year, in the coming months all physics majors can expect to be asked to explain what Special Relativity is by our friends, relatives, and enemies. In this talk, I will show how this can be done without using ANY equations.

(Note: I will NOT be explaining where E=mc² comes from since that's an equation.)

Semiconductor laser is the fundamental device in modern optoelectronics and photonics. To significantly improve the laser performance, reducing the dimensionality of its active region was proposed. The idea was initially applied to quantum well lasers, which successfully replaced the bulk lasers in most commercial applications. Further improvement in performance is anticipated from lasers with even lower dimensionality, such as quantum wire and especially quantum dot (QD) lasers. The discrete carrier spectrum in QDs appears ideally suitable for lasing generation with low threshold and high temperature stability. Thus, the QD lasers form a novel class of injection lasers that promise radically enhanced operating characteristics. The use of QDs as an active medium in diode lasers is a dramatic example of nanotechnology applied to devices of high commercial interest.

This presentation reviews a comprehensive theory of interband (bipolar) edge-emitting QD lasers developed in original works of the speaker. Current challenges, such as nonuniformity of QDs, parasitic recombination outside QDs, intrinsic nonlinearity of the light-current characteristic and critical sensitivity to structure parameters, will be discussed. Different approaches to optimization of the QD laser design will be outlined. A radically new design strategy will be discussed for the improvement of temperature stability and power characteristics, which is based on suppressing the recombination outside the active region, and can be realized in tunneling-injection- and bandgap-engineered-QD lasers. Other QD device concepts, such as vertical cavity surface-emitting lasers and intraband (unipolar) lasers, will also be shortly discussed.

Despite many years of research, a method to precisely and quantitatively determine cancer disease state remains elusive. Current practice for characterizing solid tumors involves the use of varying systems of tumor grading and staging and thus leaves diagnosis and clinical staging dependent on the experience and skill of the physicians involved. Although numerous disease markers have been identified, no combination of them has yet been found that produces a quantifiable and reliable measure of disease state. In this talk, I examine the potential for using measures of physical complexity as components of a yet to be determined “disease time” vector that would more accurately quantify disease state. These measures are applied to two different types of cancer, progressive rat hepatoma and glioblastoma multiforme (human brain tumors). Recent results will be presented and their implications discussed.

Quantum phase transitions are transitions occurring at zero temperature as a function of a parameter like pressure, chemical composition or magnetic field. In recent years, they have become one of the central paradigms in condensed matter physics, and they are believed to underlie a number of exciting low-temperature phenomena like exotic superconductivity and non-Fermi liquid behavior. At quantum phase transitions, impurities and other forms of quenched disorder can have very peculiar and surprisingly strong effects: In many random quantum Ising magnets the critical point is of infinite-randomness type, i.e., counter-intuitively, the disorder strength increases without limit under coarse graining. This gives rise to activated exponential scaling rather than the usual power-law scaling. In addition, rare disorder fluctuations lead to strong thermodynamic singularities, called the quantum-Griffiths singularities, in the vicinity of the actual transition. In metallic systems the effects of these rare fluctuations can be even stronger, leading to a destruction of the phase transition by smearing.

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. In this talk, we discuss the theory of coherent phonon generation in semiconductors. We first discuss the generation of coherent optic phonons in bulk matereials. We then focus on the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also dicuss the THz radiation emitted from these phonons and possible ways to control it.

We review recent progress in determining physical hadron properties through extrapolation from the relatively large masses where current lattice calculations are possible. In some cases it is possible to obtain remarkably accurate results and we will discuss the strange magnetic moment of the proton as an example. We also reflect on insights concerning how one might most effectively build a model of hadron structure based on the lessons learnt from these studies. Finally we explain how this work can be used in laboratory tests of whether or not the fundamental "constants" of Nature may in fact vary with time.

The identification of the non-baryonic component of the dark matter is one of the most urgent problems in cosmology. This is a very exciting problem for experimentalists, because of the rapid development of innovative new dark matter detection techniques. Weakly interacting massive particles (WIMPs), especially the neutralinos predicted by supersymmetry, are leading candidates to be the dark matter, and the search for these particles is now being intensely pursued in experiments such as CDMS, DAMA, and ZEPLIN. A particularly interesting technique for future experiments involves the use of bubble nucleation in superheated liquids to discriminate against backgrounds from environmental radioactivity. It appears this approach can be used to construct very large detectors, with sensitivity to WIMP- nucleon couplings at least 2-3 orders of magnitude below current limits.

Vibrational lifetimes of hydrogen- and deuterium-related stretch modes in crystalline silicon are measured by high-resolution infrared absorption spectroscopy and pump-probe transient bleaching technique using the Jefferson Lab. Free-Electron Laser (FEL). The lifetimes are found to be extremely dependent on the defect structure, ranging from 2 to 295 ps. Against conventional wisdom, we find that lifetimes of Si-D modes typically are longer than for the corresponding Si-H modes. Our results show that the multi-phonon coupling strength depends strongly on the structure of the defect, i.e., highly distorted interstitial-type defects have larger coupling constants and couple to lower frequency modes than vacancy-type defects. The potential implications of the results on the physics of electronic device degradation are discussed.

Many materials have optical (and infrared and microwave) properties which are entirely different when they are prepared in the form of small particles or composites with characteristic scale much smaller than the wavelength. In this talk, I will discuss work in my group and elsewhere on a wide range of electromagnetic properties of materials which are structured on a nanometer scale. I will give examples from linear optical properties of composite materials; enhanced nonlinear optical response of nanoscale materials; and methods of controlling optical response of composites using magnetic fields and liquid crystals. I will also discuss propagating electromagnetic waves which can travel along chains of gold nanoparticles. Finally, I will discuss recent work on the temperature-dependent optical properties of gold/DNA nanocomposites, and how this relates to their structure.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of three interferometers of km-scale length located in Livingston, LA and Hanford, WA. As the detector construction has been completed and its commissioning is well under way, LIGO has started its quest to detect gravitational waves, extremely small ripples in the fabric of spacetime that originate from the universe's most violent events, such as supernova explosions and black-hole formation. This talk presents the LIGO detectors' performance through the first three science runs, the first scientific results and expected reach for Advanced LIGO. Particular emphasis will be given to the search for unmodeled bursts of gravitational waves, its challenges and the analysis methods we are implementing in our "eyes-wide-open" search.

The applications of semiconductors are primarily based on the tunability of their fundamental properties. In the case of electronics, the key element is the tunability of charge carrier concentrations, and thus conductivity, with applied electrical bias. For eletro-optical devices, such as electro-optical modulators, the basic mechanism is the tunability of excitonic interband transition energy, again with applied bias. With recent developments in the field of spintronics, ferromagnetic semiconductors have been intensely studied. Combining ferromagnetism and semiconducting properties naturally led to many interesting situations for both fundamental understanding of magnetism and device applications. In this talk, I will discuss III-V ferromagnetic semiconductors, and magnetic properties that can be tuned with techniques commonly used for semiconductors.

The elucidation of the double helical structure of DNA fifty years ago initiated extensive research on both the self-recognition capabilities and electronic properties of this biomolecule. Motivated by the prospect of utilizing DNA in nanoelectronic applications, the past several years have sustained a resurgence of both experimental and theoretical reports regarding the electronic pro-perties of DNA. So far, literature reports vary widely in their choice of experimental para-meters and transport mechanisms, and reach a broad range of conclusions, portraying DNA molecules as insulating, semiconducting, or even proximity-induced superconducting. In this talk, I will address experimental conditions under which we have measured charge transport in long lambda-DNA molecules with reasonable reproducibility. Chemical modification to lambda-DNA leading to interesting current-voltage characteristics will be discussed. Lastly, experimental results that highlight the importance of the double helical structure to charge transport through DNA molecules, will be presented.

During the last few years, network approaches have shown great promise as a new tool to analyze and understand complex systems as disparate as the world-wide web, scientific collaborations, sexual contacts, and the power-grid of the Western US. However, in order to fully characterize complex networks, we need to look beyond their topology and incorporate their dynamical aspects. In this seminar I will discuss recent insights into the principles governing the global functional utilization of the cellular metabolism. In particular, the network utilization is highly uneven: while most metabolic reactions have small fluxes, the metabolism's activity is dominated by an interconnected sub-network of reactions with very high fluxes. For the bacteria H. pylori and E. coli and the yeast S. cerevisiae, the metabolism responds to changes in growth conditions by reorganizing the rates of select reactions predominantly within this high-flux backbone. Furthermore, these metabolic networks are organized around a highly lethal, evolutionary conserved metabolic core, which is surrounded by increasingly species-specific and conditionally-active reactions.

It is possible that superstrings, as well as other one-dimensional "branes", could have been produced in the early universe and then expanded to cosmic size today. I discuss the conditions under which this will occur, and the signatures of these strings. Such cosmic superstrings could be the brightest objects visible in gravitational wave astronomy, and might be distinguishable from gauge theory cosmic strings by their network properties.

Over 50 species of vertebrates are known to be sensitive to the earth’s magnetic field. Despite this wealth of information, magneto-reception remains the only sensory ability for which there is no conclusive evidence concerning the underlying transduction mechanism(s). In amphibians and birds, magnetic “compass” orientation is mediated by a light-dependent mechanism that is sensitive to the axis, but not polarity, of the magnetic field and is tuned to a narrow range of intensities. These functional properties are consistent with theoretical models involving a photo-induced radical pair reaction (e.g., Ritz et al. 2000. Biophysics J. 78:707). Radio frequency (RF) fields are predicted to affect magnetically sensitive radical-pair reactions, with the most effective frequencies being those that are in resonance with the hyperfine couplings of the radical pair (~1-40 MHz). The results of recent experiments indicate that magnetic compass orientation in both birds and amphibians is disrupted by broad band oscillating fields (0.1 to 10-15 MHz) with intensities ranging from 85 nT down to as little as ~1 nT, as well as by a 7 MHz (345 nT) field. These effects are specific to the magnetic compass, and depend on the relative alignments of the oscillating and static fields, providing support for a radical pair mechanism and arguing against the involvement of alternative mechanisms involving single-domain or super-paramagnetic particles of biogenic magnetite. Behavioral experiments are now underway using C57 BL/6 strains of inbred mice (1) to investigate the role of a specialized class of photopigments (“cryptochromes”) in magnetoreception, and (2) to determine the threshold, bandwidth and nature of the effects of discrete radio frequencies on the biophysical process underlying the magnetic compass.

Nanostructures commonly involve multiple scales, requiring a variety of classical and quantum simulation techniques. We have been studying semiconductor nanostructures, in which the electron and hole quasiparticles behave quantum mechanically in structures with sizes from 1 nm to 100 nm. While some computational techniques from chemistry and solid state science may be adapted to these new quantum problems, issues such as thermal effects and quantum correlation invite new technical developments. I will describe our path integral quantum Monte Carlo (PIMC) techniques, which use Feynman path integrals to sample the many-body thermal density matrix. These simulations allow quantum many-body simulations of nanostructures including many material details for direct comparison to experiments. The path integral formalism requires us to look at quantum problems differently than textbook approaches, and I'll give some examples, such as the simulating spins, radiative recombination rates, and energy dependent effective masses. Finally, I'll discuss the prospects for PIMC in quantum chemistry, with some tests of these techniques on small atoms and molecules.

It has recently been recognized that a large number of complex networks share statistically similar features, in particular fat-tail, or power-law degree distributions. These include networks from a wide range of areas, such as the Internet, the metabolic pathway of the cell, protein interaction networks, protein conformations, the world-wide-web and the sex-web, to name a few. Naturally arises then the question whether this structural similarity is a coincidence, or there is a universal mechanism, or reason for its occurrence. Here I will introduce the notion of gradient flow networks which can be used to investigate the effects of network topology on the efficiency of flow processing. This will lead to a selection argument for spontaneously evolving networks and it will allow us to show that scale-free structures will ensure efficient processing in the large network limit, consistent with the observations, for example, on the Internet structure. I will then discuss the applicability of this approach to biological networks, and give a simple resolution to the Levinthal paradox of protein folding using the notion of gradient networks.

The study of spin-dependent phenomena in semiconductors forms the subject of intense current research in semiconductor physics, with a particular focus on phenomena that can produce a spin-polarized carrier population. We will present experimental studies of spin-polarized charge transport in narrow-gap semiconductor quantum wells, featuring a high mobility and strong spin-orbit interaction, which offer possibilities for spin manipulation in nanoscale geometries. We describe and demonstrate a method to create spin-polarized ballistic electrons through spin-orbit interaction in a two-dimensional electron system in an InSb/InAlSb heterostructure. The method utilizes intentional spin-flip scattering from a lithographic barrier towards spin manipulation. Furthermore, narrow-gap semiconductors in various geometries exhibit remarkable magnetotransport properties, among which we will discuss localization phenomena in modulated potentials, and magnetoresistance effects in hybrid semiconductor/metal systems. The examples above show that the burgeoning synthesis of spin electronics and nanoscale electronics in novel materials presents a fruitful and increasingly compelling research direction.

Molecular electronics shows great promise in the search to extend the functionality of today?s semiconductor diodes and transistors to the nanoscale. In this context, we present charge transport measurements in two systems, namely DNA and organic semiconductors. These bio-organic and organic materials offer, apart from potential applications, new prospects to correlate chemical properties with ensuing electronic characteristics. In duplex DNA, we show that chemical modifications introduced to the DNA backbone profoundly affect the shape of the current-voltage characteristics. Moreover, single strands transport charge to a much lesser extent than the double helical structure. Hence, the extent of π - π interactions, influenced by modifications to the DNA molecule, play an important role in the electronic properties of DNA. Field-effect geometries are well suited to the electronic study of organic semiconductors. We have utilized a field-effect transistor geometry to study the conventional organic semiconductor pentacene, as well as study, in preliminary experiments, single crystals of thymine, a DNA base. For pentacene, a novel stepped field-effect geometry allows the observation of both electron and hole transport, leading to new insight in charge injection mechanisms in organic materials.

The chemotaxis network in E. coli is the best studied signal-transduction network of any living organism. The network enables E. coli to swim toward attractants such as amino acids or sugars, and away from repellents. The cells perform chemotaxis by detecting temporal changes in their chemical environment and transducing this information into a decision to swim straight or change direction (tumble). The system is remarkable for its high sensitivity and robust adaptation over a wide range of external chemical concentrations. Motivated by recent in vivo FRET studies [1], we model the chemosensing system as a mixed array of interacting receptors akin to an Ising lattice. Our results support and extend the robust-adaptation model of Barkai and Leibler [2], in which receptor complexes function as two-level systems.

1. V Sourjik and HC Berg, PNAS 99(1), 123-127 (2002); Ibid, Nature 428(6981), 437-441 (2004).
2. N Barkai and S Leibler, Nature 387(6636), 913-917(1997).

Network approaches have recently shown great promise as a tool to analyze and understand complex systems as disparate as the world-wide web, scientific collaborations, sexual contacts, and the power-grid of the Western US. In this seminar, I will present recent results on the interplay between structure and function in cellular metabolic networks. In particular, the network utilization is highly uneven: while most metabolic reactions have small fluxes, the metabolism's activity is dominated by an interconnected sub-network of reactions with very large fluxes. For the bacteria H. pylori and E. coli and the yeast S. cerevisiae, the metabolism responds to changes in growth conditions by reorganizing the rates of select reactions predominantly within this high-flux backbone while keeping the overall distribution of metabolic fluxes unchanged. Additionally, these metabolic networks are organized around a highly lethal, synchronized, evolutionary conserved metabolic core.

With the advances in material theories and computational techniques, first-principles simulations have become established as a complementary tool to experiments in the design and characterization of materials.

In this talk, I will describe some of our recent work on understanding and predicting material properties from first principles. I will discuss designing new semiconductors for optoelectronic applications, novel boron-based compound nanotubes that offer interesting and useful electronic properties, and electronic and structural properties of the newly discovered CoO₂ based superconductors.

Modern day electronics is reaching dimensions where quantum effects are becoming dominant. Aside from exciting device possibilities, nanoscale systems provide ideal laboratories for studying interesting physics due to the interplay between localization by strong charging and hybridization by tunable coupling with contacts or different molecular units. I will present a "bottom-up", atomistic, unified view of quantum transport through generic nanoscale systems such as molecules, nanotubes, nanowires, or spintronic elements. By combining electronic structure calculations with quantum transport based on nonequilibrium Green's functions, I will explore several experimentally measured current-voltage (I-V) characteristics of molecules on metal substrates. In this context, I will discuss the incorporation of electron-electron and electron-vibronic correlations in transport calculations. In the last part of my talk, I will extend these concepts to hybrid systems such as molecular switches or sensors on a silicon surface or a nanowire. Modeling hybrid systems is challenging, requiring schemes for formally combining quantum chemistry and surface physics, including surface reconstruction, band-bending and surface states. The upshot is that experiments on well-characterized silicon surfaces provide meaningful comparison standards for theory. I will show that several scanning tunneling spectra of buckyballs on silicon can be quantitatively explained simply by invoking variations in their bonding geometries, making it possible to deconstruct the role of contacts in conduction. Such an atomistic understanding of transport allows us to anticipate novel functionalities. As an example, I will outline our prediction of a novel doping-dependent one-sided negative differential resistance (NDR) in the I-Vs of molecules such as styrene or TEMPO on silicon, for which there now seems to be considerable experimental support. I will end my talk with some thoughts on where the field of nanoscale electronics is heading.

Semiconducting, single walled carbon nanotubes that emit light in near infrared spectral regime have great potential for a wide variety of optoelectronic applications. A detail understanding on intrinsic nature of fundamental optical excitation in nanotubes is essential to fully exploit this potential. Here in this talk, I will present the first single nanotube, low temperature photoluminescence (PL) and PL excitation (PLE) studies conducted to attain this understanding. In our PL spectra, we observed sharp (from sub-meV to a few meV), symmetric spectral lines of 1D excitons together with asymmetric peaks that show strong thermal broadening only on their steep high-energy sides. The asymmetric shape as well as the strange thermal behavior of these peaks could be explained in terms of the Fermi edge singularity effect that arises from many-body interaction of carriers that are unintentionally introduced into some nanotubes during sample preparation. Our PLE spectra show features related to direct excitations to the second electronic states as well as those related to strong phonon-assisted transitions involving excitations of one or more phonon modes together with the first electronic state. Surprisingly, these phonon replicas are as intense as the direct excitations of the second electronic state, which involve no phonons. In contrast to a small width of emission lines, most of the PLE features are characterized by tens of meV linewidths. All of these observations suggest that strong electron-phonon coupling gives rise to a significantly more complex structure of nanotube absorption spectra than it is assumed in a simple picture of optical transitions dominated by singularities in the one-dimensional energy spectrum.

Galactic nuclei are energetic domains which harbor large stellar populations and whose dynamics are often dominated by the presence of massive black holes. Observing nuclear star clusters in the electromagnetic spectrum is possible, but resolving individual stellar encounters with the central black hole is difficult at best. By contrast, low frequency gravitational waves generated by the close encounters of small objects with massive black holes will propogate freely out of galactic nuclei, carry information about the region very near to the horizon of the black hole, and should be visible to the proposed LISA observatory.

In this talk I will discuss ongoing efforts to understand the dynamics of the innermost galactic stellar population, convince you that "zooming" is a technical term, and describe some of our early predictions of what we should be able to observe with a space-based gravitational wave observatory.

Nanoscale materials have large surface-to-volume ratio compared to their bulk counterparts. The surfaces provide not only the physical boundaries for these materials, they could also be the dominating factor in determining the materials properties. Surfaces, where defects or traps exist, can be detrimental to the nanoscale materials properties, through effects such as optical quenching and enhanced scattering of free carriers. Additionally, appropriate interfacing of these nanoscale systems to the macroscopic world would enable their physical properties to be more readily measured and subsequently exploited. A fundamental understanding of surfaces and interfaces is critical for advancing nanoscience and technology. In this talk I will demonstrate the importance of surface and interface to the device characteristics through the molecular memory circuits we are developing. In the first example, a critical aspect in molecular electronics, the molecule-electrode interface effect will be discussed in the context of our study of electron transport in single molecule transistors. In our devices, Rotaxane molecules bridge nanometer scale Pt break-junctions. The result indicates the metal-molecule interface dominated the electron transport, effectively masking most of the molecular electronic structure. The second example concerns the passivation of Si(111) surface with methyl groups. Low temperature STM reveals a highly ordered structure that is commensurate with an unreconstructed 1x1 silicon surface. Furthermore, at 4.7 K, H atoms on the methyl group can be resolved as well as C-H bond orientation in each methyl group relative to the underlying silicon lattice. The implications of this study for the passivation and functionalization of Si nanowires for applications ranging from nanoscale logic circuits to label-free biomolecule sensing will be discussed.

During the past century our understanding of the structure and evolution of the universe has increased dramatically. From humble beginnings, astronomers have surveyed the vast reaches and contents of the universe, demonstrating that it is expanding, and pinned down a reasonably precise value for its age. Along the way, Albert Einstein's contributions have played a major role. His General Theory of Relativity set the stage for our exploration, and it now appears he was onto something that many researchers in his day, not even he, could accept. Recent observations have led many to conclude that the expansion of the universe is accelerating, whereas deceleration would be expected if ordinary matter dominates the contents of the universe. This surprising result implies that about 73% of the energy in the universe is in a form we know almost nothing about, beyond the name we have placed on it: "dark energy". Another 23% is "dark matter," known only from its attractive gravitational effects. Only about 4% of the content of our universe is ordinary matter, the stuff in the periodic table of elements! Interestingly, the basic story behind these conclusions is understandable using very simple physics, as this talk will show.

The physics of semiconductor junctions makes it possible to study the interplay of magnetism, superconductivity, and nonequilibrium spin. The motivation for examining these structures ranges from fundamental issues involving spin degrees of freedom to a wide range of spintronic applications [1]. This talk will focus on two topics. We first discuss spin-polarized transport in junctions with superconductors which were recently used for the first direct measurement of the spin polarization in a novel class of ferromagnetic semiconductors [1]. We next develop a theory of spin transport in inhomogeneously doped magnetic semiconductors. Using this theory we predict that a nonequilibrium spin leads to the spin-voltaic effect [2], a spin-analog of the photo-voltaic effect. The direction of the charge current, which can even flow at no applied bias, can be switched by reversal of the equilibrium magnetization or by reversal of the polarization of the injected spin. We discuss some implications of the spin-voltaic effect for spin switching and spin-controlled amplification [3].

Two-dimensional (2D) electron and hole gas systems have been a major platform for basic research in condensed-matter physics, as well as high performance electrical and optical devices. In this talk, I will discuss a one-dimensional (1D) hole gas system based on a germanium/silicon core/shell nanowire heterostructure. At room temperature, hole accumulation in the intrinsic germanium channel was observed due to the valence band offset at the Ge/Si interface. At low temperatures, conductance quantization at values close to that expected of a ballistic conductor was observed, and was attributed to the long mean free path in the hole gas and confinement of the hole gas in the radial direction. These effects showed little temperature dependence and suggested that transport in these small diameter nanowires is ballistic even at room temperature. Field-effect transistors made from these nanowires exhibited on current and transconductance among the highest reported in nanostructures, with a 100 percent device yield. The demonstration of a 1D hole gas in a flexible nanowire heterostructure opens up a number of possibilities for investigating quantum phenomena in 1D systems, as well as applications in nanoelectronics. Other nanowire heterostructures such as nonvolatile memories and metallic interconnects will also be discussed.

Everything you ever wanted to know about the history of physics (but were afraid to ask), from the viewpoint of Albert Einstein's scientific achievements.

Sequence alignment is the most prevalent computational method for functionally annotating newly found genes. It remains a crucial problem in the application of sequence alignment to distinguish between biologically significant and spurious similarities between the query sequence and a database sequence. Currently the necessity of providing a quantitative significance measure called a p-value limits database searches to very few predefined parameter choices for the sequence alignment algorithms. These parameter settings are not optimal for arbitrary query sequences. I will discuss several theoretical approaches based on the statistical physics of surface growth that allow rapid calculation of p-values for arbitrary sequence alignments.

In this talk we review some of the main issues raised by the problem of quantum gravity as well as the progress made within a particular program, called loop quantum gravity, which focuses on the importance of background independence and the construction of quantum spacetime. We describe the main aspects of loop quantum gravity and spin foam models as well as some concrete results they lead to.